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چكيده انگليسي :

130 Theoretical and Experimental Study of Behavior of Shape Memory Alloy Springs Behnam Heidari Dahooei Behnam hd68@yahoo com January 20 2014 Department of Mechanical Engineering Isfahan University of Technology Isfahan 84156 83111 IranDgree M Sc Language PersianSupervisor M Kadkhodaei Assoc Prof kadkhodaei@cc iut ac irAbstract Shape memory alloys SMA s are a class of smart materials with the intrinsic properties ofshape memory effect SME and pseudoelasticity PE due to their solid solid martensitic phasetransformations under various thermomechanical loadings Owing to these features SMA s havebeen used in many engineering fields in various forms including spring as actuator elements Tofabricate SMA elements with a desirable shape e g springs hooks and Belleville washers fromelementary products e g bars wires ribbons and sheets specific treatments called shapesetting are required Shape setting is accomplished by deforming an SMA specimen to a specifiedshape constraining the configuration and conducting appropriate heat treatments In this work SMA helical springs are produced by using two sets of NiTi Ti 55 87 at Ni wires one ofwhich showing SME and another showing PE at the ambient temperature Annealing is performedwith the use of a tube furnace under inert atmospheric Ar Different pitches as well as annealingtemperatures are tried to investigate the effect of such parameters on the thermomechanicalcharacteristics of the fabricated springs Phase transformation temperatures of the products aremeasured by differential scanning calorimetry DSC and are compared with those of the originalwires Mechanical properties of wires and springs are achieved by tensile and compression tests atdifferent temperatures Compression tests are also carried out and stiffness of each spring isdetermined The different desired pitches are so that some springs experienced phase transitionduring loading while the others did not The former showed a varying stiffness with the appliedload but the latter act as passive springs with a predetermined stiffness Stiffness and phasetransformation temperatures of the made springs are concluded to be influenced by the annealingtemperature as well as the adjusted pitches Also the material parameters for modeling are achievedfrom the experimental results In this study a model is presented to investigate both the SME and PE of the SMA springs An enhanced one dimensional constitutive model is assumed to describe the shear stress strainresponse within the coils and equilibrium as well as geometric consistency is utilized to evaluatethe axial response of an SMA spring It is noteworthy that the von Mises yield criterion is used inthe extended model one dimensional model and the concepts of effective stress and strain wereproperly employed to derive shear stress strain response based on a 1 D model for axial loadings l o unl ke the p ev ou wo k ne the the Po on t o umed to e 5 no the m x mumrecoverable shear strain is considered to be equal to the axial one The numerical results are shownto be in a good agreement with experimental data indicating validity of the proposed approach Theeffects of variations in the material parameters on the axial force displacement response of an SMAspring are further investigated Key Words Shape Memory Alloy Helical Springs Shape Setting Transformation Temperatures Constitutive Equations Static Equilibrium Equations

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مطالعه ي نظري و تجربي رفتار فنرهاي حافظه دار

فنرهاي مارپيچ حافظه دار , تثبيت شكل , دماهاي استحاله , معادلات ساختاري , معادلات تعادل استاتيكي