Hygrothermal effect on interfacial properties of CFRP-steel joint modified by nanosilica

The interface between the carbon fiber reinforced polymer (CFRP) reinforcement system and steel substrates may deteriorate in service environments. This study is concerned with the effect of moisture and temperature conditions on the interfacial performance of CFRP-steel joints using neat/modified adhesive. Tensile and shear tests were conducted on both unmodified adhesive specimens and those modified with nanosilica to assess the impact of nanoparticles. Water immersion tests were carried out on adhesive specimens with different nanosilica contents to determine the water absorption law of the adhesives at different immersion temperatures. The interfacial properties of CFRP-steel bonded joints were tested before and after hygrothermal aging. The results show that nanosilica significantly influences the mechanical properties of the adhesive, as well as CFRP-steel joints. In dry conditions, tensile and shear strengths, Young's modulus and the elongation of adhesive are improved by increasing the nanosilica content up to about 1.4 wt%. A higher content of nanosilica reduces adhesive strength and modulus, while the elongation increases continuously with nanoparticles increasing to 7 wt%. The water absorption of the adhesive is significantly influenced by the content of nanosilica. More nanosilica particles lead to slower diffusion in adhesion and smaller saturated water absorption. A high immersion temperature speeds up the moisture diffusion, resulting in a larger moisture content and a more severe degradation in adhesive performance with the same immersion duration. Hygrothermal aging can be delayed by the addition of nanosilica with low content. The interfacial properties of CFRP-steel joints, including interfacial shear strength, load capacity and plasticity, are weakened by hygrothermal aging. Changing the main failure mode from cohesive failure to interfacial debonding may occur in CFRP-steel joints due to aging. The more severe deterioration is caused by longer immersion or a higher immersion temperature. However, the addition of nanosilica can enhance the durability of the aged joints in the long run by improving the water resistance, although peak shear stress decreases when the nanosilica content exceeds 1.4 wt%. The trilinear bond-slip model of reinforced adhesive joints may be reduced to a bilinear model by hygrothermal aging. The incorporation of nanosilica helps to mitigate the reduction in fracture energy due to water immersion at normal and high temperatures.