Fatigue of Prenotched Graphite Fiber Composites in Compression

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 R _p 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 10⁴ 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.

Keywords

Modmor Type I graphite fiber—4617 epoxy resin Thornel 75 graphite fiber—epoxy resin Celanese GY70 graphite fiber—R350A epoxy resin graphite fiber composites. Prenotched composites notched composites. Compression tensile bending torsion loading microbuckling. Tensile fatigue compression fatigue fatigue cracking stress defor mation strain-stress fracture moment curve. Prenotched fatigue. Compression composites. Calculations com puter SATEC SFI machine motion camera strain gages.

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