Nondestructive testing of structural material by means of ultrasonics

Plate stock of uneven and variable thickness is tested for defects by means of ultrasonics whereby the zone adjacent the front surface and up to a depth of the minimum plate thickness is detected conventionally, but under utilization of a novel method the zone of variable thickness adjacent the rear wall is tested by detecting any echo in a gating period (15) that is returned from that zone or from the rear wall, and such an echo is used to set up a supplemental gating period during which either none or the rear wall echo will occur and in the latter case the setting up of the supplemental gating period is interpreted as having resulted from the presence of a defect (for example, echo 12) so that the occurrence of the rear wall echo 9 within the supplemental gating period is registered as an indication of a defect.

BACKGROUND OF THE INVENTION 
The present invention relates to ultrasonic testing of structural materials 
particularly of workpieces made of metal for the purposes of detecting 
internal flaws, defects, inclusions or the like. 
Sheet and plate stock, for example, is tested by launching ultrasonic test 
pulses perpendicularly to one surface and by searching for reflections of 
the ultrasonic signal, for example, by a defect. The sheet stock or plate 
may be so tested in its entirety and progressively over one surface 
thereby covering the entire interior of the stock. The return signals are 
usually processed by restricting the detection of echoes which occur 
within a certain period of time only because other echoes are deemed to 
have causes other than defects. Typically, for example, the so called rear 
wall echo will always occur as the result of the reflection o the 
ultrasonic pulse by the surface opposite the surface into which the test 
pulse was launched. In the case of a uniform wall thickness (i.e., in the 
case of sheet and plate stock of a uniform thickness) the time between the 
launching of the pulse into the workpiece and the occurrence of the 
rearwall echo is quite constant. Therefore, the period of time within 
which to look for echoes from defects in the interior of the workpiece is 
well defined. The situation is different, however, when the wall thickness 
varies intentionally or otherwise. In such a case, one has to consider the 
minimum wall thickness as the criteria defining rear wall echo timing and 
any echo that occurs after the minimum period has elapsed will be 
suppressed. This means that a portion of the sheet stock in locations 
where the wall thickness exceeds the minimum will not be tested. 
Alternatively, one can detect any echo whenever it occurs and evaluate the 
total transit time to thereby exclude detected rearwall echoes from echoes 
resulting from defects. The problem could be solved, for example, through 
follow-up systems in which in very small steps and on a progressive basis 
one keeps track of the rearwall echoes to thereby delineate the wall 
thickness, and echoes occurring within the variable period delineating the 
wall thickness can then be interpreted as defects. However, tracking the 
actually occuring wall thickness just for that purpose is complicated and 
expensive. Moreover, this matter requires a high resolution pattern for 
testing the workpiece as a whole without actual improvement in test 
sensitivity. One has also tried to include so called jump detectors in 
which echoes of sequential test cycles are compared under the assumption 
that the wall thickness varies gradually, and in the case of a jump in 
transit time of echoes that jump is interpreted as a defect. However, this 
is not a reliable method and in fact nonexisting "defects" may be 
indicated. 
DESCRIPTION OF THE INVENTION 
It is an object of the present invention to provide a new and improved 
method and equipment for nondestructive testing of structual material such 
as plate stock or the like having front and rear surfaces, but a 
nonuniform thickness under utilization of ultrasonic test pulses which are 
set into the front surface and under further ultilization of echoes which 
are being detected, converted into electrical signals and processed 
further. 
It is another object of the present invention to provide a new and improved 
method for nondestructive reproducible testing of structural material and 
workpieces of variable thickness under utilization of equipment which in 
the past permitted merely testing of zones within the material equal to 
minimal wall thickness. 
In accordance with the preferred embodiment of the present invention, a 
first gating and signal detection period is provided for echo signals 
which covers at least a period in which a rear wall echo signal will 
appear; any echo signal which occurs within that first gating period is 
detected. Such an echo signal may be a defect echo or a rear wall echo, in 
either case this detection is used for generating a second gating and 
detection period; and finally a rear wall echo is detected, if it occurs 
in the second gating period as an indication of the presence of a defect. 
