Evaluation and adjustments for ASTM D 5656 standard test method for thick-adherend metal lap-shear joints for determination of the stress-strain behavior of adhesives in shear by tension loading

Abstract

Adhesive-bonded joints have been used widely for composite materials as a necessary alternative to conventional mechanical joint designs. In a bonded joint, the load is transferred from one substrate to the other mainly through adhesive shear stress. One of the greatest drawbacks to predicting the mechanical behavior of bonded joints has been the lack of reliable data on the mechanical properties of adhesives. Among many test methods that have been developed to test structural adhesives in thin film geometries, the ASTM D 5656 thick-adherend lap shear test is used frequently to determine the shear properties of adhesives while the samples are loaded in tension. Due to the nonuniformity of adhesive shear stress distribution within the joint, through both the bondedline thickness and overlap length, and the measurement method described in the test method, some errors will be introduced if corrections are not made. A finite element analysis was conducted in order to provide a clear picture of the mechanical behavior of the ASTM D 5656 specimen under loading. Based on the results from finite element analysis, the sources of error were analyzed and three correction factors were introduced to recover the adhesive shear modulus of the specimen. Suggestions of mounting the KGR-1 measurement device are also given in order to avoid some of the errors. Because results from linear finite element analysis were used, only adhesive shear modulus within the linear range is discussed in this article.