Metal ultrasonic flaw detector is a portable industrial non-destructive flaw detection equipment, which can quickly, easily, without damage, accurately perform a variety of defects within the workpiece (including longitudinal cracks, transverse cracks, loose, porosity, slag, etc.) detection, positioning , assessment and diagnosis. It can be used both in the laboratory and on the project site.
Testing Physics Foundation
Waves and sound waves
All the qualities of the media are linked with each other by elasticity. A certain point vibrates in the medium and can excite the surrounding mass. The process of propagation of vibration within an elastic medium is called a wave. Waves, there are electromagnetic waves (electric waves and light waves) and sound waves (or mechanical waves). Acoustic waves are elastic waves that can propagate in gases, liquids, and solids. It can be divided into fractional sound waves, audible sound waves, ultrasonic waves and special ultrasonic waves.
The sound waves that the human ear can hear are between 20-20000 Hz. A sound wave whose frequency exceeds 20000 Hz, which cannot be heard by human ears, is called an ultrasonic wave. The higher the frequency of sound waves, the more similar to some optical properties (such as reflection and refraction laws).
Instrument application
Widely used in boilers, pressure vessels, aerospace, aviation, power, petroleum, chemical, offshore oil, pipelines, military, shipbuilding, automotive, machinery manufacturing, metallurgy, metal processing industry, steel structure, railway transportation, nuclear power, universities, etc. industry.
Instrument principle
When the ultrasonic wave propagates in the material to be detected, the acoustic characteristics of the material and the changes in the internal structure have a certain influence on the propagation of the ultrasonic wave. The technique for understanding the change of the material properties and the structure through the detection of the influence degree and condition of the ultrasonic wave is called ultrasonic detection. Ultrasonic detection methods usually include penetration method, pulse reflection method, and serial method.
The digital ultrasonic flaw detector now usually emits ultrasound to a measured object (such as an industrial material or a human body), and then uses its reflection, Doppler effect, transmission, etc. to acquire the information inside the measured object and process it to form an image. Ultrasonic Flaw Detector The Doppler effect method uses the Doppler shift effect of an ultrasonic object when it encounters a moving object to obtain the characteristics such as the direction of movement and velocity of the object. The transmission rule is used to analyze the ultrasonic penetration. Subsequent changes in the object to draw the internal characteristics of the object, its application is still in the development stage; Ultrasonic flaw detector here is mainly introduced by the most widely used method to obtain the internal characteristics of the object through the reflection method. The reflection method is based on the principle that ultrasound will generate strong reflections when tissues are interfaced with different acoustic impedances. As we know, sound waves travel from one medium to another at the interface between the two. Reflection occurs, and the greater the difference between the media, the greater the reflection, so we can emit a penetrating ultrasonic wave that can transmit straight to an object. The ultrasonic flaw detector then receives and reflects the reflected ultrasonic wave. The sequence and amplitude of the reflected ultrasonic waves can determine the size and distribution of various media contained in the tissue and the degree of contrast between various media. The reflected ultrasonic waves can reflect The distance from the reflection surface of the reflection interface is measured, and the amplitude can reflect the characteristics of the medium, such as the degree of contrast, etc.). The ultrasonic flaw detector can judge whether there is any abnormality in the measured object. In this process, it involves many aspects, including the generation, reception, signal conversion, and processing of ultrasound. The method of generating the ultrasonic wave is to generate an excitation electric signal through a circuit and transmit it to a crystal with a piezoelectric effect (such as quartz, lithium sulfate, etc.) to vibrate to generate an ultrasonic wave; and when receiving the reflected ultrasonic wave, the piezoelectric crystal is again The electric signal is generated by the pressure of the reflected sound wave and transmitted to the signal processing circuit for a series of processing. The ultrasonic flaw detector finally forms an image for observation and judgment. Here, according to the type of image processing method (that is, converting the obtained signal into what kind of image), it can be divided into A-type display, M-type display, B-type display, C-type display, F-type display, and the like. Among them, the A-type display is to process the received ultrasonic signal into a waveform image. According to the shape of the waveform, it can be seen whether the detected object has abnormalities and defects there, how large, etc. The ultrasonic flaw detector is mainly used for industrial detection; M-type The display is a one-dimensional “spatial multi-point motion timing diagram†in which one piece of luminance-processed probe information is developed in time sequence, and is suitable for observing objects in motion inside. Ultrasonic flaw detectors such as moving organs, arterial vessels, etc. The B-type display is a two-dimensional, two-dimensional image of the internal tomographic section of the measured object that is composed of a number of side-by-side probes that have undergone luminance processing. (The B-ultrasound used in hospitals is based on this principle. The ultrasonic flaw detector is suitable for observing the static objects inside; the C-type display and the F-type display are now used less frequently. Ultrasonic flaw detector testing can not only be very accurate, but also more convenient and faster than other detection methods, and it will not cause harm to the detection objects and operators, so it has been welcomed by people more and more widely, and has a very broad Development prospects
Accuracy influencing factors
In the test object such as steel,
Metal ultrasonic flaw detector
Metal ultrasonic flaw detector
Even if it contains a variety of different alloy compositions, the speed of sound is considered to be essentially constant. In many other materials, such as many non-ferrous metals or plastics, the change in the ultrasonic propagation speed is very significant, which affects the accuracy of the measurement.
