Titanium alloys are essential structural materials for a wide variety of applications, from aerospace and energy infrastructure to biomedical equipment. But like most metals, optimizing their properties tends to involve a tradeoff between two key characteristics: strength and ductility. Stronger materials tend to be less deformable, and deformable materials tend to be mechanically weak.

Now, researchers at MIT, collaborating with researchers at ATI Specialty Materials, have discovered an approach for creating new titanium alloys that can exceed this historical tradeoff, leading to new alloys with exceptional combinations of strength and ductility, which might lead to new applications.

The findings are described in the journal Advanced Materials, in a paper by Shaolou Wei ScD ’22, Professor C. Cem Tasan, postdoc Kyung-Shik Kim, and John Foltz from ATI Inc. The improvements, the team says, arise from tailoring the chemical composition and the lattice structure of the alloy, while also adjusting the processing techniques used to produce the material at industrial scale.

Titanium alloys have been important because of their exceptional mechanical properties, corrosion resistance, and light weight when compared to steels for example. Through careful selection of the alloying elements and their relative proportions, and of the way the material is processed, “you can create various different structures, and this creates a big playground for you to get good property combinations, both for cryogenic and elevated temperatures,” Tasan says.

But that big assortment of possibilities in turn requires a way to guide the selections to produce a material that meets the specific needs of a particular application. The analysis and experimental results described in the new study provide that guidance.

The structure of titanium alloys, all the way down to atomic scale, governs their properties, Tasan explains. And in some titanium alloys, this structure is even more complex, made up of two different intermixed phases, known as the alpha and beta phases.

“The key strategy in this design approach is to take considerations of different scales,” he says. “One scale is the structure of individual crystal. For example, by choosing the alloying elements carefully, you can have a more ideal crystal structure of the alpha phase that enables particular deformation mechanisms. The other scale is the polycrystal scale, that involves interactions of the alpha and beta phases. So, the approach that’s followed here involves design considerations for both.”

In addition to choosing the right alloying materials and proportions, steps in the processing turned out to play an important role. A technique called cross-rolling is another key to achieving the exceptional combination of strength and ductility, the team found.

Working together with ATI researchers, the team tested a variety of alloys under a scanning electron microscope as they were being deformed, revealing details of how their microstructures respond to external mechanical load. They found that there was a particular set of parameters — of composition, proportions, and processing method — that yielded a structure where the alpha and beta phases shared the deformation uniformly, mitigating the cracking tendency that is likely to occur between the phases when they respond differently. “The phases deform in harmony,” Tasan says. This cooperative response to deformation can yield a superior material, they found.

“We looked at the structure of the material to understand these two phases and their morphologies, and we looked at their chemistries by carrying out local chemical analysis at the atomic scale. We adopted a wide variety of techniques to quantify various properties of the material across multiple length scales, says Tasan, who is the POSCO Professor of Materials Science and Engineering and an associate professor of metallurgy. “When we look at the overall properties” of the titanium alloys produced according to their system, “the properties are really much better than comparable alloys.”

This was industry-supported academic research aimed at proving design principles for alloys that can be commercially produced at scale, according to Tasan. “What we do in this collaboration is really toward a fundamental understanding of crystal plasticity,” he says. “We show that this design strategy is validated, and we show scientifically how it works,” he adds, noting that there remains significant room for further improvement.

As for potential applications of these findings, he says, “for any aerospace application where an improved combination of strength and ductility are useful, this kind of invention is providing new opportunities.”

The work was supported by ATI Specialty Rolled Products and used facilities of MIT.nano and the Center for Nanoscale Systems at Harvard University.

MIT faculty members Nancy Kanwisher, Robert Langer, and Sara Seager are among eight researchers worldwide to receive this year’s Kavli Prizes.

A partnership among the Norwegian Academy of Science and Letters, the Norwegian Ministry of Education and Research, and the Kavli Foundation, the Kavli Prizes are awarded every two years to “honor scientists for breakthroughs in astrophysics, nanoscience and neuroscience that transform our understanding of the big, the small and the complex.” The laureates in each field will share $1 million.

Understanding recognition of faces

Nancy Kanwisher, the Walter A Rosenblith Professor of Brain and Cognitive Sciences and McGovern Institute for Brain Research investigator, has been awarded the 2024 Kavli Prize in Neuroscience with Doris Tsao, professor in the Department of Molecular and Cell Biology at the University of California at Berkeley, and Winrich Freiwald, the Denise A. and Eugene W. Chinery Professor at the Rockefeller University.

Kanwisher, Tsao, and Freiwald discovered a specialized system within the brain to recognize faces. Their discoveries have provided basic principles of neural organization and made the starting point for further research on how the processing of visual information is integrated with other cognitive functions.

Kanwisher was the first to prove that a specific area in the human neocortex is dedicated to recognizing faces, now called the fusiform face area. Using functional magnetic resonance imaging, she found individual differences in the location of this area and devised an analysis technique to effectively localize specialized functional regions in the brain. This technique is now widely used and applied to domains beyond the face recognition system. 

Integrating nanomaterials for biomedical advances

Robert Langer, the David H. Koch Institute Professor, has been awarded the 2024 Kavli Prize in Nanoscience with Paul Alivisatos, president of the University of Chicago and John D. MacArthur Distinguished Service Professor in the Department of Chemistry, and Chad Mirkin, professor of chemistry at Northwestern University.

Langer, Alivisatos, and Mirkin each revolutionized the field of nanomedicine by demonstrating how engineering at the nano scale can advance biomedical research and application. Their discoveries contributed foundationally to the development of therapeutics, vaccines, bioimaging, and diagnostics.

Langer was the first to develop nanoengineered materials that enabled the controlled release, or regular flow, of drug molecules. This capability has had an immense impact for the treatment of a range of diseases, such as aggressive brain cancer, prostate cancer, and schizophrenia. His work also showed that tiny particles, containing protein antigens, can be used in vaccination, and was instrumental in the development of the delivery of messenger RNA vaccines. 

Searching for life beyond Earth

Sara Seager, the Class of 1941 Professor of Planetary Sciences in the Department of Earth, Atmospheric and Planetary Sciences and a professor in the departments of Physics and of Aeronautics and Astronautics, has been awarded the 2024 Kavli Prize in Astrophysics along with David Charbonneau, the Fred Kavli Professor of Astrophysics at Harvard University.

Seager and Charbonneau are recognized for discoveries of exoplanets and the characterization of their atmospheres. They pioneered methods for the detection of atomic species in planetary atmospheres and the measurement of their thermal infrared emission, setting the stage for finding the molecular fingerprints of atmospheres around both giant and rocky planets. Their contributions have been key to the enormous progress seen in the last 20 years in the exploration of myriad exoplanets. 

Kanwisher, Langer, and Seager bring the number of all-time MIT faculty recipients of the Kavli Prize to eight. Prior winners include Rainer Weiss in astrophysics (2016), Alan Guth in astrophysics (2014), Mildred Dresselhaus in nanoscience (2012), Ann Graybiel in neuroscience (2012), and Jane Luu in astrophysics (2012).

To save on fuel and reduce aircraft emissions, engineers are looking to build lighter, stronger airplanes out of advanced composites. These engineered materials are made from high-performance fibers that are embedded in polymer sheets. The sheets can be stacked and pressed into one multilayered material and made into extremely lightweight and durable structures.

But composite materials have one main vulnerability: the space between layers, which is typically filled with polymer “glue” to bond the layers together. In the event of an impact or strike, cracks can easily spread between layers and weaken the material, even though there may be no visible damage to the layers themselves. Over time, as these hidden cracks spread between layers, the composite could suddenly crumble without warning.

Now, MIT engineers have shown they can prevent cracks from spreading between composite’s layers, using an approach they developed called “nanostitching,” in which they deposit chemically grown microscopic forests of carbon nanotubes between composite layers. The tiny, densely packed fibers grip and hold the layers together, like ultrastrong Velcro, preventing the layers from peeling or shearing apart.

In experiments with an advanced composite known as thin-ply carbon fiber laminate, the team demonstrated that layers bonded with nanostitching improved the material’s resistance to cracks by up to 60 percent, compared with composites with conventional polymers. The researchers say the results help to address the main vulnerability in advanced composites.

“Just like phyllo dough flakes apart, composite layers can peel apart because this interlaminar region is the Achilles’ heel of composites,” says Brian Wardle, professor of aeronautics and astronautics at MIT. “We’re showing that nanostitching makes this normally weak region so strong and tough that a crack will not grow there. So, we could expect the next generation of aircraft to have composites held together with this nano-Velcro, to make aircraft safer and have greater longevity.”

Wardle and his colleagues have published their results today in the journal ACS Applied Materials and Interfaces. The study’s first author is former MIT visiting graduate student and postdoc Carolina Furtado, along with Reed Kopp, Xinchen Ni, Carlos Sarrado, Estelle Kalfon-Cohen, and Pedro Camanho.

Forest growth

At MIT, Wardle is director of the necstlab (pronounced “next lab”), where he and his group first developed the concept for nanostitching. The approach involves “growing” a forest of vertically aligned carbon nanotubes — hollow fibers of carbon, each so small that tens of billions of the the nanotubes can stand in an area smaller than a fingernail. To grow the nanotubes, the team used a process of chemical vapor deposition to react various catalysts in an oven, causing carbon to settle onto a surface as tiny, hair-like supports. The supports are eventually removed, leaving behind a densely packed forest of microscopic, vertical rolls of carbon.

The lab has previously shown that the nanotube forests can be grown and adhered to layers of composite material, and that this fiber-reinforced compound improves the material’s overall strength. The researchers had also seen some signs that the fibers can improve a composite’s resistance to cracks between layers.

In their new study, the engineers took a more in-depth look at the between-layer region in composites to test and quantify how nanostitching would improve the region’s resistance to cracks. In particular, the study focused on an advanced composite material known as thin-ply carbon fiber laminates.

“This is an emerging composite technology, where each layer, or ply, is about 50 microns thin, compared to standard composite plies that are 150 microns, which is about the diameter of a human hair. There’s evidence to suggest they are better than standard-thickness composites. And we wanted to see whether there might be synergy between our nanostitching and this thin-ply technology, since it could lead to more resilient aircraft, high-value aerospace structures, and space and military vehicles,” Wardle says.

Velcro grip

The study’s experiments were led by Carolina Furtado, who joined the effort as part of the MIT-Portugal program in 2016, continued the project as a postdoc, and is now a professor at the University of Porto in Portugal, where her research focuses on modeling cracks and damage in advanced composites.

