Numerical Simulation And Experimental Investigation On Crack Detection In Pipes Using Ultrasonic Guided Waves

Crack in pipe work is one of main problems in the oil, chemical and other industries. These cracks can occur at the outer or inner surface of the pipe and can lead to a serious decrease of pipe wall thickness. Traditional ultrasonic nondestructive testing techniques are single position measurements, therefore, they tend to be too slow to make that long distance inspection is almost impossible. An alternative method to pipe detection is to excite guided waves, which can propagate tens of meters along pipe, and to monitor the response of the pipe by measuring changes in the received signal. An ultrasonic inspection technique using guided waves was applied to detect and determine the exact location of crack in long steel pipes. However, many modes of guided waves are generated due to guided waves' multi-modes characteristic in the inspection. To select and excite single mode to detect the pipe can obviously simplify the analysis complexity.In this paper, numerical simulation and experimental investigation on crack detection in pipe using ultrasonic guided waves were carried out. Firstly, some main studies and progresses in this field were reviewed, and the relative theoretical basis was introduced. Secondly, based on the former theoretical introduction,the theories of guided wave in hollow circular cylinder were deduced in details. The characteristic equations of cylindrical and circumferential guided wave were obtained and the corresponding dispersion curves were numerically calculated. At the same time, guided wave modes in hollow circular cylinder were analyzed to choose the appropriate mode for guided wave detection in pipe. The simple and intuitionistic formulas were given based on pulsed echo principle, in which the effect of dispersion and transverse sensitivity were considered. By using finite element code ANSYS, a finite element model with circumferentially oriented through-thickness crack of various circumferential length, depth and axial width was built to simulate crack detection in pipe using longitudinal guided wave. 10 to 15 cycles narrow band signal modulated by HANNING window was chosen for excitation which is simulated by prescribing axial transient displacement at one end of pipe model. The echo can be received at the same end. The results of numerical simulation showed that the single crack position could be accurately identified. The circumferential length, depth and cross section area of crack could be approximately determined by reflection coefficient, which was defined as the amplitude of the reflected signal divided by the amplitude of the incident signal, as a function of them. It can also be found that the longitudinal guided wave detection is not sensitive to axial width of crack and the results are merely affected by boundary condition. The results of theoretical and numerical simulation are greatly agreed with the former investigation. Furthermore, double cracks positions were accurately identified for the first time and the results were discussed. Finally, the experimental investigation on crack detection in hollow circular cylinder is performed and the preliminary results are obtained.