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 emission

The 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 principles

AE 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, or

compression 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 detection

AE 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 Applications

The 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, delamination

8.1.2.2 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 control

8.1.2.3 Monitoring Structures

●Continuous monitoring-metal structures, mines, etc.

●Periodic testing-pressure, pipelines, bridges, cables

●Loose part detection

●Leak detection

8.1.2.4 Special Applications

●Petrochemical and chemical-storage tanks, reactor vessels, offshore

plat-forms, drill pipe, pipelines, valves, hydrotreaters

●Electric utilities-nuclear reactor vessels, piping, steam generators, ceramic

insulators, transformers, aerial devices

●Aircraft and aerospace-fatigue cracks, corrosion, composite structures, etc.

●Electronics-loose particles in eletronic components, bonding, substrate

cracking

8.1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。声发射检测是基于这样一个事实：固体材料发出的声波或超声波的声发射时，机械或热应力而发生变形或断裂点。塑性变形过程中，位错穿过材料的晶格结构产生低振幅的声发射信号，它只能在短距离在实验室条件下。【1】声发射试验方法对受力物体检测，定位，识别，和显示缺陷数据为对象的时候就是缺陷是造成的。因此，缺陷是不能用声发射方法再次检测。相反，超声检测在缺陷产生后可再来检测缺陷和识别缺陷类型特征。当它们受力超过了正常的承受范围至完全破坏间，几乎所有的材料都能产生声发射。

据说，第一个实声发射的实际应用发生在公元前6500年的陶瓷制造商听急速冷却的陶罐声响。由经验得知，有裂缝的水罐是结构性缺陷和将会过早地失效。然而，现代的声发射检测之父是德国的Josef Kaiser先生。

1950年，Kaiser发表了他的博士论文，这是第一个全面的调查的声发射。他做了两个重要的发现；第一个发现是材料会在应力作用下发出具有弹性能量的分钟脉冲。他的第二个发现是一旦施加一个给定的负载从嘈杂声发出的声发射会停止，没有进一步的发射将会发生直到超出了以前的应力水平，即使负载被撤掉后重新加载。这个所谓的〝Kaiser效应〞使时间依赖性伴随弹性材料的老化。用玻璃纤维和金属压力容器的声发射检测的原理应用到现在。

开裂的结构如飞机的机翼，管道，线路板，及工业储罐能产生声发射。他们还通过变形和裂纹扩展产生的管道，压力容器，焊接件。其他来源的声发射是金属变形和过焊在点焊操作,泄漏蒸汽阀门和漏孔,和承受应力的水泵,电机和压缩机。

8.1.1理论和原则

声发射的最初设想是作为无损检测方法对在压力容器上发生的缺陷定位。今天，声发射范围很广，可用于所有类型的过程监控以及即时检测和结构完整性研究。压力，温度，真空，机械拉伸或压缩，是最常用的应力。图8.1展示了一个噪声源，如传播裂纹，产生声波辐射四面八方。声发射传感器，通常情况下是压电换能器，可以放置在物体声发射能量到达的范围内的任何地方。通过使用三个传感器和三角测量技术，缺陷的位置可以检测出来。

信号

8.1.2应用

下列应用程序被分为四类，表明声发射方法的广泛应用和普及。

8.1.2.1材料特性的金属，陶瓷，复合材料，岩石，混凝土

●裂纹扩展

●收益率

●疲劳

●腐蚀，应力腐蚀

●纤维断裂，脱层

8.1.2.2制造期间的无损检测

●材料加工

●金属和合金中的相变

●孔和带夹杂物的淬火裂纹的检测，等。

●制作

●变形过程轧制，锻造，挤压

●焊接和钎焊缺陷检测夹杂，裂纹，未焊透

●电传打印机输出发生器，制造和检查记录，位置，电子束，等。

●用于过程控制的焊接监控

8.1.2.3监测结构

●连续监测金属结构，矿山，等。

●周期性压力试验，管道，桥梁，电缆

●松动件检测

●泄漏检测

8.1.2.4特殊应用

●石油和化工储罐，反应堆容器,，海上平台，钻管，管道，阀门，加氢处理●电力公司核反应堆容器，管道，蒸汽发生器，陶瓷绝缘体，变压器，架空装

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