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چكيده انگليسي :

113 Numerical and Experimental Investigation of Electro Thermo Mechanical Behavior of Shape Memory Alloy Thin Wires Pejman Shayanfard pejmanshayanfard@yahoo com Date of Submission 2012 03 05 Department of Mechanical Engineering Isfahan University of Technology Isfahan 84156 83111 Iran Degree M Sc Language FarsiSupervisor Mahmood Kadkhodaei kadkhodaei@cc iut ac irAbstract Shape memory alloys are a specific group of materials exhibiting special behaviors which are not seenin other materials One of these behaviors is shape memory effect which is related to fully recovery ofinelastic strain caused by loading due to heating The other outstanding behavior of these alloys ispseudoelastic effect Pseudoelastic effect is related to fully recovery of inelastic strains caused by loading in the course unloading Shape memory alloys have growing usage in several areas such as robotics aerospace biomechanics and military industries The aim of this thesis is to numerically and experimentallyinvestigate the electro thermo mechanical behavior of thin wires in the mode of shape memory effect Inorder to numerically simulate the electro thermo mechanical behavior of these alloys Mohagheghain selectro thermo mechanical model is used This model by appending the term of thermal heating is anextended version of Kadkhodaie s 1D thermo mechanical modelm which is derived in a continuumframework Thermal heating which has not being noticed in other constitutive models is important becausethe most logical and the fastest way to heat these alloys specially in actuator applications where timeresponse is of importance is applying electrical current Simulations contain strain recovery under constantand variable stresses during electrical heating A constant stress is applied with a dead load and variablestresses are applied using some bias springs In order to perform experimental tests as there was no suitablesetup required equipment for performing electro thermo mechanical tests under constant and variablestresses are made Because of the appearance of the intermediate phase during cooling in the tests it wasdecided to investigate the electro thermo mechanical behavior of these alloys in the presence of theintermediate phase In this manner the performed tests led to proposing a rhombohedral phase diagram Intermediate or rhombohedral phase plays a significant role in design of shape memory actuators andneglecting the effects of this phase may lead to considerable errors In the next part of the thesis in order toeliminate this intermediate phase samples are submerged in liquid Nitrogen to completely transformintermediate phase to martensite and therefore the initial phase in the tests would be pure martensite In aspecific pre stress time response of a sample in both steps of heating and cooling in three situations of puremartensite a combination of martensite and rhombohedral phases and pure rhombohedra are compared Also tensile tests are carried out on the samples in both pure martensite case and mixture of martensite andrhombohedral case Electro thermo mechanical tests for pure martensite case contain two steps one at aconstant stress and another at variable stresses In both constant stress and variable stress cases tests areperformed with various amount of pre stresses Since the intermediate phase just appears during coolingstage and also the model used here can only predict martensite to austenite and austenite to martensitetransformations just the forward heating stage is compared to the obtained numerical simulations Finallythe ability of the proposed model in predicting the electro thermo mechanical behavior of shape memoryalloys in the absence of intermediate phase is investigated Keywords Electro thermo mechanical model Shape memory alloy Pre stress Phase diagram Martensite Austenite Rhombohedral Intermediate phase

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بررسي رفتار الكتروترمومكانيكي سيم هايي از جنس آلياژ حافظه دار

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