Fundamentals of Nondestructive Measurement of Biaxial Stress in Steel Utilizing Magnetoelastic Effect under Low Magnetic Field

The relationship between magnetization and principal stress under a triaxial stress field is deduced theoretically from an equilibrium condition between magnetoelastic and magnetization energies calculated on polycrystalline iron. The relationship is expressed by a formula in which the intensity of magnetization parallel to the principal axis of the stress tensor decreases in proportion to the difference between the largest principal stress and the magnetizing-directional principal stress. A biaxial stress is developed in a cruciform testpiece made of mild steel, and then magnetizations in the direction of principal stress parallel and normal to the surface of the testpiece are measured using U- and C-shaped magnetic sensors, respectively. The measured results are revised considering the spread of magnetic flux. The experimental results fundamentally agree with theoretical predictions. The quantitative discrepancies are discussed.