Durability of CFRP–Steel Double–Lap Joints under Cyclic Freeze–Thaw/Wet–Dry Conditions

Abstract

The usage of carbon fiber–reinforced polymer (CFRP) to strengthen cracked steel structures can effectively improve its bear capacity, so it has been extensively used in recent years. The degradation of interfacial bonding is one of the most important factors affecting the durability of CFRP–steel structures under a freeze–thaw(F–T)/wet–dry (W–D) environment. In this study, epoxy resin adhesive (ERA) dog-bone specimens and CFRP–steel double-lap joints (bonded joints) were prepared. F–T/W–D cycles experiment and tensile tests of the ERA specimens and the bonded joints were also performed. Under F–T/W–D cycles, the main properties of the ERA specimens and the bonded joints were examined. Results indicated that fracture failure occurred in all ERA specimens. The hybrid failure modes of fiber peeling on the surface of CFRP plate and the bonded interface peeling between the CFRP plate and ERA layer primarily occurred in the bonded joints. The failure of both of them can be considered to be brittle, which was unaffected by the F–T/W–D cycles. With increased F–T/W–D cycles, the ultimate load and tensile strength of the ERA specimens initially increased and then decreased, whereas the elastic modulus initially increased and then remained unchanged. The ultimate load of the bonded joints decreased gradually. Based on the relationship between the interfacial bond-slip parameters and the number of F–T/W–D cycles, the bond–slip model of the bonded joints was established. The proposed relationship was validated by comparing with the experimental bond-slip relationships and the predicted relationships under the F–T/W–D cycles.

Three-Dimensional Study of Polymer Composite Destruction in the Early Stages

“…The process of the destruction of CFRP material is divided into several stages. The initial and intermediate stages are associated with the occurrence of damage in the binder material and at the interface between the binder and the reinforcing elements [ 1 , 39 ]. The isolated breaks of individual fibers are also visible at such moderate loads; massive fiber rupture takes place in the final stage of loading.…”

“…The results of a structural comparison before and after loading show the occurrence of a significant number of cracks: transverse Cx in 90° layers and longitudinal Cz (splits) in 0° layers. The beginning of fracture processes is usually associated with the formation of transverse cracks in layers orthogonal to the direction of the load [ 1 , 36 ]. The occurrence of longitudinal cracks along the fibers has also been observed and described earlier in microtomographic experiments [ 19 , 20 , 21 , 22 , 23 , 24 ].…”

“…Fiber-reinforced plastics are fundamental for modern technologies in aviation and aerospace engineering and the automobile industry. They are widely employed due to their notable properties, specifically their low weight and favorable mechanical data, which originate from the CFRP material architecture [ 1 , 2 , 3 ]. Carbon-fiber-reinforced polymers consist of fibers of 4–8 µm diameter that are assembled into threads and prepreg plies of 70–200 µm thickness [ 4 ].…”