GB/T 41119-2021Non-destructive testing—Micromagnetic testing—General principles (English PDF)

3.1 Micromagnetics

The magnetics that describes and studies the change of magnetic domain (the thickness of the domain wall and the range of single-domain particle size) during magnetization of ferromagnets at the mesoscopic scale.

3.2 Micromagnetic testing

A non-destructive testing method to obtain the stress distribution and its state, microstructure changes, and structural changes of materials through the micro-magnetic macro characterization of stress, phase transition, organization, structure and other features.

3.3 Effective magnetic field

The vector sum of all magnetic fields acting on the magnetic moment.

3.4 Equivalent magnetic permeability

Under the action of an environmental magnetic field or external magnetic field, the local magnetic permeability of the material that is caused by the change of microstructure, the difference in stress and tissue distribution and obtained by the calculation with the effective field method.

3.5 Static micromagnetic testing

The micromagnetic testing carried out when the magnetic moment in a magnet is in a state of equilibrium under the action of an external static magnetic field or an environmental magnetic field.

3.6 Dynamic micromagnetic testing

The micromagnetic testing that is carried out when an external dynamic magnetic field exists and the magnetic moment in a magnet is in a time-varying state under the action of the effective field.

4.1 Testing principle

Due to the changes in stress, structure, and microstructure in metal materials, under the action of external magnetization fields or environmental magnetic fields, the micromagnetic parameters of the material change, resulting in the generation of corresponding abnormal electromagnetic signals. Through the processing and analysis of abnormal signals, information such as stress distribution (magnitude and direction), material structural change (type, degree, and location), phase transition (quantity and distribution area), etc. can be obtained, which can be used for the evaluation of the mechanical property, high-and-low temperature performance and corrosion resistance of the materials.

7.1 Testing instruments

The testing instrument shall have functions such as data acquisition, digital signal conditioning and filtering, waveform display, analysis, and storage, and shall at least meet the following requirements:

a) Measure the magnetic/electromagnetic field intensity and its distribution on the surface of the tested object, and the sensitivity of the magnetic field measurement shall not be less than 0.5 mV/G;

b) The response time of dynamic electromagnetic field measurement shall not be more than 0.1 s;

c) Display the detection signal;

d) Offer the quantitative evaluation curve of the state of the tested object;

e) Automatically store the testing results, and the original data corresponding to the detection signal waveform.

7.2 Probes

The structural form, applicable materials, temperature, and the performance parameters for magnetic/electromagnetic field measurement of the probe shall be provided.

7.3 Magnetizing apparatus

According to the requirements of the static/dynamic micromagnetic testing method, it is determined that the AC magnetization or DC magnetization is adopted for magnetizing apparatus; the intensity and direction of the magnetization field shall be adjustable according to the testing object.

7.4 Maintenance and calibration of testing equipment

Written procedures shall be developed, and the periodic maintenance of testing equipment shall be carried out to ensure equipment function; testing equipment shall be calibrated at least once a year.

The calibration object shall at least include the instrument host, probe, and magnetizing apparatus (if any). The host gain and bandwidth of the instrument host shall be calibrated; the sensitivity, accuracy, and response speed of the probe shall be calibrated with a standard magnetic field, and the uniformity and intensity of the magnetizing field of the magnetizing apparatus shall be calibrated.

8.3 Testing modes

Static micromagnetic testing, dynamic micromagnetic testing, or the combined testing mode can be used.

8.4 Test records

The test records shall at least include the content specified in 10.2, and the test records and data shall be kept in accordance with the contract.

8.5 Static micromagnetic testing

The following steps shall be followed to implement static micromagnetic testing:

a) Determine the surface state of the tested object, set the probe, and scan the coverage area;

b) Determine the intensity and direction of the environmental magnetic field, as well as the form, intensity, and direction of the external magnetic field (optional);

c) Place the probe at a fixed measuring point of the tested object, and start the testing after the magnetization field is in a stable state and the change rate of the magnetic/electromagnetic field is not more than 1%;

d) Determine the scanning direction and sequence to ensure 100% coverage of the inspection area;

e) Record the complete magnetic/electromagnetic field on the surface of the tested object;

f) Determine the abnormal parts and signal distortion positions with uneven areas on the surface of the tested object, and mark them;

g) If necessary, scan and test the above-mentioned parts in the cross directions.

8.6 Dynamic micromagnetic testing

The following steps shall be followed to implement dynamic micromagnetic testing:

a) Determine the surface state of the tested object, set the sensor, and scan the coverage area;

b) Determine the form, intensity, direction, and application area of the external magnetic field;

c) Determine the magnetization loading sequence and intensity, and place the probe on the surface of the tested object;

d) Determine the measuring position and scanning method to ensure 100% coverage of the inspection area;

e) Record the complete magnetic/electromagnetic field on the surface of the tested object;

f) Determine the abnormal parts and signal distortion positions with uneven areas on the surface of the tested object, and mark them;

g) If necessary, scan and test the above-mentioned parts in the cross directions.

8.7 Real-time analysis and processing of testing data

During the testing, the distribution, intensity, and sequential variation of the magnetic field shall be analyzed in real-time. For uniaxial stress measurement, the reliability and stability of the measurement data shall be verified; for non-uniformity detection, abnormal areas shall be identified.

9.1 Stress measurement and evaluation of stress distribution

For the uniaxial stress measurement, the influencing factors such as the direction of the environmental magnetic field, expected stress direction and measurement environment, and the validity of parameters of the measured magnetic/electromagnetic field shall be first determined; then, carry out the conversion of the stress equivalent magnetic field according to the testing method and give the measurement result of the uniaxial stress.

For the evaluation of stress distribution, the magnitude, direction, and nature of stress shall be converted according to the measured magnetic field parameters, and a stress distribution map shall be given. According to the stress distribution map, determine the uneven area of stress distribution or the stress distribution curve, the maximum/minimum value of stress, and the stress distribution unevenness.

9.2 Evaluation of uneven structure

For the evaluation of the structure, the equivalent magnetic permeability of the measured magnetic field parameters shall be used as the final parameter for evaluation, and it shall be compared with the relative magnetic permeability of conventional […]