NDTnet 1998 Aug, Vol.3 No.8

Abstract

Newly developed technique for ferromagnetic materials characterization based on the analysis of Directional Diagrams (DD) of Barkhausen noise (BN) is presented. The DD of BN means the diagram reflecting the dependence of BN signal in the pick-up coil with its axis, perpendicular to the surface of tested specimen, on the angle of rotation of tangential remagnetizing field around said axis. The main reason is that while analyzing DD it becomes sometimes possible to separate the contribution in the BN signal induced by the material's structure and microstructure from one side and macrostresses, from the other, as well as to separate the contribution of shear and longitudinal stress's components.

The present report is the result of the experimental investigation of DD in the plates being deformed elastically uniaxially and bi-axially. The influence of pure shear stresses on the BN signal was as well examined.

The report is organized as follows:
i) experimental investigation of DD during uniaxial tension - compression of plates;
ii) experimental investigation of the dependence of BN on pure shear stresses;
iii) experimental investigation of DD having residual stresses in plates;
iv) computer simulation and inversion of DD for the separation between the contribution of stresses and structure to the BN signal.

All experiments were provided with plates and tubes of low carbon steel (0,2%C) which were tested by tension-compression and torsion respectively. The spectral density (SD) of BN was measured while rotating the attached bipolar electromagnet around the axis normal to the surface of the specimen under testing. The set-up parameters were optimized. All measurements were provided using metal's structure and stress analyzer INTROMAT.

We observed that while loading the specimen uniaxially (the conditions of plane stress state, for which only three components of stress tensor are valid) then in the direction of maximum shearing stresses the SD of BN is insensitive to the variation of external load. This result caused providing the experiments on torsion which confirmed the very poor sensitivity of BN signal to pure shearing stresses. Then we investigated the DD while bending the 4 mm thickness plates having residual stresses in the surface layer (after cold rolling) and it was shown that on DD there exists small region of angles where the sensitivity to the variation of deformation by bending reaches its minimum.

The different sensitivity of SD of BN to various components of stress tensor was used as the guide for developing the computer based technique for basically independent estimates of variations in structure and stresses caused by different treatments of tested material, as well as to separate the contribution of normaland shearing stresses in SD of BN.

Abstract Source:
Book of Abstracts, 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN: 87-986898-0-00

Full-Text Source:
Proceedings of the 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN:

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