Fatigue Properties of Unnotched, Notched, and Jointed Specimens of a Graphite/Epoxy Composite Available to Purchase

Within a continuing program on high tensile graphite/epoxy composite, stress-strain, axial fatigue, and compliance behavior of unnotched, notched (3-mm diameter hole), and jointed specimens made of [O₂/±45/O₂/±45/90]_s T300/914C laminates (177°C curing temperature) have been studied. In addition, the behavior of unnotched specimens cut from (1) the same laminate but with the longitudinal specimen axis now perpendicular to the zero-degree fiber direction, and (2) the high modulus fiber laminate with the same build-up was investigated.

Stress-strain curves, S-N curves, and increase-in-compliance versus percentage-of-total life curves were determined for all specimen types for stress ratios, R, ranging from R = +5.0 (compression-compression cycling, C-C) to R = +0.1 (tension-tension cycling, T-T).

An overall comparison of results from specimens with different stress raisers shows that the stress raisers diminish fatigue strength in the low-cycle range, but in the high-cycle range their influence has vanished. Effective stress concentrations were found to be different for compression and tension. During T-C cycling, increase of compliance was lowest for the fastener-filled no-load transfer joint and largest for the single-shear load transfer joint. The large compliance changes of the load-transfer specimens were attributed to increased bearing damage.

In general, the scatter in static and fatigue strength was found to be comparable with that for similar features in metals. When the plain material was loaded transversely instead of longitudinally, static and fatigue strength were lower by a factor of about 3.