An elementary laminated beam theory and linear elastic fracture mechanics have been used to analyze initial core shear failure and corresponding initial crack sizes in foam core sandwich beams. The model is not intended for application to crack growth and kinking, which are observed in experiments. Predictions from the model were compared with previous experimental results from four-point bending tests of polymethacrylimide foam core sandwich beams having either unidirectional carbon/epoxy face sheets or woven E-glass/epoxy face sheets. The model was found to work best for sandwich beams having stiff face sheets, which minimize stress concentrations in the core at the loading points. For both types of beams, the predicted Mode II core shear crack sizes corresponding to fast fracture and sharp load drop in experiments were within an order of magnitude of the average foam cell size.
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