Flexural Properties of Brittle Multilayer Materials: II, Experiments

An experimental investigation of the flexural properties of a variety of brittle multilayer materials has been conducted. The results have been compared with the predictions of a model presented in a companion paper. Three types of systems have been studied. The first consisted of glass sheets bonded together with a thermoplastic adhesive. The glass sheets had been precracked by indentation prior to bonding such that their strengths were essentially deterministic. The flexural response of this system is characterized by an initial linear elastic regime, followed by a stepwise reduction in the stress–strain curve during cracking. The second system consisted of thin glass sheets (without indents), also bonded with a thermoplastic adhesive. This system exhibited flexural characteristics similar to those found in the system with indented glass plates. In both systems, the measured stress–strain behavior was in good agreement with the model predictions, incorporating the strength characteristics of the constituent layers. The third type of system was a hybrid composite comprised of alternating layers of a brittle phase (either glass or Al2O3) and a carbon‐fiber‐reinforced epoxy composite. These systems exhibited an initial linear elastic response, followed by a regime in which multiple cracks formed in each of the brittle layers, with the exception of the one on the compressive face. This process generally occurred subject to a progressively increasing stress. None of the fiber‐reinforced layers failed during this stage. The peak stress coincided with the onset of failure of the fiber‐reinforced layers. The progressive failure of these layers was accompanied by steplike load drops, similar to those observed in the single‐phase glass multilayer specimens. The measured response of these systems was qualitatively consistent with the model predictions, though some discrepancies have been noted and their origins briefly discussed.