THE SINGLE-FILAMENT-COMPOSITE TEST - A NEW STATISTICAL-THEORY FOR ESTIMATING THE INTERFACIAL SHEAR-STRENGTH AND WEIBULL PARAMETERS FOR FIBER STRENGTH

For over two decades the single-filament-composite (SFC) test has been art important tool in the study of the failure of fibrous composites. The SFC test itself involves a single brittle fiber embedded along th e center-line of a matrix specimen of both large cross-sectional area and strain to failure. With increasing strain, the fiber fractures pro gressively, breaking into an increasing number of shorter and shorter fragments. Surrounding each break a shielded or exclusion zone develop s within which no further breaks typically occur. At some strain level 'saturation' occurs abruptly as the shielded zones finally occupy the whole fiber, thus leaving a final distribution of fiber fragments end -to-end. Two toes for the SFC test have emerged: one has been to estim ate the interfacial shear stress, tau, in the exclusion zone, sometime s called the interfacial shear strength and usually idealized as a con stant over this zone. The other has been to estimate the fiber strengt h distribution and in particular the Weibull shape and scale parameter s, rho and sigma(1), for fiber strength appropriate to some characteri stic 'gage' length, 1, such as the mean fragmentation length. In the p ast, theoretical bases for these estimates have handled the statistics of shielding in ways that have led to quite lai ge biases. The purpos e of the present paper is to use some recent theoretical advances to d evelop more sophisticated estimation procedures for tau and the Weibul l fiber strength parameters 'in situ', and thus to eliminate various e rrors in previous methods. Straightforward computer programs (written in release 3 of Maple), which calculate the various quantities in the paper, will be provided by the first or second author on request. (C) 1998 Elsevier Science Ltd. All rights reserved.