The influence of the fiber-matrix microstructure on the energy release rate of off-axis tunnel cracks in laminates

Microstructural effects on the energy release rate and the mode mixity for tunneling cracks are investigated. A detailed microstructure is based on scanning electron microscopy of a real laminate. Based on a newly developed two-dimensional off-axis finite element formulation, the dependency of the Mode I, Mode II, and Mode III energy release rates on a range of different layup orientations are investigated. First, the influence of the fiber-matrix microstructures on the energy release rates is assessed and compared to results from homogenized models that exclude explicit representation of the microstructure. A finite element-based homogenization is found to provide consistent and accurate predictions, whereas analytical homogenization methods yield greater discrepancies with respect to a basic stress-based validation case and are thus not recommended in the context of tunneling crack modeling. Secondly, realistic non-straight crack shapes are included in the model resulting in similar energy release rates as for straight cracks in the corresponding fully homogenized models. However, modeling the realistic non-straight cracks without accounting for the specific microstructure provides too low energy release rates when compared to the realistic cases with microstructures.