Effect of adhesive thickness and adherend material with a surface roughness on lap shear strength of joints in tensile and compressive loading

The lap shear strength of epoxy adhesive is determined experimentally using adhesive-bonded single lap joints with two similar adherends, aluminium alloy and steel, following ASTM standards D1002 and D905 in tensile and compressive loading. Thicknesses of the adhesive layer are 25 mu m, 50 mu m, 75 mu m, 100 mu m and 125 mu m. Both tensile and compressive shear strength increase with increase in thickness of adhesive layer up to 100 mu m, and it decreases at 125 mu m of thickness of adhesive layer. Analysis of fractured surfaces of joints reveals that cohesive mode of failure dominated for the joint with 100 mu m of thickness of adhesive layer. Compressive shear strength of the joints is higher than the tensile shear strength of joints at all the thicknesses. Further, the shear strength of joints with aluminium alloy adherends is higher than that of joints with steel adherends for both types of loading. The higher strength of aluminium alloy joints could be attributed to the surface roughness and materials of the adherends. The effect of surface roughness on shear strength is more pronounced compared to the material of adherends, i.e. aluminium alloy and steel.