Application of the MMB specimen in the measurement of mixed mode interlaminar fracture toughness

Static and fatigue DCB, MMB, and ENF tests were conducted to determine the mixed mode interlaminar fracture toughnesses of IM7/5260 carbon/bismaleimide and IM7/8320 carbon/thermoplastic materials at the following mode ratios: G(I)/G(II) = 0.0, 0.25, 1.0, 4.0, and pure mode II. The test results show that these materials exhibit significantly different fracture envelopes for both static and fatigue loadings. The performance of the MMB specimen was also analyzed using both linear and nonlinear finite element procedures to investigate the significance of large displacements in its response. This analysis studied the effects of thickness on the nonlinear response and showed that increasing thickness does not necessarily minimize the geometric nonlinear effects on strain energy release rate.