The Fiber Break Evolution Process in a 2-D Epoxy/Glass Multi-Fiber Array
Edward D. McCarthy, Jae Hyun Kim, Nathanael A. Heckert, Stefan D. Leigh, Jeffrey W. Gilman, Gale A. Holmes
The mechanical integrity of a structural composite is strongly affected by the strength and toughness of the fiber-matrix interface/interphase , with interfacial shear strength (IFSS) being the accepted quanti-fying metric. However, the value of the IFSS is not directly measurable, but can be approximated by sev-eral micromechanics based test methods with the value obtained being dependent on the choice of the single fiber composite (SFC) model. The most popular of these test methods is the embedded single fiber fragmentation test (SFFT) which provides the experimental data needed to estimate the IFSS: (a) mean fragment length at saturation and (b) fiber strength at the critical fragment length. Because the IFSS is used in unidirectional composite models to predict strength and failure behavior, where the interaction between fibers can be important, the validity of extrapolating from test results based upon the repeated failure of a single isolated fiber has often been questioned. In this paper, the spatial distribution of fiber breaks in a 2-D array of glass fibers is compared with break locations observed from SFFT specimens. In both cases, the break locations in each fiber were found to evolve to a uniform distribution, thereby confirming that the ordered fragment lengths from the repeated fracture process conforms for both SFFT and multi-fiber fragmentation test (MFFT) specimens to a cumulative distribution function (CDF) derived by Whitworth [2-5]. The array break density was also observed to be less than the break density in isolated fibers, and break locations accross array fibers were observed to be highly coordinated and mostly aligned. The attainment of uniform break distributions in the single-fiber and multi-fiber arrays suggest that the IFSS obtained from the SFFT test may be a useful metric in composite models if the effect of fiber-fiber interactions on the mean fragment length can be independently quantified.