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

Adhesive-bonded joints have been used widely for composite materials as a n ecessary alternative to conventional mechanical joint designs. In a bonded joint, the load is transferred from one substrate to the other mainly throu gh adhesive shear stress. One of the greatest drawbacks to predicting the m echanical behavior of bonded joints has been the lack of reliable data on t he mechanical properties of adhesives. Among many test methods that have be en 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 join t, through both the bondedline thickness and overlap length, and the measur ement method described in the test method, some errors will be introduced i f corrections are not made. A finite element analysis was conducted in orde r 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 introd uced 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 us ed, only adhesive shear modulus within the linear range is discussed in thi s article.