Acoustic emission is a common physical phenomenon. In the early 1950s, Kaiser, a German, made a detailed study on the acoustic emission phenomenon of various metal materials and found the irreversible effect of acoustic emission-Kaiser effect, that is, the acoustic emission phenomenon only occurred in the first loading process, and the acoustic emission generated in the second and subsequent loading processes was a common physical phenomenon. In the early 1950s, Kaiser, a German, made a detailed study on the acoustic emission phenomena of various metal materials, and found the irreversible effect of acoustic emission-Kaiser effect, that is, the acoustic emission phenomenon only occurred in the first loading process, and the acoustic emission produced in the second loading and subsequent loading process became insignificant, unless the external stress added later exceeded the maximum value of the previous loading. This effect has been widely used in industry and has become the basis of acoustic emission technology to monitor structural integrity. With the rapid development of computer and signal processing technology, acoustic emission technology is becoming more and more mature, and its application scope has covered almost all fields of national economy such as aviation, aerospace, petrochemical industry, railway, automobile, construction and electric power. First, the principle of acoustic emission detection Acoustic emission refers to a physical phenomenon that an object produces transient stress waves due to the rapid release of elastic energy when it is deformed or subjected to external forces. The frequency range of acoustic emission of various materials is very wide, from infrasound frequency, audio frequency to ultrasonic frequency, so acoustic emission is also called stress wave emission. Acoustic emission is a common physical phenomenon. If the strain energy released is large enough, an audible sound will be produced. If you break a branch, you will hear a crack. Most metal materials will also produce acoustic emission during plastic deformation and fracture, but the intensity of acoustic emission signal is too weak for people to hear directly, and it needs sensitive electronic instruments to detect it. The technology of detecting and analyzing acoustic emission signals with instruments and inferring acoustic emission sources from acoustic emission signals is called acoustic emission technology. Acoustic emission detection is a dynamic nondestructive testing method, that is, the internal structure, defects or potential defects of components or materials are kept in the process of motion change for nondestructive testing. Therefore, defects such as cracks actively participate in the detection process. If cracks and other defects are at rest, there will be no acoustic emission and acoustic emission detection will not be realized. Moreover, because the acoustic emission signal comes from the defect itself, the severity of the defect can be judged by acoustic emission method. Acoustic emission (AE) detects some electrical signals, and it is often complicated to explain the changes of defects in structures according to these electrical signals, which requires rich knowledge and other experimental means. On the other hand, there is often strong noise interference in the acoustic emission detection environment. Although there are many methods to eliminate noise in acoustic emission technology, the application of acoustic emission technology will be limited in some cases. 2. Acoustic emission detection instruments can be divided into two basic types, namely, single-channel acoustic emission detector and multi-channel acoustic emission source positioning analysis system. Single-channel acoustic emission detector generally consists of transducer, preamplifier, attenuator, main amplifier threshold circuit, acoustic emission rate counter and digital-to-analog converter. Multi-channel acoustic emission detection system is based on single channel, which adds digital measurement system, computer data processing and peripheral display system. (1) The transducer used in the acoustic emission device of transducer is similar to the transducer used for ultrasonic detection, and also consists of a shell, a protective film, a piezoelectric element, a damping block, a connecting wire and a high-frequency socket. Piezoelectric elements usually use lead zirconate titanate, barium titanate and lithium niobate. But the sensitivity is generally higher than that of ultrasonic transducers. Acoustic emission signals from crack formation and propagation are converted into electrical signals by the transducer and input to the preamplifier. (2) The acoustic emission signal of the preamplifier is converted into an electrical signal by the transducer, and its output can be as low as ten microvolts. If such a weak signal is transmitted through a long cable, it may be impossible to distinguish between signal and noise. The purpose of setting a low noise preamplifier is to improve the signal-to-noise ratio and the anti-interference ability to weak signals. The gain of the preamplifier is 40 ~ 60 dB. (3) The acoustic emission signal of the filter is a signal with a wide frequency spectrum, and the frequency range can be from several hertz to several megahertz. In order to eliminate noise, the required frequency range is selected to detect acoustic emission signals. At present, the sampling frequency range is generally kHz ~ 2 MHz zo(4). After the above processing, the signals of the main amplifier and the threshold shaper are amplified by the main amplifier, and the gain of the whole system can reach 80~ 100dB. In order to eliminate the background noise, the appropriate threshold voltage is set to remove the noise below the threshold voltage, and the signals above the threshold voltage are processed to form pulse signals, including ringing pulses and event pulses. (5) signal counting The counting of acoustic emission signals includes event counting and ringing counting. After the envelope detection of the burst signal waveform, a rectangular pulse is formed after the signal level exceeds the set threshold voltage. A rectangular pulse is called an event, and the number of these event pulses is the event count. The event count per unit time is called the event count rate, and its accumulation is called the total number of events. When the ringing waveform exceeds this threshold voltage, the exceeding part forms rectangular pulses, and counting these rectangular pulses is the ringing count. The ringing count per unit time is called the acoustic emission rate and the total number of ringing.