Fatigue Behavior of Cracked High-Strength Steel Plates Strengthened by CFRP Sheets

High strength steel is widely used in buildings, bridges, railways, and other structures. However, limited fatigue experiments have been conducted for cracked high strength steel plates with carbon fiber-reinforced polymer (CFRP) sheets. This paper describes an investigation into the fatigue behavior of unstrengthened and CFRP strengthened cracked Q345, Q460, and Q690 steel plates. First, the mechanical properties of these steels, which are dependent on the chemical composition and metallographic structure, were analyzed. Microalloying was found to refine the grain size and thereby enhance the strength. Second, 10 bare steel plates and nine strengthened steel plates were tested under fatigue loading at different stress ranges and stress levels (defined as the ratio of the maximum stress to the yield stress). For the unstrengthened specimens, the results show that the fatigue behavior at a 175.5 MPa stress range is best for Q690, followed by Q460 and then Q345. However, at the same stress level, the fatigue life of Q690 was shortest, followed by Q460 and then Q345. For the strengthened specimens, different failure modes were observed, and serious debonding occurred in Q690 steel under a 402.5 MPa stress range. CFRP strengthening technology can effectively increase the fatigue life (1.3–3.1 times) for all specimens except the Q690 specimen. The microstructure of the fracture surface was then investigated by scanning electron microscopy (SEM). Finally, a fit of material-related constants in fracture mechanics theory was obtained for the three categories of steel, and the fatigue life was predicted accurately using Paris' law.