A critical analysis of the acoustic emmission technique for NDE of pressure vessels

Abstract

As a nondestructive examination, the acoustic emission technique is used to detect the presence of discontinuities inside of pressurized components. However, doubts still exist concerning the loading procedure to accomplish the acoustic emission testing, especially, in a pressure vessel where a uniform pressure can produce a nonuniform stress distribution due to the presence of the singularities such as the nozzles and supports. The combined loading of vapor and hydrostatic pressure can also generate a nonuniform stress distribution throughout the pressure vessel. According to the Kaiser effect, a structure with a nonuniform stress distribution should have a different acoustic emission testing result when compared to a structure with a uniform stress distribution. In this present study, the necessity to perform a stress analysis prior to the acoustic emission testing is examined. Furthermore, for the purpose of the stress analysis, two approaches are discussed, the membrane stress analysis and the finite element approach.

By means of the membrane stress analysis, it is shown that the combined loading of the hydrostatic and vapor pressure does not produce a significant variation of stress throughout the spherical vessel. Actually, a computer program based on the membrane stress analysis is written to determine the stress distribution due to the combined loading. The limitation of the membrane stress analysis to handle problem with the presence of bending stress is also indicated.

The finite element approach is used to perform the stress analysis of the singularities where the bending stress is important. The finite element computer program ABAQUS is used to perform the finite element stress analysis, and the mechanical computer-aided engineering program PATRAN is also used to construct the finite element model and to interpret the stress analysis results. The convenience and the success of these computer programs to handle this kind of problem are confirmed. The application of different types of finite elements to perform the stress analysis is also discussed. Results from the experiments performed by Gill, Catching and Paine [9] to measure the stress distribution of a pad reinforced nozzle is used as a benchmark to determine the performance of these finite elements. As a consequence, concrete recommendations concerning the selections of the finite elements and the stress analysis procedure are given.

Finally, the influence of the stress distribution throughout the spherical pressure vessel on the acoustic emission is discussed, and the actual interpretation of the acoustic emission testing results based on the level of activity of acoustic emission without considering the nonuniform stress distribution throughout the structure is questioned.