Nondestructive testing utilizing horizontally polarized shear waves

Disclosed is a nondestructive test device for detecting a flaw proximate to a welded seam in a pipe, including an electromagnetic acoustic transmitting transducer for generating a horizontally polarized shear wave in the wall of the pipe, a high frequency generator operatively connected to the transmitting transducer for energizing the transducer, an electromagnetic acoustic receiving transducer for responding to a horizontally polarized shear wave within the wall, an amplifier operatively connected to the receiving transducer for boosting the response signal of the transducer, and an indicating instrument for receiving and displaying the amplified response signal. In the test method disclosed, a horizontally polarized shear wave is generated in the wall, the pipe is monitored to detect a reflected horizontally polarized shear wave, and a time-dependent representation of the amplitude of the reflected wave is displayed. The wave generating, monitoring, and displaying steps are repeated along a length of the pipe to provide a comprehesive flaw inspection of that length of the pipe. The times of arrival of the generated and reflected waves are correlated to determine the circumferential position of the flaw.

BACKGROUND OF THE INVENTION 
This invention relates to apparatus and methods for nondestructive testing, 
and is concerned in particular with the use of ultrasonic techniques in 
nondestructive testing. 
Nondestructive methods are preferable for test situations where the 
comprehensive evaluation of inservice machinery, assembled components, or 
new products is required. Nondestructive techniques are particularly 
desirable, for example, in testing the materials employed in structures 
such as tracked vehicle rails, vessels and conduits for pressurized gas or 
steam, and pipelines. In such applications, it is imperative that the test 
method utilized reveal any significant flaws or imperfections so that 
potentially costly equipment failures and other undesirable consequences 
may be prevented. 
Among the various techniques of nondestructive testing which are available, 
noncontact test techniques, which do not require physical contact between 
the testing apparatus and the item tested, are especially advantageous, 
since such techniques may be implemented with the additional advantages of 
high speed operation, good performance in extreme temperature 
environments, the ability to operate in inaccessible locations by remote 
control, and a need for only minimal subsequent cleanup operations. 
One particular testing application, for example, in which nondestructive, 
noncontact testing techniques are useful is the inspection of a pipeline 
built from sections of pipe which are welded together and which are 
manufactured with longitudinal welded seams. A somewhat detailed 
discussion of the pipeline testing situation will serve to illustrate the 
state of the art, and its limitations, for nondestructive testing in 
general. 
An inspection of the structural integrity of such a pipeline has commonly 
been performed at the pipe manufacturing location for the longitudinal 
weld and at the pipeline installation site for the girth welds joining the 
sections of pipe. A detailed x-ray examination of the girth welds is often 
performed before a pipeline is buried, but inspection of the longitudinal 
welds is typically deferred until a hydraulic pressure test is performed 
on a sample section of the installed pipeline. A failure in the pressure 
test can frequently be traced to cracks or other defects in or near the 
weld which escaped detection in earlier inspections or which were formed 
in the pipe during delivery, storage, or handling of the pipe prior to 
installation in the pipeline. 
Since the hydraulic test is relatively expensive and further is not 
performed until after the completion of the pipeline construction phase, 
the cost of an earlier nondestructive inspection could be justified where 
such a test would ensure the detection of any defects in or near the 
longitudinal pipe weld which were large enough to cause a failure of the 
hydraulic pressure test. 
Noncontact ultrasonic testing procedures and apparatus are known which 
potentially could be used in such a testing environment. Electromagnetic 
acoustic transducers (EMATs), for example, some of which are disclosed in 
U.S. Pat. Nos. 3,850,028 and 4,127,035, may be utilized to generate an 
ultrasonic wave in an electrically conductive or magnetic material through 
the interaction of a static magnetic field and a dynamic electromagnetic 
field. Cracks or other defects which are present in the material will 
affect the transmission or reflection characteristics of the waves in the 
material. Those changes may be measured, by an EMAT or other suitable 
transducer, and utilized in characterizing the part as acceptable or 
unacceptable for its intended use. Ultrasonic testing procedures have been 
adapted for use in pipeline inspections, as disclosed, for example, in 
U.S. Pat. No. 4,092,868. 
