Abstract
Layered structures are used in protection systems such as personal and heavy armor, windshields and also in functionally graded thermal barriers. The focus of this study is to understand the behavior of cracks in such systems, especially when the crack orientation is such that there are property changes along the crack front. Layered plates were prepared by bonding together sheets of epoxy and Polymethylmethacrylate (PMMA) using an epoxy adhesive. Among the two, epoxy has higher elastic modulus and lower fracture toughness compared to PMMA. Two different layer configurations; a single sheet of epoxy bonded to a single PMMA sheet (two layer) and a single sheet of epoxy sandwiched between two PMMA sheets (three layer) were considered. Single edge notched specimens were loaded in three point bending and the thickness averaged stress intensity factor (SIF) was estimated through photoelasticity. Subsequently, the behavior of crack-extension in these plates was also investigated. In both configurations, crack growth initiated in the epoxy layer first and extended stably before the start of crack extension in the PMMA layer. Once the crack extension started in the PMMA layer, the plate looses its structural integrity. It was observed that the onset of crack extension in the epoxy layer can be predicted using the thickness averaged SIF. A method using analysis of patched cracks is presented for estimating the load at which the plate completely looses its load carrying capacity. The estimates from this method match well with experimental results.
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