Theoretical And Experimental Study Of Magnetic Method For Residual Stress Testing

Residual stress testing has been required by the development of modern industry. NDT methods are more concerned because the testing products are promised not be damaged with these methods. A number of NDT methods which may be applied on steels have appeared over the years. The magnetic method for residual stress testing is unique to steel. In general the changes in magnetic properties that are observed are easily measurable and the testing apparatus are cheap compared with Ultrasonic, X-ray and Neutron Diffraction methods.The magnetization of ferromagnetic materials are inevitably accompanied by the change of the specimen size, which is called magnetostriction. The magnetic stress method is based on the reversible magnetostriction effect or Vilari effect. The internal stresses are determined by investigating the changes of magnetic properties. The effect of stress on magnetization should be studied back on the macro and micro processes. The macroscopic magnetic properties like permeability and magnetization are influenced by stress, which can be explained by the magnetic moments rotation and domain wall displacement of microscopic domain caused by the coupling of stress and internal or applied field. For most engineering applications, the model of continuous media are more effective than that of magnetic domain, which often results in the nonclosure description of magnetic state. The investigation from macroscopic to microscopic then back to macroscopic should be necessary for magnetic method.We assume the linear relationship between the magnetic output and strain. A new four -coefficient method is first put forward. With U-shaped sensor we first investigate the magnetic output influenced by the magnetizing conditions like frequency and electric current. Extra detailed experimental and theoretical analysis are performed on the effect of lift-off between the sensor and test piece. Specimens of mild steel and cast iron are experimented in uniaxial and biaxial tension and compression. The cruciform specimen, which is often used in magnetic methods for calibration, is analyzed with our modified formula. With magnetic domain theory, the influence of stress or strain on the magnetization is investigated. Using J-A-S model, we obtain the magnetic strain under uniaxial stress state successfully. Recently progresses have been made using the relationship between magnetic output and principal stresses difference under uniaxial stress. We believe it is strain that truly shows the coupling of deformation and magnetization. Under biaxial stress state, magnetization is more easily achieved along the direction of positive strain. The parameters of the model can be determined from the results of uniaxial tension and compression experiments. In practical applications, a four-legged sensor is used. The clamp device is improved a lot for rapid and convenient measurement. Our testing objects are high pressure thick wall pipe and a large scale steel truss. The results are satisfactory.