An Examination of the Compressive Cyclic Loading Aspects of Damage Tolerance for Polymer Matrix Launch Vehicle Hardware

The issue of fatigue loading of structures composed of composite materials is considered in a requirements document that is currently in place for manned launch vehicles. By taking into account the short lives of these parts, coupled with design considerations, it is demonstrated that the necessary coupon level fatigue data collapse to a static case. Data from a literature review of past studies that examined compressive fatigue loading after impact and data generated from this experimental study are presented to support this finding. In other studies from the literature, a stress amplitude of about 60% of the static compression after impact (CAI) strength was found to exist, below which fatigue had no deleterious effects up to one million cycles. In this study, a stress amplitude of about 80% of the static (CAI) strength was found to exist, below which fatigue had no deleterious effects up to 10,000 cycles. A launch vehicle structure should never experience one cycle above 61.4% of static CAI strength, much less 10,000 at 80%. Despite utilizing severe fatigue amplitude loading in impact damaged coupons, residual strength after fatigue was consistently higher than expected. Unrealistically high fatigue stress amplitudes were needed to fail 5 of 15 specimens, before 10,000 cycles was reached. Since a typical launch vehicle structure, such as the ARES I interstage, only experiences a few cycles near limit load, it is concluded that static CAI strength data will suffice for most launch vehicle structures.