The Research Of Measuring Residual Stress Based On The Magnetism Of Object

The production equipments are indispensable in people's daily life, agriculture or industrial activities. Most of the equipments are made of ferromagnetic materials. Then whether the safety of the equipments (bridge, building, rail) has direct effect on the economy and people's property security. The most important influence of the quality, life and safety of the projects is the residual stress produced in the hot and cold processing, or the internal stress cumulated in use.Generally, the damage of the ferromagnetic materials all experience three procedures--- stress concentration, crack production and fracture. So it's necessary to eliminate the influence of adverse residual stress, then improve fatigue strength, of judge whether there are cracks in the elements. So, in the initial period of the damage (stage of stress concentration), monitoring activity should be carried out in order to avoid the cracks and even some accidents.The study of detection technology of residual stress started from 1930s, till now, there are dozens of methods. The methods can be divided into two kinds: mechanical measurement and physical measurement, or we may call them destructive measurement and nondestructive measurement.Because the destructive measurement will destroy the elements, it has been replaced by nondestructive measurement. In nondestructive measurement, X-ray measurement has been used extensively. But this method will be limited by the depth of detection, and it will not reflect the stress distribution. Compared with X-ray measurement, ultrasonic wave has a large detection depth, and can be applied to many materials. But it has disadvantages, such as, the change of speed is very small, and it's not easy to detect. What's more, it will be influenced by the structure of the materials. Magnetic method has many advantages, such as, high speed of detection, big depth of detection and without radiation risk. Also, magnetometer is always easy to carry, so it can measure the elements in use safely.Magnetic method develops slowly. The major reason is that there are many difficulties in practical process, and the security and precision can't be satisfied. But with the development of technology, especially because of emphasis on the stress distribution, the magnetic methods develop quickly. Because the electronic measurement and X-ray measurement are restricted in the enormous elements, for example, large pressure vessels, which makes people deep into the study of magnetic method and try to use it.The article explained how to produce the stress measurement equipment based on the permeability method. It includes: the production of stress sensors, adjustable frequency, and adjustable amplitude triangle wave signal generated, D/A converter, power amplifier and alarm of stress.[1] Measurement probe is the coil that can produce alternating magnetic field and the induction coil that can detect the signals. The previous probe is made from silicon steel sheet which have low permeability, making the sensitivity of probe affected. Magnetostrictive effect due to the existence of the probe signal in the role of incentives will produce a shock. The device selects the silicon U-core which has high rate of permeability. In the U-core, there are excitation and induction coil.[2] The low-frequency signal source due to the analog circuits can't satisfy people, so the output signal has lower frequency. Therefore it's necessary to consider the use of single-chip microcomputer as the controller, complete the output of waveform data, and then output the wave signals with D/A converter. In this paper, the low-frequency signal generator is a core part of an 8-bit MCU and the D/A converter chip DAC0832. The experiment shows that signal generated from the circuit has many features, such as, frequency stability, amplitude stability, small harmonic distortion and good wave form.[3] The triangular wave which is converted by D/A conversion and by amplification operation will output small electric current, and the power can't satisfy the need. We adopt TDA2030 power amplifier as the main element with 10 times magnification, and the rate can be adjusted according to the actual situation.[4] Sensor side of the measurement probe outputs senor signal, as a result of single-chip microcomputer with A/D conversion, it's not necessary to access A/D converter chip. The resistance of protection of pressure can be directly entered into the single chip, and will be compared to the voltage and see whether the alarm light will light.The working principle is to convert the ferromagnetic changes caused by the internal stress into voltage signal that can be easy to measure. In the load testing machine, the curve between output voltage and internal stress is measured and the yield point of the sample is identified. If the output voltage is beyond the single-chip memory, then there were warning buzzer and flashing red light warning, otherwise the green light warning appears.In order to exclude the interference of pulse, there the delay procedures will be used to judge voltage. If there are interferences, the procedure and be used to determine. From the experiment, we can see that due to various technical factors such as stray fields, the probe in contact with the workplace and the work piece gap reluctance, surface roughness, the effects of temperature, the measurement results will be a discrete and with poor repeatability. Through the adoption of corresponding measures, such as external bias magnetic field, compensation the temperature of results by dedicated fixture, the accuracy and precision can be satisfied well. Permeability-based method of the measurement device can measure the internal-stress of ferromagnetic materials and the method is feasible.