Nondestructive Evaluation of the Effects of Thermal-Mechanical Aging on IM7/K3B 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 on these types of materials. Ultrasonic Lamb waves offer a promising method of evaluating stiffness in composite materials. To help assess the effect of aging, a suitably designed Lamb wave measurement system is being used to obtain bending and out-of-plane stiffness coefficients of composite laminates undergoing thermal-mechanical loading. In this study, a series of 16-ply graphite-fiber-reinforced amorphous thermoplastic polyimide laminates, IM7/K3B, were aged in load frames equipped with special environmental chambers. The samples were subjected to both high and low strain profiles. The bending and out-of-plane shear stiffnesses as well as time-of-flight measurements for samples that have undergone up to 4,993 fatigue cycles of combined thermal and mechanical aging are reported. The Lamb wave generated elastic stiffness results show decreases of up to 20% with extended aging.