Circumferential Guided Waves in Elastic and Viscoelastic Multilayered Annuli

Open Access
- Author:
- Van Velsor, Jason Kenneth
- Graduate Program:
- Engineering Science and Mechanics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- July 10, 2009
- Committee Members:
- Joseph Lawrence Rose, Dissertation Advisor/Co-Advisor
Joseph Lawrence Rose, Committee Chair/Co-Chair
Bernhard R Tittmann, Committee Member
Clifford Jesse Lissenden Iii, Committee Member
Ghassan Chehab, Committee Member - Keywords:
- pipe inspection
ultrsound
circumferential guided waves
dispersion curves
viscoelastic coatings - Abstract:
- People all over the world rely on vast pipeline infrastructures whether it be directly, such as for fuel to cook and heat their homes, or indirectly, such as for delivery to the farmer who must harvest and process the food that will eventually end up on their table. In many cases these pipelines, or at least portions of them, have been in operation for more than half a century. This long-term use, coupled with a lack of integrity assurance programs in the early part of the 20th century, has resulted in the overall degradation and, in many cases, failure of the pipeline. Such failure can be detrimental to the people in the immediate vicinity of the pipeline and the surrounding habitat, and result in significant clean-up, reparation, and litigation costs to the operator. For this reason, most operators have now implemented pipeline integrity and assessment programs, which generally rely quite heavily on non-destructive testing methods. One of the most common inspection technologies for gas transmission pipeline is the Magnetic Flux Leakage (MFL) In-Line Inspection (ILI) tool. These tools travel through the inside of the pipeline, saturating the pipe wall with a magnetic field, which subsequently “leaks” from the wall in areas of corrosion. While this technology has been found to be a reliable detection tool for most corrosion geometries, it has relatively limited sizing capabilities. For this reason, there is currently significant interest in the use of guided ultrasonic waves for improved inspection capabilities. In a similar fashion to the MFL tools, these ultrasonic tools travel through the inside of the pipe, usually transmitting and receiving ultrasound in the circumferential direction of the pipe. As wave mechanics can be a very complex subject, many tool developers, and even researchers, equate the propagation of the circumferential wave to that which travels in a plate. While this may be a valid assumption in some cases, it is not in all cases; particularly as radius decreases and wall thickness increases. Furthermore, nowhere in the present body of literature regarding circumferential waves, has anybody made provisions in their model for the presence of coating layers, even if assuming elastic. This primary goal of the work presented here is the development of more accurate modeling tools for circumferential guided waves. The generalized characteristic equations for circumferentially propagating shear-horizontal and Lamb [type] waves are derived and the Global Matrix Method is used to extend the solutions to multiple layers. Dispersion curves are presented for several cases of single-layer and multilayered annuli and some physical insights into their meaning are provided. Wave structures are also examined for satisfaction of the appropriate boundary condtions. The elastic-viscoelastic correspondence principle is employed to allow the use of the elastic solutions for viscoelastic materials, via incorporation of complex material moduli. The Semi-Analytical Finite Element approach is enlisted to calculate the dispersion solutions for elastic/viscoelastic multilayered annuli and to obtain solution in the dispersion spaces for which the fully analytical calculation was found to fail. Excellent agreement is found between the analytical and semi-analytical approaches. The differences between the dispersion curves for elastic multilayered annuli and elastic/viscoelastic annuli are examined. Two experimental demonstrations are provided to validate the accuracy of the newly development theoretical models and to provide examples of how the theoretical models may be employed to develop new non-destructive testing methodologies. In particular, it is shown how a circumferential shear-horizontal wave is influenced by coating thickness and how these effects may be used to detect disbonds in protective coating layers. Multiple disbond detection features are identified that have resulted directly from consideration of the theoretical models. Another practical example is included in an appendix and employs a circumferential Lamb [type] wave for the estimation of the remaining wall thickness of a gradually thinned annulus. This study is completed using finite-element analysis as appropriate test specimens were not available and proper fabrication would be impractical. It should be noted that there are many other potential applications for the models developed in this work. Other examples include the inspection of gas storage well casings and boiler and heat exchanger tubing. The modeling tools will be especially useful in the development of defect detection, identification, and sizing methods. Future work may involve the modification of the modeling tools to study the influences of soil layers and liquid contents.