Flaw growth thresholds in composites

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The objective of this research program is to propose experimental and modeling methodologies to determine endurance limit for damage onset in composite based on fracture mechanics and fatigue crack initiation monitoring using wave mode propagation based on Modal Acoustic Emission new approaches (MAE) [3-8]. Through an analysis of the guided wave mode propagation and signal contents, the approach will be developed to determine the endurance limit related to the onset of delamination and cracks initiation in composite materials, as function of cyclic loading and environmental conditions related to temperature and humidity. Analytical and numerical modeling tools will be performed to predict delamination onset and growth. Crack propagation of the starting delamination will be modeled by the Virtual Crack Closure Technique and Cohesive Zone method. Experimental procedures and analytical model will be proposed to determine composite stiffness loss induced by delamination initiation and propagation, and to measure delamination toughness over a wide range of mode I / mode II ratios as well as for pure mode I and mode II loadings. The total strain energy release rate G and its mode I (GI) and mode II (GII) components will be evaluated for the mixed-mode bending (MMB) test configuration using a fracture mechanics approach. Toughness test results and modal acoustic emission data related to crack initiation will be used to define the delamination initiation criteria envelope for the composite. These failure envelopes will enable predictions of delamination onset in composite materials.