Development and Characterization of a Self-Stressing Shape Memory Alloy (SMA)-Fiber Reinforced Polymers (FRP) Composite Patch for Repair of Cracked Steel Structures

Prestressed carbon fiber reinforced polymer (CFRP) patches are emerging as a promising alternative to traditional methods to repair cracked steel structures and civil infrastructure. However, existing prestressing methods require the use of heavy and complex fixtures to apply prestressing forces, which is impractical in many applications. In this research a self-stressing shape memory alloy (SMA)-fiber reinforced polymer patch is presented that can be used to prestress civil infrastructure, with a target application of repairing cracked steel structures. The self-stressing patch consists of nickel titanium niobium (NiTiNb) SMA wires embedded in a fiber reinforced polymer (FRP). The self-stressing patch can be bonded to the steel member in the vicinity of a crack. The prestressing force is generated by restraining the shape memory effect of the embedded NiTiNb SMA. The self-stressing patch is thermally activated and therefore does not require heavy equipment, but rather only a heat source or electrical power supply during the activation of the wires. This dissertation presents the development of the self-stressing patch and the characterization of the static and fatigue behavior of the patch. Different SMA and epoxy materials were tested to identify their thermomechanical properties and to select suitable materials for the patch. The bond behavior between two different types of SMA wires, superelastic Nitinol and NiTiNb, and FRP was evaluated experimentally. Based on the experimental observations an empirical model is proposed to quantify the minimum required embedment lengths between superelastic Nitinol and CFRP. The debonding mechanism between the NiTiNb and FRP was examined numerically using the finite element method (FEM). A trilinear cohesive zone model (CZM) was established, which incorporates cohesive and frictional components, to predict the pull-out behavior of NiTiNb wires embedded in FRP. The monotonic and fatigue behavior of the self-stressing patch were also characterized experimentally. This dissertation presents an empirical model that can be used to predict the fatigue degradation of the prestressing force in the patch. The research findings indicate that the self-stressing patch is able to generate a sustained recovery stress of 390 MPa. Patches for which the maximum applied loads in a fatigue cycle did not cause debonding of the SMA wires from the FRP exhibited fatigue lives up to 2 million cycles with less than 20% degradation of the prestressing force. The results suggest that the patch is a promising alternative to traditional methods of repairing structures with prestressed FRP patches.