An alternative method of determining the local interfacial shear strength from force–displacement curves in the pull-out and microbond tests

The traditional way of determining the local interfacial shear strength (IFSS), τd, between the fiber and the matrix in microbond and pull-out test includes the measurement of the debond force, Fd, at which interfacial crack initiates, and subsequent τd calculation using the shear-lag analysis. However, for some fiber–matrix pairs the debond force, which manifests itself as a ‘kink’ in the force–displacement curve, cannot be reliably measured. We propose a new approach, based on measuring the maximum pull-out force, Fmax, and the initial post-debonding force, Fb, which does not require the knowledge of the Fd value. The comparison of the τd values calculated for several fiber–matrix pairs using three different methods of data reduction demonstrated that the new method yielded results very similar to the traditional one. Thus, it can be recommended as an alternative technique if the debond force is indeterminable.