Durability of adhesively bonded CFRP-aluminum alloy joints subjected to coupled temperature and alternating load

Abstract

The degradation of carbon fiber reinforced polymers (CFRPs) adhesive structures with exposure to combined temperature and alternating load has become a critical issue for safety considerations of CFRPs utilized in automotive, railway and other industries. However, the aging mechanism is not clearly understood. This paper presents an experimental study on the durability of adhesively bonded CFRP-aluminum alloy (CFRP-Al) joints subjected to a coupled temperature and alternating load. The effect of the CFRP surface roughness on adhesive performance was investigated, and then the optimal bonding scheme was determined. The joint performance was measured at −40 °C, −10 °C, 25 °C (room temperature, RT), 50 °C and 80 °C, and the durability tests of joints with exposure to separate alternating loads, temperatures and the combined effects were conducted with a special loading apparatus. The failure modes were investigated using visual observation and scanning electron microscopy (SEM) to explain the failure mechanism. The influences of temperature and alternating loads on the strength of joints were statistics analysis by the two-way analysis of variance (ANOVA). The results show that the adhesive property of the CFRP can be effectively improved by sandpaper polishing, and with decreasing roughness, the bearing capacity of the CFRP-Al joint increases. The failure strength of joints decreases with increasing temperature, especially when the test temperature is close to or greater than the glass transition temperature (Tg) of the adhesive. As the number of cycles of the alternating load increase, the joint strength declines slowly and then quickly. Exposure to a continuous increasing temperature also results in a decrease in the joint strength; this decrease is mainly due to the fiber/matrix interface damage of the CFRP at 80 °C, while it is determined by the degradation of the adhesive layer at other temperatures. Under the coupled action of temperature and loading, the damage occurs prevailingly in the adhesive layer, and the harmful effect of the alternating load is more significant in a high temperature environment.