【 Abstract 】 : This paper starts from the physical nature of ultrasound, and then discusses the principle of ultrasonic detection of products combined with ultrasonic and detection technology, that is, based on the linear energy loss (attenuation) and reflection on the interface when ultrasonic passes through the object, and then analyzes the reflected beam or the transmission beam under certain conditions. To determine whether there is a crack or defect and the size of the defect, location, etc. The advantages and disadvantages of ultrasonic testing are analyzed in detail, and the application of ultrasonic testing in the quality testing of oil well pipe, steel casting, steel structure welding, space grid structure welding is introduced one by one.

【 Key words 】 : Application of ultrasonic nondestructive testing

First, the physical nature of ultrasound

Ultrasonic wave is a frequency higher than 20000HZ sound wave, it has good directionality, strong penetration ability, easy to obtain more concentrated sound energy, and can be used for distance measurement, speed measurement, cleaning, welding, crushing, sterilization and so on. It has many applications in medicine, military, industry and agriculture. Ultrasound is named because its lower frequency is approximately equal to the upper limit of human hearing.

Ultrasonic is used for non-destructive testing, mainly because of the following characteristics:

(1) When ultrasonic waves propagate in the medium, they will be reflected at the interface;

(2) Ultrasonic directivity is good, the higher the frequency, the better the directivity;

(3) Ultrasonic propagation energy is large, the penetration of various materials is strong.

In recent years, the research shows that the characteristics of ultrasonic speed, attenuation, impedance and scattering provide rich information for the application of ultrasonic, and become the conditions for the wide application of ultrasonic.

The main electronic equipment for pulsed ultrasonic detection consists of a voltage supply (for excitation of a piezoelectric transducer (pulser)) and a display device (for analysis of the pulse receiving the ultrasonic wave). Figure 1 shows a typical block diagram of the basic device system for ultrasonic flaw detection. Ordinary light and X-ray waves produce electromagnetic energy, while ultrasonic waves produce mechanical energy that includes vibrations or oscillations of molecules or atoms of an object. Ultrasonic waves behave like audible sounds: they can travel through solids, liquids, and gases, but not through a vacuum. Currently, the interaction of ultrasonic waves between materials and cracks can be successfully modeled using various simulation techniques.

When the ultrasonic wave propagates in an infinite medium, it will always propagate forward without changing its direction. However, when encountering a heterogeneous interface (that is, a heterogeneous interface with a large difference in acoustic impedance), reflection and transmission will occur. That is, a part of the ultrasonic wave is reflected back to the first medium at the interface, and the other part enters the second medium through the medium interface. There are many wave modes of ultrasonic wave propagating in the medium, and the most commonly used ones are P-wave, shear wave, surface wave and plate wave. The longitudinal wave can be used to detect the defects of inclusions, cracks, shrink pipes, white spots and delamination in metal ingot, billet, medium and thick plate, large forgings and simple parts. Shear wave can be used to detect the circumferential and axial cracks, scratches, porosity, slag inclusion, cracks, underpenetration and other defects in the weld. Surface wave can be used to detect surface defects on simple parts. Defects in thin plates can be detected by using plate waves. Shear wave is also widely used in ultrasonic inspection, its propagation is somewhat similar to the vibration form of a rope formed by regular shaking at one end of the rope, and the molecules and atoms vibrate up and down in a plane perpendicular to the direction of wave propagation; Surface wave is only sometimes used in ultrasonic inspection, it travels along a plane or relatively thick curved surface; Plane waves are only used in some cases of ultrasonic detection, and only propagate on the surface of materials whose thickness is only a few wavelengths. The reflection of the ultrasonic wave at the interface is more related to the physical state of the material, but less related to the physical properties of the material itself.

Second, the generation and detection of ultrasonic waves

In most cases, ultrasonic waves used for nondestructive testing are generated and detected by piezoelectric converters. A piezoelectric transducer requires a coupler to transmit ultrasonic waves between the transducer and the specimen. The piezoelectric converter has a piezoelectric crystal (such as barium titanate, lead zirconate, lead titanate, etc.) that can quickly change shape when energized. When they are rapidly pressurized, the opposite happens, creating an electromagnetic field. The generation and detection of ultrasonic waves for non-destructive testing can also be achieved by other methods. One of them is the use of non-contact air-coupled converters, which are based on microelectromechanical systems (MEMS); Another method is to generate non-contact laser ultrasonic waves through the rapid thermal expansion and melt corrosion of the specimen surface, and use laser interferometer or air coupled transducer to detect the generated ultrasonic waves. In addition, magnetic metals can also be detected by ultrasonic detection, mainly using non-contact electro-mechanical acoustic wave converters (referred to as EMATS) to simultaneously generate and detect ultrasonic waves. Ultrasound usually checks for cracks and defects by one or more of the following methods.

(1) Detection by reflected sound waves at material boundaries or defect interfaces.

