Possibility of estimation of mechanical stresses in nickel by the magnetic ponderomotive method

Abstract

The paper considers a technique for assessing mechanical stresses in coatings by the magnetic ponderomotive method, based on the use of the magnitude of the magnet breakaway force and its relation to the magnetic characteristics of nickel (magnetic susceptibility), which change under the action of internal stresses. It is shown that the existing mechanical stress measuring equipment using X-ray, magnetic or ultrasonic methods have specimen size limitations or have low measurement locality. At the same time, a magnetic ponderomotive method for controlling mechanical stresses is presented, which does not have the indicated disadvantages. The change in the magnetic breakaway force on the thickness, applied and residual stresses caused by elastic and plastic deformations in the sample is theoretically substantiated. It is shown that the presence of residual stresses in the sample changes the nature of the dependence of the magnet breakaway force on the applied stresses. A calculation algorithm has been developed that allows measuring the magnet breakaway force to obtain the ratio between the applied and residual stresses. It is shown that the ratio between residual and applied stresses can be used to assess the degree of proximity to the critical stresses levels that lead to failure. Calibration surfaces are constructed for the dependence of applied stresses on magnetic separation force and thickness, corresponding to elastic-plastic deformation, as well as leveled calibration surfaces, corresponding to elastic deformation in the presence of residual stresses in the material. Experiments have been carried out on real samples and coatings, that determine the degree of approximation of internal stresses in the material to critical levels that may lead to destruction. It is shown that when comparing the readings of calibrated gauges with a certified tool, the largest deviation occurs at low stress values due to the inverse dependence of the breakaway force on the stress level.