FRPs are composite materials that consist of polymers that are reinforced with glass, aramid, carbon or other types of fibers. They are applied in manifold infrastructure fields including construction, marine and aerospace industry.
Their application in civil engineering projects has been increasing over the past years as they rapidly develop and begin to replace concrete and steel in certain cases. Technological advancements in the production of FRPs have enabled the materials to be relatively light, durable and present high strength and stiffness.
During the past 60 years, researchers have been trying to quantify the damage of the composite materials during mechanical loading. In particular, they focused on measuring the impact of the fiber and resin contact, but its quantification was a challenging task.
The scientific team from the U.S. National Institute of Standards and Technology (NIST), introduces a new method that utilizes small particles, known as "mechanophores", which either light up or alter their color when a crack propagates within the fiber-resin bond. “We can see when the fiber starts to break. We now have a way to quantify the damage,” Dr. Jeff Gilman, co-author of the study and a Research Polymer Chemist at NIST, stated.
Being microscopic, the mechanophores are not noticeable in the first place but they may be placed anywhere inside the composite resin. The research team utilizes specialized methods that include manipulating microscopic images to assess the material's performance.
The results of the first set of analyses revealed an unexpected behavior of the FRPs. Despite it was previously believed that when a fiber cracks, the damage is accumulated near the tip of the crack, scientists found out that it propagates through the material. Therefore, damage was discovered in locations far from the initial 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. It’s like we knew about the earthquake but didn’t know about the tsunami that follows after it,” Jeremiah Woodcock, lead author of the study and a Research Chemist at NIST, stated.
The research team revealed another significant finding. It seems that the current testing procedures cause internal damage to the composite material and, thus, developers underestimate their strength and performance resulting in oversizing and cost increase.
If the new system is widely utilized, the evaluation of damaged structures (e.g. wind turbines made out of FRPs) could be conducted more efficiently and the procedure would be less expensive.