Potential of the acoustic emission method for estimating production pipeline metal damage

Abstract

The article considers reasons for the use of acoustic emission together with kinetic identification for estimating the degree of production pipeline metal damage, operating under low-cycle loading conditions. A proposed criterion makes it possible to detect achievement of a level of metal damage above 60-70%.The contemporary concept of technical diagnostics and nondestructive monitoring is based on methods for detecting macrodefects in the form of disruption of continuity, reducing the strength of a structural area [1]. If during a short pipeline operating time formation of malfunctions depends on loading conditions and the bahavior of defects, present during manufacture and performance of repair work, then metal damage during prolonged operation under conditions of cyclic (low-cycle) loading, occurs intensely throughout the whole volume of metal, especially in areas of stress and strain concentration. In order to consider the dynamics of a change in mechanical properties of the metal, it is necessary to develop a nondestructive rapid evaluation method for metal damage. The main method for this may be acoustic emission (AE), making it possible to detect sources with energy corresponding to movement of individual groups of dislocations. Recording and analysis of acoustic emission signals, caused by dislocation processes, makes it possible to assess the degree of dislocation mobility, which specifies the degree of metal embrittlement. It is possible to initiate the work of dislocation sources by metal plastic deformation during indentation.Evaluation of the compatibility of results of analyzing AE parameters with tension and indentation are provided in Fig. 1.Comparison of the main spectral characteristics of an AE signal with indentation and uniaxial tension of steel 20 specimens has shown [2] that with both material loading schemes the AE spectral composition is qualitatively identical, i.e., signals of the same type are recorded. This confirms an assumption about single material deformation mechanisms with tension and indentation. The feature detected made it possible to move to a nondestructive method for evaluating processes that occur during metal plastic deformation.During indentation in undamaged metal, the AE signal spectral portrait consisted of one clear peak. With an increase in damage within the spectral portrait additional second peak (maximum) appears, distant from the main peak by not less than 10 kHz. With a further increase in damage, the second peak becomes clearer, and shifts in the direction of higher frequency [3]. With a ratio of height of the second peak to that of the first (Fig. 2) more than 0.1, the degree of metal damage is 60-70% [4].This relationship was followed both for the basic metal and welded joint metal. Thus, the criterion obtained makes it possible to evaluate the degree of metal cyclic damage and to determine the stage of damage.