In her tests, Furtado used the group’s techniques of chemical vapor deposition to grow densely packed forests of vertically aligned carbon nanotubes. She also fabricated samples of thin-ply carbon fiber laminates. The resulting advanced composite was about 3 millimeters thick and comprised 60 layers, each made from stiff, horizontal fibers embedded in a polymer sheet.

She transferred and adhered the nanotube forest in between the two middle layers of the composite, then cooked the material in an autoclave to cure. To test crack resistance, the researchers placed a crack on the edge of the composite, right at the start of the region between the two middle layers.

“In fracture testing, we always start with a crack because we want to test whether and how far the crack will spread,” Furtado explains.

The researchers then placed samples of the nanotube-reinforced composite in an experimental setup to test their resilience to “delamination,” or the potential for layers to separate.

“There’s lots of ways you can get precursors to delamination, such as from impacts, like tool drop, bird strike, runway kickup in aircraft, and there could be almost no visible damage, but internally it has a delamination,” Wardle says. “Just like a human, if you’ve got a hairline fracture in a bone, it’s not good. Just because you can’t see it doesn’t mean it’s not impacting you. And damage in composites is hard to inspect.”

To examine nanostitching’s potential to prevent delamination, the team placed their samples in a setup to test three delamination modes, in which a crack could spread through the between-layer region and peel the layers apart or cause them to slide against each other, or do a combination of both. All three of these modes are the most common ways in which conventional composites can internally flake and crumble.

The tests, in which the researchers precisely measured the force required to peel or shear the composite’s layers, revealed that the nanostitched held fast, and the initial crack that the researchers made was unable to spread further between the layers. The nanostitched samples were up to 62 percent tougher and more resistant to cracks, compared with the same advanced composite material that was held together with conventional polymers.

“This is a new composite technology, turbocharged by our nanotubes,” Wardle says.

“The authors have demonsrated that thin plies and nanostitching together have made significant increase in toughness,” says Stephen Tsai, emeritus professor of aeronautics and astronautics at Stanford University. “Composites are degraded by their weak interlaminar strength. Any improvement shown in this work will increase the design allowable, and reduce the weight and cost of composites technology.”

The researchers envision that any vehicle or structure that incorporates conventional composites could be made lighter, tougher, and more resilient with nanostitching.

“You could have selective reinforcement of problematic areas, to reinforce holes or bolted joints, or places where delamination might happen,” Furtado says. “This opens a big window of opportunity.”

Titanium alloys are essential structural materials for a wide variety of applications, from aerospace and energy infrastructure to biomedical equipment. But like most metals, optimizing their properties tends to involve a tradeoff between two key characteristics: strength and ductility. Stronger materials tend to be less deformable, and deformable materials tend to be mechanically weak.

Now, researchers at MIT, collaborating with researchers at ATI Specialty Materials, have discovered an approach for creating new titanium alloys that can exceed this historical tradeoff, leading to new alloys with exceptional combinations of strength and ductility, which might lead to new applications.

The findings are described in the journal Advanced Materials, in a paper by Shaolou Wei ScD ’22, Professor C. Cem Tasan, postdoc Kyung-Shik Kim, and John Foltz from ATI Inc. The improvements, the team says, arise from tailoring the chemical composition and the lattice structure of the alloy, while also adjusting the processing techniques used to produce the material at industrial scale.

Titanium alloys have been important because of their exceptional mechanical properties, corrosion resistance, and light weight when compared to steels for example. Through careful selection of the alloying elements and their relative proportions, and of the way the material is processed, “you can create various different structures, and this creates a big playground for you to get good property combinations, both for cryogenic and elevated temperatures,” Tasan says.

But that big assortment of possibilities in turn requires a way to guide the selections to produce a material that meets the specific needs of a particular application. The analysis and experimental results described in the new study provide that guidance.

The structure of titanium alloys, all the way down to atomic scale, governs their properties, Tasan explains. And in some titanium alloys, this structure is even more complex, made up of two different intermixed phases, known as the alpha and beta phases.

“The key strategy in this design approach is to take considerations of different scales,” he says. “One scale is the structure of individual crystal. For example, by choosing the alloying elements carefully, you can have a more ideal crystal structure of the alpha phase that enables particular deformation mechanisms. The other scale is the polycrystal scale, that involves interactions of the alpha and beta phases. So, the approach that’s followed here involves design considerations for both.”

In addition to choosing the right alloying materials and proportions, steps in the processing turned out to play an important role. A technique called cross-rolling is another key to achieving the exceptional combination of strength and ductility, the team found.

Working together with ATI researchers, the team tested a variety of alloys under a scanning electron microscope as they were being deformed, revealing details of how their microstructures respond to external mechanical load. They found that there was a particular set of parameters — of composition, proportions, and processing method — that yielded a structure where the alpha and beta phases shared the deformation uniformly, mitigating the cracking tendency that is likely to occur between the phases when they respond differently. “The phases deform in harmony,” Tasan says. This cooperative response to deformation can yield a superior material, they found.

“We looked at the structure of the material to understand these two phases and their morphologies, and we looked at their chemistries by carrying out local chemical analysis at the atomic scale. We adopted a wide variety of techniques to quantify various properties of the material across multiple length scales, says Tasan, who is the POSCO Professor of Materials Science and Engineering and an associate professor of metallurgy. “When we look at the overall properties” of the titanium alloys produced according to their system, “the properties are really much better than comparable alloys.”

This was industry-supported academic research aimed at proving design principles for alloys that can be commercially produced at scale, according to Tasan. “What we do in this collaboration is really toward a fundamental understanding of crystal plasticity,” he says. “We show that this design strategy is validated, and we show scientifically how it works,” he adds, noting that there remains significant room for further improvement.

As for potential applications of these findings, he says, “for any aerospace application where an improved combination of strength and ductility are useful, this kind of invention is providing new opportunities.”

The work was supported by ATI Specialty Rolled Products and used facilities of MIT.nano and the Center for Nanoscale Systems at Harvard University.

MIT faculty members Nancy Kanwisher, Robert Langer, and Sara Seager are among eight researchers worldwide to receive this year’s Kavli Prizes.

A partnership among the Norwegian Academy of Science and Letters, the Norwegian Ministry of Education and Research, and the Kavli Foundation, the Kavli Prizes are awarded every two years to “honor scientists for breakthroughs in astrophysics, nanoscience and neuroscience that transform our understanding of the big, the small and the complex.” The laureates in each field will share $1 million.

Understanding recognition of faces

Nancy Kanwisher, the Walter A Rosenblith Professor of Brain and Cognitive Sciences and McGovern Institute for Brain Research investigator, has been awarded the 2024 Kavli Prize in Neuroscience with Doris Tsao, professor in the Department of Molecular and Cell Biology at the University of California at Berkeley, and Winrich Freiwald, the Denise A. and Eugene W. Chinery Professor at the Rockefeller University.

Kanwisher, Tsao, and Freiwald discovered a specialized system within the brain to recognize faces. Their discoveries have provided basic principles of neural organization and made the starting point for further research on how the processing of visual information is integrated with other cognitive functions.

Kanwisher was the first to prove that a specific area in the human neocortex is dedicated to recognizing faces, now called the fusiform face area. Using functional magnetic resonance imaging, she found individual differences in the location of this area and devised an analysis technique to effectively localize specialized functional regions in the brain. This technique is now widely used and applied to domains beyond the face recognition system. 

Integrating nanomaterials for biomedical advances

Robert Langer, the David H. Koch Institute Professor, has been awarded the 2024 Kavli Prize in Nanoscience with Paul Alivisatos, president of the University of Chicago and John D. MacArthur Distinguished Service Professor in the Department of Chemistry, and Chad Mirkin, professor of chemistry at Northwestern University.

Langer, Alivisatos, and Mirkin each revolutionized the field of nanomedicine by demonstrating how engineering at the nano scale can advance biomedical research and application. Their discoveries contributed foundationally to the development of therapeutics, vaccines, bioimaging, and diagnostics.

Langer was the first to develop nanoengineered materials that enabled the controlled release, or regular flow, of drug molecules. This capability has had an immense impact for the treatment of a range of diseases, such as aggressive brain cancer, prostate cancer, and schizophrenia. His work also showed that tiny particles, containing protein antigens, can be used in vaccination, and was instrumental in the development of the delivery of messenger RNA vaccines. 

Searching for life beyond Earth

Sara Seager, the Class of 1941 Professor of Planetary Sciences in the Department of Earth, Atmospheric and Planetary Sciences and a professor in the departments of Physics and of Aeronautics and Astronautics, has been awarded the 2024 Kavli Prize in Astrophysics along with David Charbonneau, the Fred Kavli Professor of Astrophysics at Harvard University.

Seager and Charbonneau are recognized for discoveries of exoplanets and the characterization of their atmospheres. They pioneered methods for the detection of atomic species in planetary atmospheres and the measurement of their thermal infrared emission, setting the stage for finding the molecular fingerprints of atmospheres around both giant and rocky planets. Their contributions have been key to the enormous progress seen in the last 20 years in the exploration of myriad exoplanets. 

Kanwisher, Langer, and Seager bring the number of all-time MIT faculty recipients of the Kavli Prize to eight. Prior winners include Rainer Weiss in astrophysics (2016), Alan Guth in astrophysics (2014), Mildred Dresselhaus in nanoscience (2012), Ann Graybiel in neuroscience (2012), and Jane Luu in astrophysics (2012).

To save on fuel and reduce aircraft emissions, engineers are looking to build lighter, stronger airplanes out of advanced composites. These engineered materials are made from high-performance fibers that are embedded in polymer sheets. The sheets can be stacked and pressed into one multilayered material and made into extremely lightweight and durable structures.

But composite materials have one main vulnerability: the space between layers, which is typically filled with polymer “glue” to bond the layers together. In the event of an impact or strike, cracks can easily spread between layers and weaken the material, even though there may be no visible damage to the layers themselves. Over time, as these hidden cracks spread between layers, the composite could suddenly crumble without warning.

Now, MIT engineers have shown they can prevent cracks from spreading between composite’s layers, using an approach they developed called “nanostitching,” in which they deposit chemically grown microscopic forests of carbon nanotubes between composite layers. The tiny, densely packed fibers grip and hold the layers together, like ultrastrong Velcro, preventing the layers from peeling or shearing apart.

