The response of notched AS4/PEEK specimens to fully reversed, tension/compression loading has been investigated by examining their fatigue lives, damage initiation and propagation, and their residual strength. AS4/PEEK specimens were subjected to R = -1 tension/compression cyclic loading at 52.4 and 64.3% of their monotonic compression strength. The results indicate a significant difference in the response of the material at the two cyclic stress levels. At the lower fatigue stresses, the predominant damage, as determined by X-ray radiography and by deplying, is characterized by matrix cracking and delamination that initiate at the notch and grow both perpendicular and parallel to the load direction. Stiffness measurements taken during the low-level fatigue history show that compression stiffness and tension stiffness degrade throughout the fatigue lifetime. Further, specimens fatigued at the lower fatigue stresses lost compressive strength as the damage developed while they gained tensile strength. Fatigue life was defined by reduction of compression strength.
Damage to specimens fatigued at higher cyclic stresses developed much more predominantly in the direction perpendicular to the loading and much less in the direction parallel to the loading. Stiffness measurements made on these specimens showed a more rapid degradation of tension stiffness than of compression stiffness throughout the fatigue life. As with specimens fatigued at the lower stress levels, residual compressive strength decreased with damage development. However, the residual tensile strength of specimens fatigued at higher stresses decreased with damage development and the fatigue failure modes were tensile.
The difference in the response of the graphite-PEEK laminates at the two cyclic stress levels suggests that damage initiation and damage propagation play different roles in defining fatigue response.