Advanced AE Technology for High-Power Microwave Radar Tubes

Abstract

Acoustic emission (AE) and micro-seismic activity are naturally occurring phenomena. Almost all materials emit sound or acoustic emission when they are sufficiently stressed. Wood and rock produce AE signals in audible frequency ranges when stressed. It was also believed that AE signal generation could exist in the ultrasonic frequency range during deformation of materials, but it was not until 1950 when Joseph Kaiser reported the first comprehensive investigations on acoustic emission phenomenon in conventional engineering materials using electronic equipment and tensile testing machines. Kaiser also observed that AE activity was irreversible. Acoustic emissions are not generated during the reloading of a material until the stress level has exceeded its previous highest value. This AE irreversible phenomenon is now known as the Kaiser Effect. He also proposed a distinction between burst and continuous AE activity. The use of piezoelectric sensors and electronic amplifiers to observe high-frequency AE activity gradually led to the definition of acoustic emission. According to the ASNT Handbook (1987), acoustic emission refers to the generation of transient elastic stress (strain) waves due to the rapid release of energy from a localized source within a material undergoing some kind of deformation. The kind of stress applied to materials under testing could be tensile, compressive or shear. The transient elastic stress waves of AE have frequencies ranging from 20 kHz (kilohertz) to 1 MHz (megahertz). Green (1980) has listed many mechanisms that produce acoustic emission activity in materials. Among them, the principal mechanisms are mechanical deformation, fracture, crack propagation, dislocation motion and multiplication, twin formation, phase transformation, corrosion, friction and internal magnetic processes. Mechanical loading is not the only way to generate AE activity (phonon signals). Thermal shock loading and electrical sparking are also known to cause AE activity. Generation of AE activity during chemical reactions has also been observed. It was realized quite earlier that AE activity appears in two types, burst signals and continuous signals.