Comparative measurement of residual stresses with the method based on the magnetic Barkhausen noise and the relaxation method
For an efficient prediction of residual stresses in a material or a machine part the non-destructive method based on the magnetic Barkhausen noise is, among others, employed in practice. An experimental system developed at home and including measuring and assessment units has proved to be very efficient in various applications of surface hardening or cold hardening, and heat treatment.
Nondestructive Determination of Stresses in Steel Components by Eddy Current Method
Nondestructive eddy current (EC) method for evaluation of stresses in ferromagnetic steel components based on the stress-induced magnetic anisotropy (MA) changes was investigated. The proposed method is based on the rearrangement of the magnetic domains under stresses due to the reverse magnetostrictive effect (Villari effect). For evaluation of stresses special EC probe was developed. Investigations were carried out with the application of the conventional testing machine for tensile loading and the specially developed experimental set-up for four-point loading for creation the adjusted tensile and compressive stresses on different sided of the loaded specimen. From results obtained some specific properties were determined: the magnetic-mechanical hysteresis existence and the invariance feature in relation to the structural state of the material. The last result can create the outlook for stress measurement independently of the structural state of material as opposed to other known magnetic methods based on coercivity or Barkhausen noise measurements. Two original practical applications of the MA technique for stress determination were realized: the evaluation of the weldinginduced residual stresses and the estimation of stresses in the steel bridge framework during the launching to the pillars.
Nondestructive Determination of Residual and Applied Stress by Micro-Magnetic and Ultrasonic Methods
Ultrasonic techniques can detect volume and surface stresses and they are not limited to the investigation of ferromagnetic materials as the micromagnetic methods. The magnetic nondestructive techniques are restricted by eddy current damping to surface near regions. All ferromagnetic and magnetoelastic quantities are sensitive to the mechanical stress and microstructural state of the test specimen, whereas the ultrasonic velocity is mainly influenced by changes of the stress level and the texture. The main problem is: how to separate the different influences in the measuring quantities which is required to realize a quantitative nondestructive stress-measuring device.