Experimental observations of intersonic crack growth in asymmetrically loaded unidirectional composite plates

Some recent experimental observations of highly dynamic crack growth events in thick unidirectional composites are presented. The specimens used in th is study were 48-ply thick unidirectional graphite-epoxy composite plates w hich were either symmetrically (mode I) or asymmetrically (mode II) loaded by impact in a one-point bend configuration with an edge pre-notch machined in the fibre direction. Moderate impact speeds of up to 57 m s(-1) were us ed. The lateral shearing interferometric technique of coherent gradient sen sing in conjunction with high-speed photography was used to visualize the f ailure process in real time. Mode-I cracks propagated subsonically with cra ck speeds increasing to the neighbourhood of the Rayleigh wave speed. For a symmetric mode-II types of loading the results revealed highly unstable and intersonic shear-dominated crack growth along the fibres. These cracks pro pagated with unprecedented speeds reaching 7400 m s(-1): a speed which is m ore than three times the shear wave speed of the composite and almost equal to the dilatational wave speed of the composite along the fibres. For inte rsonic crack growth, the interferograms featured a shock wave structure typ ical of disturbances travelling with speeds higher than one of the characte ristic wave speeds in the solid. Evidence of large-scale frictional contact is also presented.