Determining stress in ferromagnetic materials from measurements

Details Determining stress in ferromagnetic materials from measurements Determining stress in ferromagnetic materials from measurements

1995-11-16

1998-10-27

O'Shea, Sandra L.

Electricity: measuring and testing

Magnetic

Stress in material measurement

G01B 724, G01R 3318

Patent

active

058282114

DESCRIPTION:

BRIEF SUMMARY
This invention relates to a method and to an apparatus for measuring stress in a ferromagnetic material.


BACKGROUND OF THE INVENTION

Steel, which is a ferromagnetic material, is a widely used construction material. To evaluate the safety of a structure it would be desirable not only to be able to detect defects (for example using ultrasonics), but also to measure the stress in the material, for example the stress around a defect which affects the likely behaviour of the defect. Hence a reliable non-destructive way of measuring the total stress (including residual, applied, static and dynamic components) would be desirable. A variety of magnetic techniques are known to have some sensitivity to stress, for example magnetoacoustic emission, Barkhausen emission, coercivity, stress-induced magnetic anisotropy, directional effective permeability, or incremental permeability over a whole magnetic cycle, although magnetic measurements are usually also affected by other material properties such as microstructure. For example EP-A-0 389 877 (Nikkoshi) teaches that stress in steel may be determined from the reversible magnetic permeability in the approach to saturation; it also states that using the variations in magnetic permeability to determine principal stresses does not enable reproducible results to be obtained. Ultrasonic measurements, X-ray diffraction and neutron diffraction can also provide information about microstructure and/or stress. It has also been suggested, for example in QT News, November 1992, that a combination of magnetic techniques may enable the effects of residual stress to be isolated.
According to the present invention there is provided a method for measuring stress in a ferromagnetic material, the method using a probe comprising an electromagnet means defining an electromagnet core and two spaced apart electromagnet poles, a first magnetic sensor between the two poles and arranged to sense magnetic flux density perpendicular to the direction of the free space magnetic field between the poles, a second magnetic sensor arranged to sense the reluctance of that part of the magnetic circuit between the poles of the electromagnet means, and means to generate an alternating magnetic field in the electromagnet means, the method comprising arranging the probe with the poles adjacent to a location on a surface of the ferromagnetic material, generating an alternating magnetic field in the electromagnet means and so also in the ferromagnetic material with a maximum amplitude well below magnetic saturation, turning the probe so the alternating magnetic field in the ferromagnetic material has in succession a plurality of different orientations, detecting the signals from the first sensor with the magnetic field in the plurality of different orientations, determining from the orientations of the magnetic field at which the maxima and minima of the signals from the first sensor occur the directions of the principal stress axes, detecting the signals from the second sensor at least when the magnetic field is aligned with the directions of the principal stress axes, processing the signals from the second magnetic sensor by backing them off with a back-off signal preset such that when the probe is adjacent to a stress-free location the backed-off signal is zero, and then resolving that component of the backed-off signal which, in the impedance plane, is perpendicular to the effect of lift-off from the surface, and using the values of the resolved component with the magnetic field aligned with the directions of the principal stress axes to determine the values of the principal stresses at that location in the material.
The first sensor would detect no signal if the material were exactly isotropic; however stress induces anisotropy into the magnetic properties of the material, and so the signals received by the first sensor are a measure of this stress-induced magnetic anisotropy (SMA). The variations in the SMA signal as the probe is rotated enable the directions of the principal stress axes to be accurate

REFERENCES:
patent: 5010299 (1991-04-01), Nishizawa et al.
Dr. Kajalainen et al., Detection of Fabrication Stresses by the Barkhausen Noise Method, University of Oulu, Finland, pp. 1-18, 1985.

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