In experiments with an advanced composite known as thin-ply carbon fiber laminate, the team demonstrated that layers bonded with nanostitching improved the material’s resistance to cracks by up to 60 percent, compared with composites with conventional polymers. The researchers say the results help to address the main vulnerability in advanced composites.

“Just like phyllo dough flakes apart, composite layers can peel apart because this interlaminar region is the Achilles’ heel of composites,” says Brian Wardle, professor of aeronautics and astronautics at MIT. “We’re showing that nanostitching makes this normally weak region so strong and tough that a crack will not grow there. So, we could expect the next generation of aircraft to have composites held together with this nano-Velcro, to make aircraft safer and have greater longevity.”

Wardle and his colleagues have published their results today in the journal ACS Applied Materials and Interfaces. The study’s first author is former MIT visiting graduate student and postdoc Carolina Furtado, along with Reed Kopp, Xinchen Ni, Carlos Sarrado, Estelle Kalfon-Cohen, and Pedro Camanho.

Forest growth

At MIT, Wardle is director of the necstlab (pronounced “next lab”), where he and his group first developed the concept for nanostitching. The approach involves “growing” a forest of vertically aligned carbon nanotubes — hollow fibers of carbon, each so small that tens of billions of the the nanotubes can stand in an area smaller than a fingernail. To grow the nanotubes, the team used a process of chemical vapor deposition to react various catalysts in an oven, causing carbon to settle onto a surface as tiny, hair-like supports. The supports are eventually removed, leaving behind a densely packed forest of microscopic, vertical rolls of carbon.

The lab has previously shown that the nanotube forests can be grown and adhered to layers of composite material, and that this fiber-reinforced compound improves the material’s overall strength. The researchers had also seen some signs that the fibers can improve a composite’s resistance to cracks between layers.

In their new study, the engineers took a more in-depth look at the between-layer region in composites to test and quantify how nanostitching would improve the region’s resistance to cracks. In particular, the study focused on an advanced composite material known as thin-ply carbon fiber laminates.

“This is an emerging composite technology, where each layer, or ply, is about 50 microns thin, compared to standard composite plies that are 150 microns, which is about the diameter of a human hair. There’s evidence to suggest they are better than standard-thickness composites. And we wanted to see whether there might be synergy between our nanostitching and this thin-ply technology, since it could lead to more resilient aircraft, high-value aerospace structures, and space and military vehicles,” Wardle says.

Velcro grip

The study’s experiments were led by Carolina Furtado, who joined the effort as part of the MIT-Portugal program in 2016, continued the project as a postdoc, and is now a professor at the University of Porto in Portugal, where her research focuses on modeling cracks and damage in advanced composites.

In her tests, Furtado used the group’s techniques of chemical vapor deposition to grow densely packed forests of vertically aligned carbon nanotubes. She also fabricated samples of thin-ply carbon fiber laminates. The resulting advanced composite was about 3 millimeters thick and comprised 60 layers, each made from stiff, horizontal fibers embedded in a polymer sheet.

She transferred and adhered the nanotube forest in between the two middle layers of the composite, then cooked the material in an autoclave to cure. To test crack resistance, the researchers placed a crack on the edge of the composite, right at the start of the region between the two middle layers.

“In fracture testing, we always start with a crack because we want to test whether and how far the crack will spread,” Furtado explains.

The researchers then placed samples of the nanotube-reinforced composite in an experimental setup to test their resilience to “delamination,” or the potential for layers to separate.

“There’s lots of ways you can get precursors to delamination, such as from impacts, like tool drop, bird strike, runway kickup in aircraft, and there could be almost no visible damage, but internally it has a delamination,” Wardle says. “Just like a human, if you’ve got a hairline fracture in a bone, it’s not good. Just because you can’t see it doesn’t mean it’s not impacting you. And damage in composites is hard to inspect.”

To examine nanostitching’s potential to prevent delamination, the team placed their samples in a setup to test three delamination modes, in which a crack could spread through the between-layer region and peel the layers apart or cause them to slide against each other, or do a combination of both. All three of these modes are the most common ways in which conventional composites can internally flake and crumble.

The tests, in which the researchers precisely measured the force required to peel or shear the composite’s layers, revealed that the nanostitched held fast, and the initial crack that the researchers made was unable to spread further between the layers. The nanostitched samples were up to 62 percent tougher and more resistant to cracks, compared with the same advanced composite material that was held together with conventional polymers.

“This is a new composite technology, turbocharged by our nanotubes,” Wardle says.

“The authors have demonsrated that thin plies and nanostitching together have made significant increase in toughness,” says Stephen Tsai, emeritus professor of aeronautics and astronautics at Stanford University. “Composites are degraded by their weak interlaminar strength. Any improvement shown in this work will increase the design allowable, and reduce the weight and cost of composites technology.”

The researchers envision that any vehicle or structure that incorporates conventional composites could be made lighter, tougher, and more resilient with nanostitching.

“You could have selective reinforcement of problematic areas, to reinforce holes or bolted joints, or places where delamination might happen,” Furtado says. “This opens a big window of opportunity.”

Titanium alloys are essential structural materials for a wide variety of applications, from aerospace and energy infrastructure to biomedical equipment. But like most metals, optimizing their properties tends to involve a tradeoff between two key characteristics: strength and ductility. Stronger materials tend to be less deformable, and deformable materials tend to be mechanically weak.

Now, researchers at MIT, collaborating with researchers at ATI Specialty Materials, have discovered an approach for creating new titanium alloys that can exceed this historical tradeoff, leading to new alloys with exceptional combinations of strength and ductility, which might lead to new applications.

The findings are described in the journal Advanced Materials, in a paper by Shaolou Wei ScD ’22, Professor C. Cem Tasan, postdoc Kyung-Shik Kim, and John Foltz from ATI Inc. The improvements, the team says, arise from tailoring the chemical composition and the lattice structure of the alloy, while also adjusting the processing techniques used to produce the material at industrial scale.

Titanium alloys have been important because of their exceptional mechanical properties, corrosion resistance, and light weight when compared to steels for example. Through careful selection of the alloying elements and their relative proportions, and of the way the material is processed, “you can create various different structures, and this creates a big playground for you to get good property combinations, both for cryogenic and elevated temperatures,” Tasan says.

But that big assortment of possibilities in turn requires a way to guide the selections to produce a material that meets the specific needs of a particular application. The analysis and experimental results described in the new study provide that guidance.

The structure of titanium alloys, all the way down to atomic scale, governs their properties, Tasan explains. And in some titanium alloys, this structure is even more complex, made up of two different intermixed phases, known as the alpha and beta phases.

“The key strategy in this design approach is to take considerations of different scales,” he says. “One scale is the structure of individual crystal. For example, by choosing the alloying elements carefully, you can have a more ideal crystal structure of the alpha phase that enables particular deformation mechanisms. The other scale is the polycrystal scale, that involves interactions of the alpha and beta phases. So, the approach that’s followed here involves design considerations for both.”

In addition to choosing the right alloying materials and proportions, steps in the processing turned out to play an important role. A technique called cross-rolling is another key to achieving the exceptional combination of strength and ductility, the team found.

Working together with ATI researchers, the team tested a variety of alloys under a scanning electron microscope as they were being deformed, revealing details of how their microstructures respond to external mechanical load. They found that there was a particular set of parameters — of composition, proportions, and processing method — that yielded a structure where the alpha and beta phases shared the deformation uniformly, mitigating the cracking tendency that is likely to occur between the phases when they respond differently. “The phases deform in harmony,” Tasan says. This cooperative response to deformation can yield a superior material, they found.

“We looked at the structure of the material to understand these two phases and their morphologies, and we looked at their chemistries by carrying out local chemical analysis at the atomic scale. We adopted a wide variety of techniques to quantify various properties of the material across multiple length scales, says Tasan, who is the POSCO Professor of Materials Science and Engineering and an associate professor of metallurgy. “When we look at the overall properties” of the titanium alloys produced according to their system, “the properties are really much better than comparable alloys.”

This was industry-supported academic research aimed at proving design principles for alloys that can be commercially produced at scale, according to Tasan. “What we do in this collaboration is really toward a fundamental understanding of crystal plasticity,” he says. “We show that this design strategy is validated, and we show scientifically how it works,” he adds, noting that there remains significant room for further improvement.

As for potential applications of these findings, he says, “for any aerospace application where an improved combination of strength and ductility are useful, this kind of invention is providing new opportunities.”

The work was supported by ATI Specialty Rolled Products and used facilities of MIT.nano and the Center for Nanoscale Systems at Harvard University.

MIT faculty members Nancy Kanwisher, Robert Langer, and Sara Seager are among eight researchers worldwide to receive this year’s Kavli Prizes.

A partnership among the Norwegian Academy of Science and Letters, the Norwegian Ministry of Education and Research, and the Kavli Foundation, the Kavli Prizes are awarded every two years to “honor scientists for breakthroughs in astrophysics, nanoscience and neuroscience that transform our understanding of the big, the small and the complex.” The laureates in each field will share $1 million.

Understanding recognition of faces

Nancy Kanwisher, the Walter A Rosenblith Professor of Brain and Cognitive Sciences and McGovern Institute for Brain Research investigator, has been awarded the 2024 Kavli Prize in Neuroscience with Doris Tsao, professor in the Department of Molecular and Cell Biology at the University of California at Berkeley, and Winrich Freiwald, the Denise A. and Eugene W. Chinery Professor at the Rockefeller University.

Kanwisher, Tsao, and Freiwald discovered a specialized system within the brain to recognize faces. Their discoveries have provided basic principles of neural organization and made the starting point for further research on how the processing of visual information is integrated with other cognitive functions.

Kanwisher was the first to prove that a specific area in the human neocortex is dedicated to recognizing faces, now called the fusiform face area. Using functional magnetic resonance imaging, she found individual differences in the location of this area and devised an analysis technique to effectively localize specialized functional regions in the brain. This technique is now widely used and applied to domains beyond the face recognition system. 

Integrating nanomaterials for biomedical advances

Robert Langer, the David H. Koch Institute Professor, has been awarded the 2024 Kavli Prize in Nanoscience with Paul Alivisatos, president of the University of Chicago and John D. MacArthur Distinguished Service Professor in the Department of Chemistry, and Chad Mirkin, professor of chemistry at Northwestern University.

Langer, Alivisatos, and Mirkin each revolutionized the field of nanomedicine by demonstrating how engineering at the nano scale can advance biomedical research and application. Their discoveries contributed foundationally to the development of therapeutics, vaccines, bioimaging, and diagnostics.

