Interface end theory and fragmentation test

Fiber fragmentation test is one of the micro-mechanical test methods to measure the interfacial shear strength in fiber composites. Its result exhibits large discrepancy with those of other three tests (fiber pull-out, micro-indentation and micro-debond tests). It's noticed that there are two questions in the fragmentation test. First, if stress singularity exists at the interface end due to the fiber breaks; second, if de-bonding occurs at the interface end as the critical length is achieved. An axisymmetric model of interface end is used to analyze the stress singularity. With the aid of an asymptotic procedure and variable separation, a characteristic equation is derived to determine the eigen-value of stress singularity. The relation of stress singularity index and Dundurs parameters is presented. It is found that the fragmentation test has the most serious stress singularity at the interface end when compared to the stress singularity indices of these four test methods. This means that the result obtained from the fragmentation test is not exactly the interface shear strength (IFSS), and is not comparable to those determined from the other three methods. The question whether de-bonding at the interface end commences when the critical length is reached is also discussed.