| Approaches for predicting interfacial failure of adhesive joints have been fairly well established. However, a systematic methodology that can be used to predict both failure of the adherend and failure of the interface has not yet been developed. The presented work uses cohesive-zone models to analyze the fracture behavior of an adhesively-bonded polymer-matrix composite. For this particular system, it has been found that the failure of the system involves competing crack paths such as cracking across the fibers, and cracking along the interface. To characterize fully the failure mechanism of the joint, cohesive-zone parameters for both the composite and the interface need to be determined. A series of experiments was designed to obtain these parameters under single-mode loading conditions (compact tension for mode-I composite, DCB for mode-I interface, and sandwich ENF for mode-II interface) and finite element calculations were performed subsequently to determine the values of these cohesive parameters by comparing the numerical results with the experimental observations. It is found that these parameters could be used without any modification to predict failure of other geometries with different dimensions under general loading conditions (single-edge notch composite specimens, DCBs with different thicknesses, and sandwich single lap-shear joints). It is shown that the toughness and the characteristic strength are often sufficient to describe fracture behavior in the presence of a crack. But when the characteristic dimensions (e.g., the initial crack length or ligament length) are very small, extra details about the cohesive law may be required. This work also demonstrates that, with the cohesive-zone approach, the engineering behavior for these geometries, such as strengths, deformation and dissipated energy, could be accurately described. In addition, it is also possible to address the phenomena such as transitions between stable and catastrophic crack growth in the composite and competing failure mechanism between composite and the interface for mode-I or mode-II composite joints. These phenomena could not be explained by conventional fracture mechanics. In summary, this study provides an effective analytical tool for the design of adhesively-bonded composite joints. |