Langer was the first to develop nanoengineered materials that enabled the controlled release, or regular flow, of drug molecules. This capability has had an immense impact for the treatment of a range of diseases, such as aggressive brain cancer, prostate cancer, and schizophrenia. His work also showed that tiny particles, containing protein antigens, can be used in vaccination, and was instrumental in the development of the delivery of messenger RNA vaccines. 

Searching for life beyond Earth

Sara Seager, the Class of 1941 Professor of Planetary Sciences in the Department of Earth, Atmospheric and Planetary Sciences and a professor in the departments of Physics and of Aeronautics and Astronautics, has been awarded the 2024 Kavli Prize in Astrophysics along with David Charbonneau, the Fred Kavli Professor of Astrophysics at Harvard University.

Seager and Charbonneau are recognized for discoveries of exoplanets and the characterization of their atmospheres. They pioneered methods for the detection of atomic species in planetary atmospheres and the measurement of their thermal infrared emission, setting the stage for finding the molecular fingerprints of atmospheres around both giant and rocky planets. Their contributions have been key to the enormous progress seen in the last 20 years in the exploration of myriad exoplanets. 

Kanwisher, Langer, and Seager bring the number of all-time MIT faculty recipients of the Kavli Prize to eight. Prior winners include Rainer Weiss in astrophysics (2016), Alan Guth in astrophysics (2014), Mildred Dresselhaus in nanoscience (2012), Ann Graybiel in neuroscience (2012), and Jane Luu in astrophysics (2012).

To save on fuel and reduce aircraft emissions, engineers are looking to build lighter, stronger airplanes out of advanced composites. These engineered materials are made from high-performance fibers that are embedded in polymer sheets. The sheets can be stacked and pressed into one multilayered material and made into extremely lightweight and durable structures.

But composite materials have one main vulnerability: the space between layers, which is typically filled with polymer “glue” to bond the layers together. In the event of an impact or strike, cracks can easily spread between layers and weaken the material, even though there may be no visible damage to the layers themselves. Over time, as these hidden cracks spread between layers, the composite could suddenly crumble without warning.

Now, MIT engineers have shown they can prevent cracks from spreading between composite’s layers, using an approach they developed called “nanostitching,” in which they deposit chemically grown microscopic forests of carbon nanotubes between composite layers. The tiny, densely packed fibers grip and hold the layers together, like ultrastrong Velcro, preventing the layers from peeling or shearing apart.

In experiments with an advanced composite known as thin-ply carbon fiber laminate, the team demonstrated that layers bonded with nanostitching improved the material’s resistance to cracks by up to 60 percent, compared with composites with conventional polymers. The researchers say the results help to address the main vulnerability in advanced composites.

“Just like phyllo dough flakes apart, composite layers can peel apart because this interlaminar region is the Achilles’ heel of composites,” says Brian Wardle, professor of aeronautics and astronautics at MIT. “We’re showing that nanostitching makes this normally weak region so strong and tough that a crack will not grow there. So, we could expect the next generation of aircraft to have composites held together with this nano-Velcro, to make aircraft safer and have greater longevity.”

Wardle and his colleagues have published their results today in the journal ACS Applied Materials and Interfaces. The study’s first author is former MIT visiting graduate student and postdoc Carolina Furtado, along with Reed Kopp, Xinchen Ni, Carlos Sarrado, Estelle Kalfon-Cohen, and Pedro Camanho.

Forest growth

At MIT, Wardle is director of the necstlab (pronounced “next lab”), where he and his group first developed the concept for nanostitching. The approach involves “growing” a forest of vertically aligned carbon nanotubes — hollow fibers of carbon, each so small that tens of billions of the the nanotubes can stand in an area smaller than a fingernail. To grow the nanotubes, the team used a process of chemical vapor deposition to react various catalysts in an oven, causing carbon to settle onto a surface as tiny, hair-like supports. The supports are eventually removed, leaving behind a densely packed forest of microscopic, vertical rolls of carbon.

The lab has previously shown that the nanotube forests can be grown and adhered to layers of composite material, and that this fiber-reinforced compound improves the material’s overall strength. The researchers had also seen some signs that the fibers can improve a composite’s resistance to cracks between layers.

In their new study, the engineers took a more in-depth look at the between-layer region in composites to test and quantify how nanostitching would improve the region’s resistance to cracks. In particular, the study focused on an advanced composite material known as thin-ply carbon fiber laminates.

“This is an emerging composite technology, where each layer, or ply, is about 50 microns thin, compared to standard composite plies that are 150 microns, which is about the diameter of a human hair. There’s evidence to suggest they are better than standard-thickness composites. And we wanted to see whether there might be synergy between our nanostitching and this thin-ply technology, since it could lead to more resilient aircraft, high-value aerospace structures, and space and military vehicles,” Wardle says.

Velcro grip

The study’s experiments were led by Carolina Furtado, who joined the effort as part of the MIT-Portugal program in 2016, continued the project as a postdoc, and is now a professor at the University of Porto in Portugal, where her research focuses on modeling cracks and damage in advanced composites.

In her tests, Furtado used the group’s techniques of chemical vapor deposition to grow densely packed forests of vertically aligned carbon nanotubes. She also fabricated samples of thin-ply carbon fiber laminates. The resulting advanced composite was about 3 millimeters thick and comprised 60 layers, each made from stiff, horizontal fibers embedded in a polymer sheet.

She transferred and adhered the nanotube forest in between the two middle layers of the composite, then cooked the material in an autoclave to cure. To test crack resistance, the researchers placed a crack on the edge of the composite, right at the start of the region between the two middle layers.

“In fracture testing, we always start with a crack because we want to test whether and how far the crack will spread,” Furtado explains.

The researchers then placed samples of the nanotube-reinforced composite in an experimental setup to test their resilience to “delamination,” or the potential for layers to separate.

“There’s lots of ways you can get precursors to delamination, such as from impacts, like tool drop, bird strike, runway kickup in aircraft, and there could be almost no visible damage, but internally it has a delamination,” Wardle says. “Just like a human, if you’ve got a hairline fracture in a bone, it’s not good. Just because you can’t see it doesn’t mean it’s not impacting you. And damage in composites is hard to inspect.”

To examine nanostitching’s potential to prevent delamination, the team placed their samples in a setup to test three delamination modes, in which a crack could spread through the between-layer region and peel the layers apart or cause them to slide against each other, or do a combination of both. All three of these modes are the most common ways in which conventional composites can internally flake and crumble.

The tests, in which the researchers precisely measured the force required to peel or shear the composite’s layers, revealed that the nanostitched held fast, and the initial crack that the researchers made was unable to spread further between the layers. The nanostitched samples were up to 62 percent tougher and more resistant to cracks, compared with the same advanced composite material that was held together with conventional polymers.

“This is a new composite technology, turbocharged by our nanotubes,” Wardle says.

“The authors have demonsrated that thin plies and nanostitching together have made significant increase in toughness,” says Stephen Tsai, emeritus professor of aeronautics and astronautics at Stanford University. “Composites are degraded by their weak interlaminar strength. Any improvement shown in this work will increase the design allowable, and reduce the weight and cost of composites technology.”

The researchers envision that any vehicle or structure that incorporates conventional composites could be made lighter, tougher, and more resilient with nanostitching.

“You could have selective reinforcement of problematic areas, to reinforce holes or bolted joints, or places where delamination might happen,” Furtado says. “This opens a big window of opportunity.”

MIT faculty members Nancy Kanwisher, Robert Langer, and Sara Seager are among eight researchers worldwide to receive this year’s Kavli Prizes.

A partnership among the Norwegian Academy of Science and Letters, the Norwegian Ministry of Education and Research, and the Kavli Foundation, the Kavli Prizes are awarded every two years to “honor scientists for breakthroughs in astrophysics, nanoscience and neuroscience that transform our understanding of the big, the small and the complex.” The laureates in each field will share $1 million.

Understanding recognition of faces

Nancy Kanwisher, the Walter A Rosenblith Professor of Brain and Cognitive Sciences and McGovern Institute for Brain Research investigator, has been awarded the 2024 Kavli Prize in Neuroscience with Doris Tsao, professor in the Department of Molecular and Cell Biology at the University of California at Berkeley, and Winrich Freiwald, the Denise A. and Eugene W. Chinery Professor at the Rockefeller University.

Kanwisher, Tsao, and Freiwald discovered a specialized system within the brain to recognize faces. Their discoveries have provided basic principles of neural organization and made the starting point for further research on how the processing of visual information is integrated with other cognitive functions.

Kanwisher was the first to prove that a specific area in the human neocortex is dedicated to recognizing faces, now called the fusiform face area. Using functional magnetic resonance imaging, she found individual differences in the location of this area and devised an analysis technique to effectively localize specialized functional regions in the brain. This technique is now widely used and applied to domains beyond the face recognition system. 

Integrating nanomaterials for biomedical advances

Robert Langer, the David H. Koch Institute Professor, has been awarded the 2024 Kavli Prize in Nanoscience with Paul Alivisatos, president of the University of Chicago and John D. MacArthur Distinguished Service Professor in the Department of Chemistry, and Chad Mirkin, professor of chemistry at Northwestern University.

Langer, Alivisatos, and Mirkin each revolutionized the field of nanomedicine by demonstrating how engineering at the nano scale can advance biomedical research and application. Their discoveries contributed foundationally to the development of therapeutics, vaccines, bioimaging, and diagnostics.

Langer was the first to develop nanoengineered materials that enabled the controlled release, or regular flow, of drug molecules. This capability has had an immense impact for the treatment of a range of diseases, such as aggressive brain cancer, prostate cancer, and schizophrenia. His work also showed that tiny particles, containing protein antigens, can be used in vaccination, and was instrumental in the development of the delivery of messenger RNA vaccines. 

Searching for life beyond Earth

Sara Seager, the Class of 1941 Professor of Planetary Sciences in the Department of Earth, Atmospheric and Planetary Sciences and a professor in the departments of Physics and of Aeronautics and Astronautics, has been awarded the 2024 Kavli Prize in Astrophysics along with David Charbonneau, the Fred Kavli Professor of Astrophysics at Harvard University.

Seager and Charbonneau are recognized for discoveries of exoplanets and the characterization of their atmospheres. They pioneered methods for the detection of atomic species in planetary atmospheres and the measurement of their thermal infrared emission, setting the stage for finding the molecular fingerprints of atmospheres around both giant and rocky planets. Their contributions have been key to the enormous progress seen in the last 20 years in the exploration of myriad exoplanets. 

