NDT characterization of brazed NiTi to carbon steel rods

Brazing of shape memory alloys (NiTi) to carbon steel is being studied for applications in civil engineering where the superelastic behavior of NiTi may be used for damping purposes. Since NiTi and steel have very different thermal and mechanical properties, brazing may be an effective and economic process. The present study aims to analyze the feasibility of a non destructive method based on eddy currents to characterize the deposited metal. Lap joints of NiTi to carbon steel were produced with a brazing alloy containing 20% Ag. The base materials had different shapes: NiTi had ribbon and wire shapes while carbon steel was in rods of 6 mm diameter. Eddy current with a helicoidal cylindrical coil was used for NDT characterization. Measurement of electrical impedance along the joint proved to be a feasible technique to characterize the morphology of brazed joints with a good resolution. The evaluation of mechanical behavior in the superelastic regime showed that the joints can undergo up to 60 cycles of load / unload without rupture, even when the maximum load of the superelastic plateau was close to the pull-out strength of the joint.