Up knowledge! Five principles of conventional nondestructive testing technology

Date：2021-09-14 14:31:39Views：1108

At present, nondestructive testing (NDT) technology is widely used. The structural integrity and safety of almost all components, especially the most critical components, need to be verified. NDT plays a very important role in it. NDT technology is required in almost all manufacturing, service, maintenance or overhaul inspection fields. Five conventional NDT principles, NDT technology does not damage parts or materials, can be tested directly on site, and has high efficiency. There are five most commonly used NDT: UT (ultrasonic testing), RT (radiographic testing), MT (magnetic particle testing), Pt (penetrant testing) and ET (eddy current testing).

Ultrasonic testing

Ultrasonic is a mechanical wave with a frequency higher than 20 kHz. The frequency commonly used in ultrasonic flaw detection is 0.5-5 MHz. This mechanical wave can propagate at a certain speed and direction in the material, and will be reflected when it meets the heterogeneous interface with different acoustic impedance (such as defect or the bottom surface of the measured object). This reflection phenomenon can be used for ultrasonic flaw detection. The most commonly used is pulse echo flaw detection. During flaw detection, the voltage sent by the pulse oscillator is applied to the probe (detection element made of piezoelectric ceramic or quartz wafer). The ultrasonic pulse sent by the probe enters the material through acoustic coupling medium (such as oil or water) and propagates in it. After encountering defects, Part of the reflected energy returns to the probe along the original path, and the probe converts it into electrical pulse, which is amplified by the instrument and displayed on the fluorescent screen of the oscilloscope tube. According to the position and amplitude of the reflected wave of the defect on the fluorescent screen (compared with the reflected wave amplitude of the artificial defect in the reference test block), the position and approximate size of the defect can be determined. In addition to the echo method, there is also a penetration method in which another probe is used to receive the signal on the other side of the workpiece. When ultrasonic method is used to detect the physical properties of materials, the characteristics of ultrasonic velocity, attenuation and resonance in the workpiece are often used.

Radiographic testing

There are many kinds of rays, among which X-rays γ Ray and neutron ray. All three kinds of rays are used for nondestructive testing, including X-ray and X-ray γ X-ray is widely used in defect detection of boiler and pressure vessel welds and other industrial products and structural materials, while neutron ray is only used in some special occasions. The main application of radiographic testing is to detect the macro geometric defects (flaw detection) in the specimen. According to different characteristics, such as the types of rays used, recording equipment, process and technical characteristics, radiographic testing can be divided into many different methods. Radiography means the use of X-rays or γ Nondestructive testing method of ray penetrating test piece with film as the equipment for recording information. This method is the most basic and widely used radiographic testing method. Radiographic testing is applicable to most materials and product forms, such as weldments, castings, composite materials, etc. The radiographic inspection film can generate an intuitive image of defects on the internal structure of the material, with accurate qualitative and quantitative. The inspection results can be directly recorded and stored for a long time. The detection rate of volumetric defects, such as pores and slag inclusion, is very high. For area defects, such as cracks and final fusion, if the camera angle is not appropriate, it is easy to miss the inspection. The limitation of radiographic testing is that the cost is very high and the radiation is harmful to human body.

Magnetic particle testing

Magnetic particle testing is to magnetize the inspected workpiece by applying a magnetic field (overall magnetization or local magnetization). There will be magnetic lines of force escaping from the workpiece surface at the surface and near surface defects of the workpiece to form a magnetic leakage field. The presence of magnetic poles can adsorb the magnetic particles applied on the workpiece surface to form aggregated magnetic marks, so as to show the existence of defects. Magnetic particle testing is widely used to detect the surface or near surface defects of magnetic materials. It is mainly used to detect welds, castings or forgings, such as valves, pumps, compressor components, flanges, nozzles and similar equipment. Radiographic or ultrasonic testing is required to detect defects on the inner surface of a deeper layer.

Penetrant testing

Penetrant testing can detect the surface defects of non-magnetic materials, which provides a supplementary means for magnetic particle testing. Penetrant testing method is to use a liquid dye on the surface of the test material and keep it on the body surface for a preset time limit. The dye can be a colored liquid that can be recognized under normal light, or a yellow / green fluorescent liquid that can appear only under special light. The liquid dye enters the crack of the opening on the material surface due to "capillary action". Capillarity always occurs during the retention of the dye until the excess dye is completely cleaned. At this time, a developer is applied to the surface of the tested material to penetrate the crack and color it, and then appear. Inspectors with corresponding qualifications can analyze the traces. Penetrant testing can be widely used to detect the surface opening defects of most non absorbent materials, such as steel, non-ferrous metals, ceramics and plastics. It can also be used to comprehensively detect the defects with complex shapes at one time. No additional equipment is required for field use. Its limitation is that the testing procedure is cumbersome, the speed is slow, the reagent cost is high, the sensitivity is lower than that of magnetic particle testing, and it is impossible to detect buried defects or closed surface defects.

Eddy current testing

Eddy current testing is a nondestructive testing method based on the principle of electromagnetic induction. It is applicable to conductive materials. If we place a conductor in an alternating magnetic field, there will be induced current in the conductor, that is, eddy current will be generated. Due to the changes of various factors of the conductor itself (such as conductivity, permeability, shape, size and defects, etc.), the induced current will change, The detection method that uses this phenomenon to judge the properties and states of conductors is called eddy current detection method. In eddy current flaw detection, the alternating magnetic field is established by the detection coil; Transfer energy to the tested conductor; At the same time, the quality information in the tested conductor is obtained by the alternating magnetic field established by eddy current. Therefore, the detection coil is a kind of transducer. The shape, size and technical parameters of the detection coil are very important for the final detection. In eddy current testing, the type of testing coil is often selected according to the tested shape, size, material and quality requirements (testing standards). There are three types of commonly used detection coils: through coil, interpolation coil and probe coil.

Ultrasonic testing and radiographic testing are mainly aimed at the internal defects of the tested object, while magnetic particle testing, penetrant testing and eddy current testing are mainly aimed at the surface and near surface defects of the tested object.