Effects of Stress Ratio, Frequency, and Loading Time on the Tensile Fatigue of Glass-Reinforced Epoxy

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Abstract
The interaction of time under load and number of tensile fatigue cycles has been investigated for 0°/90° E-glass/epoxy laminates. The frequency was varied from 0.01 to 1.0 Hz, and the stress ratio was varied from 0 to 1.0 (static fatigue). Square wave and spike loading data indicate a higher initial strength but also a higher rate of loss of initial strength per decade of cycles for waveforms with less time at maximum stress. S-N curves tend to converge at low stress to the same number of cycles to failure, regardless of waveform. Long-term results are reported for fiberglass box beams fatigued for up to 108 cycles at relatively low loads, where matrix cracking and delamination damage were studied. A brief description is also given of field failures involving crack growth in a brittle mode, normal to the fibers.

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