基于声发射技术的混凝土结构损伤性能分析(英文)

Chapter 1 Introduction 1.1 Acoustic emission monitoring technique To detect the damage of materials (i.e. metals, ceramics, composites, rocks and concrete) in members or structures, some non-destructive techniques, e.g. the laser speckle technique [1], the ultrasonic probing technique [2], the X-ray CT method [3] and the Acoustic Emission (AE) technique [4], have been widely used in Civil Engineering. Among these monitoring methods, the AE technique is a very powerful way to monitor the strain energy release due to propagation of stress waves caused by micro-cracking inside the material [5] for at least a century. In 1920, Abram Joffe observed the noise generated by deformation process of Salt and Zinc crystals. In 1948, Warren P. Mason and H. J. McSkimin et al. suggested measuring AE to observe the moving dislocations by means of the stress waves. In 1950, Josef Kaiser used tensile tests to determine the characteristics of AE in engineering materials. In 1969, Dunegan founded the first company that specializes in the production of AE equipment. Acoustic Emission (AE) technique captures the elastic stress waves coming from material damage sources, such as the micro-crack initiation and propagation, crushing, interface friction, fiber breaking etc., then converts them into electrical signals with the aid of piezoelectric transducers placed on the surface of the members or structures. Acoustic Emission can be defined as a transient elastic wave generated by the rapid release of energy within a material. As shown in Fig. 1-1, an electric circuit of AE measurement consists of a sensor, a preamplifier, a filter, a main amplifier, a cable and a detector. The AE event is a single dynamic process releasing elastic energy. The local process producing AE event is called the AE source. The electrical signal detected at the output pf the transducer is termed the AE signal. AE technique can be used to the member or structure which is under normal, service conditions or during loading over a long time. For the frequencies range of commonly used for AE monitoring, the AE sensors can detect the signal for surface movements of or less, a thousand times smaller than the size of an atom. Wave motions are defined on the basis of orientations of particles motions and directions of propagations. Being similar to the earthquake motions, AE waves consist the following waves (Fig. 1-2): (1) Longitudinal wave (Primary wave). It propagates through all media and is the fastest of elastic waves. In this case, particles vibrate along the direction in which the wave propagates. (2) Transverse wave (Secondary wave). It can propagate through solids but not through liquids and gases. In this case, particles vibrate perpendicular to the direction in which the wave propagates. (3) Other waves. Longitudinal and transverse waves are always generated and propagate in solid. While, other waves, i.e. surface and plate waves are generalized in solid following the longitudinal and transverse waves arriving at the surface.