STRENGTH DISTRIBUTION OF CARBORUNDUM POLYCRYSTALLINE SIC FIBERS AS DERIVED FROM THE SINGLE-FIBER-COMPOSITE TEST

The single-fibre-composite (s.f.c) test, in which a fibre is embedded in an epoxy matrix and the composite tested in tension, was employed to obtain the statistical strength distribution of Carborundum SiC ceramic fibres over the range of gauge lengths from 0.5 to 20 mm. The raw s.f.c. test data was organized into three independent forms: the number of fibre breaks versus applied stress; the fibre fragment length distribution at the end of the test; and the fibre strength versus fragment length during testing. The data was interpreted using two different models of the fibre/epoxy-matrix interface, and it was found that a constant shear stress model could not self- consistently fit a II of the s.f.c. data, whereas an elastic interface model provided good fits to all of the data. The applicability of the elastic interface model was supported by the absence of interfacial debonding and the rough fibre/matrix interface, which promoted mechanical interlocking. The s.f.c. test derived strength of sigma(0) = 1500 MPa at a gauge length of 20 mm, with a Weibull modulus of m = 9, agreed fairly well with independent tension test results obtained on 254 mm length samples. Obtaining self-consistent fits to all of the manifestations of the s.f.c. data requires careful testing and analysis, but the present work demonstrates that the s.f.c. test can be a powerful tool for the accurate and independent assessment of fibre strengths at small gauge lengths.