Effect of moisture-heat coupling on mechanical behavior of nano-SiO2 adhesives and CFRP-steel lap joints

The accelerated aging tests are carried out to examine the effects of moisture-heat coupling on the mechanical properties of nano-SiO2 adhesive specimens and the bonding behavior of CFRP-steel lap joints. The microstructure of the cross-section on the failed tensile specimen is observed by scanning electron microscopy (SEM). The results indicate that the glass transition temperatures (Tg,s, Tg,t) of the adhesive decrease when the aging time of water bath increases at 25 °C, while the glass transition temperatures increase at 55 °C and 70 °C due to the re-curing effect of elevated temperature. Then, the shape of the stress–strain curve remains constant when the adhesive specimens take the water bath at 25 °C and 55 °C, respectively. This shape varies obviously at 70 °C, characterized by an obvious decreasing strength and increasing ductility, respectively. Therefore, the moisture-heat temperature is recommended to be less than Tg,s for the adhesive in practical applications. Moreover, the water bath resulting in an apparent plasticization phenomenon around the pores that is the nano-SiO2 pulled out from resin matrix. In addition, the shear strength of CFRP-steel lap joint decreases because moisture-heat aging results in a decreasing bonding effect at the interfaces of CFRP-adhesive layer and steel-adhesive layer, respectively. After a water bath, the shear stress of the lap joint concentrates, and the toughness of interface decrease, resulting in a reduction of transfer length of shear stress.