Rehabilitation of Fatigue Sensitive Steel Elements Using Shape Memory Alloy/Fiber Reinforced Polymer (SMA/FRP) Composite Overlays
This dissertation presents the details of an experimental and numerical program that was conducted to evaluate the effectiveness of a new type of shape memory alloy (SMA)/fiber reinforced polymer (FRP) patch for repairing fatigue-sensitive steel elements. A nickel-titanium-niobium SMA was combined with carbon fiber reinforced polymer (CFRP) to develop a patching system to reinforce fatigue-sensitive steel elements. This thermally-activated system provides a more simple, practical and accessible method for prestressing composite patches than conventional prestressing approaches. Experimental results indicate that the composite increased the average fatigue life of fatigue sensitive elements by 15 and 26 times at stress ranges of 217 and 155 MPa, respectively. A digital image correlation (DIC) system was employed to study the interfacial debonding between the patch and substrate during fatigue loading. The influence of near-crack debonding on the strengthening effects was further studied using finite element (FE) methods. A linear elastic fracture mechanics (LEFM)-based numerical framework was established to perform a fatigue crack growth (FCG) analysis of steel elements that are reinforced with the SMA/FRP composite patches. The small crack propagation was taken into account using an equivalent initial flaw size (EIFS) method. The numerical framework was validated using the experimental results and the validated model was used to conduct a parametric study. The numerical study indicated two primary mechanisms by which the patches improved the fatigue lives of the repaired details: (i) activation of the SMA induced compressive stresses in the steel near the crack thereby increasing the critical crack length at which the element failed, and (ii) the CFRP provided a crack bridging effect which reduced the stress range in the steel locally near the crack thereby reducing the crack propagation rate. As a result, the synergistic effect between the SMA and FRP substantially extends the fatigue life of steel element. It was also found that the patch was more effective at increasing the fatigue lives of elements that were subjected to lower stress ranges. The research findings also indicated that early installation of the SMA/FRP patch results in much higher fatigue life improvement. However, installation of the patch at late stage of crack propagation, such as in an emergency repair can also significantly increase the fatigue life of cracked components providing an opportunity to mobilize a more permanent solution. Collectively, the research findings demonstrate that SMA/FRP patches are a promising new technology for repair of cracks in steel and other metallic structures.