ACFM (Alternating current field measurement) alternating current electromagnetic field detection technology is a new type of non-destructive testing and diagnosis technology. It is used to detect crack defects on the surface and near-surface of metal components. It can measure the length of cracks and calculate the depth of cracks. It has non-contact measurement, It is less affected by the surface of the workpiece. This detection technology is more and more widely used in underwater non-destructive testing of offshore facilities.
background
In the detection of underwater structures of offshore oil and natural gas installations and offshore platform equipment, due to the particularity of the detected objects and the environment, the probability of omission and misjudgment in conventional detection methods is greatly increased, resulting in huge economic losses and environmental damage. Pollution, for example: Between 1977 and 1998, there were 20 natural gas pipeline accidents in Canada, of which 9 leaked and 11 ruptured, all caused by stress corrosion cracking (SCC-Stress Corrosion Crack). And, as in-service pipeline structures continue to age, the potential for accidents increases. ACFM technology is produced in this case. It is developed from the alternating voltage drop (ACPD-Alternating Current Potential Drop) technology, which combines the advantages of ACPD technology without calibration measurement and eddy current detection. One of the non-destructive testing methods for surface cracks.
In the late 1980s, ACFM technology was first used for the inspection of weld quality in key parts of underwater structures and the inspection of surface-coated metal structures. In 1997, Petrobras applied ACFM technology to offshore oil platforms. structural inspection. With the recognition of the value of surface fatigue crack detection without removing the coating and the further development and maturity of this technology, it has begun to be widely used in petrochemical, nuclear, steel and railway industries, civil structures such as bridge inspection, aerospace and other fields.
Fundamental
The theoretical basis of ACFM technology is the principle of electromagnetic induction. When a special coil (excitation coil) passing alternating current is close to the conductor, the alternating current generates an alternating magnetic field in the surrounding space, and the induced current on the surface of the workpiece (conductor) under test is due to The skin effect is concentrated on the surface of the workpiece. When there is no defect in the workpiece, the induced current lines are parallel to each other, and the surface of the workpiece exists due to the uniform magnetic field; if there is an oil defect in the workpiece, the change in resistivity will inevitably affect the current distribution, and the current line will be deflected near the defect. The magnetic field on the workpiece surface is distorted. The strength of this magnetic field can reflect the size of the crack. The effect of defects on current and magnetic field is shown in Figure 1-4. In the electromagnetic field generated on the surface of the workpiece, ACFM technology is to detect the induced magnetic field to detect cracks, because the permeability of the magnetic field is stronger than that of the electric field, and the attenuation is slower than that of the electric field. The electromotive force can detect the induced magnetic field.
The induced current flow model on the measured workpiece is established, and the magnetic field component is divided into three components Bx (parallel to the workpiece surface and perpendicular to the current direction), By (parallel to the current direction) and Bz (perpendicular to the workpiece surface) for analysis, As shown in Figure 1. When there is no defect, the induced current flows uniformly along the Y axis, the magnetic field component (Bx) in the X direction is uniform and undisturbed, and the magnetic field components (By and Bz) in the other two directions are zero. Therefore, away from the crack, the current field is uniform and the currents are parallel to each other. When the current flows to the crack, due to the inhomogeneity of the material and the principle of the continuity of the current at the interface, the current line will deflect to both ends of the crack and the bottom surface of the crack, so that the current line at the center of the crack becomes sparse, the current density decreases, and the two ends of the current line become sparse. The current lines converge, causing changes in the magnetic field on the surface of the workpiece.
In the measurement of ACFM, the following two mutually orthogonal magnetic field changes are generally measured:
1. The magnetic induction intensity Bz perpendicular to the surface of the workpiece, as shown in Figure 1, due to the large resistance on the crack surface, the current is deflected to both ends and the bottom surface of the crack. On the right half of the crack, the general trend of the current is clockwise, producing magnetic lines of force pointing to the surface of the workpiece; on the left half of the crack, the general trend of the current is counterclockwise, producing magnetic lines of force pointing to the bottom of the workpiece, the former is defined as positive and the latter is negative, as shown in Figure 1, so that the crack length can be measured by Bz.
2. Magnetic induction intensity Bx parallel to the surface of the workpiece and the direction of the crack: As shown in Figure 1, due to the deflection of the current to both ends and the bottom surface of the crack, the current intensity of Liu Jing's crack surface decreases. In the middle of the crack, the current line is the most sparse, Bx The minimum value of Bx corresponds to the deepest crack, so the depth of the crack can be measured. ACFM technology is to show people the existence of defects in a visual way, so that people have a more intuitive understanding of the data.
Our company currently has a professional team of senior engineers and engineers. The main inspectors are composed of a team of engineers who have been engaged in non-destructive testing for many years, with strong technical ability and rich experience. All inspectors have professional inspection qualifications, including non-destructive testing (ASNT/ISO9712 II/III and national special equipment non-destructive testing qualification II/III), American Association for Materials Performance and Protection (AMPP-Coatings Inspector/Senior Coatings Inspector), welding inspection (CWI, SCWI), International Welding Engineer IWE, API 653, API 510, API 570, API 571, API Quality Engineer, Senior Mechanical Engineer, etc.
The following inspection services can be provided for enterprises:
Penetrant Testing (PT), Magnetic Particle Testing (MT), Ultrasonic Testing (UT), Phased Array (PAUT), Time-of-Flight Diffraction TOFD, Radiographic Testing (RT), Visual Testing (VT), Eddy Current Testing (ECT), Leak Testing (LT), Magnetic Leakage Detection (MFL), Acoustic Emission Detection (AE), Digital Imaging (DR), Real-time Imaging Detection (CR), Alternating Current Field Measurement(ACFM), etc.