(2) By ultrasonic switching time or transmission time detection.

(3) Through the attenuation degree of ultrasonic detection.

(4) Detect the spectral response characteristics of transmitted signals or refracted signals.

Third, the advantages and disadvantages of ultrasonic inspection

Advantages:

1, the penetration rate is high, the penetration rate can reach several meters in many workpieces, and the axial rate can reach 6 meters.

2, high sensitivity, can detect extremely small defects.

3. High accuracy when detecting the position of the defect on the inner wall of the workpiece, estimating the size of the defect, and identifying the orientation, shape and nature of the defect.

4, only one surface contact detection.

5, electronic operation, almost can display defects at the same time, suitable for instant analysis of test results, automatic operation, fast scanning, online production monitoring and process control.

6, can be scanned from the front surface of the workpiece to the back surface.

7. No damage operation.

8. Portable.

9, digital output, you can use the computer to analyze defects and material properties.

Cons:

1, manual operation requires the staff to have professional technology and careful inspection.

2, in order to develop the appropriate detection procedures require a variety of technical knowledge.

3, can not detect rough surface, irregular shape, volume is too small or uneven material workpiece.

4, can not detect defects that happen to occur in the thin layer immediately adjacent to the surface of the workpiece.

5, the coupling medium usually needs to transmit ultrasonic energy between the converter and the workpiece.

6. Actual reference standards for calibration equipment and identification of defects are required.

Fourth, the principle of ultrasonic flaw detector

Non-destructive testing is a test means to check the surface and internal quality of the inspected part without damaging the working condition of the workpiece or raw material. There are many kinds of ultrasonic flaw detectors, but in the actual process of flaw detection, the pulse reflection ultrasonic flaw detector is the most widely used. Generally, in uniform materials, the existence of defects will cause material discontinuity, which often causes the acoustic impedance at the interface of the medium will be reflected, and the size of the reflected energy is related to the difference between the acoustic impedance of the medium on both sides of the interface and the orientation and size of the interface. Pulsed reflection ultrasonic flaw detector is designed according to this principle.

When the ultrasonic wave propagates in the material to be tested, the acoustic characteristics and changes in the internal organization of the material have a certain impact on the ultrasonic wave propagation. The technology of understanding the material properties and structural changes through the detection of the ultrasonic wave affected degree and condition is called ultrasonic detection. Ultrasonic detection methods usually include penetration method, pulse reflection method, series method and so on.

Digital ultrasonic flaw detectors now usually emit ultrasound to the measured object (such as industrial materials, human body), and then use its reflection, Doppler effect, transmission, etc., to obtain the information inside the measured object and process it to form an image. The Doppler effect method of ultrasonic flaw detector is to obtain the characteristics of the moving direction and speed of the object by using the Doppler frequency shift effect of ultrasound when it meets the moving object. The transmission rule is derived from the analysis of the changes of the object after ultrasonic penetration, and its application is still in the development stage. Ultrasonic flaw detector here mainly introduces the most widely used method to obtain the internal characteristic information of objects by reflection. Reflection method is based on the principle that ultrasound will have strong reflection when passing through the interface of different acoustic impedance tissues. As we know, sound waves will be reflected at the interface between the two when propagating from one medium to another, and the greater the difference between the media, the greater the reflection will be. So we can send a penetrating ultrasonic wave that travels in a straight line to an object, The ultrasonic flaw detector then receives the reflected ultrasonic waves and according to the sequence and amplitude of these reflected ultrasonic waves, it can determine the size and distribution of various media contained in this tissue and the degree of contrast and difference between various media (among which the sequence of reflected ultrasonic waves can reflect the distance between the reflective interface and the detection surface). The amplitude can reflect the size of the medium, the degree of contrast difference and other characteristics), and the ultrasonic flaw detector can determine whether the measured object is abnormal. In this process, there are many aspects involved, including ultrasonic generation, reception, signal conversion and processing. The method of generating ultrasonic wave is to generate an exciting electrical signal through the circuit to the piezoelectric effect of the crystal (such as quartz, lithium sulfate, etc.),

Make it vibrate to produce ultrasonic waves; When receiving the reflected ultrasonic wave, the piezoelectric crystal will be subjected to the pressure of the reflected sound wave and generate an electrical signal and pass it to the signal processing circuit for a series of processing, and the ultrasonic flaw detector will finally form an image for people to observe and judge. Here, according to the image processing method (that is, what form of image will be converted into the obtained signal), it can be divided into A-type display, M-type display, B-type display, C-type display, F-type display, etc. The A-type display is to process the received ultrasonic signal into A waveform image, and according to the shape of the waveform, it can be seen whether there are anomalies and defects in the measured object and where, how big, etc. The ultrasonic flaw detector is mainly used for industrial testing; M-type display is to expand the detection information processed by brightness in time order to form a one-dimensional "spatial multi-point motion sequence diagram", which is suitable for observing objects in motion, such as moving organs, arteries, etc. The B-type display is a two-dimensional "anatomical image" that reflects the internal sectional plane of the measured object by combining many strips of detection information side by side with brightness processing (B-ultrasound used in hospitals is made by this principle). The ultrasonic flaw detector is suitable for observing the internal static object; The C-type display and F-type display are now used less.

