The effect of adhesive modulus on the performance of SMC lap shear joints

Abstract

This study examines the effect of Young's modulus of an adhesive on its performance in a single overlap specimen when the adherend is a sheet molding compound (SMC). A series of adhesives with various moduli were fabricated from an epoxy resin/flexibilizer/taic/curative combination with moduli between 21 MPa and 6.89 GPa. The strength at break increased by 50% from around 2.41 MPa to 3.79 MPa when the adhesive's modulus was reduced from 6.89 GPa to 21 MPa. Joint failure was predominantly catastrophic failure in the SMC. Finite element analysis (FEA) for a model lap shear joint produced similar results. The analysis showed that the principal stress in the SMC, at the interface, was substantially reduced when the modulus of the adhesive was much lower than the SMC's modulus. Based on experimental results and FEA, the Young's modulus of the adhesive should be approximately 400 times smaller than the modulus of the SMC when the thicknesses of the SMC and adhesive are 2.54 mm and 760 μm, respectively. Consequently, the load needed to initiate failure in the SMC was increased. These results illustrate the importance of optimizing the material properties of the adhesive to achieve higher performance.