Eddy current testing is an alternating magnetic field generated by alternating current acting on the conductive material to be tested, which can induce eddy currents. If there is a defect in the material, it will interfere with the generated eddy currents and create an interference signal. By detecting its interference signal with an eddy current flaw detector, the state of the defect can be known. There are many factors that affect eddy currents. Eddy currents are rich in signals that are related to many factors in the material. How to separate the useful signals from the many signals is a difficult problem for the current eddy current researchers. Some progress has been made over the years and some problems can be solved under certain conditions. However, it is far from meeting the requirements of the site and needs to be vigorously developed.
The presence or absence of defects such as cracks on the surface of mechanical parts is important, as this can cause serious quality problems. In order to ensure the safety and reliability of the machined parts, visual inspection alone cannot meet the requirements, so the inspection requirements for the machined surface of the workpiece have increased. Therefore, in order to achieve the required quality of the production line, a complete inspection system that uses non-contact measurement methods to inspect the produced parts in a short time is indispensable.
Using the eddy current of conductors for various inspections is a kind of electromagnetic induction inspection, and its typical application is eddy current flaw detection. The inspection for defects such as surface cracks of conductive workpieces is briefly described here.
1. The characteristics of eddy currents
It performs flaw detection on workpieces made of magnetic bodies and conductive non-magnetic bodies (aluminum alloy, stainless steel, copper, etc.) in a non-contact manner. In addition to cracks, the eddy current of the eddy current flaw detector can also detect the following defects: casting voids and holes on the surface, grinding burns, scratches, black skin residues, local material changes or the presence of foreign objects (material classification inspection) and local hardness changes (examination of material composition and structure), etc.
2. The principle of eddy current
The alternating current passes through the coil, creating a magnetic field around it (represented by magnetic lines of force) and acting on nearby conductors. The coil magnetic field is variable, so the magnetic field passing through the conductor changes over time in both direction and magnitude. At this time, the conductor hinders the change of the magnetic field to generate electricity, which is called electromagnetic induction.
The electric field generated in the conductor of the eddy current flaw detector generates a current (alternating current) under the action of the changing conductor magnetic field lines, which is called eddy current according to the shape of the current. The magnitude and distribution of eddy currents generated in conductors vary with frequency, conductor conductivity, permeability, size and shape of the test body, coil shape, coil size, coil current and its distance from the conductor, and the shape of the housing.
If there is a crack on the surface of the plate-shaped workpiece, the eddy current will flow away from the defect area with the crack (reducing the load of the coil) and the magnetic field will change. In this way, the change of conductor properties caused by damage such as cracks and the distribution of eddy currents change accordingly and the magnetic lines of force generated by eddy currents also change accordingly. Therefore, as long as the change of the magnetic field lines is detected, it is possible to know whether there is damage or the properties of other conductors.
Eddy current flaw detector is an intelligent instrument with impedance plane display and automatic sector area alarm using microcomputer technology. It has strong functions and is extremely simple and convenient to operate. It effectively separates the inner and outer wall defects, so it provides users with reliable flaw detection results.