Ultrasonic flaw detector detection can not only be very accurate, but also more convenient and fast than other detection methods, and will not cause harm to the detection object and operator, so it has been more and more popular and has a very broad development prospect.

Five, the characteristics of ultrasonic flaw detector

1, the detection speed is fast

The digital ultrasonic flaw detector can generally automatically detect, calculate and record, and some can automatically compensate the depth and automatically set the sensitivity, so the detection speed is fast and the efficiency is high.

2, high detection accuracy

Digital ultrasonic flaw detector performs high-speed data acquisition, quantization, calculation and discrimination of analog signals, and its detection accuracy can be higher than that of traditional instruments. Recording and file inspection, digital ultrasonic flaw detectors can provide inspection records up to defect images.

3, high reliability and good stability

The digital ultrasonic flaw detector can comprehensively and objectively collect and store data, and conduct real-time processing or post-processing of the collected data, analyze the signal in time domain, frequency domain or image, and also grade the quality of the workpiece through pattern recognition, reducing the influence of human factors and improving the reliability and stability of retrieval. The main functions that can be achieved are:

(1) Automatic calibration: automatically test the probe's "zero", "K value", "frontier" and the material's "sound speed";

(2) Automatically display defect echo position such as: depth d, level p, distance s, amplitude, equivalent dB, aperture ￠free switching scale;

(3) Automatically record the inspection process and can be played back dynamically;

(4) Automatic gain, echo envelope, peak memory function;

(5) The flaw detection parameters can be automatically tested or preset;

(6) Digital suppression, does not affect the gain and linearity;

(7) Multiple independent inspection channels, can freely input and store inspection standards of any industry, on-site inspection does not need to carry test blocks;

(8) Free storage and playback of waveforms and data;

(9) DAC and AVG curves are automatically generated and can be segmented, sampling points are not limited, and can be corrected and compensated;

(10) Free input of industry standards;

(11) Communicate with computers to achieve computer data management, and export the flaw detection report in Excel format and A4 paper;

(12) Real-time clock record: real-time inspection date, time tracking record, and storage;

(13) Gain compensation: Db attenuation caused by factors such as surface roughness, curvature and remote flaw detection of thick workpieces can be corrected;

Six, the application of ultrasonic flaw detector

1, oil well pipe ultrasonic automatic flaw detection system

The application of the ultrasonic automatic flaw detection system not only ensures the accuracy and high efficiency of the oil well pipe detection objectively, but also reduces the production cost of the flaw detection and reduces the influence of many human factors in the flaw detection. The design of the system is compatible and versatile, and can meet the requirements of different production. Moreover, the dynamic playback of the defect waveform effectively overcomes the defect type difficult to identify in the ultrasonic automatic flaw detection. However, compared with foreign advanced detection equipment, there is still a certain gap, the system should increase the focus on improving the performance of the ultrasonic sensor and signal processing part of the research, to drive the research and development of electronic circuits, in order to further improve the detection efficiency and accuracy of automatic detection and stability, reduce the error rate of defects.

2, cast steel ultrasonic inspection and defect identification

The location, quantitative and qualitative evaluation of steel casting defects are the requirements of product production for inspection personnel. In the actual detection process, there are considerable difficulties in the qualitative evaluation of defects, mainly because the reflection characteristics of defects to ultrasonic waves depend on the orientation of the defects, geometric shape, length and thickness relative to the ultrasonic propagation direction, the surface roughness of the defects, the types and properties of the defects, and so on. It is also related to the characteristics and display mode of the ultrasonic detection system used. Therefore, the ultrasonic response of defects obtained during ultrasonic detection is a comprehensive response.

3, the application of ultrasonic flaw detection in steel structure welding

Ultrasonic inspection is used for full penetration welds, and the inspection ratio is calculated as a percentage of the length of each weld, and is not less than 200mm. For the local inspection of the weld if it is found that there are not allowed defects, the inspection length should be increased at the extension of both ends of the defect, the increased length should not be less than 10% of the length of the weld and should not be less than 200mm, when there are still not allowed defects, should be 100% inspection of the weld.

4, the application of ultrasonic non-destructive testing technology in the welding quality inspection of space grid structure

Ultrasonic nondestructive testing technology is used to study the regularity and particularity of tube-ball joint seam, tube-tube butt joint and cross-line joint weld, tube-plate head joint weld and common defects of space grid structure. Limited to the particularity of the structure, generally only one side and one side of the steel pipe can be inspected. The probe is circular arc probe, and the front edge of the small chip probe is short five. Easy to detect the root of the weld with a single wave.