Prediction of load-displacement behavior of Fe-SMA-to-steel joints with localized bonding defects: analytical model and experimental validation

Interfacial integrity between Fe-SMA and steel substrate is of pivotal significance for Fe-SMA strengthened steel structures, but the bonding interface is prone to defects in practice. This study presents a novel analytical solution to investigate the influence of localized defects on the debonding behavior of Fe-SMA-to-steel joints. Closed-form derivations based on a generalised four-linear (GFL) bond-slip model were obtained to predict the interfacial slip, interfacial shear stress, Fe-SMA axial stress, and load-displacement response throughout the failure process with thirteen distinct debonding states. The GFL model demonstrated versatility by simplifying to established bond-slip laws (triangular, trapezoidal, or trilinear) for Fe-SMA-to-steel interfaces. Shear tests on single-lap joints with and without interfacial defects were conducted, and predictions are validated against both self-conducted experiments and experimental data given in the literature. The predictions showed good agreement with experimental load-displacement responses for specimens with various defect sizes. Notably, the solution successfully captures the snap-back, which is often difficult to observe in conventional displacement-controlled tests. Experimental results indicated that a central defect covering 33 % of the bonded region causes an approximately 26 % load drop at high load levels. Parametric analysis revealed that defect size primarily governed the post-peak instability, while its influence on ultimate load capacity was limited by the effective bonded length. The ultimate capacity was governed by the far bonded segment when the defect is near the loaded end, whereas a defect near the free end produces a marked post-peak load drop followed by recovery to a second peak comparable to the first. The proposed analytical framework provides a practical tool for engineers to assess the impact of bonding defects on the structural performance of Fe-SMA strengthened systems.