A simplified analytical model to simulate martensite reorientation and plasticity in shape memory alloy ring couplers

Abstract

Recent studies on Shape Memory Alloy rings have been undertaken at the European Organization for Nuclear Research (CERN) to develop smart and leak-tight couplers for Ultra High Vacuum systems of particle accelerators. A special thermo-mechanical process (training) is needed to provide SMA rings with proper functional properties, that is to allow thermal mounting, dismounting, and leak tight coupling within a given service temperature window. Low temperature ring expansion is a crucial part of the training process as it gives suitable size, shape recovery properties, and thermal stability range to the SMA element. An analytical model, based on simplified elastic-plastic axisymmetric concepts, has been developed and implemented in a commercial software to simulate isothermal SMA rings expansions. It is particularly useful to predict the final size of a martensitic SMA coupler as a function of the initial dimensions and of the pre-deformation parameters. The effectiveness of the model has been demonstrated by analyzing the stress/deformation field occurring in a wide range of ring geometries for different load cases including martensite reorientation and plasticity. The predictions of the analytical model have been systematically compared with those obtained by axisymmetric finite element (FE) analyses based on elastic-plastic constitutive models and experimental measurements.

NiTi shape memory alloy pipe couplers for ultra-high vacuum systems: development and implementation

“…The curve clearly shows three distinct regions. They can be directly related to the typical stress-strain curve of a martensitic NiTi [29,30]: an almost linear trend mainly attributed to the elastic response of the material; a plateau, which is ascribed to the detwinning phenomenon and another almost linear trend, which is primarily due to the elastic deformation of detwinned martensite coupled with further reorientation and plastic mechanisms occurring at higher stresses.…”

“…The material used in this investigation is a binary NiTi alloy (50.8Ni-49.2Ti at.%); its TTs are reported in figure 2. The selection of this alloy was based on preliminary experimental studies on NiTi samples [29,30].…”

“…%) was selected in this study for ring manufacturing. Based on preliminary studies [29,30], proper thermo-mechanical ring training procedures were performed in order to tune the mechanical and functional response of the rings to be suitable for room temperature vacuum system applications. Ad-hoc NiTi-based connectors of different size (30-135 mm) were designed and developed by finite element (FE) and experimental analyses.…”