Comparison of Lap Shear Test Specimens

Abstract

The single lap shear test specimen, a method for measuring the shear strengths of structural adhesives, was evaluated in a combined experimental and analytical study. The shear strengths of two structural adhesives were measured with the conventional thin adherend lap shear specimen and with several thick adherend configurations. Shear strengths obtained from thin specimens were too low: failure was controlled by the tensile (peel) strength of the adhesive/adherend interface rather than by the adhesive shear strength. Equally significant was the finding that the relative shear strengths of the two adhesives measured with the thin specimen configuration were opposite the results from the thick adherend specimens. The apparent shear strengths from thick adherend test specimens increased with increasing adherend thickness, an expected result; however, they decreased with increasing adhesive thickness, a trend which did not agree with published analyses. This anomaly is explained by finite element analyses of the shear and normal stress gradients in the adhesive layer.