A New Device For Measuring Residual Stress In Ferromagnetic Materials

The application of the ferromagnetic materials is very extensive in the world. Whether these equipments are safe or not will affect the economical operation and people's property in local place and the whole country. The damage of the ferromagnetic components generally goes through the process of stress concentration→crack(sew)→rupture. In the manufacturing process of various machines, the residual stress will always exist in the components of the machines. When the machine is being used, the work stress and residual stress often overlap in the component, which leads to the second distortion of components and the redistribution of residual stress, so the life of the equipment will be shortened. More seriously, it will cause malignant accidents. 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 of the residual stress began in the 30's of the 20th century, and there have had several methods of measuring residual stress till now. These methods may be summarized mostly to two types: the method of mechanical measure and physical measure, or called the destructive method and non-destructive method.The destructive methods are being replaced by non-destructive methods because of the damage to the materials. Of the NDT methods, X-ray detection has been widely applied, but X-ray detection limits the probing depth and can't describe the internal stress distribution clearly. Compared with the X-ray detection, ultrasonic detection can probe more depth and can be applied for more materials. However, a very small ultrasonic velocity changes, the measurement is not easy, and the structure of the material has great impact to the result. Compared with non-magnetic method, the magnetic method can detect more quickly, more depth and without radiation hazard. General portable magnetometer is able to be used for real-time detection safely. Moreover, general magnetometer can be carried easily, so it can measure the components in use safely.To measure the residual stress by the magnetometer has developed slowly. One of the reasons is that there exist some difficulties in the practical process. Also, the accuracy and precision can't satisfy the requirements. But nowadays magnetometer develops fast because of the development of scientific technology, also because more emphasis has been put on the internal stress of the outsize components. Both the electrometric method and X-ray detection will be limited seriously in the using process of the outsize components. This makes people to study the problem of magnetometer deeply and lead it into the practical period.Because magnetic permeability testing has studied the measure equipment of internal stress, it can cause the change of the permeability caused by the internal stress and transform them into pressure signals and then exports them. Measure the pressure and the internal stress and make a relative curve. Then contrast the output pressure with the relation curve. The output pressure can be transformed into the internal stress.From the experiment we can infer that the result will cause the appearance of the discreteness and also the repeatability is bad. This was caused by many technical factors, such as stray field, the reluctance between the components and probe, the coarseness of the surface of components, the measure temperature, and so on. We can put bias magnetic field, special fixture to compensate the temperature. With these measures we can effectively satisfy the requirements of accuracy of measurement. The appliance of the measurement can measure the internal pressure of the magnetic materials. This method is feasible and this design is rational.