ULTRASONIC GUIDED WAVES AND WAVE SCATTERING IN VISCOELASTIC COATED HOLLOW CYLINDERS

Open Access
Author:
Luo, Wei
Graduate Program:
Engineering Science and 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
  • Eduard S Ventsel, Committee Member
  • Qiming Zhang, Committee Member
Keywords:
  • acoustics
  • wave mechanics
  • guided wave
  • finite element analysis
  • phased array focusing
  • ultrasonics
  • viscoelasticity
  • wave scattering
  • nondestructive evaluation
Abstract:
Over a million miles of piping is used in the USA in almost every industry that calls for a large scale transportation and distribution of energy or product, like natural gas, oil, water, etc. Pipeline safety is crucial in that defective pipelines can lead to catastrophic failure, property damage and high replacement costs. To preserve the integrity of these pipelines, viscoelastic coatings are widely used on the pipes. However, pipe aging and exposure to a variety of changing environmental conditions reduces the protection effectiveness consequently leading to the occurrence of defects. An effective non-destructive evaluation (NDE) method is needed to provide the current pipeline status to the pipeline operators for any further decisions on repair or replacement actions. Ultrasonic guided waves, with a long range propagation capability, are becoming useful in new solutions for pipeline inspection. It is much more efficient and economical than other commonly used NDE methods like point-by-point bulk waves and magnetic flux leakage. Long pipes can be inspected from a simple sensor position. Among the two methods for the long rang guided wave pipeline inspection, almost decade old axisymmetric waves and recently developed phased array focusing, the latter presenting itself with a tremendous improvement in terms of penetration power, detection sensitivity, and inspection distance. However, guided wave inspection potential in coated pipe has not yet been studied in detail. Many important questions need answers, like focusing feasibility in coated pipes, wave scattering possibilities study for effective inspection of 3-D defects, and quantitative evaluation of inspection distances under various coating conditions. Since a large percentage of the pipelines are covered with viscoelastic coatings, a thorough study of guided waves in viscoelastically coated pipes is strongly called for. In this work, guided wave propagation, scattering and focusing in coated pipes are studied comprehensively for the first time with numerical, analytical and experimental methods. A three-dimensional finite element tool utilizing ABAQUS/Explicit was developed for quantitatively and systematically modeling guided wave behavior in pipes with different viscoelastic materials. A whole process, from experimental measurement to theoretical modeling has been established, including in-situ coating property measurement, transformation of measured properties to model inputs, specific wave mode generation, and output data processing and analysis. With the help of this new powerful modeling tool, it is very exciting to find that guided waves can still be focused very well in a coated pipe for the frequency studied, although there is an amplitude loss due to the viscoelastic nature of the coating materials. The quantitative evaluation of the energy increment and the subsequent inspection distance increment from axisymmetric loading and focusing was studied. Wave scattering from planar and corrosion like defects were investigated under both axisymmetric and phased array focused loading with both longitudinal and torsional waves. It was found that axisymmetric waves had a small possibility of finding small corrosion like defects while focusing had a much higher chance. Defect sizing potential was also studied based on an observation of the wave interaction with defects and the mode conversion that occurred thereafter. Moreover, in order to minimize the attenuative effect from the coatings, a parametric study of coating property effects on wave attenuation was conducted making use of the attenuation dispersion curves. An improved mathematical root search algorithm was utilized for highly viscoelastic materials. This lead to a decision on the best choice of either coating to be used or in the case of existing coatings the best set of sensor and instrumentation parameters to do the test. Appropriate coating properties, frequency range, and wave type were recommended for future work in the pipeline industry. In addition, an experimental method of property measurement for field coating materials was developed as a means of providing inputs to computer models. It was found that coating properties had a wide variation suggesting the need of in-situ coating property measurements for any subsequent modeling work based on field coated pipes. Finally, a detailed criterion to improve the inspection potential of coated pipes is recommended.