Fatigue design criteria for strengthening metallic beams with bonded CFRP plates

Different failure criteria used to prevent fatigue crack initiation in metallic members are described. A series of closed-form analytical solutions were developed to predict the fatigue resistance of metallic beams after strengthening with carbon fiber-reinforced polymer (CFRP) laminates. The model was used to determine Young’s modulus, pre-stress level and dimensions of the CFRP laminates such that the metallic detail is shifted from a ‘finite-life’ regime to the ‘infinite-life’ regime. This method is capable of predicting changes in both the stress range and the mean stress after strengthening. The results show that increasing the stiffness and pre-stress levels of the CFRP laminate can affect the fatigue life of the retrofitted member through different mechanisms. The latter preserves the alternating stress and decreases the mean stress level, whereas the former decreases both the mean and alternating stresses proportionally. To validate this model, a series of fatigue tests were performed on five steel beams, including one reference beam and four strengthened beams. The beams were strengthened with normal modulus (NM), high modulus (HM) and ultra-high modulus (UHM) laminates. Finally, a design example for the fatigue strengthening of a typical riveted metallic girder is presented. The developed analytical model was used to find the most effective fatigue strengthening solution under different cyclic load scenarios. Althoughthe major focus in this paper is on steel members, it also describes others, such as wrought iron and cast iron