Acoustics emission diagnostics of bridges

The paper describes the methodology of acoustic emission (AE) diagnostics of the metal steel beam of the bridge over the Dniester River. The bridge was built in the early twentieth century and renovated in 1956. The bridge length is 261,9 m. The main bearing structures of the bridge are continuous welded beams. For diagnosing the technical condition of beams, AE methodology developed in the Physico-Mechanical Institute of NAS of Ukraine was applied. It consists in the preliminary study of specific character of elastic waves propagation in materials the bridge made, and the application of the developed portable eight channels AE measurement system SKOP-8. The tests were conducted under static and dynamic loading. Linear and planar location techniques were applied in order to identify sites of nucleation and development of fracture. By using AE data obtained in fullscale tests, the analysis of technical state of bearing structures of the bridge was performed and proposals concerning their further operation were made. Method of acoustic emission (AE) became available for commercial use over 40 years ago. Its application is increased greatly in various industries around the world. Initially, it was tested in fields, where existing methods of non-destructive testing were nonperforming or where it promised economic benefits. Late 1970 early 1980, when AE researches in academic and industrial centres of the USA, Europe, Japan started up, qualitative changes in approaches to use this method occurred [1]. Academic researches focused on rare occasions of establishing relationships between AE signals shapes (AE parameters) and the sources characteristics or mechanisms of fracture. At the same time, AE researches of industrial structures accumulated amount of data that not only enabled establishing zone location of sources, but also demonstrated relationships between AE signals and fracture mechanisms, corrosion processes, technical state of building elements, showing features of elastic waves generated in composite structures and their elements that operated at elevated pressure. The progress of computer technology made possible the efficient analysis of stored data, creating preconditions for manufacturing specialized AE equipment designed for operation at power plants, refineries, chemical plants, pipelines, airplanes, etc. The structures of the successful AE diagnostics were compressed gas tanks, storage tanks for liquids, concrete waste storages, reinforced concrete and steel bridges, pipelines for oil and gas, offshore platforms, ropes, elements of power equipment (pipelines of boilers, fuel rod ferrules, heat exchangers, high pressure tanks), airplane components (fuselage skin, turbine engine blades, protective coating of combustion chamber), etc. Problems of application of AE method are worsened when the structure has a complex geometry, weld and other types of connections. Therefore, it remains urgent to develop new and improve existing means of processing the information, detected by transducers, whose data processing rate and reliability is still insufficient for large-scale implementation of AE diagnostics of various test units. Effective application of this method depends greatly on the modes of results presentation, automated data processing that would enable to reduce the number of staff, decrease the influence of "human factor". These difficulties associated with unknown dependencies of parameters of AE signals on the nature of the origin and propagation of material fracture, especially taking into account that the various fracture mechanisms are still not studied completely. Despite these difficulties, AE diagnostics is steadily gaining its proper place among other methods of non-destructive testing. Today, when many metal consuming, technologically sophisticated and environmentally dangerous structures exhausted their life, is extremely urgent need to establish their technical condition and remaining life. AE method successfully performs this function. Examples of its successful application presented in this paper. Acoustic emission signals caused by local subcritical crack growth. For estimation of AE signals radiation caused by subcritical crack growth at local areas of its front, we consider an elastic halfspace with a flat mode I macrocrack bounded by a smooth contour L (see Fig. 1). Fig. 1. The schematic of local growth of internal crack. Let at the time t=0 in local area, where stresses (or deformations) achieve certain critical value, due to application of external tensile forces to a body, a microcrack nucleates closely a contour of this macrocrack . As a result of unloading of the surfaces of this newly formed microcrack from an initial level down to zero, elastic waves are radiated. They reach the inspected object surface and can be detected by an AE transducer. For simplification of the problem, we replace this microcrack with a penny-shaped mode I crack equal to it by the area. Let us suppose also, that radius of this penny-shaped microcrack is much less than the radius of macrocrack contour curvature. In this case, instead of the above-mentioned problem we may to consider the problem of a sudden nucleation of a penny-shaped microcrack near the front of a through semi-infinite macrocrack. Let us estimate resulting components of the dynamic displacement field. Consider now dynamic problem of growth of a semi-infinite through crack in homogeneous isotropic elastic body. For this problem the following angular dependencies ) (  i of radiation field were obtained in [2] ( is the angle in polar coordinate system Or, which origin coincides with this semi-infinite through crack tip). They correspond to longitudinal and transverse waves that are valid for the wave front region. 2 , 1 , ) ( ) ( ) ( ) (        i P C V i i i h i . (1) Here the functions         cos 1 1 i c i c c V , (2) describe the change in angular dependence of radiation caused by the crack edge propagation with velocity cc for a component of longitudinal (i=1) and shear (i=2) waves. Functions                cos cos 1 cos 1 1 i i R i i s K c c c c C , (3)