国外声发射技术

国外声发射技术的原理及应用（后带翻译）8.1 Acoustic emissionThe acoustic emission(AE) testing method is a unique nondestructive testing(NDT) method where the material being inspected generates signals that warn of impending failure. Acoustic emission testing is based on the fact that solid materials emit sonic or ultrasonic acoustic emissions when they are mechanically or thermally stressed to the point where deformation or fracturing occurs. During plastic deformation, dislocations move through the material’s crystal lattice structure producing low-amplitude AE signals ,which can be measured only over short distances under laboratory conditions.【1】The AE test method detects, locates, identifies, and displays flaws data for the stressed object the moment the flaw is created. Therefore, flaws can not be retested by the AE method. In contrast, ultrasonic testing detects and characterizes flaws after they have been created. Almost all materials produce acoustic emissions when they stressed beyond their normal design ranges to final failure.It has been said that the first practical use of AE occurred in about 6500 BC as pottery makers listened to the cracking sounds made by clay pots that had been allowed to cool too quickly. By experience the potters learned that cracked pots were structurally defective and would fail prematurely. However, the father of modern AE testing was Josef Kaiser of Germany.In 1950, Kaiser published his Ph.D.thesis, which was the first comprehensive investigation of acoustic emissions. He made two important discoveries; the first was that material emits minute pulse of elastic energy when placed under stress. His second discovery stated that once a given load was applied and the acoustic emission from that noise had ceased, no further emission would occur until the previous stress level was exceeded, even if the load was removed and later reapplied. This so-called 〝Kaiser effect〞can be time dependent for materials with elastic aging. The principle is used in present-day AE proof testing of fiberglass and metallic pressure vessels.Cracking in structures such as aircraft wings, pipes, circuit boards, and industrial storage tanks generates acoustic emissions. They are also generated by deformation and crack propagation in pipes, pressure vessels, and weldments. Other sources of AE are nugget formation and overwelding in spot-welding operations, leaks in steam valves and traps, and bearing failure in pumps, motors, and compressors.8.1.1 Theory and principlesAE was originally conceived as an NDT method for locating flaws as they occurred in pressure vessels. Today AE encompasses a much wider scope and can be applied to all types of process monitoring as well as real-time flaw detection and structural integrity studies. Pressure, temperature, vacuum, mechanical tension, orcompression are the most commonly applied stresses. Figure 8.1 illustrates that an acoustic noise source, such as a propagating crack, generates sound waves that radiate outward in all directions. AE sensors, typically piezoelectric transducers, can be placed anywhere on the object within range of AE energy waves. By using three transducers and triangulation techniques, the location of the flaw can be detected.Figure8-1 AE signal generation, transmission, and detectionAE sensors are low-noise detectors that operate in the ultrasonic frequency range of 10kHz to 2 MHz. Physical motions as small as 1×10-12can be detected. AE sensors can hear the breaking of a single grain of metal or a single fiber in a fiber-reinforced composite material. AE sensors can also detect the sound of a tiny bubble of gas from a pinhole leak as it arrives at the surface of a liquid. With the capability to detect small AE signals or large AE signals such as are caused by brittle crack advance, AE technology warns of impending structural damage and can monitor costly and critical processes.Sudden movements in solid materials generate AE waves that may be detected at distances of a few inches to distances of several hundred feet, depending on the properties of the material being tested. Sudden subcritical local failures in materials under stress are the classical sources of AE. AE technology provides an early warning system to prevent catastrophic failures, to assess structural integrity, and to enhance safety in a wide range of structures from fiberglass tanks to bucket trucks, from bridges and aircraft to high-pressure gas cylinders. AE is also used for a wide range of process applications such as leak detection, particle impacts, electrical discharges, and a variety of friction-related processes.The recent high-tech explosion has opened up a whole new range of technical capabilities for the AE method. The frequency domain is readily accessible and full waveform capture capabilities enable advanced, real-time, and post-test analysis of waveforms that were previously reduced in real-time to a small set of half-a-dozen features measured with bulky electronic circuits.【2】PAC’s REACT department makes use of these new technological advances to develop customer-based AE applications.8.1.2 ApplicationsThe following applications are broken down into four categories that demonstrate the wide-range use and popularity of the acoustic emission method.8.1.2.1 Behavior of Materials-metals, Ceramics, Composites, Rocks, Concrete●Crack propagation●Yielding●Fatigue●Corrosion, stress corrosion●Fiber fracture, delamination8.1.2.2 Nondestructive Testing During Manufacturing Processes●Materials processing●Phase transformation in metals and alloys●Detection of pores, quenching cracks inclusions, etc.●Fabrication●Deforming processes-rolling, forging, extruding●Welding and brazing defects detection-inclusion, cracks, lack of penetration●TIG, MIG, spot, electron beam, etc.●Weld monitoring for process control8.1.2.3 Monitoring Structures●Continuous monitoring-metal structures, mines, etc.●Periodic testing-pressure, pipelines, bridges, cables●Loose part detection●Leak detection8.1.2.4 Special Applications●Petrochemical and chemical-storage tanks, reactor vessels, offshoreplat-forms, drill pipe, pipelines, valves, hydrotreaters●Electric utilities-nuclear reactor vessels, piping, steam generators, ceramicinsulators, transformers, aerial devices●Aircraft and aerospace-fatigue cracks, corrosion, composite structures, etc.●Electronics-loose particles in eletronic components, bonding, substratecracking8.1声发射声发射（AE）的测试方法是一个独特的无损检测（NDT）方法,用在被检材料发出即将到来的实效警告信号。