Kanwisher, Langer, and Seager bring the number of all-time MIT faculty recipients of the Kavli Prize to eight. Prior winners include Rainer Weiss in astrophysics (2016), Alan Guth in astrophysics (2014), Mildred Dresselhaus in nanoscience (2012), Ann Graybiel in neuroscience (2012), and Jane Luu in astrophysics (2012).

To save on fuel and reduce aircraft emissions, engineers are looking to build lighter, stronger airplanes out of advanced composites. These engineered materials are made from high-performance fibers that are embedded in polymer sheets. The sheets can be stacked and pressed into one multilayered material and made into extremely lightweight and durable structures.

But composite materials have one main vulnerability: the space between layers, which is typically filled with polymer “glue” to bond the layers together. In the event of an impact or strike, cracks can easily spread between layers and weaken the material, even though there may be no visible damage to the layers themselves. Over time, as these hidden cracks spread between layers, the composite could suddenly crumble without warning.

Now, MIT engineers have shown they can prevent cracks from spreading between composite’s layers, using an approach they developed called “nanostitching,” in which they deposit chemically grown microscopic forests of carbon nanotubes between composite layers. The tiny, densely packed fibers grip and hold the layers together, like ultrastrong Velcro, preventing the layers from peeling or shearing apart.

In experiments with an advanced composite known as thin-ply carbon fiber laminate, the team demonstrated that layers bonded with nanostitching improved the material’s resistance to cracks by up to 60 percent, compared with composites with conventional polymers. The researchers say the results help to address the main vulnerability in advanced composites.

“Just like phyllo dough flakes apart, composite layers can peel apart because this interlaminar region is the Achilles’ heel of composites,” says Brian Wardle, professor of aeronautics and astronautics at MIT. “We’re showing that nanostitching makes this normally weak region so strong and tough that a crack will not grow there. So, we could expect the next generation of aircraft to have composites held together with this nano-Velcro, to make aircraft safer and have greater longevity.”

Wardle and his colleagues have published their results today in the journal ACS Applied Materials and Interfaces. The study’s first author is former MIT visiting graduate student and postdoc Carolina Furtado, along with Reed Kopp, Xinchen Ni, Carlos Sarrado, Estelle Kalfon-Cohen, and Pedro Camanho.

Forest growth

At MIT, Wardle is director of the necstlab (pronounced “next lab”), where he and his group first developed the concept for nanostitching. The approach involves “growing” a forest of vertically aligned carbon nanotubes — hollow fibers of carbon, each so small that tens of billions of the the nanotubes can stand in an area smaller than a fingernail. To grow the nanotubes, the team used a process of chemical vapor deposition to react various catalysts in an oven, causing carbon to settle onto a surface as tiny, hair-like supports. The supports are eventually removed, leaving behind a densely packed forest of microscopic, vertical rolls of carbon.

The lab has previously shown that the nanotube forests can be grown and adhered to layers of composite material, and that this fiber-reinforced compound improves the material’s overall strength. The researchers had also seen some signs that the fibers can improve a composite’s resistance to cracks between layers.

In their new study, the engineers took a more in-depth look at the between-layer region in composites to test and quantify how nanostitching would improve the region’s resistance to cracks. In particular, the study focused on an advanced composite material known as thin-ply carbon fiber laminates.

“This is an emerging composite technology, where each layer, or ply, is about 50 microns thin, compared to standard composite plies that are 150 microns, which is about the diameter of a human hair. There’s evidence to suggest they are better than standard-thickness composites. And we wanted to see whether there might be synergy between our nanostitching and this thin-ply technology, since it could lead to more resilient aircraft, high-value aerospace structures, and space and military vehicles,” Wardle says.

Velcro grip

The study’s experiments were led by Carolina Furtado, who joined the effort as part of the MIT-Portugal program in 2016, continued the project as a postdoc, and is now a professor at the University of Porto in Portugal, where her research focuses on modeling cracks and damage in advanced composites.

In her tests, Furtado used the group’s techniques of chemical vapor deposition to grow densely packed forests of vertically aligned carbon nanotubes. She also fabricated samples of thin-ply carbon fiber laminates. The resulting advanced composite was about 3 millimeters thick and comprised 60 layers, each made from stiff, horizontal fibers embedded in a polymer sheet.

She transferred and adhered the nanotube forest in between the two middle layers of the composite, then cooked the material in an autoclave to cure. To test crack resistance, the researchers placed a crack on the edge of the composite, right at the start of the region between the two middle layers.

“In fracture testing, we always start with a crack because we want to test whether and how far the crack will spread,” Furtado explains.

The researchers then placed samples of the nanotube-reinforced composite in an experimental setup to test their resilience to “delamination,” or the potential for layers to separate.

“There’s lots of ways you can get precursors to delamination, such as from impacts, like tool drop, bird strike, runway kickup in aircraft, and there could be almost no visible damage, but internally it has a delamination,” Wardle says. “Just like a human, if you’ve got a hairline fracture in a bone, it’s not good. Just because you can’t see it doesn’t mean it’s not impacting you. And damage in composites is hard to inspect.”

To examine nanostitching’s potential to prevent delamination, the team placed their samples in a setup to test three delamination modes, in which a crack could spread through the between-layer region and peel the layers apart or cause them to slide against each other, or do a combination of both. All three of these modes are the most common ways in which conventional composites can internally flake and crumble.

The tests, in which the researchers precisely measured the force required to peel or shear the composite’s layers, revealed that the nanostitched held fast, and the initial crack that the researchers made was unable to spread further between the layers. The nanostitched samples were up to 62 percent tougher and more resistant to cracks, compared with the same advanced composite material that was held together with conventional polymers.

“This is a new composite technology, turbocharged by our nanotubes,” Wardle says.

“The authors have demonsrated that thin plies and nanostitching together have made significant increase in toughness,” says Stephen Tsai, emeritus professor of aeronautics and astronautics at Stanford University. “Composites are degraded by their weak interlaminar strength. Any improvement shown in this work will increase the design allowable, and reduce the weight and cost of composites technology.”

The researchers envision that any vehicle or structure that incorporates conventional composites could be made lighter, tougher, and more resilient with nanostitching.

“You could have selective reinforcement of problematic areas, to reinforce holes or bolted joints, or places where delamination might happen,” Furtado says. “This opens a big window of opportunity.”

To save on fuel and reduce aircraft emissions, engineers are looking to build lighter, stronger airplanes out of advanced composites. These engineered materials are made from high-performance fibers that are embedded in polymer sheets. The sheets can be stacked and pressed into one multilayered material and made into extremely lightweight and durable structures.

But composite materials have one main vulnerability: the space between layers, which is typically filled with polymer “glue” to bond the layers together. In the event of an impact or strike, cracks can easily spread between layers and weaken the material, even though there may be no visible damage to the layers themselves. Over time, as these hidden cracks spread between layers, the composite could suddenly crumble without warning.

Now, MIT engineers have shown they can prevent cracks from spreading between composite’s layers, using an approach they developed called “nanostitching,” in which they deposit chemically grown microscopic forests of carbon nanotubes between composite layers. The tiny, densely packed fibers grip and hold the layers together, like ultrastrong Velcro, preventing the layers from peeling or shearing apart.

In experiments with an advanced composite known as thin-ply carbon fiber laminate, the team demonstrated that layers bonded with nanostitching improved the material’s resistance to cracks by up to 60 percent, compared with composites with conventional polymers. The researchers say the results help to address the main vulnerability in advanced composites.

“Just like phyllo dough flakes apart, composite layers can peel apart because this interlaminar region is the Achilles’ heel of composites,” says Brian Wardle, professor of aeronautics and astronautics at MIT. “We’re showing that nanostitching makes this normally weak region so strong and tough that a crack will not grow there. So, we could expect the next generation of aircraft to have composites held together with this nano-Velcro, to make aircraft safer and have greater longevity.”

Wardle and his colleagues have published their results today in the journal ACS Applied Materials and Interfaces. The study’s first author is former MIT visiting graduate student and postdoc Carolina Furtado, along with Reed Kopp, Xinchen Ni, Carlos Sarrado, Estelle Kalfon-Cohen, and Pedro Camanho.

Forest growth

At MIT, Wardle is director of the necstlab (pronounced “next lab”), where he and his group first developed the concept for nanostitching. The approach involves “growing” a forest of vertically aligned carbon nanotubes — hollow fibers of carbon, each so small that tens of billions of the the nanotubes can stand in an area smaller than a fingernail. To grow the nanotubes, the team used a process of chemical vapor deposition to react various catalysts in an oven, causing carbon to settle onto a surface as tiny, hair-like supports. The supports are eventually removed, leaving behind a densely packed forest of microscopic, vertical rolls of carbon.

The lab has previously shown that the nanotube forests can be grown and adhered to layers of composite material, and that this fiber-reinforced compound improves the material’s overall strength. The researchers had also seen some signs that the fibers can improve a composite’s resistance to cracks between layers.

In their new study, the engineers took a more in-depth look at the between-layer region in composites to test and quantify how nanostitching would improve the region’s resistance to cracks. In particular, the study focused on an advanced composite material known as thin-ply carbon fiber laminates.

“This is an emerging composite technology, where each layer, or ply, is about 50 microns thin, compared to standard composite plies that are 150 microns, which is about the diameter of a human hair. There’s evidence to suggest they are better than standard-thickness composites. And we wanted to see whether there might be synergy between our nanostitching and this thin-ply technology, since it could lead to more resilient aircraft, high-value aerospace structures, and space and military vehicles,” Wardle says.

Velcro grip

The study’s experiments were led by Carolina Furtado, who joined the effort as part of the MIT-Portugal program in 2016, continued the project as a postdoc, and is now a professor at the University of Porto in Portugal, where her research focuses on modeling cracks and damage in advanced composites.

In her tests, Furtado used the group’s techniques of chemical vapor deposition to grow densely packed forests of vertically aligned carbon nanotubes. She also fabricated samples of thin-ply carbon fiber laminates. The resulting advanced composite was about 3 millimeters thick and comprised 60 layers, each made from stiff, horizontal fibers embedded in a polymer sheet.

She transferred and adhered the nanotube forest in between the two middle layers of the composite, then cooked the material in an autoclave to cure. To test crack resistance, the researchers placed a crack on the edge of the composite, right at the start of the region between the two middle layers.

