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.

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.

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.

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.”

Imagine it’s 2050 and you’re on a cross-country flight on a new type of airliner, one with no fuel on board. The plane takes off, and you rise above the airport. Instead of climbing to cruising altitude, though, your plane levels out and the engines quiet to a low hum. Is this normal? No one seems to know. Anxious passengers crane their necks to get a better view out their windows. They’re all looking for one thing.

Then it appears: a massive antenna array on the horizon. It’s sending out a powerful beam of electromagnetic radiation pointed at the underside of the plane. After soaking in that energy, the engines power up, and the aircraft continues its climb. Over several minutes, the beam will deliver just enough energy to get you to the next ground antenna located another couple hundred kilometers ahead.

The person next to you audibly exhales. You sit back in your seat and wait for your drink. Old-school EV-range anxiety is nothing next to this.

Electromagnetic waves on the fly

Beamed power for aviation is, I admit, an outrageous notion. If physics doesn’t forbid it, federal regulators or nervous passengers probably will. But compared with other proposals for decarbonizing aviation, is it that crazy?

Batteries, hydrogen, alternative carbon-based fuels—nothing developed so far can store energy as cheaply and densely as fossil fuels, or fully meet the needs of commercial air travel as we know it. So, what if we forgo storing all the energy on board and instead beam it from the ground? Let me sketch what it would take to make this idea fly.

Beamed Power for Aviation

Fly by Microwave: Warm up to a new kind of air travel

For the wireless-power source, engineers would likely choose microwaves because this type of electromagnetic radiation can pass unruffled through clouds and because receivers on planes could absorb it completely, with nearly zero risk to passengers.

To power a moving aircraft, microwave radiation would need to be sent in a tight, steerable beam. This can be done using technology known as a phased array, which is commonly used to direct radar beams. With enough elements spread out sufficiently and all working together, phased arrays can also be configured to focus power on a point a certain distance away, such as the receiving antenna on a plane.

Phased arrays work on the principle of constructive and destructive interference. The radiation from the antenna elements will, of course, overlap. In some directions the radiated waves will interfere destructively and cancel out one another, and in other directions the waves will fall perfectly in phase, adding together constructively. Where the waves overlap constructively, energy radiates in that direction, creating a beam of power that can be steered electronically.

How far we can send energy in a tight beam with a phased array is governed by physics—specifically, by something called the diffraction limit. There’s a simple way to calculate the optimal case for beamed power: D1 D2 > λ R. In this mathematical inequality, D1 and D2 are the diameters of the sending and receiving antennas, λ is the wavelength of the radiation, and R is the distance between those antennas.

Now, let me offer some ballpark numbers to figure out how big the transmitting antenna (D1) must be. The size of the receiving antenna on the aircraft is probably the biggest limiting factor. A medium-size airliner has a wing and body area of about 1,000 square meters, which should provide for the equivalent of a receiving antenna that’s 30 meters wide (D2) built into the underside of the plane.

If physics doesn’t forbid it, federal regulators or nervous passengers probably will.

Next, let’s guess how far we would need to beam the energy. The line of sight to the horizon for someone in an airliner at cruising altitude is about 360 kilometers long, assuming the terrain below is level. But mountains would interfere, plus nobody wants range anxiety, so let’s place our ground antennas every 200 km along the flight path, each beaming energy half of that distance. That is, set R to 100 km.

Finally, assume the microwave wavelength (λ) is 5 centimeters. This provides a happy medium between a wavelength that’s too small to penetrate clouds and one that’s too large to gather back together on a receiving dish. Plugging these numbers into the equation above shows that in this scenario the diameter of the ground antennas (D1) would need to be at least about 170 meters. That’s gigantic, but perhaps not unreasonable. Imagine a series of three or four of these antennas, each the size of a football stadium, spread along the route, say, between LAX and SFO or between AMS and BER.

Power beaming in the real world

While what I’ve described is theoretically possible, in practice engineers have beamed only a fraction of the amount of power needed for an airliner, and they’ve done that only over much shorter distances.

