Experimental And Theoretical Study On Electrical Resistance Sensing Characteristics Of Super-fine Shape Memory Alloy Wires

Structural health monitoring and structural vibration control are multidisciplinary and frontier research directions of civil engineering.By means of real-time monitoring of building structures and improving their seismic performance,the security of building structures can be guaranteed.The monitoring information of the structural health monitoring system provides effective feedback signals for the structural vibration control,which is an indispensable part of the building lifecycle management(BLM).Structural health monitoring system is composed of intelligent structures.Intelligent structures can judge the changes of the surrounding environment by means of the selfsensing ability of intelligent materials,and make prompt responses to the changes of the external environment.As an intelligent material that integrates sensing and driving capabilities,shape memory alloy(SMA)has many excellent characteristics,such as shape memory effect,superelasticity,corrosion resistance,fatigue resistance and high energy density.What’s more,SMA has excellent resistance sensing properties and large deformation ability,and its resistance changes sensitively with macroscopic deformation.In practical applications,the resistance of SMA can be obtained by applying voltage and current without additional measurement equipment,so it has great development potential in the field of structural health monitoring and structural vibration control.In this paper,the mechanical properties and resistance characteristics of superfine Ni Ti SMA wires with a diameter of 25 μm were experimentally studied.The stress-strain relationship and strain sensing ability of superfine SMA wires were mainly analyzed.The back propagation(BP)neural network constitutive model and temperature-strain-resistance model of superfine SMA wires were established.The major research contents are as follows:(1)The mechanical properties of superfine Ni Ti SMA wires at different temperatures were experimentally studied.The effects of training strain amplitude,cycle times and loading rate on the mechanical properties of superfine SMA wires were performed by uniaxial tensile test.The test results show that the superfine SMA wires need 40 cycles of training to achieve stable mechanical properties.The training amplitude has a great influence on the mechanical properties of superfine SMA wires.High strain amplitude training will increase the volume fraction of detwined martensite in martensite phase material,resulting in the reduction of detwining process.Superfine SMA wires have large surface area and are easy to exchange heat with the environment,so the loading rate has little effect on their mechanical properties.(2)The resistance characteristics of superfine Ni Ti SMA wires were experimentally studied.The resistance of SMA filaments during uniaxial tension was measured by four-wire method.The effects of test temperature,strain and loading rate on the resistance characteristics were mainly analyzed.The experimental results show that superfine SMA wires have good strain-resistance linear correlation and can respond sensitively.During the tensile process,the resistance change rate of superfine SMA wires was the largest in the high-temperature austenitic phase;According to the test data,the calculation formula of resistance change rate was derived,which was simple and effective.(3)By simulating the engineering application environment,the influence of static aging on the mechanical properties and resistance stability of superfine SMA wires was experimentally investigated.During the static treatment,the range of temperature was the same as the central plains of China,and the static time was 15 days.The test results show that the mechanical properties of martensite phase and austenite phase of SMA superfine wires are affected to different degrees,and the recovery methods to eliminate the static effect are proposed.What’s more,the influence of static aging on the resistance characteristics of superfine SMA wires can be ignored.(4)The temperature-strain-resistance model and constitutive model of superfine SMA wires were established based on BP neural network.The BP neural network model was established by MATLAB,and the test data of mechanical properties and resistance characteristics of superfine SMA wires were organized into samples.The BP neural network train the samples by the method of mechanical learning,and continuous error feedback is carried out to improve the accuracy of the model.The results show that the BP neural network temperature-strain-resistance model and constitutive relation model of superfine SMA wires can predict rapidly and accurately,which lays a solid foundation for the application of superfine SMA wires in the field of structural health monitoring.