“In fracture testing, we always start with a crack because we want to test whether and how far the crack will spread,” Furtado explains.

The researchers then placed samples of the nanotube-reinforced composite in an experimental setup to test their resilience to “delamination,” or the potential for layers to separate.

“There’s lots of ways you can get precursors to delamination, such as from impacts, like tool drop, bird strike, runway kickup in aircraft, and there could be almost no visible damage, but internally it has a delamination,” Wardle says. “Just like a human, if you’ve got a hairline fracture in a bone, it’s not good. Just because you can’t see it doesn’t mean it’s not impacting you. And damage in composites is hard to inspect.”

To examine nanostitching’s potential to prevent delamination, the team placed their samples in a setup to test three delamination modes, in which a crack could spread through the between-layer region and peel the layers apart or cause them to slide against each other, or do a combination of both. All three of these modes are the most common ways in which conventional composites can internally flake and crumble.

The tests, in which the researchers precisely measured the force required to peel or shear the composite’s layers, revealed that the nanostitched held fast, and the initial crack that the researchers made was unable to spread further between the layers. The nanostitched samples were up to 62 percent tougher and more resistant to cracks, compared with the same advanced composite material that was held together with conventional polymers.

“This is a new composite technology, turbocharged by our nanotubes,” Wardle says.

“The authors have demonsrated that thin plies and nanostitching together have made significant increase in toughness,” says Stephen Tsai, emeritus professor of aeronautics and astronautics at Stanford University. “Composites are degraded by their weak interlaminar strength. Any improvement shown in this work will increase the design allowable, and reduce the weight and cost of composites technology.”

The researchers envision that any vehicle or structure that incorporates conventional composites could be made lighter, tougher, and more resilient with nanostitching.

“You could have selective reinforcement of problematic areas, to reinforce holes or bolted joints, or places where delamination might happen,” Furtado says. “This opens a big window of opportunity.”

To save on fuel and reduce aircraft emissions, engineers are looking to build lighter, stronger airplanes out of advanced composites. These engineered materials are made from high-performance fibers that are embedded in polymer sheets. The sheets can be stacked and pressed into one multilayered material and made into extremely lightweight and durable structures.

But composite materials have one main vulnerability: the space between layers, which is typically filled with polymer “glue” to bond the layers together. In the event of an impact or strike, cracks can easily spread between layers and weaken the material, even though there may be no visible damage to the layers themselves. Over time, as these hidden cracks spread between layers, the composite could suddenly crumble without warning.

Now, MIT engineers have shown they can prevent cracks from spreading between composite’s layers, using an approach they developed called “nanostitching,” in which they deposit chemically grown microscopic forests of carbon nanotubes between composite layers. The tiny, densely packed fibers grip and hold the layers together, like ultrastrong Velcro, preventing the layers from peeling or shearing apart.

In experiments with an advanced composite known as thin-ply carbon fiber laminate, the team demonstrated that layers bonded with nanostitching improved the material’s resistance to cracks by up to 60 percent, compared with composites with conventional polymers. The researchers say the results help to address the main vulnerability in advanced composites.

“Just like phyllo dough flakes apart, composite layers can peel apart because this interlaminar region is the Achilles’ heel of composites,” says Brian Wardle, professor of aeronautics and astronautics at MIT. “We’re showing that nanostitching makes this normally weak region so strong and tough that a crack will not grow there. So, we could expect the next generation of aircraft to have composites held together with this nano-Velcro, to make aircraft safer and have greater longevity.”

Wardle and his colleagues have published their results today in the journal ACS Applied Materials and Interfaces. The study’s first author is former MIT visiting graduate student and postdoc Carolina Furtado, along with Reed Kopp, Xinchen Ni, Carlos Sarrado, Estelle Kalfon-Cohen, and Pedro Camanho.

Forest growth

At MIT, Wardle is director of the necstlab (pronounced “next lab”), where he and his group first developed the concept for nanostitching. The approach involves “growing” a forest of vertically aligned carbon nanotubes — hollow fibers of carbon, each so small that tens of billions of the the nanotubes can stand in an area smaller than a fingernail. To grow the nanotubes, the team used a process of chemical vapor deposition to react various catalysts in an oven, causing carbon to settle onto a surface as tiny, hair-like supports. The supports are eventually removed, leaving behind a densely packed forest of microscopic, vertical rolls of carbon.

The lab has previously shown that the nanotube forests can be grown and adhered to layers of composite material, and that this fiber-reinforced compound improves the material’s overall strength. The researchers had also seen some signs that the fibers can improve a composite’s resistance to cracks between layers.

In their new study, the engineers took a more in-depth look at the between-layer region in composites to test and quantify how nanostitching would improve the region’s resistance to cracks. In particular, the study focused on an advanced composite material known as thin-ply carbon fiber laminates.

“This is an emerging composite technology, where each layer, or ply, is about 50 microns thin, compared to standard composite plies that are 150 microns, which is about the diameter of a human hair. There’s evidence to suggest they are better than standard-thickness composites. And we wanted to see whether there might be synergy between our nanostitching and this thin-ply technology, since it could lead to more resilient aircraft, high-value aerospace structures, and space and military vehicles,” Wardle says.

Velcro grip

The study’s experiments were led by Carolina Furtado, who joined the effort as part of the MIT-Portugal program in 2016, continued the project as a postdoc, and is now a professor at the University of Porto in Portugal, where her research focuses on modeling cracks and damage in advanced composites.

In her tests, Furtado used the group’s techniques of chemical vapor deposition to grow densely packed forests of vertically aligned carbon nanotubes. She also fabricated samples of thin-ply carbon fiber laminates. The resulting advanced composite was about 3 millimeters thick and comprised 60 layers, each made from stiff, horizontal fibers embedded in a polymer sheet.

She transferred and adhered the nanotube forest in between the two middle layers of the composite, then cooked the material in an autoclave to cure. To test crack resistance, the researchers placed a crack on the edge of the composite, right at the start of the region between the two middle layers.

“In fracture testing, we always start with a crack because we want to test whether and how far the crack will spread,” Furtado explains.

The researchers then placed samples of the nanotube-reinforced composite in an experimental setup to test their resilience to “delamination,” or the potential for layers to separate.

“There’s lots of ways you can get precursors to delamination, such as from impacts, like tool drop, bird strike, runway kickup in aircraft, and there could be almost no visible damage, but internally it has a delamination,” Wardle says. “Just like a human, if you’ve got a hairline fracture in a bone, it’s not good. Just because you can’t see it doesn’t mean it’s not impacting you. And damage in composites is hard to inspect.”

To examine nanostitching’s potential to prevent delamination, the team placed their samples in a setup to test three delamination modes, in which a crack could spread through the between-layer region and peel the layers apart or cause them to slide against each other, or do a combination of both. All three of these modes are the most common ways in which conventional composites can internally flake and crumble.

The tests, in which the researchers precisely measured the force required to peel or shear the composite’s layers, revealed that the nanostitched held fast, and the initial crack that the researchers made was unable to spread further between the layers. The nanostitched samples were up to 62 percent tougher and more resistant to cracks, compared with the same advanced composite material that was held together with conventional polymers.

“This is a new composite technology, turbocharged by our nanotubes,” Wardle says.

“The authors have demonsrated that thin plies and nanostitching together have made significant increase in toughness,” says Stephen Tsai, emeritus professor of aeronautics and astronautics at Stanford University. “Composites are degraded by their weak interlaminar strength. Any improvement shown in this work will increase the design allowable, and reduce the weight and cost of composites technology.”

The researchers envision that any vehicle or structure that incorporates conventional composites could be made lighter, tougher, and more resilient with nanostitching.

“You could have selective reinforcement of problematic areas, to reinforce holes or bolted joints, or places where delamination might happen,” Furtado says. “This opens a big window of opportunity.”

To save on fuel and reduce aircraft emissions, engineers are looking to build lighter, stronger airplanes out of advanced composites. These engineered materials are made from high-performance fibers that are embedded in polymer sheets. The sheets can be stacked and pressed into one multilayered material and made into extremely lightweight and durable structures.

But composite materials have one main vulnerability: the space between layers, which is typically filled with polymer “glue” to bond the layers together. In the event of an impact or strike, cracks can easily spread between layers and weaken the material, even though there may be no visible damage to the layers themselves. Over time, as these hidden cracks spread between layers, the composite could suddenly crumble without warning.

Now, MIT engineers have shown they can prevent cracks from spreading between composite’s layers, using an approach they developed called “nanostitching,” in which they deposit chemically grown microscopic forests of carbon nanotubes between composite layers. The tiny, densely packed fibers grip and hold the layers together, like ultrastrong Velcro, preventing the layers from peeling or shearing apart.

In experiments with an advanced composite known as thin-ply carbon fiber laminate, the team demonstrated that layers bonded with nanostitching improved the material’s resistance to cracks by up to 60 percent, compared with composites with conventional polymers. The researchers say the results help to address the main vulnerability in advanced composites.

“Just like phyllo dough flakes apart, composite layers can peel apart because this interlaminar region is the Achilles’ heel of composites,” says Brian Wardle, professor of aeronautics and astronautics at MIT. “We’re showing that nanostitching makes this normally weak region so strong and tough that a crack will not grow there. So, we could expect the next generation of aircraft to have composites held together with this nano-Velcro, to make aircraft safer and have greater longevity.”

Wardle and his colleagues have published their results today in the journal ACS Applied Materials and Interfaces. The study’s first author is former MIT visiting graduate student and postdoc Carolina Furtado, along with Reed Kopp, Xinchen Ni, Carlos Sarrado, Estelle Kalfon-Cohen, and Pedro Camanho.

Forest growth

At MIT, Wardle is director of the necstlab (pronounced “next lab”), where he and his group first developed the concept for nanostitching. The approach involves “growing” a forest of vertically aligned carbon nanotubes — hollow fibers of carbon, each so small that tens of billions of the the nanotubes can stand in an area smaller than a fingernail. To grow the nanotubes, the team used a process of chemical vapor deposition to react various catalysts in an oven, causing carbon to settle onto a surface as tiny, hair-like supports. The supports are eventually removed, leaving behind a densely packed forest of microscopic, vertical rolls of carbon.

The lab has previously shown that the nanotube forests can be grown and adhered to layers of composite material, and that this fiber-reinforced compound improves the material’s overall strength. The researchers had also seen some signs that the fibers can improve a composite’s resistance to cracks between layers.