NASA holds the record from an experiment in 1975, when it beamed 30 kilowatts of power over 1.5 km with a dish the size of a house. To achieve this feat, the team used an analog device called a klystron. The geometry of a klystron causes electrons to oscillate in a way that amplifies microwaves of a particular frequency—kind of like how the geometry of a whistle causes air to oscillate and produce a particular pitch.

Klystrons and their cousins, cavity magnetrons (found in ordinary microwave ovens), are quite efficient because of their simplicity. But their properties depend on their precise geometry, so it’s challenging to coordinate many such devices to focus energy into a tight beam.

In more recent years, advances in semiconductor technology have allowed a single oscillator to drive a large number of solid-state amplifiers in near-perfect phase coordination. This has allowed microwaves to be focused much more tightly than was possible before, enabling more-precise energy transfer over longer distances.

In 2022, the Auckland-based startup Emrod showed just how promising this semiconductor-enabled approach could be. Inside a cavernous hangar in Germany owned by Airbus, the researchers beamed 550 watts across 36 meters and kept over 95 percent of the energy flowing in a tight beam—far better than could be achieved with analog systems. In 2021, the U.S. Naval Research Laboratory showed that these techniques could handle higher power levels when it sent more than a kilowatt between two ground antennas over a kilometer apart. Other researchers have energized drones in the air, and a few groups even intend to use phased arrays to beam solar power from satellites to Earth.

A rectenna for the ages

So beaming energy to airliners might not be entirely crazy. But please remain seated with your seat belts fastened; there’s some turbulence ahead for this idea. A Boeing 737 aircraft at takeoff requires about 30 megawatts—a thousand times as much power as any power-beaming experiment has demonstrated. Scaling up to this level while keeping our airplanes aerodynamic (and flyable) won’t be easy.

Consider the design of the antenna on the plane, which receives and converts the microwaves to an electric current to power the aircraft. This rectifying antenna, or rectenna, would need to be built onto the underside surfaces of the aircraft with aerodynamics in mind. Power transmission will be maximized when the plane is right above the ground station, but it would be far more limited the rest of the time, when ground stations are far ahead or behind the plane. At those angles, the beam would activate only either the front or rear surfaces of the aircraft, making it especially hard to receive enough power.

With 30 MW blasting onto that small of an area, power density will be an issue. If the aircraft is the size of Boeing 737, the rectenna would have to cram about 25 W into each square centimeter. Because the solid-state elements of the array would be spaced about a half-wavelength—or 2.5 cm—apart, this translates to about 150 W per element—perilously close to the maximum power density of any solid-state power-conversion device. The top mark in the 2016 IEEE/Google Little Box Challenge was about 150 W per cubic inch (less than 10 W per cubic centimeter).

The rectenna will also have to weigh very little and minimize the disturbance to the airflow over the plane. Compromising the geometry of the rectenna for aerodynamic reasons might lower its efficiency. State-of-the art power-transfer efficiencies are only about 30 percent, so the rectenna can’t afford to compromise too much.

A Boeing 737 aircraft at takeoff requires about 30 megawatts—a thousand times as much power as any power-beaming experiment has demonstrated.

And all of this equipment will have to work in an electric field of about 7,000 volts per meter—the strength of the power beam. The electric field inside a microwave oven, which is only about a third as strong, can create a corona discharge, or electric arc, between the tines of a metal fork, so just imagine what might happen inside the electronics of the rectenna.

And speaking of microwave ovens, I should mention that, to keep passengers from cooking in their seats, the windows on any beamed-power airplane would surely need the same wire mesh that’s on the doors of microwave ovens—to keep those sizzling fields outside the plane. Birds, however, won’t have that protection.

Fowl flying through our power beam near the ground might encounter a heating of more than 1,000 watts per square meter—stronger than the sun on a hot day. Up higher, the beam will narrow to a focal point with much more heat. But because that focal point would be moving awfully fast and located higher than birds typically fly, any roasted ducks falling from the sky would be rare in both senses of the word. Ray Simpkin, chief science officer at Emrod, told me it’d take “more than 10 minutes to cook a bird” with Emrod’s relatively low-power system.

