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چكيده انگليسي :

Investigation of Creep and Stress Relaxation in Shape Memory Alloy Wires Fateme Zare f zare@me iut ac ir September 17 2013 Department of Mechanical Engineering Isfahan University of Technology Isfahan 84156 83111 IranDegree M Sc Language FarsiSupervisor M Kadkhodaei Assoc Prof kadkhodaei@cc iut ac irAbstractIn shape memory alloys SMAs the shape memory effect and superelastic characteristics appearowing to the martensitic transformation MT In practical applications of SMAs they aresubjected to various thermomechanical loadings When an SMA is subjected to a mechanical load especially at high strain rates its temperature varies due to thermomechanical coupling in theresponse of these materials In thermomechanical loadings the mechanical loading unloading andthermal heating cooling processes in which the MT is completed are called full loop loadings orcomplete loop loadings If the loading directions vary during the MT before the MT is completed the loading processes are called subloop loadings or internal loop loadings Creep and stressrelaxation are induced by the MT according to variations in temperature under subloop loadings Thus if strain is kept constant during the transformation temperature change will cause stress todecrease during loading and to increase during unloading A decrease in stress under constant strainindicates stress relaxation and an increase in stress indicates stress recovery Similarly if stress iskept constant during the transformation temperature change will cause strain to increase duringloading and to decrease during unloading An increase in strain under constant stress indicatescreep and a decrease in strain indicates creep recovery In this thesis a 1 D thermo mechanicalmodel is proposed and equations are extracted in continuum mechanics framework for stress controlled and strain controlled loadings to study stress relaxation and creep in SMA wires Numerical analysis is done by developing a code in MATLAB To study stress relaxation and creepin an SMA wire three steps are considered for stress relaxation in the first step strain controlledloading at the specified strain rate is done up to a particular strain during transformation In thesecond step strain is kept constant and decrease in stress is seen indicating stress relaxation In thelast step the applied strain is again increased at the same strain rate as the first step until thetransformation is completed Similarly to study stress recovery loading followed by incompleteunloading is first done Then strain is kept constant leading to increase in the stress of an SMAwire At last unloading continues to the complete removal of the applied strain For creep firstly stress controlled loading is done up to a particular stress during transformation In the second step stress is kept constant and increase in strain is seen indicating creep In the last step the appliedstress is again increased until the transformation is completed In the same way to study creeprecovery loading followed by incomplete unloading is first done Then stress is kept constantleading to decrease in the strain At last unloading continues to the complete removal of theapplied stress Various relaxation and creep experiments were also performed on NiTi wires Forthis aim fabricating some equipment was required Thus the second part of this thesis correspondsto the manufacture of this equipment Finally various relaxation and creep experiments at differentambient temperatures and applied strain rates were done and the empirical findings were found tocoincide with the numerical results The results indicate that increase in the applied strain rate aswell as radius of an SMA wire gives rise to more pronounced stress relaxation and stress recovery Moreover increase in the applied strain rate and decrease in the radius of an SMA wire gives riseto more pronounced creep and creep recovery Key WordsShape Memory Alloy Wire Thermomechanical Stress relaxation Stress recovery creep creep recovery

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بررسي خزش و وارفتگي تنش در سيم هاي آلياژ حافظه دار

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