Bond strength measurement in composites—Analysis of experimental techniques

Abstract Two of the most common techniques used to measure fiber-matrix interfacial shear strength, the single-fiber fragmentation test and the microbond, have been analyzed and compared. Photoelastic and finite element analyses were performed to obtain the stress distribution at the fiber-matrix interface and its dependence on the loading and geometrical parameters. The effect of a penny-shaped crack in the fiber, in the fiber fragmentation test is shown to be one of the parameters governing the interfacial failure mode. It is also shown that loading conditions, meniscus formation, and fiber free length have a large effect on the distribution of interfacial stresses in the case of the microbond, which may explain the large observed scatter of experimental results. Furthermore, it is shown that the effect of the stress distribution is highly non-uniform, thus making the calculation of shear stress very inaccurate when single averages are considered. It appears that the single-fiber fragmentation test is more reliable than the microbond test because of its simplicity and the smaller number of parameters involved in its analysis.