Legal challenges would surely come, though, and not just from the National Audubon Society. Thirty megawatts beamed through the air would be about 10 billion times as strong as typical signals at 5-cm wavelengths (a band currently reserved for amateur radio and satellite communications). Even if the transmitter could successfully put 99 percent of the waves into a tight beam, the 1 percent that’s leaked would still be a hundred million times as strong as approved transmissions today.

And remember that aviation regulators make us turn off our cellphones during takeoff to quiet radio noise, so imagine what they’ll say about subjecting an entire plane to electromagnetic radiation that’s substantially stronger than that of a microwave oven. All these problems are surmountable, perhaps, but only with some very good engineers (and lawyers).

Compared with the legal obstacles and the engineering hurdles we’d need to overcome in the air, the challenges of building transmitting arrays on the ground, huge as they would have to be, seem modest. The rub is the staggering number of them that would have to be built. Many flights occur over mountainous terrain, producing a line of sight to the horizon that is less than 100 km. So in real-world terrain we’d need more closely spaced transmitters. And for the one-third of airline miles that occur over oceans, we would presumably have to build floating arrays. Clearly, building out the infrastructure would be an undertaking on the scale of the Eisenhower-era U.S. interstate highway system.

Decarbonizing with the world’s largest microwave

People might be able to find workarounds for many of these issues. If the rectenna is too hard to engineer, for example, perhaps designers will find that they don’t have to turn the microwaves back into electricity—there are precedents for using heat to propel airplanes. A sawtooth flight path—with the plane climbing up as it approaches each emitter station and gliding down after it passes by—could help with the power-density and field-of-view issues, as could flying-wing designs, which have much more room for large rectennas. Perhaps using existing municipal airports or putting ground antennas near solar farms could reduce some of the infrastructure cost. And perhaps researchers will find shortcuts to radically streamline phased-array transmitters. Perhaps, perhaps.

To be sure, beamed power for aviation faces many challenges. But less-fanciful options for decarbonizing aviation have their own problems. Battery-powered planes don’t even come close to meeting the needs of commercial airlines. The best rechargeable batteries have about 5 percent of the effective energy density of jet fuel. At that figure, an all-electric airliner would have to fill its entire fuselage with batteries—no room for passengers, sorry—and it’d still barely make it a tenth as far as an ordinary jet. Given that the best batteries have improved by only threefold in the past three decades, it’s safe to say that batteries won’t power commercial air travel as we know it anytime soon.

Any roasted ducks falling from the sky would be rare in both senses of the word.

Hydrogen isn’t much further along, despite early hydrogen-powered flights occurring nearly 40 years ago. And it’s potentially dangerous—enough that some designs for hydrogen planes have included two separate fuselages: one for fuel and one for people to give them more time to get away if the stuff gets explode-y. The same factors that have kept hydrogen cars off the road will probably keep hydrogen planes out of the sky.

Synthetic and biobased jet fuels are probably the most reasonable proposal. They’ll give us aviation just as we know it today, just at a higher cost—perhaps 20 to 50 percent more expensive per ticket. But fuels produced from food crops can be worse for the environment than the fossil fuels they replace, and fuels produced from CO₂ and electricity are even less economical. Plus, all combustion fuels could still contribute to contrail formation, which makes up more than half of aviation’s climate impact.

The big problem with the “sane” approach for decarbonizing aviation is that it doesn’t present us with a vision of the future at all. At the very best, we’ll get a more expensive version of the same air travel experience the world has had since the 1970s.

True, beamed power is far less likely to work. But it’s good to examine crazy stuff like this from time to time. Airplanes themselves were a crazy idea when they were first proposed. If we want to clean up the environment and produce a future that actually looks like a future, we might have to take fliers on some unlikely sounding schemes.

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.”