FURTHER DEVELOPMENT OF A LOW CHANNEL PHASED ARRAY FOR MATERIAL DEFECT IMAGING

Open Access
Author:
Bohenick, Mark Adam
Graduate Program:
Engineering Science and Mechanics
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
December 21, 2007
Committee Members:
  • Bernhard R Tittmann, Thesis Advisor
Keywords:
  • stepped
  • linear phased array
  • non-destructive evaluation
  • NDE
  • high temperature
  • defect detection
  • high pressure
Abstract:
As lighter, stronger, and more affordable materials are available for industrial applications in hazardous environments, the effect of the environment on the material during operations must be studied. A material must perform adequately and degrade gracefully. Sudden failure is unacceptable. Current methods to qualify materials are expensive and do not provide real-time results. A method to monitor material characteristics and degradation while the material is in the hazardous environment is desirable. Development of a Non-Destructive Evaluation device that can function inside a high temperature, high pressure environment would provide a tool for monitoring a material specimen in real-time. A limitation is that the number of inputs to the environment should be minimized. The device should provide real-time data on the specimen’s degradation. Surface and interior defects such as film growth, blistering, cracking, and corrosion should be able to be characterized. In addition, geometric changes such as shape, size, and warping should be monitored. An ultrasonic linear phased array provides the capability to examine a specimen without mechanical scanning. Linear phased arrays are essentially multiple ultrasonic transducers that are arranged along a line. Electronic time-delays provide the ability to scan a beam of acoustic energy across the specimen. Reflected pressure waves are received by the array and are used to produce real-time images of the specimen. In order to limit the number of inputs to the environment, a stepped linear phased array design was used. A physical offset is designed into the device between multiple linear phased arrays. Wire inputs to the environment connect an element on each step of the array. The physical offset allows for the signals of each step to be separated in time. This allows additional sets of elements so that additional portions of a material specimen can be investigated with the same number of wires. Three prototype devices were manufactured. Each has four channels; four piezoelectric elements on each step forming a linear phased array. Prototype 1 has four steps and its elements have a central frequency of 2.25 MHz. Prototype 2 has four steps and a central frequency of 10 MHz. Prototype 3 has six steps and a central frequency of 10 MHz. The ability of the prototypes to monitor degradation of a material specimen is examined through bench-top experiments modeling defects and geometric changes to a material specimen. Beam steering and focusing were simulated using a ultrasonic phased array simulation program, Field II, to determine optimum array dimensions. The current prototypes cannot sufficiently monitor a specimen for defects. Large surface defects can be detected but smaller individual surface defects cannot be sufficiently resolved. The prototypes were unable to detect internal defects. Warping or bending of the specimen in the vertical plane has been able to be examined by the prototypes. The stepped linear phased array design parameters can be optimized to develop a device that can provide all the functionality desired. The higher frequency of Prototypes 2 and 3 did not show a marked increase in resolution or effectiveness. Sparse arrays and data analysis may be required to further optimize the stepped linear phased array design.