Fracture Mechanics Applied to Adhesive Joints

Abstract

Since the advent of synthetic adhesives in the middle of the last century, the growth in their usage has been so phenomenal that a handbook, the International Plastics Selector-Adhesives. published by D.A.T.A. in 1991, lists some 5000 different adhesive formulations that are commercially available. Despite their wide usage, much is not well understood about adhesives and adhesive joint failure. A few observations relevant to this discussion of Fracture Mechanics Analysis of Adhesive Joints are: (a) The stress distribution in such joints is never as simple as often assumed, and the usually reported “standard test results” often completely ignore the most important aspects of the stress distribution. (b) Crack growth does not always initiate at the point of maximum stresses in an adhesive joint. (c) The locus of adhesive crack growth is often somewhat removed from the adhesive-substrate bond-line, but there are instances where it closely follows this bond-line. (d) Adhesive Fracture Mechanics analysis can provide significant information on the resolution of questions raised in these observations. This presentation will review some of the work in these areas by graduate students in our laboratory. Some of the cited work was accomplished by previous lab students. Other parts of the cited work are from ongoing research by current students. The presentation will endeavor to demonstrate that: (a) Adhesive Fracture Mechanics can be used to quantitatively predict the strength of adhesive joints and, more particularly, to elucidate the effects of such factors as adherend thickness, adhesive thickness (and variation), amount of overlap, geometric configuration, material discontinuities, etc. (b) Other factors being equal, crack growth tends to initiate at locations of maximum energy release rate irrespective of whether or not these are points of maximum stresses. (c) The preferred locus of crack growth is along those paths for which the energy release rate is a maximum. In each of these cases, results of a fracture mechanics analysis will be presented along with experimental verification of the analyses.