Ultrasonic testing is suitable for nondestructive testing of metal, non-metal and composite materials. The defect location is accurate, the cost is low, the speed is fast, and the equipment is portable.
A principle and brief introduction
Ultrasonic flaw detection is a method using the ultrasonic energy to penetrate the depth of the metal material and from one section into the other section. A method of checking the defects of the parts at the edge of the interface is used to check the defect. When the ultrasonic beam is from the probe to the metal inside the surface of the part, the reflection wave occurs respectively when the defect and the bottom surface of the part are encountered. Pulse waveforms can be formed on the screen, and the location and size of the defects can be judged by these pulse waveforms.
According to its principle, ultrasonic testing can be divided into defect echo method, penetration method and resonance method. According to the waveform, it can be divided into longitudinal wave, transverse wave, surface wave and plate wave. Longitudinal waves are used to detect defects in metal ingot, billets, medium plates, large forgings, and simple parts of the shape. The transverse waves are the defects in the circumferential and axial cracks, scratches, welds, slags, cracks, and non penetration in the detection of pipe materials, and the surface wave on the surface of a casting with simple shape can be detected. A plate wave can detect defects in a thin plate.
Two. Detection process
A variety of detection techniques can be used in ultrasonic testing. In the process of implementation, each detection technology has its special problems to be considered, and its detection process has its own characteristics. But all kinds of ultrasonic testing technology have common technical problems. Its detection process can be roughly divided into the following steps:
1. Preparation of the specimen
In order to improve the reliability of the test results, the material grades, properties, manufacturing methods and process characteristics of the tested parts should be dealt with, and the types and causes of the defects, the maximum possible orientation and size of the defects, the stress state of the tested parts and the inspection standards are understood.
2, the determination of testing conditions, including the choice of ultrasonic detector, probe, test block and so on.
The selection of the incident direction should make the center line of the sound beam and the defect extension plane, especially the defect surface perpendicular to the maximum direction of the force, as close as possible to the vertical, and to obtain the maximum signal of the defect. In addition, it is difficult to avoid the discrimination of the defect by the reflection or variant signal in order to avoid the shape and structure of the workpiece. The direction should also be chosen in the absence of such interference signals. It should be checked from both positive and negative sides when necessary.
The selection of the probe is also particularly important. As one of the most important tools for ultrasonic testing, there are many kinds of probes and different structural types. Before testing, the probe should be selected according to the shape, attenuation and technical requirements of the subjects. The selection of probes includes selection of probe type, frequency, wafer size and refraction angle of inclined probe (K value). According to the shape of the workpiece and the position and direction of the defect, the method of detection is selected. Once the method is determined, what type of probe should be used is also determined.
3. Adjustment of testing instruments
When the instrument starts to be used, the horizontal and vertical linearity of the instrument is measured. The time base scale can be adjusted proportionally to represent the horizontal distance, depth or sound distance of the pulse echo.
Two principles are generally considered in scanning, one is to ensure that the entire inspection area of the specimen is covered with enough sound beam to avoid leakage, and the two is that the incident direction of the sound beam in the scanning process is always in accordance with the required requirements. The general standard stipulates that the scanning speed should not exceed 150mm / S. In the process of scanning, the probe should be given appropriate and consistent pressure to keep the probe moving smoothly. In the course of scanning, the direction of the probe should be strictly in accordance with the rules of the scanning method (the oblique probe should be paid attention to).
Because the change of the moving direction of the probe will change the sensitivity of the defect detection due to the change of the direction of the incident wave for the single probe detection, the reflection or transmission wave can not be received by the other probe for the double probe detection. Therefore, in order to avoid omission, each scan should have a certain percentage of beam coverage.
5. Assessment of defects
The specific assessment method of defects is omitted here. Interested friends can check the corresponding national standards.
6. The compilation of results and reports
Three, advantages and disadvantages
1. The main advantages
The penetration ability is strong, the depth of detection can be up to several meters; the sensitivity is high, and the reflector with equal air gap reflection of about a few millimeters of the diameter can be found; it is more accurate in determining the direction, size, shape, and so on of the internal reflector; it can provide the defect inspection results immediately; the operation is safe and the equipment is portable.
2. Main shortcomings
Ultrasonic flaw detection is not intuitionistic to the display of defects. It is difficult to be affected by the main and objective factors. It is difficult to inspect the rough, irregular, small, thin or heterogeneous materials, and it is difficult to make a precise qualitative and quantitative characterization of the found defects; it is not suitable for the structure with the cavity.
32 common problems of NDT
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1. What is nondestructive testing / nondestructive testing?
(1) nondestructive testing is a testing method for the inspection of the surface and internal quality of the tested parts without damaging the working state of the workpiece or raw material.
(2) NDT: NondestructiveTesting (abbreviated NDT)
2. What are the commonly used methods of flaw detection?
According to NASA’s investigation and analysis, NDT can be divided into six categories and about 70 species. But in practical application