This investigation concerns the validity and feasibility of using moduli reduction to monitor the effect of fatigue damage in graphite epoxy composites.

Five laminate orientations were considered, [O]₄, [90]₄, [±45]_s, [0,90]_s, [0,90,±45]_s, and four inplane-stiffness properties were monitored for each. The stiffness parameters were E_xx, E_yy, G_xy, and v_xy, and were measured using a longitudinal tension test, a rail shear test and a transverse bend test. Nondestructive testing techniques such as C-scan and edge replication were also performed to aid in the observation of damage development.

Results describe the response of each laminate orientation in tension-tension fatigue, including a record of changes in the stiffness properties at intervals during fatigue.

Longitudinal stiffness (E_xx) and shear stiffness (G_xy) were shown to significantly decrease for the [0,90,±45]_s, laminate following fatigue loading. The inplane stiffness properties for the other four laminates remain essentially unchanged following fatigue loading. Matrix cracking and delamination appears to contribute to the stiffness reductions that occur in the [0,90,±45]_s laminate.