GUIDED WAVE FOCUSING POTENTIAL IN HOLLOW CYLINDERS

Open Access
Author:
Zhang, Li
Graduate Program:
Engineering Mechanics
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 05, 2005
Committee Members:
  • Joseph Lawrence Rose, Committee Chair
  • Bernhard R Tittmann, Committee Member
  • Clifford Jesse Lissenden Iii, Committee Member
  • Nicholas John Salamon, Committee Member
  • Edward C Smith, Committee Member
Keywords:
  • pipe inspecton
  • Guided wave
  • phased array
  • focusing
Abstract:
Ultrasonic non-destructive evaluation is generally used in industry because of its excellent defect detection potential. The ultrasonic waves propagating in a bounded waveguide are called guided waves. Long range ultrasonic guided waves have intrigued investigators for years because of an ability to test structures over a long distance from a single sensor position. Guided waves have much more complex wave behavior than bulk waves in infinite media. Plane-strain guided waves in a single layer plate with traction free boundaries are Lamb waves. The Lamb type guided waves in hollow cylinders spread out and wrap around the circumference when propagating in an axial direction. Therefore, except for the guided waves with axisymmetric energy distributions in a tube, there are an infinite number of non-axisymmetric wave modes called flexural modes that have similar particle behavior as the axisymmetric mode in the same group. Focusing at a certain point in a hollow cylinder is implemented by utilizing the non-axisymmetric energy distributions of flexural wave modes. There are two types of focusing in pipes: natural focusing and phased array focusing. When a partially loaded excitation is used to generate guided waves in cylindrical shells, the excited flexural modes lead to ultrasonic energy naturally focused at certain positions. This special natural focusing phenomenon can be used to improve guided wave inspection results in hollow cylinders. By applying input time delays and amplitudes for a multi-channel ultrasonic signal generation system, one can focus the guided waves at a pre-selected position. The input parameters of the ultrasonic phased array are calculated by implementing a deconvolution computation of the displacement angular profiles excited by one excitation channel. Because the angular profiles of the circumferential displacement distributions in a hollow cylinder are decided by the excitation sources, focusing results are strongly affected by the excitation conditions. A source influence for focusing potential in pipes must be considered. Research shows that excitation focusing potential in hollow cylinders depends highly on the frequencies and sizes of the excitation transducers. An infinitely long transducer along the axis only generates axisymmetric guided waves, although a short axial transducer length has little influence on the focusing results. A transducer with a small circumferential length usually leads to drastic variations of energy distributions in the circumferential direction. These sharp angular profiles could lead to a failure of the phased array focusing technique. If a low frequency guided wave group propagates in a pipe with a small diameter, the flexural modes may be cut-off or have much different velocities from the axisymmetric mode. A single axisymmetric mode can never achieve focusing. Hence, contour charts of focusing potential at different frequencies and with various circumferential excitation lengths are used as directories of the focusing procedures. Because the geometry changes or material inhomogeneities in a hollow cylinder could affect the angular profiles and focusing potential, three-dimensional finite element analysis (FEA) was employed to simulate guided wave propagation and scattering in these regions. Numerical simulations show that guided waves are successfully focused beyond small defects and anisotropic welds. However, seam welds and elbows have extreme effects on the angular profiles: ultrasonic guided waves often tend to concentrate in a seam weld and elbows force a guided wave group to be separated into different groups. Therefore, a time reversal technique is utilized to achieve focusing through these media. For the phased array used as a time-reversal mirror (TRM), the input signal is the time reversed signals, which are then received from a transmitter or reflector at a predetermined focal position. Theoretical and experimental results show the power of focusing in the long range ultrasonic guided wave inspection of pipe. Therefore, the research of focusing potential in hollow cylinders becomes a significant achievement for applying pipeline integrity evaluations.