Southwest Research Institute® (SwRI®) News
DAISY Flaw Detection and Imaging System
San Antonio, Texas -- Sept. 17, 1987 -- A system that facilitates the detection and sizing of potentially hazardous fatigue cracks in cladded pressure vessels used in the chemical and nuclear power industries has been selected to receive an I-R 100 Award, recognizing it as one of the 100 most significant technical developments of 1986.
The winning development, known as DAISY, is a flaw detection and imaging system designed for use with an innovative technique of ultrasonic inspection. Developed at Southwest Research Institute® (SwRI®), it automates the acquisition of inspection data and the presentation of signal patterns ("images") representing material flaws, thus providing improved means of analyzing signals obtained during examination.
The selection of award-winners was announced by Research and Development magazine in Chicago on Thursday, Sept. 17.
Three SwRI staff members are co-developers of DAISY: Dr. George J. Gruber, who has the formal title of Institute scientist; Dennis R. Hamlin, staff engineer; and Harry L. Grothues, manager.
DAISY was developed to address a particular problem in industry's use of reactor pressure vessels, metal containers in which processes occur under pressure. For protection against corrosion, these vessels are often lined with an inner layer of resistant metal that is different than the base material. Unfortunately, the interface between the cladding and the base metal scatters ultrasonic waves beamed into the vessel wall to search for flaws, severely limiting the ability of standard inspection methods to provide reliable detection and accurate sizing of service-induced cracks.
The inability to "see" such flaws, a serious problem concerning both plant safety and operational economy, led to development at Southwest Research Institute of an innovative family of ultrasonic transducers. While standard procedure relies on variations in amplitude of one kind of ultrasonic wave, known as a shear wave, the new approach makes simultaneous use of Shear and Longitudinal waves to Inspect for Cracks, and is referred to by the acronym SLIC. Instead of relying solely on signal amplitude measurements, examination with SLIC transducers emphasizes measurement of ultrasonic pulse flight time, and recognition of pulse patterns, particularly a pattern of paired pulses known as a doublet.
The problem of interface area noise interference with signals is overcome by using different types of SLIC transducer. One type (SLIC-40) successfully enhances signal-to-noise ratio for crack detection through interaction of longitudinal waves beamed in from two different directions. Another (SLIC-50) derives sizing information from longitudinal wave signals returned from a crack's upper extremity and from mode-converted shear wave signals from the lower end. Both the multibeam and multimode transducers produce return signals that include pairs of associated pulses that move together across the inspection instrument screen as a test area is scanned, and the distance between pulses in these doublets is linearly related to crack depth. (Use of distinctive, recognizable doublets is emphasized in operation of DAISY, which is an acronym for Doublet Acquisition and Imaging System.)
While the SLIC technology is successful in detecting and sizing interface region cracks, it was noted that application requires considerable knowledge, experience, and judgment.
"If these transducers, and the associated novel time-of-flight and pattern recognition techniques are to be of real use in a variety of applications to difficult-to-inspect structures, the examination process must be aided significantly by a computer," said Institute Scientist George Gruber.
"In accomplishing this purpose, DAISY takes the use of these proven transducer and technique combinations out of the realm of expert interpretation, and makes them practical for use by less-skilled ultrasonic examiners. Much of the task of sophisticated echo-pattern recognition and interpretation is removed from the operator in a time-saving, cost-effective, and objective manner.
"Accurate depth estimates for the detected cracks can be derived from permanent color graphic records, and growth of cracks between inspections can be determined," Gruber said. "The assistance of DAISY is pressure vessel evaluations can therefore help plant owners avoid premature repair or retirement."
For more information about the DAISY Flaw Detection and Imaging System, contact Joe Fohn, Communications Department, Southwest Research Institute, P.O. Drawer 28510, San Antonio, Texas, 78228-0510, Phone (210) 522-4630, Fax (210) 522-3547.