Prediction of in situ strengths in composites: Some considerations

The classic formulation that relates the fracture toughness with the in situ strength of the ply implicitly assumes that a fracture process zone fully develops within the ply. This assumption, reasonable for conventional composite laminates, may not be appropriate when the ply thickness is very small or the fracture process zone very large. In the following it is shown how considering the R-curve of the material, the in situ strength for the cases when the fracture process zone cannot develop completely can be correctly computed. Closed form solutions are found for the in situ strengths, and for their maximum values that are obtained when the ply thickness approaches zero.