Fatigue Testing Techniques for Evaluating the Effects of Environment on Composite Materials

Composite materials are finding an increasing usefulness in aircraft structures which are subject to long-term, uncontrolled weathering conditions. In order to obtain data that are representative of actual operating conditions, cantilever bending fatigue testing of plus or minus 45F fiberglass epoxy specimens and graphite epoxy composite specimens was initiated on outdoor and indoor test equipment. After trying several unsuccessful modifications to the test procedure, it became obvious that state-of-the-art, metal testing techniques did not yield valid data. It was decided to abandon the free-vibration approach. An indoor shaker table was modified to produce a fixed deflection on the specimen with a system of actuator arms. Tests were run to determine the fatigue life at different magnitudes of deflection using this system, and they proved to be successful.