Why Should the "Alternative" Method of Estimating Local Interfacial Shear Strength in a Pull-Out Test Be Preferred to...

One of the most popular micromechanical techniques of determining the local interfacial shear strength (local IFSS, τ_d) between a fiber and a matrix is the single fiber pull-out test. The τ_d values are calculated from the characteristic forces determined from the experimental force⁻displacement curves using a model which relates their values to local interfacial strength parameters. Traditionally, the local IFSS is estimated from the debond force, F_d, which corresponds to the crack initiation and manifests itself by a "kink" in the force⁻displacement curve. However, for some specimens the kink point is hardly discernible, and the "alternative" method based on the post-debonding force, F_b, and the maximum force reached in the test, F_max, has been proposed. Since the experimental force⁻displacement curve includes three characteristic points in which the relationship between the current values of the applied load and the crack length is reliably established, and, at the same time, it is fully determined by only two interfacial parameters, τ_d and the interfacial frictional stress, τ_f, several methods for the determination of τ_d and τ_f can be proposed. In this paper, we analyzed several theoretical and experimental force⁻displacement curves for different fiber-reinforced materials (thermoset, thermoplastic and concrete) and compared all seven possible methods of τ_d and τ_f calculation. It was shown that the "alternative" method was the most accurate and reliable one, while the traditional approach often yielded the worst results. Therefore, we proposed that the "alternative" method should be preferred for the experimental force⁻displacement curves analysis.