Although the change was not visible to the naked eye, a red laser and a microscope built and designed by NIST were used to take photos inside the composite, showing even the most minute breaks and fissures to its interior, and revealing points where the fiber had fractured.

Activation of MP at the interface of a single fiber bundle silk epoxy by mechanical deformation. a) Uniaxial tensile response of single fiber bundle composites after the application of moderate strain (ε = 0.20) and critical strain (ε = 0.64). b–d) Corresponding TP-FLIM images of fiber composites. b) Prior to deformation, minimal fluorescence is observed. c) At ε = 0.20, a uniform, low intensity fluorescence is observed with minimal visible damage to the fiber surface. d) At ε = 0.64, the sample fractures, the fluorescence intensity is heterogeneous but overall brighter than at moderate strain, and numerous microcracks can be seen on the surface of the fiber bundle. Credit: Woodcock et al. Click to enlarge.

Polymer based composite materials are ubiquitous in our physical world. Natural examples include wood, crustaceans’ exoskeleton, and bone; while man-made examples range from applications in aerospace structures and transportation vehicles, to biomaterials, and sporting goods. In fact, demand for lightweight, energy saving, composites are growing faster than industry’s current ability to respond; this mismatch between supply and demand is due not only to the large variety and volume of composites required, but also the unprecedented coupled expectations for performance.

The materials comprising polymer composites are diverse. The reinforcing phase, whose dimensions can range from the nanometer scale to the meter scale, can be mineral, glass, carbon, or biomaterial in origin. The polymer matrices can be equally varied in make-up: they can be protein or polysaccharide, as in natural composites, or fabricated from any of the myriad of commercially available polymers. A unifying characteristic across all of these related classes of materials is the presence of an interface where the components meet.

… Fiber reinforced polymer composites (FRPC) are an interesting and important class of composites, because they combine the beneficial attributes of the two components: namely, the strength of the fiber and the toughness of the polymer matrix. However, this synchronistic effect is only possible when there is efficient stress transfer across the interface from the matrix into the ber. The quality of the interface determines the efficiency of stress transfer. The nature of the interface is determined by several factors, such as the composition of the two components, their morphology, physical properties, and the nature of association between them. … Without a fundamental understanding of interfacial properties, robust design rules and accurate predictive models necessary to address the growing need for composites cannot be developed.

—Woodcock et al.

While there are many ways to measure the macroscopic properties of composites, for decades the challenge has been to determine what was happening inside, at the interface, said researcher Jeffrey Gilman, who led the team doing the work at NIST.

One option is optical imaging. However, conventional methods for optical imaging are only able to record images at scales as small as 200-400 nanometers. Some interfaces are only 10 to 100 nanometers in thickness, making such techniques somewhat ineffective for imaging the interphase in composites. By installing the RS probe at the interface, the researchers were able to “see” damage exclusively at the interface using optical microscopy.

The NIST research team is planning to expand their research to explore how such probes could be used in other kinds of composites. They also would like to use such sensors to enhance the capability of these composites to withstand extreme cold and heat. There’s a tremendous demand for composites that can withstand prolonged exposure to water, too, especially for use in building more resilient infrastructure components such as bridges and giant blades for wind turbines.

The research team plans to continue searching for more ways that damage sensors such as the one in this study could be used to improve standards for existing composites and create new standards for the composites of the future, ensuring that those materials are safe, strong and reliable.

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.

—Jeffrey Gilman

This research was funded through collaborative research agreements with both the Air Force Office of Scientific Research and the Army Research Office.

Resources

• J.W. Woodcock, R. Beams, C.S. Davis, N. Chen, S.J. Stranick, D.U. Shah, F. Vollrath and J.W. Gilman (2017) “Observation of Interfacial Damage in a Silk-Epoxy Composite, Using a Simple Mechanoresponsive Fluorescent Probe” Adv. Mat. Interfaces. doi: 10.1002/admi.201601018