1 LAMB WAVE STIFFNESS CHARACTERIZATION OF COMPOSITES UNDERGOING THERMAL-MECHANICAL AGING

INTRODUCTION 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 upon those materials. Analytical methods investigating the effects of intense thermal heating combined with mechanical loading have been investigated by Griffis, et al. [1] and Chen, et al. [2]. Schubbe and Mall [3] and Castelli, et al. [4] have experimentally investigated both in-phase (maximum temperature at maximum load) and out-of-phase (maximum temperature at minimum load) thermal-mechanical cycling on composites. Both authors found that the damage mechanisms and fatigue lives were dependent on test parameters as well as stress levels. Castelli, et al. [4] identified matrix dominated failure modes for out-of-phase cycling and fiber dominated damage modes for in-phase cycling. In recent years, ultrasonic methods have been developed that can measure the mechanical stiffness of composites. To he