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.