High Cycle Fatigue Testing

Applications such as rotor systems or aero-engines are designed for high cycle fatigue loading. In other applications the loading is vibrational not mechanical and undergo giga cycles. Clearly it is not feasible to test structure out to these cycles prior to certification. MERL has collaborated in using durability analysis along with its multi-station fatigue machines to validate damage onset predictions at low to high fatigue cycles against structural tests. These predictions can then be extrapolated to make predictions under high cycle fatigue conditions.

Most applications have a variable fatigue loading typically known as a spectrum, particularly in the aerospace industry. Spectrum truncation considers the high fatigue threshold behaviour to reduce fatigue test time. Advantage is taken of composites relative insensitivity to low load cycles. The fatigue life is dominated by the higher fatigue stresses. MERL uses its knowledge of threshold testing in composites in a spectrum truncation fatigue approach assuming that the composite material is brittle and there are no fatigue load sequence effects. Therefore, removing the low load cycles will not affect the fatigue life nor the damage initiation and growth processes. The fatigue threshold of the material is generally used to identify the loading cycles that can be truncated.