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Measuring principle of ultrasonic flaw detector
Sep 20, 2018

Measuring principle of ultrasonic flaw detector

The following small series talk about the measurement principle of ultrasonic flaw detector

When the ultrasonic wave propagates in the material to be tested, the acoustic properties of the material and the change of the internal structure have a certain influence on the propagation of the ultrasonic wave. The technique of understanding the material properties and structural changes by detecting the degree and condition of the ultrasonic wave is called ultrasonic testing. Ultrasonic testing methods generally include a penetrating method, a pulse reflecting method, a tandem method, and the like.

Digital ultrasonic flaw detectors usually emit ultrasound on the object to be measured (such as industrial materials, human body), and then use its reflection, Doppler effect, transmission, etc. to obtain information inside the measured object and process it to form an image.

Measuring principle of ultrasonic flaw detector: Doppler effect method

Is to use the Doppler shift effect that occurs when the ultrasonic encounters a moving object to derive the characteristics of the moving direction and speed of the object;

Measuring principle of ultrasonic flaw detector: transmission method

The internal characteristics of the object are obtained by analyzing the changes after the ultrasonic wave penetrates the object to be measured, and its application is still in the development stage;

Measuring principle of ultrasonic flaw detector: reflection method

The ultrasonic flaw detector here mainly introduces the method of obtaining the internal characteristic information of the object by the reflection method.

The reflection method is based on the principle that ultrasonic waves will have strong reflections when organizing interfaces through different acoustic impedances. As we know, sound waves are transmitted from one medium to another at the interface between the two. Reflection will occur, and the greater the difference between the media, the larger the reflection will be, so we can emit an ultrasonic wave that is strong and capable of linear propagation to an object. The ultrasonic flaw detector then receives the reflected ultrasonic wave and according to The order and amplitude of the reflected ultrasonic waves can determine the size and distribution of the various media contained in the tissue and the degree of contrast between the various media (the ultrasonic waves reflected back can reflect The distance from the reflection surface to the detection surface, the amplitude can reflect the characteristics of the size of the medium, the degree of contrast difference, etc., and the ultrasonic flaw detector determines whether the object to be measured is abnormal. In this process, there are many aspects involved, including the generation, reception, signal conversion and processing of ultrasonic waves.

The method for generating ultrasonic waves is to generate an excitation electric signal through a circuit to transmit a crystal having a piezoelectric effect (such as quartz, lithium sulfate, etc.) to vibrate to generate ultrasonic waves; and when receiving the reflected ultrasonic waves, 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, and an image is formed behind the ultrasonic flaw detector for observation and judgment.

Here, the type of the image processing method (that is, the image into which the obtained signal is converted) can be further classified into an A-type display, an M-type display, a B-type display, a C-type display, an F-type display, and the like.

The A-type display processes the received ultrasonic signal into a waveform image. According to the shape of the waveform, it can be seen whether there are abnormalities and defects in the measured object, and how large, etc., the ultrasonic flaw detector is mainly used for industrial detection;

The M-type display is a chronological expansion of a luminance-processed detection information to form a one-dimensional "space multi-point motion timing diagram" suitable for observing internal moving objects, ultrasonic flaw detectors such as moving organs and arteries. Blood vessels, etc.

The B-type display is a two-dimensional "anatomical image" that combines a plurality of brightness-processed detection information side by side to reflect the internal tomographic section of the measured object (the B-mode used in hospitals is made by this principle). ), the ultrasonic flaw detector is suitable for observing objects that are static inside;

The C-type display is also an image display. The abscissa and ordinate of the flaw detector screen are mechanically scanned to represent the position of the probe on the surface of the workpiece. The amplitude of the received signal of the probe is expressed by the brightness of the spot. Therefore, when the probe moves on the surface of the workpiece, the flat image of the internal defect of the workpiece is displayed on the screen, but the depth of the defect cannot be displayed.

Type C display and F type display are used less.

Ultrasonic flaw detector detection 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 object and the operator. Therefore, it is more and more popular and popular. Development prospects.

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