Nondestructive Evaluation of Stress and Damage of Materials by Magnetoacoustic Effects

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The purpose of the subject is to develope nondestructive evaluation technique for stress and damage by use of magnetoacoustic effects which can be seen in many structural materials. The obtained results during the term of the project are the followings.1.Derivation of Governing Equations for Deformable Magnetic MaterialsWe first derived the constitutive equations for deformable materials under magnetic field, taking into account magnetoelastic interactions. On the vasis of the presented theory, it is showed that the magnetoacoustic phenomena, the magnetization curve and the magnetostriction depend on the stress. Such predictions were verified by loading and acoustoelastic tests for steel and nickel under the magnetic field.2.Measurement of Stress and Magnetization by Ultrasonic Shear WavesWe developed the measurement technique for stress and magnetization by the speeds and the polarizations of ultrasonic shear waves. When the stess is applied to an anisotropic material, the acoustical … More anisotropy is given by superposition of the stress and the material anisotropies. Thus by subtracting the speed change due to the material anisotropy from the obtained data, it becomes possible to separate the contribution by stress to the speed change of the ultrasonic waves. By a similar method, we can measure the magnetization vector from the speed change of ultrasonic shear waves polarized in each direction.3.Stress Dependence of Magnetoacoustic Emission(MAE)We showed that MAE generated during cyclic magnetization depends on the stress due to the magnetoelastic couplings. By means of the dependence, the stress can be nondestructively estimated by measuring the count of MAEs during a cycle of the magnetization. For a typical ferromagnetic material, we proved that the count of MAEs changes according ot the angle between the stress and the magnetic field and the difference of compression/tension and elasticity/plasticity in the stress. From the obtained result, it is found that MAE can be expected as a promising nondestrucrive method for stress and damage of magnetic materials which can be applied in advance to fracture of damage processes. Less