A New Kind Of Device Of Residual Stress Testing Sensor Based On Magnetic Effective

Based on the basic theory of Ferromagnetism,we studied the relative topics about application of magnetic stress measurement systematically, optimized the structure of the sensor,chosen the right magnetic core and winding. Have designed a inspecting instrument with the character of miniaturization, high sensitivitye and strong signal. We also made some instructive efforts for the theory research and application of magnetic stress measurement.The main achievements and innovation are listed as below:(1) Simply elaborated on stress detection technology. Analysised the current advantages and disadvantages of non-destructive testing technology. Raised the benefits of stress permeability measurement: to prevent fatigue damage accidents; to facilitate the early diagnosis of ferromagnet. In addition, high measurement speed with no radiation risk, the advantages of more convenient to carry. Thus it can be used in secure and real-time measurements.(2) Summarized the systematic theory of ferromagnet magnetization . Including the classification , magnetic characteristic of ferromagnet, the mechanism of magnetization, magnetostriction effect, the effects of magnetic reverse , magneto-elastic effect and inverse effect and so on. We also elaborated on the stress associated with the magnetic measurement of mechanical properties of metals. Introduced the concept and characteristic of external force, internal force, stress and strain. Described the formation reason and the impact of the material of the residual stress, stress concentration and the phenomenon of dislocation.(3) Designed Stress permeability detection system. Based on the similar approach in the past , made several kinds of probes of different shape and material for a large number of experiment, and experimental results of the repeatability and accuracy compared carefully. We found that the permeability of the silicon-based probe is not high enough , it has affected the sensitivity of the probe. And because the existence of magnetostrictive effect, the probe under the incentive signal will shock lightly. The device finally selected a very high permeability and low magnetostriction coefficient of the cobalt iron boron amorphous alloy of silicon as the core material of the probe. We also determined the number of turns of the coil through the calculation and experiment.(4) Improved the approach of the the workpiece and the probe's surface to reduce the test error. The probe and the workpiece constituted the closed magnetic circuit , due to the detected workpiece and the probe surface must exist crack, and there is certain surface roughness on detected workpiece. Thus this circuit are constituted by the peak convex pieces of material and the composition of the air gap. The gap between the probe and the workpiece internally generated demagnetization field, formed very large reluctance. When we place or rotate the probe, the reluctance of the gap will change largely. The reluctance in air is bigger than magnetic material on the dozens or even hundreds of times, so the experimental phenomenon will be concealed by the gap. This is also a major cause of poor reproducibility and the magnetic stress measurement readings are sometimes unstable. The use of home-made jig and the magnetic glue combination methods to deal the surface of the probe and the sample is a very good solution to settle this problem, and improved the efficiency of the measurement.(5) studied the impact of the excitation frequency for the test results to improve the accuracy of the measurement. Too high frequency signal will arouse the "skin effect", to detect the combination of shallow depths. Low frequency signal is too susceptible to interference; At the same time, in order to further reduce the interference, it is necessary to avoid the frequency 50Hz, and the difference frequency, sum frequency and the multiple frequency. Finally, according to the needs of different circumstances, we selected 11Hz, 17Hz, 37Hz frequency in the experiments.(6)ln order to avoid the measuring results which is impacted by stray field, we supplied the bias magnetic field to improve the accuracy of our test system , so that the process of measuring totaled at one of the static field strength to overcome the stray field on the measurement results impact. Put our self-made bias coils with external power supply at both ends of the sample. They can provide a stable magnetic field. In order to determine the magnetic measurement of the effects of stress, we will gain access to different bias coil current strength to do a series of experiments. As can be seen from the experimental results, if the plus bias magnetic field strength is greater the induced voltage changes in the volume of the will be smaller, the rate of duplicate experiments will be better , but lower sensitivity. Also, we should prevent bias current to be excessive heat and consider the result of electromagnetic suction. Finally,we chose the value of the we selected the the value of 1.8A as the bias current. In this field region, the induced voltage can be more precise reflection of the stress on the sample of magnetism.(7) Studied the permeability method of stress measurement system to detect the depth of sample. We used 37Hz, 17Hz and 11Hz as the incentive frequency to in the experiment. As can be seen from experimental data, the impact of excitation frequency to detect the depth of the workpiece is very great. By adjusting the excitation frequency can adjust the effective depth of the stress test in order to know the changing of stress with depth. As can be seen from the experimental data, the probe can detect the depth of silicon at least in more than 1mm.(8) Discussed the stress on the impact of the coercivity in the elastic deformation region and plastic deformation of the ferromagnet, from the basic structure of magnetic materials, and used magnetic domain wall movement and magnetic moment rotation theory. It was found that in the elastic deformation region, because of the rotating magnetic domain magnetic moment, coercivity increases with decreasing external force. But in the region of plastic, because the domain wall in the expansion encountered nail bar location, the coercivity grows almost in proportion with the increase of stress. The measurement which is the same as the result detected by Hall sensor proves the effectiveness and stability of the sensor.