A simple strength of materials analysis was developed for a double cantilever beam (DCB) specimen to account for geometric nonlinearity effects due to large deflections and T-tabs. A new DCB data analysis procedure was developed to include the effects of these nonlinearities. The results of the analysis were evaluated by performing DCB tests on materials having a wide range of fracture toughnesses. The composite materials used in the present study were T300/5208, IM7/8551-7, and AS4/PEEK.
Based on the present analysis, for a typical load-point deflection/crack length ratio of 0.3 (for AS4/PEEK), T-tabs and large deflections lead to a 19 and 6% effect, respectively, in the computed Mode I strain energy release rate. Design guidelines for DCB specimen thickness and T-tab height were also developed in order to keep effects due to these nonlinearities within 2%.
Based on the test results, for both hinged and tabbed specimens, the effects of large deflection (compared to linear DCB results) on the Mode I fracture toughness (GIc) were almost negligible (less than 1%) in the case of T300/5208 and IM7/8551-7; however, for AS4/PEEK, the effect was from 2 to 3%. The effects of T-tabs (compared to a linear DCB with tabs) on GIc were more significant, with T300/5208, IM7/8551-7, and, AS4/PEEK showing a 5, 15, and 20% effect, in that order.