The effect of nonlinear viscoelasticity on interfacial shear strength measurements

Experimental evidence demonstrates that diglycidyl ether of bisphenol-A (DG EBA)/meta phenylenediamine (m-PDA) epoxy wsin matrix used in the single fib er fragmentation tests exhibits nonlinear stress strain behavior in the reg ion where E-glass fiber fracture occurs. rn addition, strain hardening afte r the onset of yield is observed. Therefore, linear elastic shear-lag model s and the Kelly-Tyson model are inappropriate for the determination of the interfacial shear strength for this epoxy resin system. Using a strain-depe ndent Secant modulus in the Cox model, the calculated interfacial shear str ength is shown to be relatively lower by at least 15% than the value determ ined using a linear elastic modulus. This decrease is consistent with numer ical simulations which show the linear elastic approximation:over predicts the number of fragments in the fragmentation test. In addition, the value o btained by the strain-dependent secant modulus is approximately 300% relati vely higher than the value predicted by the Kelly-Tyson model.