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Acoustic emission refers to the phenomenon in which strain energy is released in the form of elastic waves when a material or component deforms or fractures under external forces or internal stresses, or when there are changes in its internal defect conditions. Acoustic emission is a common physical phenomenon; if the strain energy released is large enough, it can produce audible sounds. For example, when a branch is broken, a audible sound is produced as the branch bends, indicating that the internal condition of the branch is changing. At the moment of breakage, a louder sound is heard, suggesting that more strain energy is released than what was required for the deformation. Acoustic emission also occurs when metals are deformed or fractured; for example, bending a piece of tin near the ear produces a humming sound, which is generated as the tin deforms under stress. During fracture toughness tests on metallic materials, a popping sound can be heard when the crack transitions from stable propagation to unstable propagation. When cracks expand stably, they can also produce sounds that are inaudible to the human ear. Whether the human ear can hear sounds generated inside materials or components mainly depends on the intensity and frequency of the sounds. The human ear cannot hear the acoustic emissions generated during the plastic deformation of most metals, mainly because the intensity of such acoustic emissions is relatively low in metals like copper, iron, aluminum, and steel. Basic principles and characteristics: The basic principle of acoustic emission testing is that external factors such as force, temperature, and electromagnetic fields cause an object to emit sound, and by analyzing this sound, the state of the object or changes in its internal structure can be determined. Since acoustic emission occurs during the process of changes in internal structures, acoustic emission testing is a dynamic non-destructive testing method; that is, it is a non-destructive testing method performed while the structure, defects, or potential defects within a component or its internal structure are undergoing changes. Characteristics of acoustic emission testing: (1) Acoustic emission testing is a dynamic non-destructive testing method ； (2) Compared with ultrasonic testing, acoustic emission testing does not require moving the transducer, making it simpler to operate ； (3) Acoustic emission testing is hardly restricted by materials. Acoustic emission occurs in various materials such as metals, composite materials, plastics, wood, and rocks ； (4) Acoustic emission has an irreversible effect ； (5) It is difficult to distinguish acoustic emission generated by plastic deformation from that generated by crack propagation. Range of application (1) Dynamic detection of plastic deformation in materials ； (2) Monitor the development and fracture process of fatigue cracks ； (3) Monitor the occurrence of welding cracks during the welding process ； (4) Evaluate the safety of equipment during hydrostatic testing, periodic inspections, and operation. Acoustic emission testing procedure during hydrostatic testing (1) Preparation: Understand the structure of the equipment, its installation, the pressure testing process, as well as the pressure application devices; identify the locations of welds and the potential weak points where problems may occur. (2) Placement of acoustic emission transducers: The number of elements in the array, its size, and the positions where the transducers should be placed are determined based on the material, structure, and dimensions of the equipment, as well as the number of channels in the acoustic emission instrument used and the requirements regarding the arrangement of the transducers. (3) Standard acoustic emission instrument: Adjust parameters such as the overall gain and threshold voltage of the acoustic emission detection system based on the acoustic emission characteristics of the material, the intensity of the simulated acoustic emission source, and background noise. (4) During the test, record the number of acoustic emission events, the amplitude or energy of the acoustic emission signals, and the location of the acoustic emission sources from 30 seconds before pressure increase until the pressure is maintained. (5) Evaluation: Infer the source of acoustic emission based on the results of acoustic emission testing. Classify the acoustic emission sources according to relevant standards.