It is the general term of all technical means to determine the technical status (such as qualified or not, residual life, etc.) of the subject subject by using the characteristics of sound, light, magnetism and electricity, and on the premise that the performance of the tested object is not damaged or affected, whether there is any defect or inhomogeneity in the object under inspection, and the size, position, nature and quantity of the defect are given.
Nondestructive testing is a test method to inspect the surface and internal quality of the inspected parts without damaging the work piece or the working state of raw materials.
The common methods of flaw detection include: X-ray, ultrasonic, magnetic particle, eddy current, etc γ The physical detection methods such as ray inspection, penetrant inspection (fluorescent inspection and coloring inspection) etc.
Compared with destructive testing, NDT has the following characteristics. It is non-destructive, because it will not damage the performance of the object being tested when it is tested; The second is comprehensive. Because the detection is non-destructive, it can be used to detect the object under test when necessary, which is not available for destructive detection; The third is full process, destructive testing is generally only applicable to the detection of raw materials, such as tensile, compression, bending, etc. generally used in mechanical engineering. Destructive inspection is conducted for raw materials for manufacturing. For finished products and articles in use, destructive testing is not allowed unless they are not ready to continue service, But the non-destructive testing does not damage the performance of the tested object. Therefore, it can not only test raw materials, intermediate process links, and final products, but also test the equipment in service.
Through NDT, the defects in the material or the inner and surface of the workpiece can be found, the geometric characteristics and dimensions of the workpiece can be measured, and the internal composition, structure, physical properties and state of the material or workpiece can be measured. NDT can be applied to product design, material selection, processing and manufacturing, finished product inspection, in-service inspection (maintenance) and other aspects, and can play an optimized role in quality control and cost reduction.
NDT also helps the safe operation and / or effective use of the product. Through the detection of the internal defects of the products, the following aspects are improved: 1. Improve the manufacturing process; 2. Reduce manufacturing cost; 3. Improve the reliability of the product; 4. Safe operation of the equipment. NDT scope:
Inspection of weld surface defects. Check the welding quality of weld surface crack, incomplete penetration and welding leakage. 2. Internal cavity inspection. Check surface cracks, peeling, pull wires, scratches, pits, bulges, spots, corrosion and other defects. 3. Status check. When some products (such as worm gear pump, engine, etc.) work, the endoscope inspection shall be carried out according to the items specified in technical requirements. 4. Assembly inspection. When required and needed, the assembly quality is checked by using Yatai photoelectric industry video endoscope; After assembly or a certain process is completed, check whether the assembly position of each component meets the requirements of drawing or technical conditions; Whether there is any assembly defect. 5. Check the excess. Check the residual internal debris and foreign matters in the inner cavity of the product.
Ultrasonic testing (UT)
Principle of ultrasonic testing: ultrasonic is a kind of mechanical wave, and mechanical vibration and fluctuation are the logistics basis of ultrasonic detection. The object moves periodically around a certain equilibrium position along a straight line or curve, which is called mechanical vibration.
The propagation process of vibration is called fluctuation. The wave is divided into two categories: mechanical wave and electromagnetic wave. Mechanical wave is the process of mechanical vibration propagation in elastic medium. Ultrasonic is a kind of machine wave.
Ultrasonic detection is a method to inspect the defect of parts by using ultrasonic energy penetrating into the depth of metal materials and entering another section from one section to another. When the ultrasonic beam passes through the probe from the surface of the part to the metal, it will reflect wave separately when it encounters the defect and the bottom of the part, and forms a pulse wave on the fluorescent screen, The position and size of defects are determined by these pulse waveforms.
Ultrasonic detection classification: there are many kinds of wave types when ultrasonic wave propagates in medium. The commonly used ones are longitudinal wave, shear wave, surface wave and plate wave. The defects of inclusions, cracks, shrinkage tubes, white spots and delamination in ingots, billets, plates, forgings and simple shapes can be detected by longitudinal wave; The defects of circumferential and axial cracks, scratches, porosity, slag inclusion, cracks and incomplete penetration in the weld can be detected by using the shear wave; Surface defects on simple casting can be detected by surface wave; The defects in the thin plate can be detected by plate wave.
