Warren F. Knoff

Relationship between the tensile and shear strength of aramid fibres

The tensile failure morphology of aramid fibres is often highly fibrillated [1-4] and analogous to that of a uniaxially oriented fibre-reinforced composite which fails in tension via matrix shear failure initiated at fibre ends. A simple hypothesis regarding the relationship between tensile and shear strength of these fibres can be proposed by considering the individual fibres to be micro-composites and applying the concepts of engineering composite mechanics. The problem of stress concentrations in a macro-composite and the stress transfer process at fibre ends have been studied extensively [5-7]. Although details of these studies are different, in all of them, the magnitude of the shear stress concentrations between a fibre end and its neighbouring fibres is proportional to the tensile stress applied to the structure. I f aramid fibre failure occurs when the shear forces at a discontinuity exceed the shear strength of the bond between the fibrous subcomponents, then, for the case of elastic shear stress/ strain behaviour, the fibre tensile strength should be directly proportional to the shear strength. The experimental work described below was undertaken to test the above hypotheses. The filament tenacity, shear modulus and shear strength were determined for a series of 13 aramid yarn samples. Filament shear strength was estimated by combining the results of two fibre torsion tests as described below. The ultimate axial shear strain was estimated by measuring the filament strength as a function of twist and determining the threshold torsional strain (diameter × twist) on the filament surface at which strength loss began. The ultimate axial shear strain was then taken to be equal to the threshold torsional surface strain. This method was based on previous work with Kevlar (DuPont registered trademark) aramid fibres [8] in which the onset of strength loss and nonrecoverable deformation upon twisting was observed to correspond to the appearance of axial TA B LE I Tenacity and torsional properties of aramid fibres