Experimental And Constitutive Model Investigation Of NiTi SMA Mechanical Properties

Shape memory alloys（SMA）is a class of novel smart materials that combines the functions of sensing and driving,and is widely used in engineering structures,such as active restraint reinforcement and vibration control,due to its unique shape memory effect and transformation psedoelasticity.In order to make full use of SMA and lay the theoretical foundation of application,domestic Ni Ti SMA which includes austenitic Ni Ti SMA wire and martensitic Ni Ti SMA wire at ambient temperature were chosen for experimention.The characteristics of SMA phase recovery stress and transformation psedoelasticity under different coupling of constraint factor combination were studied using methods of experimental,theoretical analysis and computer-numerical simulation.The specific research work and content are as follows:（1）The constraint recovery stress properties experimental using martensitic Ni Ti SMA wire at ambient temperature was studied,during the test specimens conventional thermal triggering method was improved,and the effects of pre-strain heat recovery stress cycles output characteristics of SMA wire constraint was investigated.The results show:6%pre-strain Ni Ti SMA wire can be constrained to obtain more favorable recovery stress at lower pre-strain,smaller energy input and narrower transformation hysteresis condition.Thus,in the design of Ni Ti SMA actuator,6%prestrain can be considered the best effective.After five times of thermal cycle,the experimental curve gradually reaches stability;（2）Based on Ni Ti SMA wire full heat cycle set obtained by the 6%prestrain test,the recovery stress hysteresis BP neural network model with temperature and complete thermal cycle times as input and constrained recovery stress as output was established.The results showed:The numerical results of neural network hysteresis model are in good agreement with the experimental data,error distribution of neural network hysteresis model is more concentrated.The mean absolute error of constraint recovery stress simulation results of the 4th,6th and 9th cycles of thermal is 2.9%,4.9%and 2.1%,respectively.The average absolute error is within 5%indicating that the neural network model hysteresis Ni Ti SMA wire restraint stress responses prediction accuracy,and has certain engineering guiding significance;（3）Transformation psedoelasticity mechanical properties experimention of Ni Ti martensitic SMA wire at ambient temperature was studied,the effects of training cycles,strain rate and strain amplitude to the SMA hysteretic behavior were considered,the variation of mechanical parameters related were analyzed.The results showed that:a clear hysteresis curve of hardening can be found clearly after the fourth cycle of training,a stabilized phase pseudo-elasticity will be showed after 50 cycles of training.With increasing strain amplitude,lap linear energy increases.With the strain rate increasing loading,the martensitic transformation is gradually inclined upwardly internet,loading segment deformation modulus significantly increased.The rate change of strain hysteresis curve gradually become stabilized when the rate exceeds 5.0×10^-3sec^-1;（4）Based on the observation of mechanical properties of SMA wire,the Graesser macro phenomenological constitutive model was extended under the frame of general visco-plasticity.The difference of characteristic parameters in the martensitic forward/reverse phase transition and the nonlinear hardening behavior under large strain amplitude were considered by the new model.Through the internal variable evolution equation introduced,the superelastic SMA macroscopic phenomenological constitutive model considering the influence of strain amplitude and strain rate was established.The superelastic SMA hysteretic curve was simulated by Matlab/Simulink module,and the predicted results were compared with the experimental results.The results show that the strain amplitude-strain rate dependent SMA macroscopic phenomenological constitutive model can accurately describe the superelastic behavior of SMA in the process of stress-induced phase transition,and can also reflect the influence of strain rate and strain amplitude on the hysteretic properties of SMA.