A team at the National Institute of Standards and Technology has developed a tool to monitor changes in composite structures used in areas like aerospace, infrastructure and wind turbines than could signal future damage to the fiber-reinforced polymers as they age.
"This gives us the ability to develop better, more fatigue-resistant composites," NIST chemist Jeff Gilman. "We can see when the fiber starts to break. We now have a way to quantify the damage."
The team of NIST composites specialists added small molecules that become fluorescent after the impact of mechanical force. These molecules, called "mechanophores" change color or light up, helping identify tiny nanometer-size openings or cracks between the fiber and resin, according to NIST.
Now NIST has taken this technology one step further by incorporating the mechanophore throughout the composite resin. This allows scientists to use microscopy imaging techniques to measure FRP damage. The approach uses a minute amount of fluorescent die called rhodamine that NIST said causes no material changes in the physical properties of the FRP material.
That makes it possible to do field testing for fatigue inexpensively, and on a regular basis. Structures like wind turbines could be scanned for interior cracks frequently, even years after they've erected, NIST said.
Initial work with this new tool also revealed a surprise about FRP damage. When a fiber breaks, it sends out a kind of "shock wave" that moves throughout the material, said Jeremiah Woodcock, the lead author of a new paper about the mechanophore published in Composites Science and Technology.
"We thought that when we looked at the results, there'd be a halo of light around the crack, showing the fluorescence of the mechanophore," Woodcock said. Instead, the researchers found that damage happens in places are very remote from the point of the fiber fracture. "It's like we knew about the earthquake but didn't know about the tsunami that follows after it," he said.
The NIST research also discovered that existing test methods were unintentionally damaging the material's strength — which, in turn, had led designers and engineers to overdesign FRPs. So NIST said that using the mechanophore could bring down energy and manufacturing costs, increasing the ways composites are used in industry.