Changes in the stiffness of graphite epoxy laminates were experimentally investigated as indicators of fatigue damage. Five laminate orientations were considered: [0]₄, [90]₄, [±45]₈, [0.90]₈, [0.90,±45]₈, and four inplane stiffness properties were monitored for each. The stiffness components were E_xx, E_yy, G_xy, and , 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]₈ laminate following fatigue loading. The inplane stiffness properties for the other four laminates remain essentially unchanged following fatigue loading. Matrix cracking and delamination appear to contribute to the stiffness reductions that occur in the [0.90,±45]₈ laminate, although the damage state and attendant stifness changes are not simple functions of the response of the constituent laminae.