Factors affecting the detection accuracy of ultrasonic flaw detectors: detection object, temperature, surface roughness, magnetic field
Release time:
2021-08-20
Factors that affect the detection accuracy of ultrasonic flaw detectors: detection object, temperature, surface roughness, magnetic field, attached material, shape characteristics of defects, acoustic impedance of defects, and surface characteristics of defects (such as whether it is smooth).
material influence
In a test object such as steel, even if it contains a variety of different alloy components, its sound velocity is considered to be basically constant. In many other materials, such as many non-ferrous metals or plastics, the ultrasonic propagation velocity changes very significantly, which will affect the accuracy of the measurement. If the material of the object to be detected is not isotropic, the speed of sound will be different in different directions. In this case, the average value of the speed of sound in the detection range must be used for calculation. The average value is obtained by measuring a reference test block whose sound velocity is equivalent to the average sound velocity of the block to be tested.
The influence of temperature
The speed of sound of the material will change with the change of the temperature of the material. If the calibration of the instrument is carried out in a relatively low temperature environment, but the instrument is used in a relatively high temperature environment, the detection result will deviate from the true value in this case. To avoid this influence of temperature, the method is to preheat the reference test block before calibrating the instrument to reach the same temperature as the operating environment; or multiply the measurement result by a temperature influence factor.
The influence of surface roughness
The surface roughness of the tested parts has an influence on the flaw detection. The roughness increases, the influence increases. Rough surface will cause systematic errors and accidental errors. In each measurement, the number of measurements should be increased at different positions to overcome this accidental error.
The influence of attached substances
Before flaw detection, the attached material must be removed to ensure that the probe of the instrument is in direct contact with the surface of the test piece.
The influence of the magnetic field
The strong magnetic field generated by various electrical equipment around will seriously interfere with the inspection work.
All ultrasonic flaw detectors detect defects and locate based on the measurement of ultrasonic echo signals. Whether the speed of sound in the test object is constant is an important factor that affects the accuracy of the test result. Therefore, to achieve higher detection accuracy, a relatively constant ultrasonic propagation velocity in the test object is required.
Defect assessment method
In the current flaw detection practice, there are basically two different defect evaluation methods:
1. If the diameter of the sound beam is smaller than the defect range, the sound beam can be used to detect the defect boundary and determine its range.
2. If the beam diameter is larger than the defect range, the defect large echo response must be compared with the artificial defect large echo response used for comparison.
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