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Smart structures using Shape Memory Alloys

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Solid to solid phase transformation is a unique feature of Shape Memory Alloys (SMA) which makes it popular for many applications. Large recoverable strain, super elasticity and shape memory effect are the characteristics which make Ni-Ti an extraordinary material for structures to control seismic response. Two phases of this material Austenite and Martensitic have two different crystalline forms showing different behaviors in these phases. The phase transformation is attained by change in temperature or stress. SMA is able to revert back to its original shape and recover residual strains by transforming its phase using temperature change. Shape memory effect of two different alloys NiTi and Fe based SMA is used to enhance the structural properties of a steel Moment Resisting Frame as well as a RCC structure. The yielded steel beams need to be treated after the occurrence of earthquake. Shape memory alloy is proposed to be used at plastic hinge regions to recover the deflection due to its super elasticity property, thus creating a ‘psudo plastic hinge’, one capable of undergoing large strains and recovering them completely with no residual strain. Structural behavior of a steel frame with Shape Memory Alloy at beam column connection has been studied for lateral loading. Also the response of the frame was checked for time history analysis using past earthquake data. Comparison of the time history analysis response of bare frame (Steel only connections) and frame with use of SMA at connection suggested excellent performance of frame equipped with SMA . To check the performance of the frame for loading unloading cycle, incremental lateral loading is applied to the frame up to maximum load and then it is unloaded completely. The SMA equipped frame shows significant recovery of the residual displacement. The reduction in residual displacement of the SMA equipped frame is also seen in case of time history analysis. Though Ni-Ti SMAs show a little more recovery in residual displacement, cost comparison shows using Fe based SMA in Civil Engineering industry will be beneficial for the maximum utilization of material with lesser cost. In the concrete structures reinforced with conventional steel, the earthquake energy dissipation is provided through yielding of reinforcement and plastic deformation. Shape memory alloy (SMA) is considered as a replacement to conventional steel reinforcement. Pushover analysis is carried out for a Reinforced Concrete moment resistance frame with the traditional steel reinforcement replaced partially with Nickel-Titanium (Nitinol) SMA . Partial replacement of traditional steel reinforcement by SMA shows better performance.

This talk is part of the Engineering Department Structures Research Seminars series.

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