Silk sensor to detect micro stresses in composite materials

Credit: NIST.

Nowadays the demands being placed on materials are increasingly stringent, with new technology comes a need for more environmentally friendly and efficient products that are more durable, cheaper, and lightweight. There are more stresses placed on materials, and they also need to be “smart” in many cases, allowing sensors to be fitted in them. So it makes sense that manufacturers would want to be able to monitor the status and condition of the material while it is being used.

A solution that allows for manufacturers and designers to analyze the result of stresses while the material is in production or even when it has been produced is therefore of great value. It is this motivation that was in mind when the NIST (National Institute of Standards and Technology) recently created a composite that contains a probe which senses damage. The material is made from silk and a form of resin called epoxy.

Mechanophore is the name of the probe. It is hoped that the abovementioned tasks (such as quality testing of new products) can be made more efficient with its use. It contains a dye which changed appearance depending on pressures applied to it. The chemical (rhodamine spirolactam) was applied to silk material which was embedded in a composite of the resin. So the more pressure and strain the overall mixture was put under, the more the colour change of the rhodamine spirolactam molecules was visible under the light of red lasers which was then picked up by a microscope specially built for the purpose. This allowed an extremely detailed analysis of the pressure the material was under. This allows the ability to inspect in detail exactly where the strain points are.

These findings were released in the research publication Advanced Materials Interfaces. It was an advance on the research of Oxford University researchers who developed the silk from Bombyx mori silk worms. That team at Oxford was led by Professor Fritz Vollrath. They also used a polymer made with fiber-reinforced materials added. In this way these composite materials get the best from both materials: the resilience of the polymer with the strength of the fibre. The interface between the different “ingredients” in the composite is the critical feature, since many manufacturers now use very thin composite layers it is hard to measure the pressures and performance of these thin layers. The new NIST innovation makes this more practical.

This was explained by the head f the NIST research project, Jeffrey Gilman: “There have long been ways to measure the macroscopic properties of composites. But for decades the challenge has been to determine what was happening inside, at the interface. We now have a damage sensor to help optimize the composite for different applications. If you attempt a design change, you can figure out if the change you made improved the interface of a composite, or weakened it.”

His team are searching for new ways to apply this innovation to other manufacturing processing with other products, and in other types of composites. They are also working to engineer the sensors to withstand greater extremes in pressure, temperature, and moisture level. It is hoped that these new developments will set a new standard for composite material production.

The new probe is a big leap in capability over previous options for stress testing of composites. Optical imaging is often used, but it doesn’t allow for analysis of extremely thin composite interfaces since it is effective only to around 200 nanometres thick materials, while many interfaces are ten times thinner. The new NIST innovation can detect those changes at infinitesimally thin interfaces.

More information can be found at: NIST.

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