Study of failure mechanisms in bimaterial and multiphase material systems is motivated by the industrial importance of multiphase components made from metals, ceramics and polymer composite materials. An understanding of failure modes in these systems is relevant in applications ranging from advanced composite aircraft structures to packaging of electronic devices. Failure in such systems can be due to a variety of mechanisms: macro- and microcracking of a brittle matrix, cracking along interfaces between dissimilar materials, delaminations and so on depending on the specifics of the material system. Under a prior research grant from the NSF, the PI's research group has developed a compact Polariscope/Shearing Interferometer (PSI) that can be used as either a polariscope or a shearing interferometer. This single device allows one to obtain stress states from optically anisotropic materials (such as Homolite and epoxy) as well as essentially optically isotropic materials (such as PMMA and glass). The range of material mismatch parameters that can be provided by combining optically isotropic and anisotropic model materials essentially spans the range that is typically obtained in technologically important bimaterial systems, consequently results from the model material combinations have wide ranging applicability. Direct optical data regarding crack initiation from statically-loaded bimaterial systems have been obtained. In this work, full-field optical interferometric information will be obtained about the stress state around quasi-statically propagating interfacial and sub-interfacial cracks. The optical interferometers previously developed for monitoring crack initiation will be used in this study in conjunction with a relatively high-speed video recorder (6000 frames/see) to monitor quasi-static propagation. Simultaneous with the video recording of the optical data, load and load-point displacement data will also be recorded using a load cell and a linearly-varying displacement transducer mounted on the loading device, and these will be used in numerical modeling that can be of use in interpreting the experimental data. A series of experiments on bimaterial and multilayer sandwich specimens will be conducted. Phenomena of particular interest are: * the interfacial crack propagation regime where the crack propagates entirely on the interface between two dissimilar materials; * the crack kinking regime where the crack starts from the interface but kinks into the material with lower toughness; * the sub-interfacial crack propagation regime after the crack kinks from the interface and continues in the material with lower toughness; * the crack tunneling mechanisms in sandwiched layers. The data gathered, and the analysis performed, should provide us with additional information to gain a better understanding of crack kinking and crack tunneling phenomena, in addition to substantiating the current state of knowledge of issues of interfacial crack initiation and crack paralleling.
