Electromagnetic techniques for materials characterization

Most structural components are made of ferromagnetic steel. The mechanical properties of steel are determined by the microstructure, texture, residual stress state, and texture are influenced, for example, by a heat treatment or hardening process. Microstructural changes are changes in the dislocation density, the phase content, and grain size. These microstructural changes determine essentially the mechanical technological properties of steels, for instance, yield strength, hardness, toughness, and residual stresses. Knowledge of micro- and macroresidual stresses is necessary to evaluate the stress condition of a component. From the various kinds of residual stresses, residual stresses of the first kind can arise due to forming under applied mechanical stress or due to the different colling rates of different cross-sections of homogeneous materials. Residual stresses of the second kind forms due to the different thermal expansion coefficients between precipitates and matrix in a two phase material. Residual stresses of a third kind appear when the lattice parameter of the second phase particles embedded coherently in the matrix and the lattice parameter of the matrix are different. Destructive test procedures for determination of microstructure state and mechanical properties, ie, mechanical hardness, need a sample preparation. X-ray techniques are well known for residual stress and texture determination, but they are time consuming and therefore expensive. With the understanding of ferromagnetic properties, especially micromagnetic theory, we are able to explain interactions between lattice imperfections and Bloch walls. Most of these studies were performed on single crystals and unalloyed polycrystalline materials. Since these studies, micromagnetism has been a most successful tool to design new magnetic nondestructive techniques. Since the early 1940s, there have been basic investigations on microstructure and residual stress analysis by magnetic techniques. Since the 1950s and 1960s, when the theoretical bases of micromagnetism were laid, nondestructive methods have been derived from irreversible, magnetostrictive active Bloch wall movements. Most micromagnetic nondestructive techniques started in the 1960s.At the end of the 1960s, developments began on the evaluation of microstructure and stress states by means of the inductive Barkhausen noise effect. Further investigations were carried out by Pawlowski and Rulka and Tiitto. This research resulted in development of the first prototype testing devices and industrial applications. Developments since 1980 use the micromagnetic multiparameter NDE-approach for materials characterization. This technique enables a nondestructive materials characterization even under rough environmental conditions, also in the industrial praxis. Electromagnetic techniques are able to indicate nondestructively and quickly changes of residual stresses, texture, microstructure states, and mechanical properties, and are, therefore, very useful tools for materials characterization and damage assessment of in-service engineering components. Various magnetic methods for this task have been reviewed by Jiles and Altpeter et al. Out of all available methods, magnetic Barkhausen noise, incremental permeability, upper harmonics, and dynamic magnetostriction are very promising techniques for materials characterization