Interfacial Normal Strength Evaluation in Unidirectional Glass/Epoxy Composites

In this study, the cruciform geometry was utilized to evaluate the interfacial normal strength in unidirectional glass/epoxy composites using single fiber specimens. These model specimens were incrementally loaded in tension to failure using a specially built loading device mounted on the straining stage of a microscope. The initiation and location of interfacial debonding was observed in situ by microscopic examination and inspection of photoelastic fringe patterns and stress-strain data. Analytical modeling using 3-D FEM revealed that the stress concentration factor (value of radial stress at the interface normalized by the externally applied stress) was at maximum and remained fairly constant (within 10 percent) over two-thirds of the central loading region. As a result, interface debonding always occurred in the interior of the sample (region initially free of stress singularities), thus avoiding the influence of free-edge effects on measured debond strengths.