Correlation Of The Magnetic And Mechanical Properties Of Steel, Their Use As An Inspection Tool For Early Prediction Of Defects
In this paper correlation of the magnetic and mechanical properties of steel will be explained and the mathematical equations relating these properties will be described. This relation was first identified by James Joule in 1842 when he observed that feeromagnetic materials change their shape or dimensions during the process of magnetization Internally, ferromagnetic materials have a structure that is divided into domains , each of which is a region of uniform magnetic polarization. When a magnetic field is applied, the boundaries between the domains shift and the domains rotate; both of these effects cause a change in the material's dimensions. The reason that a change in the magnetic domains of a material results in a change in the materials dimensions is a consequence of magnetocrystalline anisotropy, that it takes more energy to magnetize a crystalline material in one direction than another. If a magnetic field is applied to the material at an angle to an easy axis of magnetization the material will tend to rearrange its structure so that an easy axis is aligned with the field to minimize the free energy of the system. Since different crystal directions are associated with different lengths this effect induces a strain in the material this phenomena was named Joule effect and now it's known as Magnetostriction.
The inverse magnetostrictive effect (also known as magnetoelastic effect or Villari effect) is the name given to the change of the magnetic susceptibility of a material when subjected to a mechanical stress.
In his study about correlation of the magnetic and mechanical properties of steel Burrows made an early prediction about the future of this correlation and the important role that it will play in studying materials mechanical properties " Magnetic testing in conjunction with mechanical testing may be expected to be of considerable value in determining mechanical properties".
The mechanical properties that have been studied in connection with the magnetic characteristics are hardness, toughness, elasticity, tensile strength, and fatigue resistance.
In this paper I , will present , some of NDT methods that were developed based on these effects and their use as an inspection tool for early prediction of defects, their limitations and extent will be also shown.