Interfacial normal strength evaluation in unidirectional glass/epoxy composites

In this study, the cruciform geometry was utilized to evaluate the interfac ial normal strength in unidirectional glass/epoxy composites using single f iber 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 photoela stic fringe patterns and stress-strain data. Analytical modeling using 3-D FEM revealed that the stress concentration factor (value of radial stress a t the interface normalized by the externally applied stress) was at maximum and remained fairly constant (within 10 percent) over two-thirds of the ce ntral loading region. As a result, interface debonding always occurred in t he interior of the sample (region initially free of stress singularities), thus avoiding the influence of free-edge effects on measured debond strengt hs.