Fatigue of Prenotched Graphite Fiber Composites in Compression

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Abstract
Direct observation of compressive fatigue of prenotched unidirectional graphite fiber composites reveals that fatigue cracking extends transverse to the fiber direction, via microbuckling. An elasto-plastic-strain hardening model of the material behavior, based upon experimental measurements of material properties, is used together with a finite element computation to obtain the stress and strain distribution near the notch tip. The calculated radius Rp of the plastic zone at the notch tip is shown to correlate with the observed fatigue crack propagation rates via Fatigue cracking is seen to decelerate with crack extension, indicating a residual bearing strength of newly generated compressive fracture surfaces of approximately 10 4 Ib/in.2 The implications of compressive fatigue in composites design are discussed. It is pointed out that prevention of axial cracking in the composite is a key element in preventing compressive fatigue.

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