Fracture of Composite Compact Tension Specimens

Abstract

Fracture experiments were carried out on compact tension specimens of unidirectional and cross-ply S-glass/epoxy and graphite/epoxy. Fracture toughness values were determined by the compliance calibration technique and by measuring the area under the load-displacement curve. In unidirectional specimens, crack extension was always parallel to the fibers and was dependent on crack length. Toughness did not vary significantly with fiber orientation relative to the load direction in unidirectional S-glass/epoxy. Tests on cross-ply S-glass specimens were not valid because crack propagation did not occur; instead, a zone containing a system of superficial parallel cracks and other damage developed, which extended with increasing load. Cross-ply graphite specimens, on the other hand, did appear to give valid test results although the cracks propagated were not always straight and other damage mechanisms were also present. Toughness values for cross-ply graphite were approximately two orders of magnitude higher than for unidirectional specimens due chiefly to the fracture resistance of fibers transverse to the crack. Toughness values determined by the compliance calibration method were consistent with reported values obtained by other methods.