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The test methods to determine interlaminar fracture toughness of composite materials require values to be determined at delamination initiation from the thin insert. In woven composites, the location of the insert relative to the yarns will vary which may effect the toughness values at initiation. This work determined the effect of the location of the insert relative to the yarns on interlaminar fracture toughness using the double cantilever beam (DCB) and endnotched flexure (ENF) specimens under quasi-static and fatigue loads. The specimens were configured so that the insert end was placed at the edge of and in the center of a yarn and in a mixed position where the insert was at the edge in one ply above and in the center of the ply below. Quasi-static and fatigue tests were conducted for each configuration. The static tests on the DCB resulted in “stick-slip” type fracture, where the delamination grew rapidly across a transverse yarn to the beginning of the next transverse yarn and stopped until the load increased sufficiently to cause further stick-slip fracture. As the delamination propagated down the beam, the crack branched either side of the transverse yarns eventually resulting in the complete ply bridging the delamination. This resulted in an increase in G1c values. Delamination growth in the ENF static tests also produced crack branching and ply bridging that resulted in stable delamination growth. For both specimen types, the lowest values of Gc were obtained in the specimens with the insert in the center of the yarn and the highest Gc with the insert at the edge of a yarn. The scatter in the fatigue data prevented a clear indication of which insert position resulted in the lowest number of cycles to delamination onset with applied Gmax.
Head, Materials Engineering Research Laboratory,
Stock #: CTR10010J