The need for lightweight materials for a variety of applications has resulted in the use of structural adhesives 10 bond prototype structures. Adhcsivcs developed to accommodate the stringent requirements of these high-technology applications are usually deficient in one or two of three very crucial properties: strength, moisture resistance, and toughness. So far, advances in adhesive formulation that have ameliorated one of these deficiencies have generally adversely affected the others. Hence a considerable amount of effort is being expended in the search for strong, moisture resistant, and tough adhesives.As adhesives become tougher and less brittle, evaluating their performance in terms of fracture parameters becomes more complicated. Linear elastic fracture mechanics (LEFM), which is widely used to characterize these materials, does not fully describe adhesive performance as more and more ductility or plastic deformation is introduced. In the study reported here, wc introduce the energy separation method for characterizing the fracture resistance of adhesives and compare it with currently used elastic and plastic fracture parameters such as G and the J integral. Both neat and bonded % CT plan specimens were tested and compared in this work.KEY WORDS: elastic-plastic fracture, structural adhesive, neat bonded fracture, fracture toughness, energy separation During a research program to develop a strong, moisture-resistant, ambient-temperaturecuring, structural adhesive, we found a consistent reduction in ductility (as measured by neat material tensile tests) as strength and moisture resistance improved. This effect was primarily due to the higher crosslink densities in the modified stock epoxy adhesives we were using. Our immediate concern, however, was the fracture resistance of the modified epoxies and how to reliably characterize it so that it could be related to joint performance (e.g., correlate modifications to adhesive chemistry with adhesive fracture parameters).A review of the fracture characterization options for our anticipated tougher adhesives indicated that the most useful parameters would be the elastic energy released, the plastic energy dissipated, and the potential energy of the test specimen at any crack extension. Lincar elastic fracture mechanics (LEFM), which is widely used to characterize these materials, would not be applicable as ductility or plastic deformation increased.The J integral [/] was explored because of its application to nonlinear elastic material behavior. For the adhesive geometry of interest the HRR singularity field could not be expected, but the J integral parameter still generally provides a useful experimental parameter to evaluate the relative material toughness of materials falling outside the LEFM regime.
