Measuring and testing – Vibration – By mechanical waves
Reexamination Certificate
2001-11-14
2003-07-15
Moller, Richard A. (Department: 2856)
Measuring and testing
Vibration
By mechanical waves
Reexamination Certificate
active
06591679
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention pertains generally to the field of non-destructive testing of materials, and in one more particular manifestation to a method of examining conduits to more rapidly and precisely detect and measure flaws.
2. Description of the Related Art
There are many industrial and commercial applications where a material is most desirably tested, prior to being placed in service or subsequent thereto. In such instances, non-destructive testing methods are required which enable rapid and reliable testing and evaluation. Ultrasonic examination is one such method which has been applied successfully, particularly with metal materials, though not as well as still desired.
One particular industrial application where ultrasonic testing has proven to be of great value is the testing of heat exchanger tubes such as are used in electric power plants. Plant efficiency and consequent profits can be reduced by removing heat exchanger tubes from service. However, placing or leaving a defective tube in operation in a nuclear power plant could result in radioactive contamination. Using ultrasonic examination, the tubes may be tested even when they are of great length and generally independent of whether they are straight, bent or coiled. Testing may be done prior to placing tubes into service, to identify processing-related discontinuities that have arisen during manufacturing, or after tubes are in service to detect service-related discontinuities. One example of a service related discontinuity is an Outside Diameter Stress Corrosion Crack (ODSCC) which may extend from the outer diameter of a tube towards the inside diameter.
While discontinuities may require replacement of tubes, not all discontinuities are actually detrimental to continued operation. Consequently, analysis will most preferably be conducted to determine whether a reflector exceeds a critical dimensional limit, or is instead deemed acceptable. Detecting service-related discontinuities in advance of a failure is highly desired, which enables timely replacement or taking the tubes out of service. These tubes are frequently not readily removed from service, and so are most preferably tested prior to installation, and then at intervals between periods of use on location. With such timely detection and sizing of reflectors, the tubes will only be installed when satisfactory, and later replaced or taken out of service only when necessary.
Components used in ultrasonic examination and applicable to various degree to the present invention are known in the industry. These components may, for example purposes, consist of ultrasonic signal generator and receive instrumentation, a search unit containing at least one ultrasonic transducer, cabling, data recording equipment and data analysis software. The signal generator creates high-frequency electric pulses that are transmitted through the cabling to the search unit. The search unit will preferably contain at least one piezoelectric crystal or equivalent transducer that converts high-frequency electric pulses into ultrasonic mechanical vibrations. A liquid which has a relatively high efficiency of transmission, will typically serve to couple the transducer to the material to be tested.
Typically, ultrasonic energy generated by the transducer is transmitted by compression wave to the material, and will strike the material at a particular angle of incidence. Generally, a normal angle of incidence will result in reflection from the material back to the transducer, and further reflection from the transducer leading to a bouncing back and forth. However, when the angle of incidence is different from normal (perpendicular) to the surface, part of the energy is refracted in the tube wall, and the incident compression wave is converted into a shear wave within the material. The angle of fraction is governed by Snell's law and depends on the wave velocity of the liquid and the material under test.
In the case of a cylindrical tube or other material with parallel surfaces of the wall, the refracted shear wave will continue to propagate in the material, in the absence of defects and surface irregularities, by successively bounding between outer and inner surfaces. The propagation of ultrasonic energy in material without parallel wall sides or which is not cylindrical can also be predicted and is contemplated herein, but is not specifically addressed herein to avoid further complicating an understanding of the operation of the present invention. In all cases, the refracted shear wave will continue to propagate in the material until dissipated by various mechanisms such as scatter, attenuation, refraction and diffraction.
When a shear wave encounters a defect or material discontinuity, the refracted shear wave interacts with the defect differently. The defect acts as an internal reflector, and so disrupts the internal propagation and dissipation. There are normally two detectable interactions between refracted shear wave and reflectors that are particularly important to the present invention. One is the corner reflection or echo, and the other is the tip echo.
When the ultrasonic wave hits the root of the crack, the corner formed by the tube wall and the crack root will reflect a portion of the energy. This echo, referred to generally as the corner echo, travels back to the transducer for conversion into a corner signal. Typically, this corner signal is relatively strong and readily detected. There will be a measurable amount of time between generation of the wave and receipt of the echo at the transducer. The amount of time delay is directly related to the distance of travel of the wave in the material, and so the location of the reflector may be readily calculated.
When the ultrasonic wave hits the tip of a crack, the wave front will bend around the tip of the crack. This phenomenon is known as diffraction. The diffracted wave will produce a radial propagating wave with its center at the crack tip, producing a tip echo that is detected by the transducer and converted into a tip signal. The tip signal is generally a weaker signal than the corner signal, and can be much more difficult to distinguish from background noise. Nevertheless, and like the corner signal, there will be a time delay between generation of wave and receipt of echo which can be used to calculate the location of the tip.
After the ultrasonic waves are reflected back to the transducer, or to another receiver, the receiver converts the wave into an electrical signal. This signal is typically presented or displayed as an A-scan, which plots time on one axis (typically the X-axis) and signal amplitude on the other axis. Where the X-axis represents time, the horizontal distance between any two signals represent the material distance between the two conditions causing the signals. Using one prior art technique, an inspector moves a search unit along a material under test, while simultaneously interpreting the A-scan signals on a portable ultrasonic instrument. The corner and tip signals are identified, and the separation in arrival time between these two signals, represented by an X-axis displacement between the two signals, is used to calculate the depth of the reflector. This type of inspection requires tremendous training and expertise to accurately interpret the A-scan displays, and a great deal of dexterity and patience to thoroughly evaluate a reflector. Consequently, the prior techniques have not produced by intuitive and rapid sizing technique.
As an improvement thereto, computer aided examinations have been devised by the present inventors to include the acquisition and storage of signal time delay, amplitude and transducer position through a large number of transducer positions. The data is then analyzed either in real time or later, using data analysis software. This computer aided examination allows the data to be analyzed in different ways and by different persons. However, the examination has heretofore consumed more time and has been more difficult than
Henry Gary L.
Kenefick Steven A.
Electric Power Research Institute Inc.
Moller Richard A.
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