The critical stress or strain causing the onset of matrix cracking in composite laminates has been referred to as in situ transverse lamina strength. A majority of past studies have considered relatively brittle composites exposed to room temperature environments and static load conditions. In the current work, fracture mechanics analysis and test data for a toughened composite material that has a resin-rich interlaminar layer (RIL) were used to investigate in situ strength. Exposure to a range of environmental conditions was considered.
A parametric analysis study was performed to judge the effects of laminate and material variables. A finite thickness effect, indicating an interaction between ply group thickness and effective flaw size, was found dominant. The magnitude of the effect was directly related to RIL stiffness. In situ strength was found to decrease with decreasing RIL stiffness. Experiments with five different laminate layups and six different environmental conditions confirmed the analysis. This work indicates the need to use a fracture mechanics model of actual lamina microstructure and heterogeneous properties to predict in situ strength in materials with RIL.