A combined tension-bending test for assessing mixed-mode delamination growth predictions

A combined tension-bending (CTB) test is described for assessing delamination growth in laminated composite materials. The test specimen consists of a flat laminate with two symmetrically located delaminations. The specimen is subjected to axial tension by a standard load frame, and bending loads are applied through a dead weight configuration. The CTB test is somewhat more difficult to use than existing standardized tests, however, and is therefore not developed with the intention of general delamination toughness testing. Rather, the test may be used to assess the accuracy of mixed-mode I-II delamination growth predictions. That is, one may use this test, along with fracture toughness data obtained from standardized tests, to evaluate the predictive accuracy of any delamination growth prediction methodology. Such a test is important, as recent experimental evidence suggests that the commonly used classical approach to delamination growth prediction has relatively poor accuracy. It is shown that a simple closed-form expression may be used to determine energy release rate and mode mix for the CTB test, and that a wide range of mode mixities may be achieved by varying the location of the delaminations or the ratio of the in-plane and bending loads. The test is used to determine the toughness of an interlayer-toughened graphite/epoxy material under a variety of combined in-plane and bending loading conditions. It is observed that, in many instances, poor predictions of fracture by the classical approach are obtained. When the CTB test data is used to determine toughness, it is found that for high mode I loading configurations, a toughness lower than that observed in double cantilever beam testing (G_Ic) is obtained. At present, it is unclear whether this result is an artifact of the test method, or whether under certain conditions a delamination toughness lower than G_Ic is in fact possible. If this were proved to be the case, then this finding has important ramifications in delamination tolerant design of structures. Work is continuing in an effort to resolve this issue.