Subsonic and Intersonic Dynamic Crack Growth in Unidirectional Composites

Some recent experimental observations of highly dynamic crack growth events in thick unidirectional graphite fiber-reinforced epoxy matrix composite plates are presented. The composite plates were symmetrically (mode-I) and asymmetrically (mode-II) loaded in a one-point bend configuration with an edge pre-notch machined in the fiber direction. The lateral shearing interferometric technique of coherent gradient sensing (CGS) was used in conjunction with high-speed photography. Symmetric, mode-I cracks initiated at 1300 m/s and subsequently accelerated up to the Rayleigh wave speed but never exceeded it. For asymmetric, Mode-II types of loading, the results reveal highly unstable and intersonic, shear-dominated crack growth along the fibers. The intersonic cracks propagated with unprecedented speeds reaching 7400 m/s, more than three times the shear wave speed of the composite, and featured a shock wave structure typical of disturbances travelling with speeds higher than one of the characteristic wave speeds in the solid.