1449. Interphase fracturing of composite bodies: S Mostovoy and E J Ripling, Rep AD-639,317, Sep 1966 ( Materials Res Lab, Inc Richton Park, Illinois)

Abstract

Abstract : Cracking location for short time tests in the Dow epoxy resin (DER) 332, TEPA adhesive was found to depend on composition. At specific ratios of resin to hardener the crack path changed from center of bond (CoB) to interface (IF), depending on adherend material and surface treatment. For compositions where CoB fractures predominate, pre-cracked specimens exposed at zero stress, to either atmospheric humidity or liquid water, showed reinitiation toughness values higher than those seen in prior running or subsequent extension. This toughening is probably a result of the compressive stresses developed in the epoxy by water absorption around the crack tip. Static fatigue exposure of normally CoB pre-cracked specimens was found to cause significant slow crack extension at loads well below those required for rapid extension in short time tests. Debonding occurred starting at one IF directly in line but separated from the pre-existing crack. Propagation rates increased as either applied load or humidity was increased. Dynamic fatigue exposure of similarly CoB pre-cracked specimens also showed slow IF crack extension from the initial flaw. Rates of crack extension per cycle for this loading condition were found to be linear, increasing as stress amplitude or humidity was increased. Effects of a given maximum load and humidity were much more severe in the dynamic fatigue case than in the static. (Author)