Characterization of a Graphite/Epoxy Composite Under In-Plane Shear Fatigue Loading

To examine the in-plane shear behavior of graphite/epoxy material in fatigue, the three-rail shear test specimen, normally used for static testing, is modified by inserting notches on the edge of the specimen. The performance of the notched specimen shows great improvement in both static and fatigue loading conditions. Therefore, by using the notched specimen, a more reliable experimental data base can be obtained.

The main objective of this research is to fully characterize a graphite/epoxy composite under in-plane shear fatigue loading by using the modified notched specimen. To fully characterize the material under in-plane shear, both static and fatigue tests are performed. Fatigue life, residual strength, and residual stiffness of AS4/3501-6 are measured. Experimental results from both static and fatigue loading show a very clear mode of failure under in-plane shear in the gage area of test specimens. To measure the fatigue life, a cyclic load is applied to the specimen and tests are continued until catastrophic failure is achieved. To measure the residual strength and residual stiffness, the fatigue loading is stopped after a fixed number of cycles (for example, 100, 1000, and so forth) and the residual properties are measured by applying a static load and destroying the specimen. Nonlinear stress-strain behavior of the material under static loading conditions is observed and the effect of fatigue loading on the nonlinearity is studied and discussed. To study the effects of mean stress on the fatigue behavior of the material, fatigue tests are performed under different stress ratios (k = σ_min/σ_max). The fatigue life, residual strength, and residual stiffness of the material are presented in normalized forms that can be used to predict the fatigue behavior of the composite material, under arbitrary state of stress and stress ratio, based on the presented experimental data base. Thus, the behavior of AS4/3501-6 graphite/epoxy composite under in-plane shear fatigue loading is fully characterized.