"This gives us the ability to develop better, more fatigue-resistant composites. We can see when the fiber starts to break. We now have a way to quantify the damage."Jeff Gilman, NIST

A team at the US National Institute of Standards and Technology (NIST) has developed a tool to monitor changes in widely used composite materials known as fiber-reinforced polymers (FRPs), which can be found in everything from aerospace and infrastructure to wind turbines. The new tool, integrated into these materials, can help measure the damage that occurs as they age.

"This gives us the ability to develop better, more fatigue-resistant composites," said NIST chemist Jeff Gilman. "We can see when the fiber starts to break. We now have a way to quantify the damage."

Since the 1960s, scientists have been experimenting with ways to make FRPs lighter and stronger, which has often meant testing the bond between fiber and resin. As reported in a previous paper, the NIST team experimented with adding small molecules that fluoresce after the impact of mechanical force. By changing color or lighting up, these molecules, called ‘mechanophores’, can help to identify tiny nanometer-sized openings or cracks between the fiber and resin.

The NIST team has now taken this technology to the next level by incorporating mechanophores throughout the composite resin. Although not noticeable to the naked eye, this latest approach allows scientists to use special microscopy imaging techniques to measure FRP damage. The approach involves incorporating a minute amount (less than 0.1% mass) of a fluorescent dye called rhodamine that causes no appreciable changes in the FRP's physical properties.

If the new mechanophore is embedded in structures made of FRP, field testing for fatigue could be done inexpensively and on a regular basis. Structures like wind turbines could frequently be scanned easily for interior cracks, even years after they've been erected.

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, explained Jeremiah Woodcock, the lead author of a paper on this work in Composites Science and Technology. In the past, it was believed that most of the damage was happening at the point of breakage.

"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, they found that damage occurs in places that are very remote from the point of fiber fracture. "It's like we knew about the earthquake but didn't know about the tsunami that follows after it."

The NIST mechanophore research also found that existing testing was unintentionally damaging the material's strength, leading designers and engineers to overdesign FRPs. Using the mechanophore could, therefore, bring down energy and manufacturing costs, and expand the ways these materials are used in industry.

This story is adapted from material from NIST, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.