Application of Coating Thickness Gauge in Nondestructive Testing
Non-destructive testing technology is a promising subject with strong theoretical comprehensiveness and great emphasis on practical aspects. It involves the physical properties of materials, product design, manufacturing process, fracture mechanics and finite element calculation and many other aspects. In the chemical, electronic, electric power, metal and other industries, in order to achieve the protection or decoration of various materials, methods such as spraying, non-ferrous metal covering, phosphating and anodizing are usually used. The concept of coating, coating, sticking or chemically generated film, we call it "cladding". The thickness measurement of the cladding has become the most important procedure for the quality inspection of the finished product by the users of the metalworking industry. It is the means by which the product reaches the standard.
At present, the thickness of the coating layer is generally determined according to a unified standard around the world. The selection of non-destructive testing methods and instruments for the coating layer is more important with the gradual progress in the research on the physical properties of materials. The non-destructive testing methods for coatings mainly include: wedge cutting method, optical cutting method, electrolysis method, thickness difference measurement method, non-destructive testing resource network weighing method, X-ray fluorescence method, β-ray reflection method, capacitance method, magnetic measurement method method and eddy current measurement. Most of these methods except the last five have to damage the product or the surface of the product, which is a destructive test, the measurement method is cumbersome and the speed is slow, and it is mostly suitable for sampling inspection. X-ray and beta-ray reflection methods can be non-contact and non-destructive measurement, but the device is complex and expensive, and the measurement range is small. Because of the radioactive source, the user must comply with the radiation protection regulations, and it is generally used for the thickness measurement of each layer of metal coating. Capacitance methods are generally only used for insulating coating thickness testing of very thin electrical conductors. Magnetic measurement method and eddy current measurement method, with the increasing progress of technology, especially after the introduction of microprocessor technology in recent years, the thickness gauge has taken a big step towards miniature, intelligent, multi-functional, high-precision and practical. The resolution of the measurement has reached 0.1μm, and the accuracy can reach 1%. It has the characteristics of wide application range, wide measuring range, easy operation and low price. It is the most widely used instrument in industry and scientific research. The thickness measurement by non-destructive testing method neither damages the coating nor the substrate, and the detection speed is fast, so a large number of detection work can be carried out economically. Our company now introduces several conventional thickness measurement methods as follows.
Magnetic measurement principle
1. The principle of magnetic attraction The thickness gauge can measure the thickness of the coating by using the suction force between the magnetic probe and the magnetically conductive steel to be proportional to the distance between them. This distance is the thickness of the coating. , so as long as the difference between the permeability of the coating and the substrate is large enough, the measurement can be performed. In view of the fact that most industrial products are stamped from structural steel and hot-rolled and cold-rolled steel sheets, magnetic thickness gauges are the most widely used. The basic structure of the measuring instrument is a magnetic steel, a tension spring, a ruler and a self-stop mechanism. When the magnetic steel and the object to be measured are attracted, a spring gradually elongates and the pulling force gradually increases. When the pulling force of the steel is greater than the suction force, the pull-down force is recorded at the moment when the magnetic steel is detached, and the coating thickness can be obtained. Generally speaking, according to different models and different ranges and suitable occasions. The characteristics of this instrument are simple operation, strong and durable, no power supply and calibration before measurement, and low price, which is very suitable for workshops for on-site quality control.
2. Magnetic induction principle The magnetic induction principle of the thickness gauge is to measure the thickness of the coating by using the size of the magnetic flux that flows into the iron substrate through the non-ferromagnetic coating. The thicker the coating, the smaller the magnetic flux. Because it is an electronic instrument, it is easy to calibrate, can perform various functions, expand the range, and improve the accuracy. Since the test conditions can be reduced a lot, it has a wider application field than the magnetic suction type. When the probe around the coil on the soft iron core is placed on the object to be measured, the instrument automatically outputs the test current. The magnitude of the magnetic flux affects the magnitude of the induced electromotive force. The instrument amplifies the signal and then indicates the thickness of the coating. The early products were indicated with a meter, and the accuracy and repeatability were not good. Later, a digital display type was developed, and the circuit design was also increasingly perfected.