If the material of the object to be inspected is not isotropic, the speed of sound will be different in different directions. In this case, the average value of the speed of sound within the detection range must be calculated. The average value is obtained by measuring a reference block whose speed of sound corresponds to the average speed of sound of the block to be tested.
Effect of temperature
The sound speed of a material changes as the temperature of the material changes. If the calibration of the instrument is performed in a relatively low temperature environment, and the instrument is used in a relatively high temperature environment, the detection results will deviate from the true value in this case. To avoid this effect of temperature, warm up the reference block before calibrating the instrument to achieve the same temperature as the operating environment, or multiply the measurement result by a temperature influence factor.
Effect of surface roughness
The roughness of the flawed part has an effect on the flaw detection. The roughness increases and the effect increases. Rough surfaces can cause systematic errors and accidental errors. Each measurement should increase the number of measurements at different positions to overcome this accidental error.
Accuracy influencing factors
Material effects
In the test object such as steel,
Metal ultrasonic flaw detector
Metal ultrasonic flaw detector
Even if it contains a variety of different alloy compositions, the speed of sound is considered to be essentially constant. In many other materials, such as many non-ferrous metals or plastics, the change in the ultrasonic propagation speed is very significant and thus affects the accuracy of the measurement.
If the material of the object to be inspected 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 calculated. The average value is obtained by measuring a reference block whose speed of sound corresponds to the average speed of sound of the block to be tested. .
Effect of temperature
The sound speed of a material changes as the temperature of the material changes. If the calibration of the instrument is performed in a relatively low temperature environment, and the instrument is used in a relatively high temperature environment, the detection results will deviate from the true value in this case. To avoid this effect of temperature, warm up the reference block before calibrating the instrument to achieve the same temperature as the operating environment, or multiply the measurement result by a temperature influence factor.
Effect of surface roughness
The roughness of the flawed part has an effect on the flaw detection. The roughness increases and the effect increases. Rough surfaces can cause system errors and accidental errors. At each measurement, the number of measurements should be increased at different locations to overcome this accidental error.
Effect of attached substances
Adhesion must be removed before testing to ensure that the instrument probe is in direct contact with the surface of the part under test.
magnetic field
The strong magnetic fields generated by various electrical equipment around will seriously interfere with the inspection work.
technical problem
Sensitivity
The sensitivity in ultrasonic flaw detection generally refers to the ability of the entire flaw detection system (instrument and probe) to find the smallest flaw. The smaller the defect, the higher the sensitivity.
The sensitivity of the instrument's probe is often measured by the sensitivity margin. Sensitivity margin refers to the total amount of attenuation required to attenuate the echo required by the reflector when the maximum output of the instrument (gain, emission intensity, attenuation, and suppression is 0). The large sensitivity margin indicates high sensitivity of the instrument and probe. The sensitivity margin is related to the overall performance of the instrument and probe, so it is also called the comprehensive sensitivity of the instrument and the probe.
2. Dead zone and starting pulse width
The blind zone is the minimum distance from the detection surface to where the defect can be found. No defect in the blind zone can be found.
The starting pulse width refers to the starting pulse duration when the screen height exceeds 20% of the vertical amplitude at a certain sensitivity. The starting pulse width is related to the sensitivity, the sensitivity is high, and the starting pulse width is large.
3. Resolution
The resolution of the instrument and the probe is the ability to distinguish two adjacent defects on the screen. The smaller the distance between adjacent two defects that can be distinguished, the higher the resolution.
4. Signal to noise ratio
The SNR is the ratio of the smallest useful signal amplitude to the unwanted noise clutter amplitude on the screen. High signal-to-noise ratio, less clutter, and good for flaw detection. The signal-to-noise ratio is too low, which can easily lead to missed detection or misjudgment. In severe cases, it can not even detect flaws. The flaw detection can be characterized by fraud, and the production of substandard products.
If you do not have professional testing tools, the following simple detection methods can help you identify the authenticity:
1. When the probe is not connected, adjust the gain to the maximum and the waveform on the screen cannot exceed 10% of the screen. If it exceeds, the instrument fails.
2, see if the vertical line is qualified, method
3, there are some indicators need special test blocks. It is recommended that new instruments be sent to the provincial measurement and testing institutes for identification, so as not to be fooled.
4, the price is extremely low.
5. Pay attention to the production process and documentation. General ultrasonics require professional training.
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