چكيده به لاتين
Delamination has been introduced as the most crucial failure mechanism in laminated composites due to the absence of reinforcement through the thickness. The limited knowledge available on the phenomenon of interlaminar crack growth has led to a conservative design that results in the loss of some of the benefits of composites. In order to fully exploit the useful properties of composites, we need to scrutinize this phenomenon by researchers in this field. The main contribution of this study is to characterize the toughening mechanisms which increase the fracture toughness of laminated composites. In order to investigate this relatively complex phenomenon, [0]12, [0/90]6 and [05/90/06] carbon/epoxy laminates will be examined with two analytical and experimental approaches. Using the SEM imaging, fractography analysis of the failure surfaces was done. In the experimental part, the results indicate an increase in the fracture toughness of the cross-ply laminates comparison with unidirectional ones. According to the observations, it can be said that the reason for this increase is the difference in the form and manner of occurrence of energy absorption and dissipation mechanisms (the so-called toughening mechanisms), including the zig-zag crack propagation and fiber bridging, between different stacking sequences. There are no observed zigzag cracks in the unidirectional laminate, and the number of bridged fibers is much lower than cross-ply ones. In the analytical section, using the previous studies, the distribution of force is considered for the bridging zone. Then, using simple beam analysis, a relationship between the various parameters involved in crack growth is presented, and finally, the amount of energy dissipated by the fiber bridge mechanism is calculated using the integral J method. In the last part of this section, a method is proposed to quantify the effect of the zigzag crack growth mechanism, which is estimated to be about a 10% increase in the amount of fracture toughness. By removing the effects of energy dissipation mechanisms from the R-curve extracted from the test, it is observed that the hardening behavior of the graph is dropped to an acceptable level, which can confirm the materiality hypothesis of fracture toughness parameter in the laminated composite.
