Fracture Scaling Parameters of Inhomogeneous Microstructure in Composite Structures

Abstract

The influence of inhomogeneous microstructure on fracture properties is presented in this study. Previous tests have shown an improvement in the notched tensile strength of laminates manufactured with fiber placement versus hand layup for the same materials and laminate configuration. Fiber placement results in structures with a more inhomogeneous microstructure than in conventional hand layup manufacturing. Equivalent continuum modeling was employed to study this phenomenon. A specialty, couple stress (Cosserat Theory) finite element was developed to explore the influence of inhomogeneities in regions of high stress gradients such as notches. The results indicate that the addition of the couple stresses mitigates singularities at notch tips. In particular, it was shown that geometry alone is not sufficient for proper scaling if inhomogeneities exist. A two-parameter failure criterion, based on critical strain energy release rates and an inhomogeneity parameter, is proposed. Good correlations were found between predictions and experimental results.