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