Heat Activated SMA-CFRP Composites for Fatigue Strengthening of Cracked Steel Plates

Many studies exist on carbon fiber-reinforced polymer (CFRP) strengthening of cracked steel details (e.g., cracks in riveted or welded bridge connections). The majority of these studies used CFRP sheets without prestressing. The efficiency of such systems for fatigue strengthening is often limited because the high strength of CFRP materials is not utilized under service loads. However, prestressing small CFRP sheets is very difficult and requires hydraulic jacks and mechanical clamps, which makes the strengthening process very complicated or even impossible. Therefore, in this study, NiTi shape memory alloy (NiTi-SMA) wires and CFRP sheets were used to produce a heat activated prestressed SMA-CFRP composite patch. This composite patch offers both the benefits of the prestress of SMA and the high elastic modulus of CFRP. The proposed patch is relatively small (300 x 45 mm) and can be prestressed, which makes it a good solution for the strengthening of cracked bridge connections, for example, welded connections. In this study, the composite patch is used to enhance the fatigue performance of central cracked steel plates. Because the SMA wires were small (1 mm in diameter), firstly 45 SMA wires were converted into a sheet of SMA strands and then embedded into multiple CFRP layers, forming an SMA-CFRP composite patch, which offers an easy manufacturing process. Eight central cracked steel plates were tested under cyclic loading. For the specimens strengthened by prestressed SMA-CFRP composites, the fatigue lives were extended to 2.7 times (with normal modulus CFRP) and 6 times (with high modulus CFRP), respectively. A finite element (FE) simulation was employed to better understand and interpret the test results. The decreased effective stress intensity factor (SIF) range was the reason for the prolonged fatigue life of the cracked specimens. (C) 2020 American Society of Civil Engineers.