The estimation of statistical fiber strength by fragmentation tests of single-fiber composites

Both evaluation of interfacial properties and statistical strength theories for the failure of unidirectional composites require information about fiber strength distributions and, in particular, the Weibull scale and shape parameters, σL and m, at some characteristic gauge length, L, such as the critical fiber length. In this paper, the in situ strengths of E-glass fibers with five different kinds of surface treatments in epoxy matrix have been estimated by fragmentation tests on single-fiber composites (SFC). The number of fiber breaks as a function of applied stress is simulated according to the Curtin theory for SFC with various fiber strengths and interfacial properties. The Weibull scale parameter, σo, and shape parameter, m, at short gauge lengths are determined for each type of fiber through best-fit of the calculated and measured numbers of fiber breaks versus applied fiber stress. Lower values of σo and higher values of m are obtained when compared with those obtained from single-fiber tension (SFT) tests for all kinds of fibers. However, the effects of surface treatments and gauge length on the fiber strength showed similar trends for both methods. It is found that the relatively low values of interfacial shear strengths calculated from SFC data are considered to be reasonable than those from SFT tests.