Dynamic Fracture of Layered Plates Subjected to In-Plane Bending

Layered structures typically used in applications such as windshields, thermal protection systems, heavy armor, etc., have property jumps at the layer interfaces. Present study focuses on understanding crack initiation and propagation in such systems under dynamic loading particularly when the property jumps are across the crack front. Layered plates were fabricated by joining polymethylmethacrylate (PMMA) and epoxy sheets using an epoxy-based adhesive (Araldite). Single-edge notched (SEN) specimens were subjected to dynamic loading using a modified Hopkinson bar setup. High-speed imaging coupled with dynamic photoelasticity was used to record the crack-tip isochromatic fringes from which the stress intensity factor (SIF) history was obtained. In selected experiments, a pair of strain gages installed on surfaces of specimen was used to record the strain history in the layers, from which the SIF in each layer was obtained. The results indicated that, prior to crack extension, the strain in both layers was identical. The crack tips in the layers start extending at different time instants with the one in the relatively brittle epoxy layer extending first followed by the one in the PMMA layer. At low impact velocity, the delay obtained was significantly higher than that at high impact velocity. The speed of epoxy crack was lower initially due to the bridging of the crack by the uncracked portion of the PMMA layer till initiation of the crack in the PMMA layer. This effect reduced at higher impact velocity for which the delay was much lower and the cracks propagated at a higher-speed.