Difficulties have been encountered, however, when such ultrasonic test 
methods are employed in an attempt to evaluate an object which contains a 
known discontinuity or inhomogenity. Such a discontinuity might be due to 
a weld, as in the type of pipeline discussed above, or might be caused by 
some other similar aspect of the structure of an object. It has been found 
that such a discontinuity tends to cause a disturbance in the reflection 
and refraction of ultrasonic waves which is so large that the effects due 
to the presence of cracks or defects near the discontinuity tend to be 
obscured, effectively rendering such defects undetectable by prior art 
ultrasonic methods. 
Consequently, a need has developed in the art for an ultrasonic 
nondestructive testing technique which is capable of detecting flaws or 
imperfections in or near a known discontinuity or inhomogeneity in a 
material. 
SUMMARY OF THE INVENTION 
It is a general object of this invention to provide a new and improved 
method for performing a nondestructive evaluation of a test object. 
A nondestructive test method for detecting an imperfection in an object 
includes, according to the invention, the steps of: 
(a) generating a horizontally polarized shear wave in the object, and 
(b) monitoring the object to detect a reflected horizontally polarized 
shear wave, 
the method thereby being adapted to detect imperfections in the object 
which reflect a horizontally polarized shear wave, while exhibiting 
relative insensitivity to the presence in the object of a discontinuity, 
such as a weld, which does not effectively reflect a horizontally 
polarized shear wave. 
In a more particular embodiment, the method is adapted to detect a flaw 
proximate to a welded seam in a pipe, and includes the steps of: 
(a) generating a horizontally polarized shear wave in the wall of the pipe, 
(b) monitoring the pipe to detect a reflected horizontally polarized shear 
wave in the wall, 
(c) displaying a time-dependent representation of the amplitude of the 
reflected wave, the form of the time-dependent display being indicative of 
the presence or absence of a flaw in the pipe, 
(d) repeating, at sequential locations along a preselected length of the 
pipe, the steps of generating a horizontally polarized shear wave, 
monitoring to detect a reflected wave, and displaying a time-dependent 
representation, thereby accomplishing a comprehensive flaw inspection of 
the preselected length of pipe, and 
(e) correlating the times of arrival of the generated and reflected waves 
to determine the circumferential location of the flaw. 
Examples of the more important features of the invention have been broadly 
outlined above in order to facilitate an understanding of the detailed 
description that follows and so that the contributions which this 
invention provides to the art may be better appreciated. There are, of 
course, additional features of the invention, which will be described 
below and which are included within the subject matter of the appended 
claims.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
Now referring to the drawings, and first to FIG. 1, a schematic block 
diagram of a test device constructed according to the present invention is 
illustrated. A transmitting transducer 10 induces a horizontally polarized 
shear wave 12 in a pipe wall 14 when energized by a signal from a signal 
generator 16. A reflected horizontally polarized shear wave 18 is detected 
by a receiving transducer 20, the output of which is processed by an 
amplifier 22, an indicating instrument 24, and a recorder 26 to provide 
data useful in determining whether a flaw exists in the pipe. 
Although the method of this invention will be described herein with respect 
to a preferred embodiment of the invention which is adapted for the 
testing of welded pipe, it should be understood that the invention is not 
limited to applications in the pipeline testing environment, but may be 
used to advantage in many other testing environments, as will be 
appreciated by those skilled in the art. 
It is well known in the art that ultrasonic waves propagating in an elastic 
medium will be perturbed by the presence of flaws or other imperfections 
in the medium. This effect has been utilized to develop nondestructive 
test methods which employ ultrasonic waves to detect defects in materials. 
A number of specific types of ultrasonic waves, such as L (longitudinal), 
SV (vertical shear), and SH (horizontal shear), may be generated and 
propagated in a solid material. 
As a particular example of the generation of ultrasonic waves, such waves 
may be generated and detected in an electrically conductive or magnetic 
material through the use of electromagnetic acoustic transducers (EMATs). 
