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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.
composite materials, glass epoxy, material fatigue, time dependence, tension, box beam, field failures, composites
Research associate, Massachusetts Institute of Technology, Cambridge, MA
Swiss Federal Laboratories for Testing Materials and Research, Dübendorf,