A proposal to modify the Kelly-Tyson equation to calculate the interfacial shear strength (IFSS) of composites with low aspect ratio fibers

The mechanical behavior of short-fiber composites generally differs from long-fiber composites. The mechanical response is influenced by the particular distribution of tensions and by the mechanism of load transfer from the matrix to the fibers. The transfer load from the matrix to the fibers occurs through shear stresses at the surfaces of the fibers (edges or ends effects). The end effects in long fiber composites, involving a small portion of fiber, are negligible; nevertheless, for short and very short fibers composites these end effects cannot be neglected. Interfacial adhesion between the reinforcement and the matrix plays an important role, in fact it influences both physical and mechanical properties of the composites, but the experimental determination is often laborious and analytical models are frequently used to evaluate the interfacial shear strength (IFSS). In this paper a modification of the Kelly-Tyson model for the calculation of the interfacial stress for short (aspect ratio < 20) and ultra-short fibers composites (aspect ratio < 10) has been proposed to take into account the end effects that in the original model were not considered. Successively, the Bader and Bowyer model (that derives from the Kelly-Tyson model) for the evaluation of the IFSS was also modified. A few examples of calculations of the IFSS, using this modified Bader and Bowyer model, have been provided using published literature data. Furthermore, a mechanical characterization of flax fibers has been carried out and their adhesion to poly(lactic) acid (PLA) matrix were evaluated for composites containing ultrashort fibers (aspect ratio < 10). The IFSS value obtained was compared with that obtained from the single fiber fragmentation test (SFFT). It was found that a very good estimation of IFSS can be done by using this analytical model that can be easily applied with a limited number of experimental tests.