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Strengthening steel structures using carbon fiber reinforced polymer (CFRP) materials has attracted much attention in recent years owing to their potential for fatigue crack repair and their convenience in construction. However, little is known about the efficiency of this strengthening method when applied to steel plates at different crack propagation stages. An experimental study was carried out on notched steel plates strengthened using CFRP laminates. 20 specimens were tested to evaluate the fatigue performance of the strengthened steel plates with emphasis on various degrees of initial damage, simulated by different lengths of slots, including 2%, 10%, 20%, 30% and 40% of the plate width. The effects of the retrofitted configuration and CFRP stiffness were also investigated. The "beach marking" technique and crack propagation gauges were adopted to monitor the fatigue crack propagation. The experimental results were very encouraging, demonstrating that the CFRP patches could effectively slow crack growth and extend fatigue life, regardless of the initial damage levels. More effective strengthening was found by using ultra-high modulus CFRP laminates, covering the initial cracks with CFRP and repairing at an earlier stage (i.e. smaller damage level).

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Cracked cross-beam connections made of thin-walled rectangular hollow sections (RHS) are repaired with carbon fiber-reinforced polymer (CFRP) sheets. Constant amplitude fatigue experiments are conducted on the repaired specimens. A pilot test is conducted on a T-connection of square hollow sections (SHS) to explore the effective method of applying CFRP. Due to the peeling effect at the corner region of the SHS-to-SHS connection, early debonding happens in the pilot test that only results in a slight extension in fatigue life. In the following experiments of repairing RHS-to-RHS cross-beam connections, circumferential or transverse restraining CFRP patches are applied in the corner region that prevents early debonding successfully and leads to significant increase in fatigue life. Finally, an improvement is made with the addition of steel strengthening plates that increase both the flexural stiffness and fatigue life of the cracked specimens significantly. The proposed retrofitting method may be useful for repairing other similar fatigue-cracked welded tubular connections.