Advantages of ultrasonic testing: compared with X-ray detection, the flaw detection has the advantages of high sensitivity, short period, low cost, flexibility and convenience, high efficiency, harmless to human body, etc; Ultrasonic testing is suitable for the inspection of parts with larger thickness.
Scope of ultrasonic testing:
1. Inspection of weld surface defects. Check the welding quality of weld surface crack, incomplete penetration and welding leakage.
2. Internal cavity inspection. Check surface cracks, peeling, pull wires, scratches, pits, bulges, spots, corrosion and other defects.
3. Status check. When some products (such as worm gear pump, engine, etc.) work, the endoscope inspection shall be carried out according to the items specified in technical requirements.
4. Assembly inspection. When required and needed, the assembly quality is checked by using Yatai photoelectric industry video endoscope; After assembly or a certain process is completed, check whether the assembly position of each component meets the requirements of drawing or technical conditions; Whether there is any assembly defect.
5. Check the excess. Check the residual internal debris and foreign matters in the inner cavity of the product.
Magnetic particle inspection (MT)
Magnetic particle inspection uses the interaction between the magnetic flux at the defect of the workpiece and the magnetic particle. It uses the difference between the permeability of the surface and the near surface defects of the steel products (such as cracks, slag inclusion, hair lines, etc.), and the magnetic field at the discontinuous parts of these materials will change after magnetization, which will form the leakage field on the surface of the workpiece at the leakage of some magnetic flux, Thus, magnetic particle accumulation at the defect formed by magnetic particle is attracted. Under the proper illumination conditions, the defect position and shape are displayed. The magnetic particle detection is realized by observing and explaining the accumulation of these magnetic particles.
Magnetic particle inspection is a nondestructive testing method for detecting the defects on the surface or near the surface of ferromagnetic materials by the accumulation of magnetic particles in the leakage field near the defect. The magnetic work pieces made of iron and steel are magnetized. The magnetic flux leakage energy at the defect part is used to absorb the magnetic particle. According to the distribution of magnetic particle, the detection methods of surface defects and near surface defects of the detected objects are shown. The method is simple and intuitive in display. Magnetic particle inspection is the same as that of Hall element and magnetic sensitive semiconductor element, magnetic tape recording and coil induction electromotive force.
Type of magnetic particle inspection:
1. According to the different magnetization direction of workpiece, it can be divided into circumferential magnetization, longitudinal magnetization, composite magnetization and rotating magnetization.
2. According to the different magnetization current, it can be divided into DC magnetization, half wave DC magnetization and AC magnetization.
3. According to the different preparation of magnetic powder used in flaw detection, it can be divided into dry powder method and wet powder method.
4. According to the time of applying magnetic particle on the workpiece, it can be divided into continuous method and remanence method.
Magnetic particle inspection process:
1. Pretreatment
2. Magnetization
3. Apply magnetic particle or magnetic suspension
4. Observation and recording of magnetic marks
5. Defect rating
6. Demagnetization, post treatment
Penetrant testing (PT):
Penetrant inspection is a nondestructive inspection method for the surface defects of materials by capillary phenomenon. Penetrant examination includes fluorescence and coloring. Fluorescence method is to apply the liquid containing fluorescent substance on the surface of the tested part, penetrate into the surface defects by capillary action, then clean the permeation liquid from the surface, and retain the permeate in the defect for imaging.
The typical method of imaging is to spread the uniform white powder on the surface of the tested part, and suck the penetrant liquid out of the defect and extend it to the surface. At this time, the surface is illuminated with ultraviolet light in the dark, and bright fluorescence is emitted at the defect.