Selective use of rubber toughening to optimize lap-joint strength

This paper introduces a novel approach to increasing the lap joint strength, different from the traditional methods of tither increasing the lap joint area or altering the joint geometry. This: is accomplished by the selective use of rubber toughening in epoxy to optimize lap joint strength. This was accomplished in three stages. In the first stage an adduct was prepared, this was used to make bulk tensile specimens to calculate the bull, properties for various concentrations of rubber, i.e. 0, 10, and 20 parts per hundred parts of resin (epoxy). In the second stage finite element models were developed using the bulk properties previously obtained. Interfacial stresses were used to access the trends obtained by the selective use of rubber toughening at different locations of the overlap in different configurations. The modeling of adhesive joints was done using ALGOR 2-D, linear and nonlinear finite element analyses (FEA). In the third stage, tensile shear tests conducted on the lap joints validated the tl ends from the finite element models. Finite element modeling and meshing of the lap joints having 25.4 and 50.8 mm adhesive overlap lengths were completed. Different configurations of rubber toughened and untoughened adhesive were tried in these two overlaps. The validation was done by lap joint tests conducted on an Instron mechanical tester coupled with an extensometer. Comparable strengths were obtained fur completely toughened overlap and the configuration where only the edges of the adhesive overlap were toughened and the region in-between was untoughened. Also, the nonlinear FEA was shown to represent the experimental results more closely than the linear approach.