Crack Deflection Near a Ply Interface in a Composite Laminate

Fiber reinforced composites (FRCs) make up a large part of modern aircraft structures today and will continue this trend in the future aircraft, leading to increasingly significant fuel savings. One of the most attractive characteristics of FRC is the ability to tailor its mechanical properties with different composite layup designs. For a composite laminate with multidirectional plies, matrix cracking within off-axis plies is usually the first failure event upon loading. Historically, matrix cracking has often been investigated at a ply level with homogenized material properties. While such approach has its own value, it provides little insight into the mechanisms at the microscale where failure actually initiates and progresses. There are three stages of matrix cracking in general: the initiation, the intermediate stage when macro-sized crack forms through coalescence of individual micro cracks and final stage when macrocrack propagates towards ply interface. Realizing the limitation of traditional approaches discussed above, significant research efforts have recently been devoted to investigating matrix cracking at the constituent level mostly through the utilization of representative volume elements (RVEs). Despite the progress made, however, most of these studies focus on the initial and intermediate stages of the matrix cracking process, leaving unknown the behavior at the subsequent stage when a matrix crack approaches a ply interface. Existing fracture analyses based on homogenizing the plies into orthotropic layers typically predict a straight-line path of the transverse ply crack, which is in contradiction with the experimental findings where matrix crack branching and deflection near ply interfaces is often observed.

The present work aims to close the gap by investigating the behavior when macro-sized matrix crack approaches a ply interface and by clarifying the underlining mechanism governing the potential crack branching and deflection, with a focus on how microstructural nonuniformity affects the crack progression. In the current study, a cross-ply laminate [0/90]_s is modeled in a 2-D finite element (FE) model. An initial matrix crack is introduced in the 90° layers away from the 0/90 interface. The initial matrix crack is allowed to propagate towards either of the two interfaces. The 0° layers and a part of the initial matrix crack are modelled using homogenized layer properties to simplify the model. The nonuniformly distributed fibers are modeled explicitly close to the 0/90 interface in order to focus on the influence of this nonuniformity on the crack deflection process. Existing experimental findings have shown that the matrix cracking process is essentially the coalescence of individual fiber/matrix interfacial debond cracks. In the current study, a fracture mechanics-based approach is adopted to simulate the fiber/matrix interfacial debonding and the coalescence of individual debonds. The fiber/matrix interfacial debond crack is assumed to kink of out the interface when the driving force for debond growth is the lowest on the fiber surface and the kinked crack is assumed to propagate in a direction that would maximize the energy release rate of the crack tip. Preliminary results have shown a clear influence of the non-uniform fiber distribution on the predicted crack path and the ongoing work is generating more realizations of the non-uniform fiber distribution near a ply interface to further understand its influence on matrix cracking path as it approaches the ply interface.