A Method For Measuring Residual Stress In Ferromagnetic Materials Based On The Changing Of Magnetism

The application of the ferromagnetic materials such as rails, oil pipelines, steel pressure containers, steel structure bridges and various steel structures is very extensive in the world. The safety of these equipments is much important to people. The damage of the ferromagnetic components generally goes through the process of stress concentration →crack(sew) →rupture. Residual stress is a tension or compression which exists in the bulk of a material without application of an external load (applied force, displacement of thermal gradient). In the manufacturing process of various machines, the residual stress always exists in the components of the machines. When the machine is being used, the work stress and residual stress often overlap in the components, which leads to distortion of components again and redistribute residual stress, So the life of the equipment will be shortened.. And then, to void the crack and the accident of rupture, the stress concentration of the equipments should be monitored at the initial stage of damage. The research on the measuring technique to the residual stress, Began in the 30's of the 20th century, and there have had several tens of methods of measuring residual stress till now. These methods may be summarized mostly to two types: the method of releasing stress and physics method, or called the destructive method and non-destructive method. The method of releasing stress is to separate the part in which the residual stress exists, and release the residual stress in part. Through measuring the deflection of the part at this time, we can get the residual stress by elasticity. This kind of method is highly reliable and accurate, but it will destroy the equipment and components to a certain extent, so it is limited in application to some degree. The physical method is a kind of non-destructive method. It makes use of the relationship between the physical character and stress of measured material to evaluate stress. It mainly includes X-ray diffraction methods, magnetic methods, ultrasonic waves method and so on. In these two types of measurement, people have made a deal of researches on drilling hole method and X-ray diffraction method, which have already been highly developed. Especially, the drilling hole method has already been introduced as a standard by ASTM. But these two kinds of methods also have some defects, the test depth of X -ray diffraction method is under 30μm, the drilling hole method is a destructive method. When the ferromagnetic materials are affected by the residual stress, the microstructure and parameter such as magnetic domain, magnetic permeability and magnetic resistance ...etc. have changed. The changes of these parameters can be represented by the changesof induced voltage in the coils. So we can measure the changes of induced voltage in the coils to get the residual stress. This is the principle of magnetic method. We have developed a new technique to measure residual stress according to magnetic method, and have designed the transducer successfully, by which we have made the elemental experiments .The result of the experiments attests to the feasibility of the technique, and the rationalization of the transducer. The technique has a good future. The design of transducer is the basis of our work .We have consulted a great deal of national and international transducers and designed a new one. The new transducer has two major characteristics. Firstly, it can fit for the different ferromagnetic materials, whereas previously peculiar transducer is only fit for that kind of material. Secondly, it can carry out absolute measurement of the residual stress, but most of the other transducers merely carry out relative measurement The transducer is made up of slug, measure point, reference point, drive coils and pickup coils. The measured sample is placed to the measure point, and the standard sample that has the same material as the measured one and has no residual stress is placed to reference point. There are two uniform pickup coils on each slug near the measured point and reference point. These two coils havethe same material, wire size, and the number of turns. After the AC current flows to the drive coils, the measured sample and standard sample are magnetized by the changed magnetic field, which is induced by AC current at the same time, so they radiate magnetic field. As a result, voltage is produced in two pickup coils at the same time. The induced voltage in the pickup coils at the measured point contains the information of stress, which makes the induced voltage change. Since the standard sample at the reference point has no stress, the induced voltage in the standard sample does not change. And the difference of the voltage in two pickup coils is the one that responds to the stress in the measured sample. Thus we carry out the absolute measurement of the residual stress. The width of measured point and standard point is alterable, so we may select the width according to the material, at last we can find a proper width to some material, which means that the transducer adapts to various ferromagnetic materials We have designed the measurement system, and made a great number of experiments to measure residual stress by the transducer with the magnetic method. Through experiments we have discovered the laws between the variety of induced voltage in the pickup coils and the residual stress of ferromagnetic materials. When magnetic field is vertical to the stress, along with the compressive stress increasing, the induced voltage increases linearly; when the magnetic field run parallel with the stress, along with compressive stress decreasing, the induced voltage decreases...