Evaluation of interfacial normal strength in a SCS-0/epoxy composite with cruciform specimens

In this study, the cruciform geometry is utilized to evaluate the interfacial normal strength in unidirectional SCS-0/epoxy composites by using single-fiber specimens. These model specimens are incrementally loaded in tension to failure with a specially built loading device mounted on the straining stage of a microscope. The initiation and location of interfacial debonding is observed in situ by microscopic examination and inspection of photoelastic fringe patterns. The model specimens are also loaded to failure in an MTS machine while strain and acoustic emission activity are continuously monitored. Analytical modeling by the 3-D finite-element method reveals that the radial stress at the interface is the dominant stress component and remains fairly constant (within 10%) over two-thirds of the central loading region. As a result, interface debonding always occurs in the interior of the sample (region initially free of stress singularities), thus avoiding the influence of free-edge effects on measured debond strengths.