Evaluation and Characterization of Functionally Graded Adhesive Joints: Experimental and Numerical Analyses

Epoxy resins have exhibited exceptional performance in engineering applications, particularly as a replacement for traditional mechanical joints in adhesive bonding. This study evaluates the suitability of two innovative adhesives, SikaPower^®-1511 and SikaPower^®-1548, in various graded configurations. The thermal curing behavior of the adhesives was analyzed using differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR). Shear tests and finite element simulations were employed to investigate the strength performance and interfacial stress distribution of four adhesive configurations, including single and graded joints in single lap adhesive joints. The results show that SikaPower^®-1548 reveals a slower heat-curing rate and achieves an average shear limit load of 9 MPa, outperforming the more rigid SikaPower^®-1511, which reaches 4 MPa. Ultimate load predictions indicate that the shear strength of the 1511-1548-1511 graded configuration is slightly lower than that of SikaPower^®-1511, with a decrease of 8.86%. In contrast, the 1548-1511-1548 configuration demonstrates a significant improvement, achieving a 32.20% increase in shear strength, along with a 13.12% reduction in peel stress field intensity at the interface end and a 12.21% reduction in shear stress field intensity. Overall, the experimental and simulation results highlight the significant advantages of graded joints over traditional single joints in alleviating stress concentrations and enhancing joint strength. Additionally, the research confirms the potential of epoxy resins in advanced engineering applications, providing a reliable theoretical foundation and technical guidance for the design of graded adhesives.