NiTi Super Elastic Shape Memory Alloys for Energy Dissipation in Smart Systems for Aerospace Applications

摘要

Shape Memory Alloys (SMAs) have attracted the attention of a wide range of researchers from several disciplines. This is because they possess very special and unusual properties and have potential for use in numerous applications. The nuances and subtleties associated with these unusual properties indeed pose challenges for the discerning researcher and designer. Even more exciting and demanding is to conceive, build and test a device exploiting the intrinsic property of these materials for aerospaceudapplications where the weight and volume budgets are most stringent. Such an effort would involve material characterization, arriving at the design envelope of the material and effectively integrating it into the device. The SMAs exhibit complex non-linear thermo–mechanical behavior. They exhibit hysteresis both in the thermal and stress loading domains. SMAs can undergo large pseudo elastic deformations (typically 4 – 6%) in the low temperature martensite phase at low stresses. These deformations are completely recovered on heating to the high temperature austenite phase. This effect which is thermally induced is known as the thermal Shape Memory Effect. There is also a large pseudo elastic deformation which results from the application of stress in SMAs.udThis is obtained by stressing the SMA (loading) in the austenite phase and inducing stress induced martensite (SIM). During the process of inducing the SIM, pseudo elastic deformations of the order of 6 – 8% accrue. These ‘large pseudo elastic’ deformations are completely recovered when the stress is removed from the material. The removal of stress (unloading) takes the material back to the austenite phase. The path taken during unloading is different from the path taken during loading resulting in a stress hysteresis.udAssociated with the stress hysteresis is the dissipation of energy in these materials which is several orders higher compared to conventional materials such as steels,udaluminium or other metal systems. The objective here is to develop an alternate method to build an efficient smart landing gear device with superior energy dissipating features exploiting the large energy dissipating characteristics of Super Elastic (SE) SMAsudapplicable to a wide variety of vehicles. It is pertinent here to mention that polymeric carbon composites are gaining increased acceptance for airframes and other structural subsystems due to their superior stiffness and strength properties. The focus of this study is to systematically investigate the energy dissipation capability of NiTi SMA Systems and effectively blend them with carbon composites to realize efficient landingudgear systems. The heat treatment given to the SE SMA materials hold the key to the control of their mechanical properties. Like other smart materials SE SMAs possess bifunctional properties. Associated with the energy dissipation characteristics is the sensory function of SE SMA. The study covers the effect of heat treatment on theudenergy dissipation characteristics, evaluation of the sensory behavior, effects of strain rate on energy dissipation and other related mechanical properties. It also details the design, fabrication, analysis and testing aspects associated with integrating the NiTi SMA with the polymeric carbon composite based smart landing gear system that is applicable to a large category of ground and air vehicles.