In their new study, the engineers took a more in-depth look at the between-layer region in composites to test and quantify how nanostitching would improve the region’s resistance to cracks. In particular, the study focused on an advanced composite material known as thin-ply carbon fiber laminates.

“This is an emerging composite technology, where each layer, or ply, is about 50 microns thin, compared to standard composite plies that are 150 microns, which is about the diameter of a human hair. There’s evidence to suggest they are better than standard-thickness composites. And we wanted to see whether there might be synergy between our nanostitching and this thin-ply technology, since it could lead to more resilient aircraft, high-value aerospace structures, and space and military vehicles,” Wardle says.

Velcro grip

The study’s experiments were led by Carolina Furtado, who joined the effort as part of the MIT-Portugal program in 2016, continued the project as a postdoc, and is now a professor at the University of Porto in Portugal, where her research focuses on modeling cracks and damage in advanced composites.

In her tests, Furtado used the group’s techniques of chemical vapor deposition to grow densely packed forests of vertically aligned carbon nanotubes. She also fabricated samples of thin-ply carbon fiber laminates. The resulting advanced composite was about 3 millimeters thick and comprised 60 layers, each made from stiff, horizontal fibers embedded in a polymer sheet.

She transferred and adhered the nanotube forest in between the two middle layers of the composite, then cooked the material in an autoclave to cure. To test crack resistance, the researchers placed a crack on the edge of the composite, right at the start of the region between the two middle layers.

“In fracture testing, we always start with a crack because we want to test whether and how far the crack will spread,” Furtado explains.

The researchers then placed samples of the nanotube-reinforced composite in an experimental setup to test their resilience to “delamination,” or the potential for layers to separate.

“There’s lots of ways you can get precursors to delamination, such as from impacts, like tool drop, bird strike, runway kickup in aircraft, and there could be almost no visible damage, but internally it has a delamination,” Wardle says. “Just like a human, if you’ve got a hairline fracture in a bone, it’s not good. Just because you can’t see it doesn’t mean it’s not impacting you. And damage in composites is hard to inspect.”

To examine nanostitching’s potential to prevent delamination, the team placed their samples in a setup to test three delamination modes, in which a crack could spread through the between-layer region and peel the layers apart or cause them to slide against each other, or do a combination of both. All three of these modes are the most common ways in which conventional composites can internally flake and crumble.

The tests, in which the researchers precisely measured the force required to peel or shear the composite’s layers, revealed that the nanostitched held fast, and the initial crack that the researchers made was unable to spread further between the layers. The nanostitched samples were up to 62 percent tougher and more resistant to cracks, compared with the same advanced composite material that was held together with conventional polymers.

“This is a new composite technology, turbocharged by our nanotubes,” Wardle says.

“The authors have demonsrated that thin plies and nanostitching together have made significant increase in toughness,” says Stephen Tsai, emeritus professor of aeronautics and astronautics at Stanford University. “Composites are degraded by their weak interlaminar strength. Any improvement shown in this work will increase the design allowable, and reduce the weight and cost of composites technology.”

The researchers envision that any vehicle or structure that incorporates conventional composites could be made lighter, tougher, and more resilient with nanostitching.

“You could have selective reinforcement of problematic areas, to reinforce holes or bolted joints, or places where delamination might happen,” Furtado says. “This opens a big window of opportunity.”

To save on fuel and reduce aircraft emissions, engineers are looking to build lighter, stronger airplanes out of advanced composites. These engineered materials are made from high-performance fibers that are embedded in polymer sheets. The sheets can be stacked and pressed into one multilayered material and made into extremely lightweight and durable structures.

But composite materials have one main vulnerability: the space between layers, which is typically filled with polymer “glue” to bond the layers together. In the event of an impact or strike, cracks can easily spread between layers and weaken the material, even though there may be no visible damage to the layers themselves. Over time, as these hidden cracks spread between layers, the composite could suddenly crumble without warning.

Now, MIT engineers have shown they can prevent cracks from spreading between composite’s layers, using an approach they developed called “nanostitching,” in which they deposit chemically grown microscopic forests of carbon nanotubes between composite layers. The tiny, densely packed fibers grip and hold the layers together, like ultrastrong Velcro, preventing the layers from peeling or shearing apart.

In experiments with an advanced composite known as thin-ply carbon fiber laminate, the team demonstrated that layers bonded with nanostitching improved the material’s resistance to cracks by up to 60 percent, compared with composites with conventional polymers. The researchers say the results help to address the main vulnerability in advanced composites.

“Just like phyllo dough flakes apart, composite layers can peel apart because this interlaminar region is the Achilles’ heel of composites,” says Brian Wardle, professor of aeronautics and astronautics at MIT. “We’re showing that nanostitching makes this normally weak region so strong and tough that a crack will not grow there. So, we could expect the next generation of aircraft to have composites held together with this nano-Velcro, to make aircraft safer and have greater longevity.”

Wardle and his colleagues have published their results today in the journal ACS Applied Materials and Interfaces. The study’s first author is former MIT visiting graduate student and postdoc Carolina Furtado, along with Reed Kopp, Xinchen Ni, Carlos Sarrado, Estelle Kalfon-Cohen, and Pedro Camanho.

Forest growth

At MIT, Wardle is director of the necstlab (pronounced “next lab”), where he and his group first developed the concept for nanostitching. The approach involves “growing” a forest of vertically aligned carbon nanotubes — hollow fibers of carbon, each so small that tens of billions of the the nanotubes can stand in an area smaller than a fingernail. To grow the nanotubes, the team used a process of chemical vapor deposition to react various catalysts in an oven, causing carbon to settle onto a surface as tiny, hair-like supports. The supports are eventually removed, leaving behind a densely packed forest of microscopic, vertical rolls of carbon.

The lab has previously shown that the nanotube forests can be grown and adhered to layers of composite material, and that this fiber-reinforced compound improves the material’s overall strength. The researchers had also seen some signs that the fibers can improve a composite’s resistance to cracks between layers.

In their new study, the engineers took a more in-depth look at the between-layer region in composites to test and quantify how nanostitching would improve the region’s resistance to cracks. In particular, the study focused on an advanced composite material known as thin-ply carbon fiber laminates.

“This is an emerging composite technology, where each layer, or ply, is about 50 microns thin, compared to standard composite plies that are 150 microns, which is about the diameter of a human hair. There’s evidence to suggest they are better than standard-thickness composites. And we wanted to see whether there might be synergy between our nanostitching and this thin-ply technology, since it could lead to more resilient aircraft, high-value aerospace structures, and space and military vehicles,” Wardle says.

Velcro grip

The study’s experiments were led by Carolina Furtado, who joined the effort as part of the MIT-Portugal program in 2016, continued the project as a postdoc, and is now a professor at the University of Porto in Portugal, where her research focuses on modeling cracks and damage in advanced composites.

In her tests, Furtado used the group’s techniques of chemical vapor deposition to grow densely packed forests of vertically aligned carbon nanotubes. She also fabricated samples of thin-ply carbon fiber laminates. The resulting advanced composite was about 3 millimeters thick and comprised 60 layers, each made from stiff, horizontal fibers embedded in a polymer sheet.

She transferred and adhered the nanotube forest in between the two middle layers of the composite, then cooked the material in an autoclave to cure. To test crack resistance, the researchers placed a crack on the edge of the composite, right at the start of the region between the two middle layers.

“In fracture testing, we always start with a crack because we want to test whether and how far the crack will spread,” Furtado explains.

The researchers then placed samples of the nanotube-reinforced composite in an experimental setup to test their resilience to “delamination,” or the potential for layers to separate.

“There’s lots of ways you can get precursors to delamination, such as from impacts, like tool drop, bird strike, runway kickup in aircraft, and there could be almost no visible damage, but internally it has a delamination,” Wardle says. “Just like a human, if you’ve got a hairline fracture in a bone, it’s not good. Just because you can’t see it doesn’t mean it’s not impacting you. And damage in composites is hard to inspect.”

To examine nanostitching’s potential to prevent delamination, the team placed their samples in a setup to test three delamination modes, in which a crack could spread through the between-layer region and peel the layers apart or cause them to slide against each other, or do a combination of both. All three of these modes are the most common ways in which conventional composites can internally flake and crumble.

The tests, in which the researchers precisely measured the force required to peel or shear the composite’s layers, revealed that the nanostitched held fast, and the initial crack that the researchers made was unable to spread further between the layers. The nanostitched samples were up to 62 percent tougher and more resistant to cracks, compared with the same advanced composite material that was held together with conventional polymers.

“This is a new composite technology, turbocharged by our nanotubes,” Wardle says.

“The authors have demonsrated that thin plies and nanostitching together have made significant increase in toughness,” says Stephen Tsai, emeritus professor of aeronautics and astronautics at Stanford University. “Composites are degraded by their weak interlaminar strength. Any improvement shown in this work will increase the design allowable, and reduce the weight and cost of composites technology.”

The researchers envision that any vehicle or structure that incorporates conventional composites could be made lighter, tougher, and more resilient with nanostitching.

“You could have selective reinforcement of problematic areas, to reinforce holes or bolted joints, or places where delamination might happen,” Furtado says. “This opens a big window of opportunity.”

To save on fuel and reduce aircraft emissions, engineers are looking to build lighter, stronger airplanes out of advanced composites. These engineered materials are made from high-performance fibers that are embedded in polymer sheets. The sheets can be stacked and pressed into one multilayered material and made into extremely lightweight and durable structures.

But composite materials have one main vulnerability: the space between layers, which is typically filled with polymer “glue” to bond the layers together. In the event of an impact or strike, cracks can easily spread between layers and weaken the material, even though there may be no visible damage to the layers themselves. Over time, as these hidden cracks spread between layers, the composite could suddenly crumble without warning.

Now, MIT engineers have shown they can prevent cracks from spreading between composite’s layers, using an approach they developed called “nanostitching,” in which they deposit chemically grown microscopic forests of carbon nanotubes between composite layers. The tiny, densely packed fibers grip and hold the layers together, like ultrastrong Velcro, preventing the layers from peeling or shearing apart.

In experiments with an advanced composite known as thin-ply carbon fiber laminate, the team demonstrated that layers bonded with nanostitching improved the material’s resistance to cracks by up to 60 percent, compared with composites with conventional polymers. The researchers say the results help to address the main vulnerability in advanced composites.

