Ivan Tomáš

Measurement of Flat Samples With Rough Surfaces by Magnetic Adaptive Testing

Magnetic nondestructive tests of structural degradation of ferromagnetic construction materials must be often carried out on flat samples whose magnetic circuit is artificially closed by an attached soft magnetic yoke. Quality of magnetic contact between the sample surface and faces of the yoke has a strong influence on the measured signal, and especially with unpolished surfaces, it is difficult to be re- produced. The presented experiment suggests a partial solution of the problem via application of nonmagnetic spacers inserted between the sample and the yoke. The spacers distort and decrease the signal, but they also decrease fluctuation of the magnetic contact quality and smooth scatter of the measured data. As the goal of the nondestructive test is not absolute measurement of magnetic parameters of the material, but relative comparison of some convenient magnetic features, which relate to degradation of microstructure of the sample, application of the spacers proves to be not only acceptable, but it is very helpful.

Temperature Dependence of Magnetic Descriptors of Magnetic Adaptive Testing

The method of Magnetic Adaptive Testing (MAT) was applied for investigation of ductile cast iron samples having matrix and graphite structure. This method is typical by its low required magnetization of samples, because it is based on the measurement of minor magnetic hysteresis loops and calculates magnetic descriptors for characterization of the sample. Results of the non-destructive magnetic tests were compared with the results of destructive mechanical measurements of Brinell hardness and also with the results of non-destructive eddy current measurements. Linear, sensitive correlation with low scatter of values was found between the magnetic and the mechanical characteristics. If the parameters of MAT measurement are properly chosen, then they do not depend on temperature in the T = 20-180°C range, in contrast to the results of eddy current measurements. Based on these results, MAT is suggested as a highly promising non-destructive alternative of destructive tests for monitoring structural changes in cast iron and in other ferromagnetic objects.

Flake Graphite Cast Iron Investigated by a Magnetic Method

The method of magnetic adaptive testing (MAT) was applied for the investigation of flake graphite cast iron samples having various metallic matrix and graphite structures. MAT is typical by its low required magnetization of samples, because it is based on the measurement of families of minor magnetic hysteresis loops. The flat samples were magnetized by an attached yoke and sensitive descriptors of their magnetic/structural state were obtained from the evaluation of the measured data. Metallographic examination of the matrix and graphite structures was performed and results of the nondestructive magnetic tests were compared with these data. A very good correlation was found between the magnetic descriptors and the graphite morphology. With these results, MAT is suggested as a highly promising nondestructive alternative of destructive tests for monitoring structural changes in cast iron and other ferromagnetic objects.

Nondestructive indication of fatigue damage and residual lifetime in ferromagnetic construction materials

A new revolutionary attitude toward investigation of fatigue damage in cyclically loaded steel samples is reported. The measurement is based on the method of magnetic adaptive testing, which-–in contrast to traditional magnetic hysteresis investigations-–picks up the relevant information from systematic measurement and evaluation of whole minor magnetic hysteresis loops and their derivatives. Satisfactory correlations between nondestructively measured magnetic descriptors and actual lifetime of the fatigued material were found. The presented method is able to serve as a powerful tool for indication of changes, which occur in the structure of the inspected objects during their industrial service lifetime, as long as they are manufactured from ferromagnetic materials.

Complex Characterization of Degradation of Ferromagnetic Materials by Magnetic Adaptive Testing

Tensile tests of plastically deformed specimens were performed in order to investigate the relation between variation of magnetic characteristics and residual plastic strain. Deformed steel (CSN 12050) specimens were investigated by the method of magnetic adaptive testing (MAT), measuring and evaluating series of minor magnetic hysteresis loops. It was shown that an efficient combination of the MAT parameters yields a reliable and unambiguous correlation with residual strain of the specimens even though all relations between the strain and each of the individual MAT-parameters were nonmonotonous.

Optimization of fatigue damage indication in ferromagnetic low carbon steel

Fatigue damage was investigated by the method of magnetic adaptive testing (MAT), which is based on the systematic measurement and evaluation of minor magnetic hysteresis loops. A large number of magnetic measurements were performed on a single reference series of low carbon steel flat samples, which were fatigued by cyclic bending in an identical way, up to an increasing level of fatigue damage. The measurements of the magnetic properties of these samples were repeated under varied conditions, including speed of magnetization of the samples, sample temperature during the measurement, choice of the evaluated signal, frequency of the voltage sampling, and range of the applied amplitudes of the magnetizing field/current. Special attention was turned to the influence of the thickness of the non-ferromagnetic spacers positioned between the surface of the samples and the flat fronts of the attached magnetizing yokes. On one hand, the spacers decrease the values of the induced signal and its derivatives, but on the other hand they substantially increase the reproducibility of the measurement and positively influence the shapes of the resulting degradation curves. Optimum conditions for the magnetic measurement of the fatigue damage were searched, found, and recommended. The results indicate the reliable applicability of MAT to detect early stages of the material fatigue, and to predict its residual lifetime.

Nondestructive Inspection of Ductile Cast Iron by Measurement of Minor Magnetic Hysteresis Loops

Systematic measurement of minor magnetic hysteresis loops of traditional hysteresis tests requires substantially lower magnetization of samples and offers higher sensitivity of detection of changes in the ferromagnetic materials’ microstructure. The keynote idea of this method, called Magnetic Adaptive Testing (MAT) is utilization of sensitive correlations between the varied microstructure of the magnetized material and the corresponding, highly sensitive modifications of some of the minor hysteresis loops. The paper presents some of the recent results of the MAT measurement performed on specially prepared series of cast iron samples. Results of the non-destructive magnetic tests were compared with the destructive mechanical measurements of Brinell hardness and linear correlation was found between them. A very good correlation was also found between magnetic descriptors and conductivity and chill/ferrite area fraction. Based on these results, Magnetic Adaptive Testing is suggested as a highly promising non-destructive method for monitoring structural changes in different types of ferromagnetic materials.

Adaptive testing for nondestructive evaluation of degraded materials

Abstract Method of Adaptive testing (AT) is described, as a way of optimization of experimental evaluation of degraded materials, which are measured nondestructively, indirectly via a convenient physical process. The general AT is illustrated by a well-explored special case of magnetic adaptive testing (MAT) of strained steel. Degradation curves obtained from AT are regularly more sensitive and experimentally friendly than traditional descriptors of the used physical process itself. Any of the traditional descriptors of the physical process, being optimized for description of the process itself, is very probably not optimized for description of the investigated material degradation.