An EMAT operates in a manner analogous to an electric motor, i.e., when an 
electromagnetic field is applied to a conducting material in the presence 
of a D.C. or static magnetic field, eddy currents generated within the 
material will interact with the static field to exert forces on the 
material. By selecting an A.C. current of the appropriate frequency to 
create the electromagnetic field, the forces exerted may be used to 
generate ultrasonic waves at that frequency in the material. This 
phenomena may also be utilized in reverse for detection purposes, since an 
eddy current will be generated when the acoustic wave causes the 
conductive material to move in the presence of a D.C. magnetic field. This 
eddy current will induce a current in the coil of an EMAT. If the material 
being tested is also ferromagnetic, magnetostrictive forces will act in 
the material and will tend to further enhance the efficiency of an EMAT. 
Typical EMAT designs which may be used in ultrasonic testing applications 
are disclosed in U.S. Pat. Nos. 4,092,868 and 4,127,035, the teachings of 
which are incorporated herein by reference. 
The prior art techniques utilizing ultrasonic waves for materials testing, 
however, have proven inadequate for the detection of flaws located near an 
inhomogeneity in the tested material. Steel pipe, for example, such as 
that commonly used in oil and gas pipelines, is manufactured from flat 
stock which is bent into a cylindrical shape and joined by a weld, the 
weld thus forming a longitudinal seam along the pipe. In conventional 
ultrasonic testing techniques, it has been found that such a pipe weld 
will strongly reflect the ultrasonic energy propagated in the pipe. 
Consequently, reflections from cracks or other imperfections located in 
the vicinity of the weld are likely to be obscured by the large response 
caused by the weld. 
It is an outstanding feature of this invention to provide a method and 
apparatus for ultrasonic testing by which imperfections located near an 
inhomogeneity in a material may be detected. This invention is founded on 
the discovery that horizontally polarized shear (SH) ultrasonic waves 
experience minimal reflection when passing through a symmetric 
discontinuity, such as a weld, in a material, while SH waves are reflected 
relatively well from a flaw in the material. This effect appears to arise 
from the nonsymmetric geometry of a flaw which enters the material from 
one side. This characteristic may be used to advantage by designing an 
ultrasonic testing apparatus to selectively detect only horizontally 
polarized (SH) shear waves. The presence of reflected SH waves will then 
indicate that a symmetry breaking defect has been detected. 
The usefulness of this discovery has been proven in tests on an 
experimental section of pipe which contained a number of identified flaws. 
A 36 inch diameter section of welded pipe 79 inches in length, with a wall 
thickness of 0.47 inches, was used for the test. In order to simulate 
various types of flaws in the pipe, the weld bead was ground off of both 
the inner and outer surfaces at from 0 to 50 cm from the left end of the 
pipe. A 23 cm square section of the pipe was cut out at 0-23 cm. A 
longitudinal saw cut 5.8 cm in length and through 47% of the wall 
thickness was made at 23 cm, while a similar cut 5.9 cm in length and 
through 24% of the wall was made at 39 cm. Further cuts were made in the 
weld at 74 cm (53% of wall and 6 cm long) and 90 cm (20% of wall and 4 cm 
long). A 12 cm, 25% crack at 132 cm, a 16.5 cm, 24% crack at 155 cm, and a 
17 cm, 40% crack at 178 cm were introduced into the pipe wall 
approximately 1/8 inch from the weld bead. 
A transmitter and receiver pair of periodic magnet transducers, similar to 
those disclosed in U.S. Pat. No 4,127,035, was mounted on the test pipe in 
the configuration shown in FIG. 2, where the transmitting transducer 10 is 
mounted on the pipe wall 14 20 cm from the longitudinal weld bead 28, and 
receiving transducer 20 is mounted on the wall 35 cm from the transmitting 
transducer. Using an operating frequency of 130 KHz, the transmitting 
transducer 10 generated horizontal shear waves which propagated 
circumferentially in both directions, as indicated in FIG. 2 by the solid 
lines 30 and 32. A reflected horizontal shear wave is indicated by the 
dashed line 34. 
