Response of Thick, Notched Laminates Subjected to Tension-Compression Cyclic Loads

The fatigue response of a [(0/45/90/ −45)_s]₄ T300-5208 graphite/epoxy laminate with a drilled center hole subjected to constant-amplitude, fully reversed tension-compression loading was investigated. Damage evaluation techniques such as stiffness monitoring, penetrant-enhanced X-ray radiography, C-scan, laminate deply, and residual strength were used to establish the mechanisms of damage development as well as the relations between this damage and the stiffness, strength, and life of the laminate. Two load levels provided for fatigue lives of 10⁵ to 10⁶ cycles and significant stiffness reductions. Damage initiated at the hole as matrix cracking parallel to the fibers in all plies. Matrix cracks had a significant effect on delamination initiation and growth. Delaminations initiated near the surface in the densely cracked region at the hole and grew along major matrix cracks. Delaminations of smaller extent developed later throughout the interior of the laminate and followed similar growth patterns as those closer to the surface. Compressive properties degraded more rapidly than tensile properties. At the stress levels used in this investigation, residual tensile strength increased early in the fatigue life and remained approximately constant to near the end of life, when failure was precipitated by excessive laminate instability during the compressive portion of the loading.