A Study of the Time Dependence in Fracture Processes Relating to Service Life Prediction of Adhesive Joints and Advanced Composites

摘要

This report deals with developments in understanding adhesive fracture as related to structural bonds and composite materials. Both fracture mechanics based considerations as well as material related aspects are considered experimentally. One thrust of the program with the detailed measurements of the crack opening profile of a crack at a polymer-rigid (glass) interface. Very high resolution optical interferometry allowed a careful examination of the linearity response of the displacements in the fracture-critical area. The suprising results was that even at average strains across the bond line (in shear and in bone-normal deformation) on the order of 0.2% significant deviation from linear response is readily apparent. In view of the need to better understand the effect of moisture and temperature on the time dependence of adhesion and matrix materials hygro-thermoviscoelastic behavior of two materials was studied. Resolution of apparently complex material behavior was accomplished through a 'free volume' argument for the material behavior. Based on this result the idea was close at hand that mechanical dilatation at relatively high stresses could lead to non-linear viscoelastic behavior, a concept that was born out with surprising accuracy in an analytical and experimental study. Description of pertinent (non-trivial) equipment developments round out this report.