TAILIEUCHUNG - Flaw detection using flux leakage techniques

Principles of operation Magnetic Flux Leakage (MFL) inspection techniques are electromagnetic test methods primarily used to detect flaws or defects in high-permeability ferromagnetic metals such as carbon steel tubing, plate, wire rope and tubular parts. | FLAW DETECTION USING FLUX LEAKAGE TECHNIQUES PRINCIPLES OF OPERATION Magnetic Flux Leakage MFL inspection techniques are electromagnetic test methods primarily used to detect flaws or defects in high-permeability ferromagnetic metals such as carbon steel tubing plate wire rope and tubular parts. MFL inspection can be applied with an active or a residual magnetization technique using a DC magnetizing source. In the case of active magnetization enough flux density is created in the material to bring it to near-saturation. In residual magnetization a temporary saturation is created. In both cases detection of defects depends on a flux sensor probe detecting changes in the leakage flux that extend beyond the test piece. Figure 1 FIGURE 1 Outside diameter crack causes flux lines to leak beyond the tube. ACTIVE MFL MAGNETIZATION In Active MFL magnetization the DC magnetizing field creates an intense magnetic flux in the material some of which will extend or leak beyond it when interrupted by either a surface or internal defect. This leakage flux will generate a signal voltage in an external flux sensing probe as it is moved relative to the fixed material or as the material is moved relative to a fixed flux sensor probe. RESIDUAL MFL MAGNETIZATION Where residual MFL magnetization is used after saturation of the material has been achieved and the DC magnetizing field is de-energized its remaining or residual magnetization will create constant yet very weak leakage flux patterns for surface and near-surface defects only. As a result residual MFL techniques are most often used with dry magnetic particles for standard visual tests or magnetic particles with-dye penetrant for black-light visual inspections. Effectiveness is limited to surface and near surface defects but systems can be automated. PRINCIPLES OF SATURATION An important aspect of flux leakage technology is understanding the relationship between flux density and the applied magnetizing force. As shown in Figure 2

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• Principles of operation
• Flaw detection using flux leakage techniques
• Residual MFL magnetization
• Active MLF magnetization
• Flux leakage techniques
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