Research Of The Constant-force Elements Characteristics Based On NiTi Shape Memory Alloy Strips

Since 1963,in which the shape memory phenomenon of NiTi shape memory alloy(SMA)was found by Dr.Buehler and collaborators—members of the United States Navy ordnance laboratory,persistent and extensive research to the mechanism of shape memory effect and superelasticity closely related is performed and many commercial applications have been developed today.Now,NiTi SMA has been widely used in medical devices and industrial fields for its excellent biocompatibility,comprehensive mechanical properties,shape memory effect and superelasticity etc.In addition,its constant-force behavior,which resulted from the stress-induced martensite phase transformation,has also been used specially in related devices design.It is easy to get the behavior by stretching a NiTi SMA wire,when the stress reaches the critical stress of martensitic transformation,the constant-force behavior occurs.The devices embedded with NiTi SMA wires as constant-force elements have been used in practical applications already.However,it has many limitations for the NiTi SMA wires as a constant-force elements.First,the wire can be stretched but can not be compressed,while the major operation mode is compression in many occasions,so complex conversion mechanism is needed,even it is not available due to the narrow operating space.Second,it is difficult to change the values of the constant-force arbitrary and to fix the wires with the parts.The approaches for design constant-force elements with other profiles of NiTiSMA are mainly through substantial tryouts,there is no universal and systematic design principles for its shape design,deformation regularity,the adjustment of constant-force values etc.Therefore,NiTi SMA's applications as constant-force elements are greatly restricted.This research aims at above issues,whose goal is to develop a kind of universal design principles of NiTi SMA constant-force elements with simple geometric structures and convenient adjustment of constant-force values and manufacturing.The research contents and conclusions are as follows:1.Research on geometric structures of NiTi SMA constant-force elements by simulations.Based on the software ANSYS and ProE,the probable constant-force structures were acquired through preliminary analysis,with which a great deal of simulations and optimization are performed and the influence of each geometric parameter on the constant-force behavior is summarized.At last the universal constant-force structures design formula is summed up and design basis of NiTi SMA constant-force elements is provided.2.Research on experimental verification of NiTi SMA constant-force elements.The simulations results above have some limitations because the constitutive model of NiTi SMA in ANSYS is an approximate expression.Verification and modification for the principles are necessary so that it has actually application meanings.A great deal of heat treatment experiments and test experiments are performed and available heat treatment programme for the constant-force elements is obtained.Based on the elements test experiments,verification and modification for the simulation results are achieved.3.Research on arbitrary constant-force values design of NiTi SMA constant-force elements.In the light of various constant-force values in application,the relationship between constant-force values and the width of elements(load displacements)are studied by simulations and its design rulesare achieved.According to these,NiTi SMA constant-force elements of various constant-force values can be obtained conveniently,which establishes a foundation for the practical applications of the constant-force elements in different fields.4.Research on an application example of NiTi SMA constant-force elements.The usage of NiTi SMA constant-force elements is mainly divided into two types: one is the sustained constant-force output during working;the other is safety protection purpose,the largest output force is limited to a preset value(that is constant-force value).Here is an example of safety protection: a constant-force impact absorption mechanism.Through experiments it indicates that this mechanism can reduce the impact force substantially,and more compact than the common compression springs,thus it is especially suitable for the narrow space.Furthermore it can limit the impact force to the preset value and provide safety protection function.Therefore,in terms of security,this NiTi SMA constant-force impact absorption mechanism has unique advantages.In summary,aiming at the design problems of NiTi SMA constant-force elements,this paper provides the universal design formula of constant-force elements using SMA strips through a great deal of simulations.These constant-force elements have the advantages of easily manufacturing and treating,convenient value adjustment and reliable installation.Besides,through a large amount of experiments,an effective heat treatment programme is found.The experimental results indicate that it is in accordance with the simulation results.Therefore this work establishes a foundation for extensive applications of SMA constant-force elements in different fields.At last,an application example of the constant-force elements is provided and its effectiveness and practicability are verified through experiments.