It can thus be seen that the principal behind the invention is the 
intentional extension of a detection period for searching additionally for 
further away defects, but also for the rear wall echo. If an echo occurs 
within this preliminary detection period it cannot per se be identified as 
a defect echo or a rear wall echo but is used for the setting up of a 
further detection period. If no echo occurs within this supplemental 
detection period then the previously detected echo was the rear wall echo. 
If, however, within the supplemental detection period another echo (i.e., 
from the true rear wall) occurs then the previous echo was generated by 
reflection from a defect. That defect now is indicated indirectly by the 
supplemental detection of the rear wall echo. Moreover, it can readily be 
seen that the principal of the invention involves supplementing the 
existing circuitry such as ultrasonic test equipment whose use was 
previously restricted to the detection of flaws within the minimal wall 
thickness zone behind the front surface through which the test pulses are 
launched. The inventive principal extends the detection range further by 
means of equipment which simply can be added on to existing equipment. The 
original detection period should remain separate; the extension of 
detection covers the transit time equivalents of rear wall echoes of 
minimum and maximum thickness portions. The supplemental detection period 
is preferably equal in length to the extension period.

Proceeding now to the detailed description of the drawing, FIG. 1 
illustrates a piece of generally flat stock 1 having, however, variable 
wall thickness as represented here by an indent. The stock is 
nondestructively tested by ultrasonic test equipment which includes a test 
head 2 launching ultrasonic pulses into the material, possibly via a water 
column that bridges the physical input and output of the test head 2 with 
the solid material of which the workpiece 1 is made. The ultrasonic pulse 
launched by the test head 2 traverses the surface 3 of the workpiece 1 at 
right angles thereto. The test pulse propagates through the material and 
is reflected in part by the opposite surface 4 and returns as an echo 
signal into the test head 2 which in the meantime, for example, has been 
switched over from a transmitter mode to a receiver mode. 
It can readily be seen that in the position A of the testhead, the rear 
wall echo occurs relatively late. In the position B, however, the rear 
wall echo occurs significantly earlier. If one now assumes that the stock 
thickness and the bottom of the indent defines the minimum wall thickness 
of the sheet stock, the minimum being established by the level 5, and if 
one assumes further that echo detection by itself is restricted to echos 
occurring prior to the earliest possible rear wall echo (this condition 
exists as position B) then one can see that in the position C the defect 6 
will not be detected for the following reasons. The echo produced by the 
defect 6 occurs at a time that is earlier than the true rear wall echo 
will occur, namely a reflection on surface portion 4 when the testhead is 
in position C. However, the rule according to which rear wall echoes are 
to be excluded from the detection sequence, requires in turn that the test 
equipment be desensitized for periods after the earliest possible rear 
wall echo could have occured which is of course determined by the minimum 
wall thickness level 5. A defect situated in the material between that 
minimum wall thickness level 5, on one hand, and the true rear surface of 
the material to be tested will not be detected. 
Turning briefly to the timing diagrams of FIGS. 2, 3, and 4 it will suffice 
for the moment that the pulse diagram of FIG. 2 pertains to the position A 
of the testhead in FIG. 1. Typically, there will be a front wall or 
entrance echo 7 which follows immediately the launch, after the 
transmitted ultrasonic signal has traversed the distance between the 
testhead and the front surface 3. Reference numeral 8 delineates the range 
within which defect echoes are to be detected. A certain period thereafter 
elapses until the rear wall echo 9 arrives. At a somewhat later time, a 
second rear wall echo 11 arrives resulting from a reflection of the first 
rear wall echo in parts on the surface 3, back into the material and again 
by surface 4. 
FIG. 3 illustrates an analogous pulse and timing diagram but for the 
position B of the testhead 2 and here one can see that the rear wall echo 
9 appears immediately after the end of the detection interval 8. The 
timing distance or spacing between the two echo peaks 7 and 9 for this 
particular case defines, in terms of transit time difference, the minimum 
wall thickness which represents the restricted detection interwall. One 
can readily see that in the diagram of FIG. 4 the wall thickness has its 
normal value, but an echo peak 12 results from the reflection of the test 
pulse by the flaw or defect 6. That particular pulse will not be detected 
within the detection interval 8. We will return to these figures after 
explaining in detail the effect and operation of the inventive circuit 
which remedies the situation and makes it possible that a defect such as 6 
will in fact be detected. 
