FI89414C - Process for measuring plastic shape change and dislocation structure in magnetic material - Google Patents

Description

1 89414

METHOD OF MEASURING PLASTIC DEFORMATION AND DISLOCATION STRUCTURE IN A MAGNETIC MATERIAL

The invention relates to a method for measuring the plastic deformation and dislocation structure in a magnetic material.

During manufacture and use, different parts undergo plastic deformation of different size and quality. Plastic deformation occurs through dislocations. Deformation is caused by, for example, turning, milling, ball blasting, hardening, static and dynamic loads, creep, etc.

To measure stresses, there are residual stress measurement methods based on the magnetic properties of the material. It is characteristic of all of them that the body is cyclically magnetized and changes in e.g. the magnetic hysteresis curve or the so-called Barkhausen noise characteristics.

A problem with these known methods is that the signal to be measured is also affected by those magnetization processes which are sensitive to microstructural phenomena other than stress and deformation, e.g. inclusions, precipitates, phase boundaries and surface cracks, e.g. in EP-235 118.

The object of the invention is to eliminate the above-mentioned drawbacks. In particular, the object of the invention is to provide a new type of measuring method with which the plastic deformation state on the surface of a body can be measured and with which the dislocation structure formed during plastic deformation can be studied.

With regard to the features characteristic of the invention, reference is made to the claims.

35 When a ferromagnetic material is subjected to a load, its magnetization changes. This interaction is called a magnetoelastic or magnetomechanical 2, 9414 sex phenomenon. The direction and magnitude of the change in magnetization depend on the magnetostrictivity of the material, the stress, the previous deformation, and the initial magnetic state of the material, i.e., the difference between the hysteresis and anhysteresis states. The change in magnetization can be measured with a measuring sensor on the surface of the body.

In the method according to the invention, the signal to be measured is mainly produced by magnetization processes, 90 ° domain walls, which are particularly sensitive to plastic deformation. Consequently, the sensitivity of the method according to the invention for measuring deformation is higher than in known methods.

In the method according to the invention, the point to be measured of the material to be examined is magnetized with a constant magnetic field of a certain magnitude. In this way, the magnetic structure of the metal is tuned as sensitive as possible to the effect of plastic deformation dislocations and yield. An acoustic excitation pulse is then applied to the magnetized region 20, and the change in magnetization in the material caused by the excitation pulse is measured. From the measurement results describing the magnetization changes, the stress and deformation states of the investigated material can be analyzed.

In the method according to the invention, before magnetization, the material to be examined is demagnetized and then magnetized locally so that the magnetoelastic interaction of the area to be examined is at its maximum. Experiments have shown that the magnetoelastic interaction is greatest when the magnetizing field is about 70-90% of the value corresponding to the magnetic saturation state. This magnetization can be performed, for example, by first magnetizing the material to a value corresponding to its saturation state, whereby the extreme limits at which it operates are obtained.

The elastic excitation pulse used in the method can be caused, for example, by a spring-loaded or other 3 3 9 414 * mechanical striker. As the excitation pulse propagates in the area to be measured, the magnetization of this area changes irreversibly. This change can be measured by a suitable sensor on the surface of the object to be examined, e.g. a ferrite core coil. Individual pulses can be used as the excitation pulse or it can consist of a series of successive pulses.

If a plastic deformation has taken place in the material under test, the characteristics of the electrical voltage induced in the measuring coil are different from those in the unmodified part. The curves of the measurement pulses thus obtained vary according to the properties of the material to be examined. The pulse measured from the modified material is smaller than the original pulse. In addition, the pulse measured from the 15 modified materials has a smaller or smaller go than the original pulse.

In the method according to the invention, a reference sample known for its properties and structure can be used, whereby the pulse-like curve shape given by the reference sample can be compared with the curve shapes obtained from the materials to be examined.

The device used in the method according to the invention for measuring deformation in a magnetic material comprises a magnetizing circuit for magnetizing the material to be measured with a magnetic field of a desired magnitude, a measuring sensor for measuring magnetic flux density, an excitation pulse source To analyze.

