Abstract
An experimental program on fatigue of doubly notched [0/-60/+60] s graphite-epoxy specimens was performed which had two major objectives: 1. To adapt a photomicrographic technique, previously developed for detecting fatigue crack initiation in metals, for use with composite mate rials ; 2. To demonstrate the use of the technique in an exploratory fatigue program. Examination of enlarged images of photomicrographs, auto matically exposed at periodic interval during the tests, revealed that fatigue cracks developed early in the test. It was also evident that the test material had many manufacturing irregularities such as non-uniform ply-thicknesses and fiber distribution, incomplete ply bonding, and random voids. Two fatigue specimens sustained 10 Kc of high level cyclic loading and had a residual strength of approximately the static value; a third specimen frac tured on cycle 1 at 75% of the static strength. The inconsistent fatigue behavior may be due to the observed random distribution of manufactur ing irregularities.
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