Crack propagation paths in layered, graded composites

Layered, graded material structures exhibit anisotropies in elastic properties and failure resistance that can strongly influence the propagation of cracks, and hence overall structural integrity. This study examines the influences on propagation trajectory for cracks initially oriented parallel to the layers in layered, graded alumina-epoxy composites, produced experimentally by infiltration of layered porous alumina bodies. Notched specimens were tested under monotonic and cyclic bending loading. Finite element (FE) modelling was used to predict crack-tip stress fields and crack propagation paths. Measured initial crack deflection angles agreed with predictions from FE results, and observations from phase-shifted Moiré interferometry. Crack propagation paths showed good agreement with FE predictions, except when cracks were influenced by interfaces between layers. Influences of elastic property gradient, microstructural heterogeneity and toughness anisotropy at interfaces are addressed, and the implications for structural reliability of layered structures are discussed.