The excitation circuit used can be formed, for example, by a U-core coil, with which the material to be examined can first be demagnetized and then magnetized 35 locally so that the magnetoelastic interaction of the region is at its maximum.

An elastic wave, 4 3 9-114, which can be produced, for example, by a spring-loaded striker, is used as the excitation pulse. The acoustic wave produced by the excitation pulse source propagates in the material and causes a local irreversible magnetization change in the measured range.

5 As a measuring sensor, it is possible to use, for example, a ferrite-core coil in which the change in magnetization caused by the excitation pulse induces an electrical voltage, the properties of which depend on the stress and deformation states of the measuring range. The analyzer used in the device may be any known apparatus or arrangement by which the results can be processed and / or recorded or stored.

The device also has a measuring device, e.g. a Hall sensor, with which the magnetic flux density of the magnetic area under study can be measured, whereby the magnetizing field strength can be adjusted to correspond to the maximum magnetoelastic interaction.

When using the device, by analyzing the properties of the voltage pulse induced in the measuring coil, information is obtained about the plastic deformation state and dislocation structure of the material. If a plastic deformation has taken place in the measured part, the magnetization change caused by the excitation pulse and the voltage pulse induced in the measuring coil are also different from those in the unloaded part.

An advantage of the method according to the invention compared to the prior art is that information is obtained locally on the plastic deformation and dislocation structure in the surface layer of the material.

In the following, the invention will be described with the aid of the accompanying drawing, which schematically shows a device used in the method according to the invention.

35 The device does not in any way limit the shape of the body 1 of magnetic material to be examined, but may be plate-shaped, rod-shaped, tubular 5 b 9 41 4, etc. The device shown in the figure includes a U-core coil magnetization circuit 3, excitation pulse source 4, analyzer 5, magnetization change sensor 2 and a magnetic flux measuring device 6.

5 Use the device as follows. The device is placed on the test site of the material to be examined 1 and first the area to be examined is demagnetized by the excitation circuit 3. The area to be examined is then magnetized by the magnetizing circuit 3 so that the toelastic interaction of the area magnet-10 is at its maximum, which occurs e.g. by measuring the area to be examined by the measuring device 6 to its magnetic saturation state and then the magnetization is reduced to about 80% of this. value. The measuring device 6 15 is e.g. a Hall sensor.

Thereafter, an excitation pulse source 4, which may be e.g. a spring-loaded striker or a piezoelectric crystal, produces an acoustic wave in the test area, which propagates in the test area, causing a magnetization change that induces a The obtained voltage is applied to an analyzing device 5, with which the obtained pulsed curve shape can be examined and on the basis of this, the deformation state of the material to be examined can be determined.

The sensor shown can be used to measure deformations in a part during manufacture or use, which can be caused by, for example, turning, milling, 30 Ball blasting, hardening, static and dynamic loads, creep, etc.

The invention has been described above by way of example with the aid of the accompanying drawing, however, various embodiments of the invention are possible within the scope of the inventive idea defined by the claims.

Claims (5)

Method for measuring plastic shape change and dislocation structure in magnetic material, characterized in that - the site to be measured in the material (1) to be investigated is magnetized with a constant-sized magnetic field, an acoustic impulse is produced, - an irreversible magnetization change produced by the impulse is measured, and 2. A method according to claim 1, characterized in that the material (1) to be examined is de-magnetized before the magnetization. 20 3. A method according to claim 1 or 2, characterized in that the acoustic pulse is effected by a mechanical impact pin. Method according to any one of claims 1 to 3, characterized in that the magnetization change is measured with a detector (2) on the object to be measured, e.g. a coil with ferrite core. Method according to any of claims 1 to 4, characterized in that the material (1) to be examined is magnetized with a magnetic field, the size of which is approx. 70-90% of the value corresponding to the magnetic saturation state of the material to be examined.