GB/T 34361-2026Non-destructive testing - Test method for frequency scanning eddy current testing (English PDF)

6.4 Reference Test Block

6.4.1 The reference test block contains known features and discontinuities, and is used to set system parameters, verify the detection function of the system, and provide calibration curves.

6.4.2 The reference test block for measuring the coating thickness should have the same or similar coating and base material as the tested part, and have the same coating thickness. The number of layers and the thickness of each layer are similar, the manufacturing process is the same, and the surface curvature is basically the same.

6.4.3 For discontinuity testing, the material properties, electromagnetic characteristics, and structural shape of the reference test block should be similar to or closely resemble those of the tested piece. The location of the discontinuity... The shape should be similar to the inspected part.

6.4.4 An example of a reference test block for measuring the cover layer thickness is shown in Appendix A. An example of a reference test block for discontinuous testing is shown in Appendix B.

6.5 Extension Cable Provided that the detection system's functionality, sensitivity, and resolution meet the requirements, an extension cable can be used to connect the probe and the instrument. The use of cables must meet the following requirements.

a) Electrical performance should be compatible with the testing system;

b) The impact on the test results should be verified through comparative experiments, and the maximum permissible length should be determined based on the test frequency;

c) Before use, check the appearance integrity and connection reliability of the extension cable. If any damage or poor contact is found, replace it immediately.

d) During use, it should not be excessively bent or squeezed, and should not be exposed to high temperature or corrosive environments to prevent damage to the shielding layer or changes in impedance characteristics.

6.6 Detection System Verification The testing system should ideally be calibrated annually. This calibration should be performed before and after the testing process, during personnel changes, or when system components are repaired. Functional verification should be performed when repairing or replacing parts.

7.1 Preparations before testing

7.1.1 Basic information about the inspected item should be obtained through document review and/or physical inspection.

7.1.2 Based on the instruments used and the actual site conditions, and in accordance with the requirements of the testing process specifications, a sweep frequency eddy current testing operation manual should be prepared. Or process card.

7.1.3 Burrs, scale, rust, and other foreign matter on the surface of the inspected part that may hinder eddy current testing should be removed. Cleaning should not damage the part. The coating layer on the surface of the inspection piece.

7.1.4 The temperature and relative humidity of the testing site should be controlled within the range allowed by the testing system.

7.1.5 There should be no magnetic fields, vibrations, corrosive gases, or other interferences near the testing site that could affect the normal operation of the testing system.

7.2 Testing Procedures

7.2.1 Measurement of overburden thickness The measurement of the overburden thickness includes the following basic steps.

a) Adjust the instrument's sweep frequency range, number of sweep points, filtering, phase, gain, and other parameters according to the reference test block for measuring the coating thickness. The number and parameter adjustments should meet the following requirements. 1) Select an appropriate sweep frequency range to ensure that the eddy current field penetrates the coating layer and reaches the substrate, and that the signal exhibits good variation with thickness. Good linear decay characteristics; 2) The number of sweep points is determined based on the sweep frequency range and resolution; 3) Adjust the filtering parameters according to the electromagnetic interference conditions at the site to ensure that the signal-to-noise ratio is not less than 8dB; 4) Phase adjustment ensures that the signals corresponding to different capping layer thicknesses exhibit a good linear relationship on the horizontal axis projection; 5) The gain setting meets the sensitivity requirements, and the maximum thickness signal amplitude required for the specified sensitivity reaches the threshold height (e.g., 80%). FSH/Full Screen Height).

b) Establish a reference test block capping layer thickness calibration curve, which should meet the following requirements. 1) For single-layer capping layer thickness measurement, the known thickness of the reference test block for capping layer thickness measurement is used as the abscissa, corresponding to the frequency sweep signal. The eigenvalues of the sign are used as the ordinates, and the calibration curve is obtained by fitting the curve. 2) For multi-layer capping thickness measurement, calibration curves are created layer by layer (e.g., first using the uncapped substrate as a reference, then stacking layers sequentially). (Add the first and second cover layers for calibration) to clarify the correspondence between the thickness of each layer and the signal characteristics.

c) Based on the area, curvature, and accuracy requirements of the cover layer of the inspected part, plan and arrange the inspection points.

d) Perform point-by-point testing at the planned testing points and save the swept-frequency eddy current signal data for each testing point.

7.2.2 Discontinuous Detection Discontinuous detection includes the following basic steps.

a) Adjust the instrument's sweep frequency range, number of sweep points, filtering, phase, excitation, and gain according to the discontinuous detection reference block. parameter;

b) Based on the discontinuous inspection requirements of the inspected parts, select the corresponding artificial defect calibration inspection system on the discontinuous inspection reference block for sensitivity. The gain is increased by 2dB after ensuring that the amplitude height of artificial defects on the discontinuous detection reference block exceeds the alarm threshold.

c) Based on the signal amplitude of artificial defects of different depths on the discontinuous detection reference block, create defects of different depths and corresponding signal amplitudes. The calibration curve;

d) Place the probe on the surface of the workpiece and scan the surface of the workpiece. The scanning speed should be consistent with the probe scanning speed during calibration.

7.2.3 Creep Damage Detection of Metallic Materials Examples of creep damage detection for metallic materials are shown in Appendix C.

8.1 Evaluation of Overburden Thickness Measurement Results

8.1.1 By comparing and analyzing the calibration curve of the cover layer thickness of the reference test block with the swept-frequency eddy current signal data of each test point, the results of each test point were obtained. The thickness of the overburden layer at the measuring point.

8.1.2 The quality acceptance level shall be specified in accordance with the contract between the supplier and the buyer, or in accordance with the relevant product standards.

8.2 Evaluation of Discontinuous Detection Results

8.2.1 By comparing and analyzing the calibration curve of the reference test block for discontinuous detection and the swept-frequency eddy current signal data at each detection point, the results of each detection point are obtained. Discontinuous depth of the measuring point.

8.2.2 The inspector shall give a conclusion on whether the inspected parts are qualified or not, based on the technical specifications of the relevant products or the acceptance criteria agreed upon with the client.