This paper proposes an experimental methodology to monitor the onset of delamination in real time and uses mechanical test methods for measuring interlaminar fracture resistance to determine mixed mode fracture initiation criteria. The main objective is to introduce crack initiation monitoring by acoustic emission (AE) wave detection that can be used to measure delamination toughness in pure mode Ⅰ and Ⅱ opening and over a wide range of mode Ⅰ / mode Ⅱ ratios. Double Cantilever Beam (DCB), End-Notch Flexure (ENF) and Mixed Mode Bending (MMB) samples were used to calculate strain critical energy release rates for mode Ⅰ, mode Ⅱ and mixed mode loading. The total strain energy release rate G and its mode Ⅰ (G_Ⅰ) and mode Ⅱ (G_Ⅱ) components were evaluated using beam theory for the DCB and MMB samples and load based compliance calibration for the ENF sample. These results were compared to a finite element model using the Virtual Crack Closure Technique (VCCT). The mode Ⅰ energy was plotted against mode Ⅱ in order to outline failure envelopes, which may enable the prediction of delamination onset in composite materials.
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