Now referring to FIG. 3, a computer printout representing the shear wave 
amplitude detected by the transducer 20 for a nonflawed section of pipe is 
illustrated. The signal arriving at 150 .mu.sec corresponds to the SH wave 
which propagated directly from the transmitting transducer 10 to the 
receiving transducer 20 in a clockwise direction, as shown by line 30. Now 
referring to FIG. 4, a computer printout of the received SH wave amplitude 
is shown for a test run covering a flawed section of the pipe. As in FIG. 
3, the signal at 150 .mu.sec represents the directly propagated SH wave 
travelling in a clockwise direction. The signal at 260 .mu.sec, however, 
corresponds to the SH wave which propagated from the transmitting 
transducer in a counterclockwise direction (line 32), was reflected by a 
defect in the weld 28, and thence travelled to the receiving transducer in 
a clockwise direction. The arrival times of the two signals in FIGS. 3 and 
4 are consistent with the known propagation velocity of shear waves and 
the physical locations of the transducers relative to the weld in the 
test. 
Thus, the test demonstrates the remarkable result that a good weld bead is 
a very poor reflector of horizontally polarized shear waves. Presumably, 
the fact that the weld represents a symmetrical mass loading on the pipe 
wall causes it to interact very poorly with shearing distortions which are 
polarized in the plane of the pipe wall and parallel to the weld bead, 
such as are associated with an SH wave. The fact that the crack which 
caused the reflection at 260 .mu.sec in FIG. 4 was not symmetrically 
located relative to the center line of the pipe wall appears to cause it 
to be a strong reflector of horizontally polarized shear waves. 
To further verify the utility of this flaw detection technique, 
measurements were made of the amplitude of the SH wave signal reflected 
from the weld at each defect present in the test section of pipe and in 
the regions of good weld between the defects. Since the signal at 150 
.mu.sec can serve as a measure of the amplitude of the incident sound wave 
at any point, it was used to normalize the signal measurements. FIG. 5 
provides a plot of the relative signal amplitude which was obtained as a 
function of position along the test pipe. As can be observed from the 
plot, the defective weld locations stand out in sharp contrast to areas 
with a good weld. 
Now referring again to FIG. 1, an apparatus for practicing this invention 
is schematically illustrated. To begin a test sequence, the trigger 
circuit 36 is activated, causing the signal generator 16 to supply a high 
frequency pulse to the transmitting transducer 10. This pulse causes the 
transducer to generate a horizontally polarized shear (SH) wave, 
represented by the dashed line 12, in the pipe wall 14 travelling in both 
directions away from the transducer. The receiving transducer 20 detects 
the directly propagated portion of the generated SH wave and any reflected 
SH wave, as represented by dashed line 18. The resulting signal from the 
transducer 20 is amplified in the amplifier 22 and displayed by the 
indicating instrument 24. Instrument 24, which may typically be an 
oscilloscope, is triggered by the trigger circuit 36. A recorder 26 may 
also be provided, if necessary, to provide a record of the test results 
displayed by the instrument 24. The transducers 10 and 20 are preferably 
of the periodic permanent EMAT design, as disclosed in U.S. Pat. No. 
4,127,035. 
In conclusion, although typical embodiments of the present invention have 
been illustrated and discussed above, numerous modifications and 
alternative embodiments of the method of this invention will be apparent 
to those skilled in the art in view of this description. The invention may 
be of considerable utility, for example, in many applications which do not 
involve the testing of a pipeline. Accordingly, this description is to be 
considered as illustrative only and is provided for the purpose of 
teaching those skilled in the art the manner of performing the method of 
this invention. Furthermore, it should be understood that the forms of the 
invention depicted and described herein are to be considered as the 
presently preferred embodiment. Various changes may be made in the 
configurations, sizes, and arrangements of the components of the 
invention, as wil be recognized by those skilled in the art, without 
departing from the scope of the invention. Equivalent elements, for 
example, might be substituted for those illustrated and described herein, 
parts or connections might be reversed or otherwise interchanged, and 
certain features of the invention might be utilized independently of the 
use of other features, all as will be apparent to one skilled in the art 
after receiving the benefit attained through reading the above description 
of the invention.