Introduction
The main detection principle of magnetic leakage detection method is to magnetize the workpiece (close to saturation) to give it a certain magnetic flux density, so as to generate a leakage magnetic field at discontinuous points. The magnetic field sensor sends the output signal to the operating amplifier.
Details
After magnetization of the workpiece, the magnetic field lines deform at the surface and near surface defects, escaping from the workpiece surface to form magnetic poles, and forming a detectable leakage magnetic field. The position, shape, and size of the defects are displayed through the interaction between the leakage magnetic field and externally applied magnetic particles.
In magnetic particle inspection, the magnetic track method is one of the widely used methods. In the magnetic track method, the main inspection indicator of the equipment is lifting force. In most indicators and standards, the magnetic track lifting force is used as the standard for equipment performance control and equipment calibration. Magnetic track lifting force refers to the ability of a magnet to lift a ferrite block weighing G solely by its magnetic attraction. It is generally believed that the magnetic field strength of a magnetic track can be determined by the lifting force of the magnetic track.
The main detection principle of magnetic leakage detection method is to magnetize the workpiece (close to saturation) to give it a certain magnetic flux density, so as to generate a leakage magnetic field at discontinuous points. The magnetic field sensor sends the output signal to the operating amplifier. Due to the use of magnetic saturation state, the workpiece has a relatively high magnetic field strength and density, and the magnetic field lines are not restricted. Therefore, there is a large magnetic leakage on the surface of the workpiece, which is conducive to on-site inspection.
Magnetic sensitive sensors scan along the surface of magnetized ferromagnetic materials, pick up defect leakage magnetic fields, form defect electrical signals, and achieve the purpose of discovering defect locations and parameters.
Magnetic flux leakage non-destructive testing technology has been widely used in the detection of metal materials and the evaluation of related products due to its fast detection speed, high reliability, and low surface cleanliness of workpieces. Unlike magnetic particle testing, magnetic leakage testing does not require magnetic particle display of signals and is environmentally friendly. Due to the use of various sensitive components (such as Hall elements and coils), the detection results are directly output as electrical signals, making it easy to connect with computers for digital processing. Therefore, the detection results can be stored and reproduced, facilitating the analysis of detection signals and trend analysis of detection results.
Generally speaking, the magnitude of the leakage magnetic signal depends on four factors, namely:
1. The performance of the monitoring instrument itself includes sensors and supporting systems, pre-processing circuits, and signal analysis systems.
2. The geometric shape and characteristics of actual defects.
3. The instrument detects speed and the operating condition of the tested component (such as whether it is under force, etc.).
4. Magnetic properties of the inspected component. At present, the main methods for processing magnetic flux leakage signals include time-domain waveform analysis (including signal peak to peak and short-range energy), frequency-domain analysis, wavelet analysis, and neural networks. These methods are more focused on specific information of specific working conditions, using detection signals to compare with standard defect signals for defect analysis. They rarely consider the influence of different factors on the signal analysis results during the detection process, and lack quantitative descriptions of defect types, geometric shapes, and component working conditions.
Corrosion defect magnetic flux leakage testing is a common and effective method in recent years for detecting oil and gas pipelines and tank bottom plates. He determines the size of workpiece defects by measuring the magnetic field strength leaked from the surface of the magnetized material workpiece.
In principle, if there are no defects on the surface of the inspected workpiece and no inclusions inside, all magnetic flux will pass through the inspected workpiece; If there is a defect, it will cause an increase in magnetic resistance at and near the defect, resulting in distortion of the magnetic field near the defect. They can be divided into three parts:
1. Most of the magnetic flux bypasses defects inside the workpiece.
2. A small amount of magnetic flux passes through the defect.
3. Some of the magnetic flux leaves the upper and lower surfaces of the workpiece and bypasses the defect through air.
The third part is called magnetic flux leakage. Several Hall probes arranged between the excitation circuit (or detection probes) are used to detect changes in the leakage magnetic field, and their signal strength is closely related to the condition of the detected object. This method has been widely used for corrosion detection of oil and gas pipelines, tank bottoms, and non-destructive testing of magnetic materials such as steel wire ropes, steel plates, and steel blocks.
