Research On Ultrasonic Wave Propagation Characteristics In Thick Wall Structure And Its Application In Nondestructive Testing

In the energy industry thick wall structures are critical in containing high temperatures and pressures in applications like transporting oil or generating electricity.Some of the most widely used structures in this area include thick plates and thick wall pipes that if compromised could fail and result in casualties and economic losses.Using ultrasonic technology it is possible to nondestructively evaluate the integrity of these structures to prevent their failure.Towards these ends it is of great importance to research ultrasonic wave propagation characteristics in thick wall structures and their applications in nondestructive testing.In this dissertation the propagation characteristics of ultrasonic waves in different thick wall structures are studied systematically in order of increasing boundary complexity: half space,thick plate,thick wall pipe and double layer thick wall pipe.Theoretical basis and practical guidance for ultrasonic nondestructive testing of thick wall structures is also provided.According to the order of chapters,the main contents of this research include:(1)The elastodynamic model of wave propagation in an unbounded medium by a line load is established which transforms the problem into a plane strain problem in half space.The Cagniard-de Hoop method is introduced to simplify the solution process,and then the transient displacement analytical solution is obtained.(2)A method for calculating the sound field of linear phased array based on analytical solutions of transient wave displacement in a half space is proposed.The transient displacements of multiple array elements by the modulation function load are calculated,then the sound field directivity function is determined and verified by experiments.The results show that their overall trends are consistent.The influence of array parameters to sound field directivity is further studied,and the optimal array parameter range is obtained.(3)A method for calculating the transient displacement in an elastic thick plate by a surface modulation function point load is proposed.An ultrasonic wave point displacement measurement system based on a noncontact optical detection device is established,and the method is verified by experiments.Experimental results are in agreement with the theoretical calculation.At the same time,the propagation characteristics of Lamb waves in a thick plate and its similarities and differences with the thick wall pipe are also studied.(4)A new type of transducer,Narrow Beam Array Transducer,is developed for the circumferential Lamb wave detection in large diameter thick wall pipes.It controls the circumferential sound field of a thick wall pipe through the array.A sound field measurement system is established,the transient waveforms at different angles are obtained,and the sound field directivity is measured.(5)A thickness measurement method of the inner wall attachments in the small diameter thick wall pipes is proposed.The propagation characteristics of longitudinal modes in double layer pipes are studied,the change rule of group velocity with the attachments thickness is obtained.The group velocities of pipes with 0-2mm uniform attachment layer on the inner wall are measured by experiments,and the experimental results are consistent with the theoretical values.