Recommendations for an ASTM standardized test for determining G _IIc of unidirectional laminated polymeric matrix composites

This study was aimed at finalizing the details of a standardized test to determine the Mode II interlaminar fracture toughness of laminated composites. A large number of end-notched flexure tests were performed on glass/epoxy, two graphite/epoxies, and on graphite/bismaleimide specimens. Tests were performed directly from the preimplanted insert as well as from Mode II precracks. Results were combined with those from previous experimental and numerical studies and used to extract recommendations for the proposed standard. A compliance calibration method of data reduction was used. It was found that the results from only two compliance tests, along with the compliance from the fracture toughness test, are sufficient to characterize a specimen's compliance versus crack length relation. Furthermore, the unloading line and compliance data from the non-precracked toughness test can be used to accurately locate the new crack front, which allows determination of both the non-precracked and precracked toughness from the same specimen. It is shown that accurate and repeatable toughness values are obtained provided that the ratio of crack length to half-span length is 0. 6, that compliance tests are performed to a target maximum load corresponding to 0. 25 G_IIc at each crack length, and that toughness values are only accepted if the actual energy release rate during compliance testing is in-between 15 and 35 % of the computed value of C _IIc.