Durability of aluminium alloy adhesive joints in cyclic hydrothermal condition for high-speed EMU applications

Abstract The representative adhesive structure of high-speed electric multiple units (EMU) is the side-window, the durability of which is concerned due to the degradation caused by changeable temperature and moisture in hostile service environment. It is of great significance to evaluate the adhesive structure's durability of side-window to prevent bonding failure. In this study, aluminium alloy single lap joints (SLJ), scarf joints (SJ) and butt joints (BJ) were manufactured to conduct accelerated aging tests under cyclic hydrothermal (CH)condition, in which the temperature and moisture vary from 80 °C/95% relative humidity (RH) to −40 °C/30%RH periodically. The adhesive Sikaflex®-265 were studied by Fourier transform infrared (FTIR), thermogravimetric analysis (TGA) and Differential scanning calorimetry (DSC). The failure strength of adhesive joints was tested after different aging cycles and the scanning electron microscopy (SEM) was also employed to investigate the microscopic fracture mechanisms. It has been found that the composition, glass transition temperature (Tg), melting temperature(Tm) and thermal stability of adhesive changes after CH aging. The failure strengths of SLJ, SJ and BJ show cubic polynomial descending with aging cycles, and loss about 30% of initial strength after 60 aging cycles. Moreover, the failure mode changes from cohesive failure to mixed failure after CH exposure, which is verified by SEM analysis. The failure criteria before and after CH aging were consistent with quadratic stress criteria, and an equation for a surface describing the variations of quadratic stress criterion with different aging cycles was built, providing reference for the durability evaluation of high-speed EMU's adhesive structures.