The NIST team created its probe from a dye, rhodamine spirolactam (RS), which changes from a dark state to a light state in reaction to an applied force. In experiments, the molecule was attached to silk fibres contained inside an epoxy-based composite. As force was applied to the composite, the stress and strain activated the RS, causing it to fluoresce when excited with a laser. Although the change was not visible to the naked eye, a red laser and a microscope were used to take photos inside the composite, showing even the most minute breaks and fissures to its interior, and revealing points where the fibre had fractured.
All composites have an interface where the components meet. The resilience of that interface is critical to a composite’s ability to withstand damage. Interfaces that are thin but flexible are often favoured by designers and manufacturers, but it is challenging to measure the interfacial properties in a composite.
“There have long been ways to measure the macroscopic properties of composites,” said researcher Jeffrey Gilman, who led the team at NIST. “But for decades the challenge has been to determine what was happening inside, at the interface.”
One option is optical imaging. However, conventional methods for optical imaging are only able to record images at scales as small as 200 to 400nm. Some interfaces are only 10 to 100nm in thickness, making such techniques ineffective for imaging the interphase in composites. By installing the RS probe at the interface, the researchers could ‘see’ damage exclusively at the interface using optical microscopy.
“We now have a damage sensor to help optimise the composite for different applications,” Gilman added. “If you attempt a design change, you can figure out if the change you made improved the interface of a composite, or weakened it.”
The NIST research team plans to expand its research to explore how such probes could be used in other kinds of composites. The team would also like to use such sensors to enhance the capability of these composites to withstand extreme cold and heat. There is additional demand for composites that can withstand prolonged exposure to water, especially for use in building more resilient infrastructure components such as bridges and giant blades for wind turbines.