The meaning of the critical length concept in composites: Study of matrix viscosity and strain rate on the average fiber fragmentation length in short‐fiber polymer composites

Computer modeling is used in order to provide a theoretical understanding of the concept of critical length in composites, and of the factors influencing the critical aspect ratio. The effects of interphase and matrix properties have been investigated. We have identified the interphase parameters which minimize the critical length. Contrary to the assumption that the critical aspect ratio is related to interfacial shear strength and fiber strength only, we find a significant dependence on matrix viscosity and strain rate, for fixed interphase properties. We therefore conclude that the fragmentation test, which relates the measured critical aspect ratio to a value for the interfacial shear strength, has to be reinterpreted in terms of more parameters than those simply present in the Kelly‐Tyson formalism. Moreover, the significance of the concept of critical length for tailoring mechanical properties of composites needs to be reassessed.