“Just like phyllo dough flakes apart, composite layers can peel apart because this interlaminar region is the Achilles’ heel of composites,” says Brian Wardle, professor of aeronautics and astronautics at MIT. “We’re showing that nanostitching makes this normally weak region so strong and tough that a crack will not grow there. So, we could expect the next generation of aircraft to have composites held together with this nano-Velcro, to make aircraft safer and have greater longevity.”

Wardle and his colleagues have published their results today in the journal ACS Applied Materials and Interfaces. The study’s first author is former MIT visiting graduate student and postdoc Carolina Furtado, along with Reed Kopp, Xinchen Ni, Carlos Sarrado, Estelle Kalfon-Cohen, and Pedro Camanho.

Forest growth

At MIT, Wardle is director of the necstlab (pronounced “next lab”), where he and his group first developed the concept for nanostitching. The approach involves “growing” a forest of vertically aligned carbon nanotubes — hollow fibers of carbon, each so small that tens of billions of the the nanotubes can stand in an area smaller than a fingernail. To grow the nanotubes, the team used a process of chemical vapor deposition to react various catalysts in an oven, causing carbon to settle onto a surface as tiny, hair-like supports. The supports are eventually removed, leaving behind a densely packed forest of microscopic, vertical rolls of carbon.

The lab has previously shown that the nanotube forests can be grown and adhered to layers of composite material, and that this fiber-reinforced compound improves the material’s overall strength. The researchers had also seen some signs that the fibers can improve a composite’s resistance to cracks between layers.

In their new study, the engineers took a more in-depth look at the between-layer region in composites to test and quantify how nanostitching would improve the region’s resistance to cracks. In particular, the study focused on an advanced composite material known as thin-ply carbon fiber laminates.

“This is an emerging composite technology, where each layer, or ply, is about 50 microns thin, compared to standard composite plies that are 150 microns, which is about the diameter of a human hair. There’s evidence to suggest they are better than standard-thickness composites. And we wanted to see whether there might be synergy between our nanostitching and this thin-ply technology, since it could lead to more resilient aircraft, high-value aerospace structures, and space and military vehicles,” Wardle says.

Velcro grip

The study’s experiments were led by Carolina Furtado, who joined the effort as part of the MIT-Portugal program in 2016, continued the project as a postdoc, and is now a professor at the University of Porto in Portugal, where her research focuses on modeling cracks and damage in advanced composites.

In her tests, Furtado used the group’s techniques of chemical vapor deposition to grow densely packed forests of vertically aligned carbon nanotubes. She also fabricated samples of thin-ply carbon fiber laminates. The resulting advanced composite was about 3 millimeters thick and comprised 60 layers, each made from stiff, horizontal fibers embedded in a polymer sheet.

She transferred and adhered the nanotube forest in between the two middle layers of the composite, then cooked the material in an autoclave to cure. To test crack resistance, the researchers placed a crack on the edge of the composite, right at the start of the region between the two middle layers.

“In fracture testing, we always start with a crack because we want to test whether and how far the crack will spread,” Furtado explains.

The researchers then placed samples of the nanotube-reinforced composite in an experimental setup to test their resilience to “delamination,” or the potential for layers to separate.

“There’s lots of ways you can get precursors to delamination, such as from impacts, like tool drop, bird strike, runway kickup in aircraft, and there could be almost no visible damage, but internally it has a delamination,” Wardle says. “Just like a human, if you’ve got a hairline fracture in a bone, it’s not good. Just because you can’t see it doesn’t mean it’s not impacting you. And damage in composites is hard to inspect.”

To examine nanostitching’s potential to prevent delamination, the team placed their samples in a setup to test three delamination modes, in which a crack could spread through the between-layer region and peel the layers apart or cause them to slide against each other, or do a combination of both. All three of these modes are the most common ways in which conventional composites can internally flake and crumble.

The tests, in which the researchers precisely measured the force required to peel or shear the composite’s layers, revealed that the nanostitched held fast, and the initial crack that the researchers made was unable to spread further between the layers. The nanostitched samples were up to 62 percent tougher and more resistant to cracks, compared with the same advanced composite material that was held together with conventional polymers.

“This is a new composite technology, turbocharged by our nanotubes,” Wardle says.

“The authors have demonsrated that thin plies and nanostitching together have made significant increase in toughness,” says Stephen Tsai, emeritus professor of aeronautics and astronautics at Stanford University. “Composites are degraded by their weak interlaminar strength. Any improvement shown in this work will increase the design allowable, and reduce the weight and cost of composites technology.”

The researchers envision that any vehicle or structure that incorporates conventional composites could be made lighter, tougher, and more resilient with nanostitching.

“You could have selective reinforcement of problematic areas, to reinforce holes or bolted joints, or places where delamination might happen,” Furtado says. “This opens a big window of opportunity.”

To save on fuel and reduce aircraft emissions, engineers are looking to build lighter, stronger airplanes out of advanced composites. These engineered materials are made from high-performance fibers that are embedded in polymer sheets. The sheets can be stacked and pressed into one multilayered material and made into extremely lightweight and durable structures.

But composite materials have one main vulnerability: the space between layers, which is typically filled with polymer “glue” to bond the layers together. In the event of an impact or strike, cracks can easily spread between layers and weaken the material, even though there may be no visible damage to the layers themselves. Over time, as these hidden cracks spread between layers, the composite could suddenly crumble without warning.

Now, MIT engineers have shown they can prevent cracks from spreading between composite’s layers, using an approach they developed called “nanostitching,” in which they deposit chemically grown microscopic forests of carbon nanotubes between composite layers. The tiny, densely packed fibers grip and hold the layers together, like ultrastrong Velcro, preventing the layers from peeling or shearing apart.

In experiments with an advanced composite known as thin-ply carbon fiber laminate, the team demonstrated that layers bonded with nanostitching improved the material’s resistance to cracks by up to 60 percent, compared with composites with conventional polymers. The researchers say the results help to address the main vulnerability in advanced composites.

“Just like phyllo dough flakes apart, composite layers can peel apart because this interlaminar region is the Achilles’ heel of composites,” says Brian Wardle, professor of aeronautics and astronautics at MIT. “We’re showing that nanostitching makes this normally weak region so strong and tough that a crack will not grow there. So, we could expect the next generation of aircraft to have composites held together with this nano-Velcro, to make aircraft safer and have greater longevity.”

Wardle and his colleagues have published their results today in the journal ACS Applied Materials and Interfaces. The study’s first author is former MIT visiting graduate student and postdoc Carolina Furtado, along with Reed Kopp, Xinchen Ni, Carlos Sarrado, Estelle Kalfon-Cohen, and Pedro Camanho.

Forest growth

At MIT, Wardle is director of the necstlab (pronounced “next lab”), where he and his group first developed the concept for nanostitching. The approach involves “growing” a forest of vertically aligned carbon nanotubes — hollow fibers of carbon, each so small that tens of billions of the the nanotubes can stand in an area smaller than a fingernail. To grow the nanotubes, the team used a process of chemical vapor deposition to react various catalysts in an oven, causing carbon to settle onto a surface as tiny, hair-like supports. The supports are eventually removed, leaving behind a densely packed forest of microscopic, vertical rolls of carbon.

The lab has previously shown that the nanotube forests can be grown and adhered to layers of composite material, and that this fiber-reinforced compound improves the material’s overall strength. The researchers had also seen some signs that the fibers can improve a composite’s resistance to cracks between layers.

In their new study, the engineers took a more in-depth look at the between-layer region in composites to test and quantify how nanostitching would improve the region’s resistance to cracks. In particular, the study focused on an advanced composite material known as thin-ply carbon fiber laminates.

“This is an emerging composite technology, where each layer, or ply, is about 50 microns thin, compared to standard composite plies that are 150 microns, which is about the diameter of a human hair. There’s evidence to suggest they are better than standard-thickness composites. And we wanted to see whether there might be synergy between our nanostitching and this thin-ply technology, since it could lead to more resilient aircraft, high-value aerospace structures, and space and military vehicles,” Wardle says.

Velcro grip

The study’s experiments were led by Carolina Furtado, who joined the effort as part of the MIT-Portugal program in 2016, continued the project as a postdoc, and is now a professor at the University of Porto in Portugal, where her research focuses on modeling cracks and damage in advanced composites.

In her tests, Furtado used the group’s techniques of chemical vapor deposition to grow densely packed forests of vertically aligned carbon nanotubes. She also fabricated samples of thin-ply carbon fiber laminates. The resulting advanced composite was about 3 millimeters thick and comprised 60 layers, each made from stiff, horizontal fibers embedded in a polymer sheet.

She transferred and adhered the nanotube forest in between the two middle layers of the composite, then cooked the material in an autoclave to cure. To test crack resistance, the researchers placed a crack on the edge of the composite, right at the start of the region between the two middle layers.

“In fracture testing, we always start with a crack because we want to test whether and how far the crack will spread,” Furtado explains.

The researchers then placed samples of the nanotube-reinforced composite in an experimental setup to test their resilience to “delamination,” or the potential for layers to separate.

“There’s lots of ways you can get precursors to delamination, such as from impacts, like tool drop, bird strike, runway kickup in aircraft, and there could be almost no visible damage, but internally it has a delamination,” Wardle says. “Just like a human, if you’ve got a hairline fracture in a bone, it’s not good. Just because you can’t see it doesn’t mean it’s not impacting you. And damage in composites is hard to inspect.”

To examine nanostitching’s potential to prevent delamination, the team placed their samples in a setup to test three delamination modes, in which a crack could spread through the between-layer region and peel the layers apart or cause them to slide against each other, or do a combination of both. All three of these modes are the most common ways in which conventional composites can internally flake and crumble.

The tests, in which the researchers precisely measured the force required to peel or shear the composite’s layers, revealed that the nanostitched held fast, and the initial crack that the researchers made was unable to spread further between the layers. The nanostitched samples were up to 62 percent tougher and more resistant to cracks, compared with the same advanced composite material that was held together with conventional polymers.

“This is a new composite technology, turbocharged by our nanotubes,” Wardle says.

“The authors have demonsrated that thin plies and nanostitching together have made significant increase in toughness,” says Stephen Tsai, emeritus professor of aeronautics and astronautics at Stanford University. “Composites are degraded by their weak interlaminar strength. Any improvement shown in this work will increase the design allowable, and reduce the weight and cost of composites technology.”

The researchers envision that any vehicle or structure that incorporates conventional composites could be made lighter, tougher, and more resilient with nanostitching.

“You could have selective reinforcement of problematic areas, to reinforce holes or bolted joints, or places where delamination might happen,” Furtado says. “This opens a big window of opportunity.”