Abstract: A theoretical model of general delamination cracked laminates is established based on the classical laminate theory and the bilinear cohesive zone model. The crack propagation in Mixed-Mode Bending (MMB) specimen is studied considering both I and Ⅱ mode cracks. A crack superposition model of I and Ⅱ mode cracks is proposed, followed by the introduction of the rigid body rotational displacement of the mode I crack component. The closing effect of the central load component on the mode I crack propagation is also considered in the MMB tests. According to the mechanical responses in the cohesive zone, a piecewise general solution to the displacement functions is obtained considering that the crack could be larger than half of the specimen length. Combining the boundary and continuity conditions of the superposition model, we analyzes the crack propagation process in the MMB test, obtaining the load-displacement curves. Comparison with both predictions of the beam model and the existing test results verifies the effectiveness and accuracy of the proposed model for I-Ⅱ mixed mode crack propagation. The model is further applied to the discussion of the effect of mode mixity ratios and the closing effect. The results show that the closing effect is more intensive when the initial mixity of the mode Ⅱ crack is relatively larger, which can lead to the closure of the mode I crack. During crack propagation, the mode mixity ratio remains essentially constant when the crack length is smaller than half of the specimen length. Conversely, as the crack length exceeds half of the specimen length, the crack mode gradually degenerates to a single mode fracture with a relatively large initial mode mixity ratio due to the closing effect.

Key words: MMB tests, delamination, cohesive zones, mode mixity ratio, crack superposition