Use of Guided Acoustic Waves to Assess the Effects of Thermal-Mechanical Cycling on Composite Stiffness

The introduction of new, advanced composite materials into aviation systems requires a thorough understanding of the long-term effects of combined thermal and mechanical loading. As part of a study to evaluate the effects of thermal-mechanical cycling, a guided acoustic (Lamb) wave measurement system was used to measure the bending and out-of-plane stiffness coefficients of composite laminates undergoing thermal-mechanical loading. The system uses a pulse/receive technique that excites an antisymmetric Lamb mode and measures the time-of-flight over a wide frequency range. Given the material density and plate thickness, the bending and out-of-plane shear stiffnesses are calculated from a reconstruction of the velocity dispersion curve. A series of 16 and 32-ply composite laminates were subjected to a thermal-mechanical loading profile in load frames equipped with special environmental chambers. The composite systems studied were a graphite fiber reinforced amorphous thermoplastic polyimide and a graphite fiber reinforced bismaleimide thermoset. The samples were exposed to both high and low temperature extremes as well as high and low strain profiles. The bending and out-of-plane stiffnesses for composite samples that have undergone over 6,000 cycles of thermal-mechanical loading are reported. The Lamb wave generated elastic stiffness results have shown decreases of up to 20 percent at 4,936 loading cycles for the graphite/thermoplastic samples and up to 64 percent at 4,706 loading cycles for the graphite/thermoset samples.