The concept of ultrasound  

Ultrasound is a sound wave with a frequency higher than 20,000 Hz. It has good directionality and strong reflection ability. It is easy to obtain concentrated sound energy and has a long propagation distance in water. It can be used for flaw detection, thickness measurement, distance measurement, speed measurement, cleaning, Welding, gravel, sterilization, etc. Ultrasound is a mechanical wave. Ultrasonic waves include longitudinal waves, transverse waves, surface waves (Rayleigh waves), climbing waves, plate waves (SH waves), and diffraction waves.

The characteristics of ultrasonic waves
are mainly: "good directionality", "high energy", "strong penetrating ability", "can produce reflection, refraction, diffraction and waveform conversion on the interface", detailed as follows :
1. The directionality of the ultrasonic wave is good: the higher the frequency, the better the directionality. It radiates into the medium with a very narrow beam, and it is easy to determine the position of the defect. The ultrasonic sound beam can be concentrated in a specific direction, propagates along a straight line in the medium, and has good directivity.
2. During the propagation of ultrasonic waves in the medium, attenuation and scattering will occur. 3. Ultrasonic waves will produce reflection, refraction and wave mode conversion on the interface of different media. Using these characteristics, the reflected wave reflected from the defect interface can be obtained, so as to achieve the purpose of detecting defects. When the ultrasonic wave propagates in the medium, it has the characteristic of reflection on the heterogeneous interface. If a defect is encountered and the size of the defect is equal to or greater than the wavelength of the ultrasonic wave, the ultrasonic wave will be reflected back on the defect, and the flaw detector can display the reflected wave ; When the size of the defect is even smaller than the wavelength, the sound wave will bypass the ray and cannot be reflected;
4. The propagation energy of the ultrasonic wave is large, and the energy of the ultrasonic wave is much greater than that of the sound wave, such as the energy transmitted by the ultrasonic wave with a frequency of 1MHZ (100 Hz), It is equivalent to 1 million times the sound wave with the same amplitude and frequency of 1000HZ (Hertz). 5. The transmission loss of ultrasonic waves in solids is small, and the detection depth is large. Because ultrasonic waves will reflect and refract on heterogeneous interfaces, especially they cannot pass through gas-solid interfaces. If there are pores, cracks, delamination and other defects (gas in the defect) or inclusions in the metal, when the ultrasonic wave propagates to the interface between the metal and the defect, it will be fully or partially reflected. The reflected ultrasonic wave is received by the probe, processed by the circuit inside the instrument, and waveforms with different heights and certain intervals will be displayed on the fluorescent screen of the instrument. The depth, position and shape of the defect in the workpiece can be judged according to the changing characteristics of the waveform.

What is Ultrasonic Testing
Ultrasonic testing is to use the characteristics of the difference in acoustic performance between the tested material and the defect on the premise of not damaging the test object, using the corresponding ultrasonic testing equipment and equipment, through the reflection, refraction and diffraction of ultrasonic waves during the propagation process It is a non-destructive testing method to test the interior and surface of the detection object, analyze and evaluate the results. Ultrasonic Testing (Ultrasonic Testing) abbreviated as UT, also called ultrasonic testing, is one of the five conventional nondestructive testing methods. Nondestructive Testing (Nondestructive Testing), abbreviated as NDT, is a testing method to check the surface and internal quality of the inspected part without damaging the working state of the workpiece or raw materials. Non-destructive testing includes five testing methods including radiographic testing (RT), ultrasonic testing (UT), magnetic particle testing (MT), penetrant testing (PT) and eddy current testing (ET). It is mainly used for raw materials, parts and welds of
machinery and devices made of metal materials , and can also be used for other products such as glass. Radiographic testing is suitable for welding seams of machinery and devices made of carbon steel, low alloy steel, aluminum and aluminum alloys, titanium and titanium alloy materials, and butt ring seams of steel pipes. Rays are harmful to the human body, and direct exposure to rays and the influence of scattered rays should be avoided as much as possible. Ultrasonic testing refers to the detection of defects with a type A pulse reflection ultrasonic flaw detector, which is suitable for ultrasonic testing of raw materials, parts and welds of metal products, and ultrasonic thickness measurement. Magnetic particle testing is suitable for the detection of surface and near-surface defects of ferromagnetic material products and their parts, including dry magnetic powder, wet magnetic powder, fluorescent and non-fluorescent magnetic particle testing methods. Penetration testing is suitable for the detection of surface opening defects of metal products and their parts, including fluorescent and coloring penetrant testing. Eddy current testing is suitable for pipe testing, such as round seamless steel pipes and welded steel pipes, aluminum and aluminum alloy drawn thin-walled pipes, etc. Magnetic particle, penetrant and eddy current are collectively referred to as surface inspection.

The principle of ultrasonic testing
uses a certain method to transmit the ultrasonic detection generated by the sound source into the workpiece under test. If there is a defect in the workpiece, an interface will be formed between the defect and the workpiece material, and the acoustic impedance between the interfaces is different. When the ultrasonic Reflection will occur after encountering the interface, and the reflected ultrasonic signal will be received. By analyzing the received ultrasonic signal, the internal condition of the workpiece can be judged. This is how ultrasonic testing works. Ultrasonic flaw detection is a non-destructive testing method that uses the difference in acoustic properties of materials and their defects to the energy changes in the reflection of ultrasonic wave propagation and penetration time to test internal defects in materials. The pulse reflection method uses longitudinal waves in vertical flaw detection and transverse waves in oblique shot flaw detection. The pulse reflection method includes longitudinal wave flaw detection and shear wave flaw detection. On the oscilloscope screen of the ultrasonic instrument, the abscissa represents the propagation time of the sound wave, and the ordinate represents the echo signal amplitude. For the same homogeneous medium, the propagation time of the pulse wave is proportional to the sound path. Therefore, the existence of the defect can be judged by the appearance of the defect echo signal; the distance between the defect and the detection surface can be determined by the position of the echo signal, and the defect location can be realized; the equivalent size of the defect can be judged by the echo amplitude.