The Study On Stretching Process Of Ti-Ni Shape Memory Alloy By Acoustic Emission

Shape memory alloy is widely used in engineering,aviation and medical fields because of its excellent shape memory effect and super elasticity.Acoustic emission is a kind of high sensitivity dynamic nondestructive testing technology,which can accurately monitor the process of transformation and plastic deformation induced by stress.Using the traditional stress-strain curve analysis,experience analysis and probability entropy analysis and other analysis methods studied the process of transformation and plastic deformation under multiple loading condition.And the plastic slip point of Ti-Ni shape memory alloy was found,which can provide a basis for ensuring its maximum elasticity.The acoustic emission sources,such as transformation and plastic deformation in Ti-Ni shape memory alloy,were effectively identified by the acoustic emission process analysis at 80 ℃ and 23 ℃.The results showed that the activity of acoustic emission source was the strongest during transformation,while the activity was poor during plastic deformation.The ring counts and duration of AE signals produced during transformation were far lower than the signals produced during plastic deformation.The results about the rise time-duration correlation analysis showed that the acoustic emission source could be divided into transformation and plastic deformation.And the AE source could be divided into transformation,plastic deformation developing and yield fracture using the correlation analysis of counts-amplitude method.According to the characteristics of acoustic emission signals,a new division of the tensile process of Ti-Ni shape memory alloy at 80 ℃ and 23 ℃ was carried out.The samples of AE signals obtained from repeated tests under different temperatures were constructed as an acoustic emission signal distribution matrix D.And matrix D was quantized by probability entropy.According to the trend of entropy change,the tensile process of Ti-Ni shape memory alloy was divided.The entropy curves at 80 ℃ includes 3 isentropic platform,3 entropy increase process,1 approximate isentropic platform and 1 entropy reduction process.It quantitatively described the development and change process of austenite elastic deformation,R phase transition,martensitic transformation,plastic initiation and plastic yield fracture in the stretching process of Ti Ni shape memory alloy.The entropy curve at 23 ℃ including 1 isentropic platform,1 entropy increasing process,1 approximate isentropic platform and 1 Platform entropy entropy scattered process.And The evolution process of R phase elastic deformation,R transformation and plastic deformation during the tensile process of Ti Ni shape memory alloy was quantitatively described.According to stress-strain and the change of acoustic emission events,the relationship between the martensitic transformation and plastic dislocation in Ti-Ni shape memory alloy and the experimental parameters such as temperature,cycle time and ultimate stress were studied.The effects of temperature and plastic dislocation on the mechanical properties of Ti-Ni shape memory alloy were revealed.And it also verified the stage division of tensile process of Ti Ni shape memory alloy by parameter analysis and entropy curve analysis.The accumulation of plastic dislocation in the material is identified by acoustic emission signal.A technical attempt is made to ensure that the Ti-Ni shape memory alloy can obtain maximum stable superelasticity.