Ultrasonic multi-element transducers and methods for testing

Details Ultrasonic multi-element transducers and methods for testing Ultrasonic multi-element transducers and methods for testing

2002-03-18

2004-04-27

Kwok, Helen (Department: 2856)

Measuring and testing

Vibration

By mechanical waves

C073S628000, C073S622000

Reexamination Certificate

active

06725721

ABSTRACT:

BACKGROUND OF THE INVENTION
The present application relates to an invention for inspecting tubes, bars, pipes, and other objects using ultrasonic transducers. More particularly, the invention is concerned with increasing inspection speed while providing high sensitivity and accuracy for non-destructive testing of such objects.
Ultrasonic inspection is commonly used to detect both surface flaws such as cracks and internal flaws such as voids or inclusions of foreign material. It is also used to measure wall thickness in tubes and pipes as well as bar diameter. In what is known as the pulse-echo method, the same transducer serves both as a transmitter and a receiver of ultrasonic beams or waves used to detect such flaws and take such measurements.
In such a method, in response to an electrical pulse, the transducer produces a pressure wave referred to as an ultrasonic pulse. The pressure wave travels through a coupling medium between the transducer and the tested object. In longitudinal wave inspection, once the ultrasonic pulse reaches an interface between the coupling medium and the tested object, a portion of the pulse enters the object whereas another portion is reflected back to the transducer (i.e., a partial reflection and transmission occur). The initially reflected pulse is known as a frontwall echo. The portion of the pulse that enters the object continues until the back wall, where another partial reflection and transmission occurs. This partial reflection is known as the backwall echo. If there is an internal flaw in the tested object for instance, a portion of the ultrasonic pulse is also reflected back to the transducer at the flaw. The flaw can be located knowing the elapsed time between the different reflections. For automatic flaw testing, a gate is placed between the frontwall and backwall echoes. Any pulse within the gate area is peak detected, producing an analog output that can be recorded and that represents a flaw in the tested object. In addition, thickness measurements are made possible knowing the time difference between the backwall and frontwall echo pulses as well as the velocity of the ultrasonic wave as it travels through the medium of the tested object.
The most widely used pulse-echo process for non-destructive testing of objects such as tubes and bars is performed by using ultrasonic rotary testers. Ultrasonic transducers are mounted on a rotary testing unit of such testers, while the tube or bar to be tested is moved freely through the tester. Rotating the transducers in the tester around the tube as opposed to rotating the tube as it is moved through the tester eliminates the need for heavy machinery and high power in the case of testing large and long tubes and bars. The space between the object and transducers is generally filled with water in order to provide coupling for the ultrasonic beam. The electrical signals from the ultrasonic inspection instrument are connected to the rotating transducers by rotary capacitors. In order to detect various kinds of surface and internal flaws and to provide thickness measures, several transducers are generally mounted on the tester, each being oriented to perform a specific function.
For instance, in a longitudinal wave inspection arrangement, a transducer is typically oriented so that the ultrasonic beam is perpendicular to the surface of the tested object. In such a case, the angle of incidence is 90 degrees. Longitudinal waves are suitable for detecting inner flaws and taking thickness or diameter measurements. When the angle of incidence is not 90 degrees, a refracted shear wave occurs within the tested object. Shear waves are used to detect both surface and internal flaws. Transducers can be oriented to detect both longitudinal and transverse flaws.
To improve the detectability of irregularly shaped flaws, shear waves are generated in both clockwise and counter-clockwise directions.
FIG. 1
illustrates a setup for performing such a test with two offset transducers
10
. The incident beams
20
of transducers
10
are maintained within the same plane of a cross section that is perpendicular to longitudinal axis
50
of tube
30
. Under this setup, beams
22
and
44
travel clockwise and counter-clockwise, bouncing between the outer and inner surfaces of tube
30
until a flaw is detected and beam
20
is partially reflected back to transducer
10
. As shown in
FIG. 1
, the beam traveling clockwise, beam
22
, is reflected back from an inner diameter crack
60
, while the beam traveling counter-clockwise, beam
44
, is reflected back from an outer diameter crack
70
.
In another example, the much less common transverse cracks can be detected by angling transducer
10
in a plane containing the longitudinal axis
50
of tube
30
, without offsetting transducer
10
from its position when it emits longitudinal beam
20
. In such a case the beam would travel along the length of the tube and partially reflect back from transverse cracks.
An entire tube can be scanned if a set of transducers is rotated around the center of the tube while the tube is freely moved along its longitudinal axis. To allow for thickness measurement and to ensure full flaw detection, several transducers are mounted in the rotary tester. Transducers can be oriented generally for longitudinal wave testing and for clockwise, counterclockwise, forward-, and reverse-looking shear wave testing. In this manner, five channels are required so that each transducer can be individually driven.
The linear movement of the tube combined with the rotation of the transducers around the tube results in a helical test path around the circumference of the tube. In order to achieve 100 percent inspection, the helical traces must slightly overlap. The foregoing limits the inspection speed or test throughput rate. If the tube is moved faster, the pitch of the helix increases and the helical traces may cease to overlap. In such a case, gaps may form between separate adjacent helical traces. As a result there would be volumes left untested in the tube. On the other hand, the tester's rotational speed is mechanically limited. In addition, the rate at which a transducer emits an ultrasonic beam, namely, the pulse repetition frequency or rate, should not drop beyond a certain level given the rotational speed of a rotary tester. Moreover, the beam size of the transducer is limited by the required sensitivity of the test itself. The larger the beam, the less sensitive the beam will be to small defects and, as a result, some small defects may not be detected.
However, there has been increasing demand for higher throughput rates and higher testing sensitivities in non-destructive testing of tubes, bars, pipes, etc. With higher throughput rates being especially desirable for online testing (i.e., post manufacturing testing), inspection sensitivity and accuracy cannot be compromised.
It is possible to increase a tester's throughput rate by increasing the number of transducers mounted at different positions on the rotor. Several problems, however, are associated with increasing the number of transducers used. The number of channels used for analyzing the signals emitted and received from transducers, as well as the number of coupling capacitors required would increase accordingly, which would in turn complicate the required rotary connections. In addition, the mounting space on the rotor may be limited and transducer cross-talk can become a greater problem.
In view of the foregoing, it would be desirable to provide an ultrasonic transducer arrangement for achieving higher throughput rates and higher testing sensitivities during ultrasonic testing.
It would be further desirable to provide an ultrasonic transducer arrangement that would maximize test sensitivity given mechanical limitations in rotary testers and pulse repetition requirements.
It would be further desirable to provide an ultrasonic transducer arrangement that makes efficient use of space on rotary testers without increasing the number of channels and capacitors needed.
SUMMARY OF THE IN

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