Briefly, the principle behind the invention is to use any echo that 
subsequently to the regular detection interval 8 to open up another 
looking window and detection period. If a rear wall echo occurs within 
that particular supplemental period, then the pulse which was detected 
previously and gave rise to the setting up of this supplemental detection 
period and looking window was a defect echo; if no echo occurs within the 
supplemental detection period and looking window then the pulse which 
resulted in the last mentioned setup was in fact the rear wall echo and 
there is no defect within the zone defined by the minimum wall thickness 
and the actual wall thickness. 
Proceeding now to the description of FIG. 5, a test piece such as a piece 
of plate stock of a conceivably irregular wall thickness is 
nondestructively tested by ultrasonic test pulses using a testhead 23 
being suitably coupled to the surface of the workpiece and in progressive 
portions thereto. The test piece has a front wall 24 and a rear wall 25. 
The testhead serves as a transmitter for ultrasonic pulses as well as 
receiver in alternating transmit and receive cycles, a pair of such cycles 
constituting a test cycle. The circuit accordingly has a transmit or 
transmitter branch and a receiver branch. The transmitter branch includes 
a trigger circuit 21 which may be suitably computer controlled in 
accordance with the particular test program and a timing operation 
commensurate with a test task at hand. That particular circuit 21 issues 
trigger pulses to a transmitter 22 proper, which provides a contoured 
signal for purposes of triggering and stimulating the transmitter portion 
of the transducer 23 for purposes of launching an ultrasonic test pulse 
into the test piece through the front wall 24. A representative example of 
a defect is shown near the rear wall. Ultrasonic pulses reflected for any 
reason including front and rear wall echoes as well as echoes on defects 
as they may occur, are returned into the transducer head 23 then being 
operated in the receiver mode and converting the ultrasonic vibrations it 
receives into an electrical signal to be passed on to a suitable amplifier 
26. The circuit 26 may be comprised of several stages and has accordingly, 
for purposes of impedance matching or the like, different output branches. 
One output circuit of the amplifier 26 leads to a display monitor 27, but 
that is not essential as far as practicing the invention is concerned. The 
second output branch 31 of amplifier 26 leads to a circuit 28 to be 
described in greater detail below with reference to FIG. 6 and including 
several gating circuits. These gating circuits are cyclically operated to 
some extent in response to particular signals derived from the amplifier 
26, but the detection cycle for each test cycle begins with a trigger 
pulse which is, of course, derived from the trigger circuit 21. Reference 
numeral 29 refers generally to a flaw or defect indicating or registrating 
device which in a more simple form may simply signal absence and presence 
of a defect signal, but the circuit 29 can be a bit more involved in 
nature and may include electronic processing and storage of signals 
representing defects in order to work a redundancy control etc. 
The particular gate circuit shown in FIG. 6 includes an input line 31 from 
which measuring signals are derived from amplifier 26 in FIG. 5. The 
signal is to some extent free from noise as can be obtained through proper 
response and threshold adjustment in amplifier 26, but basically the 
signals that arrive in line 31, are comprised of echoes that have been 
returned by the reflecting surfaces in the test piece. 
The circuit 28 is comprised of two threshold circuits 81 and 82 each 
receiving the test signal from line 31 and each including a comparator or 
differential amplifier being denoted 811, and 821 respectively for the two 
circuits 81 and 82; the line 31 being accordingly connected to one input 
each of these two comparators. The second input of each of these 
comparators receives reference signals, from the circuits 812, and 822 
respectively, which are individually adjustable in order to adjust the 
response level for the two threshold circuits 81 and 82 to different 
levels. For reasons below, it can be seen that the threshold adjusted by 
the circuit 812 for the circuit 811 is lower, possibly considerably lower, 
than the threshold adjusted by means of circuit 822 for the comparator 
821. The output of comparator 811 is fed to one input of a logic 
coincidence gate such as an AND gate 83 while the output of comparator 821 
is fed to a similar AND gate 84. 
AND gate 83 has a second input which can be regarded as an enabling signal 
or gating-open signal derived from a timing circuit 85. Circuit 85 
receives a trigger signal from the circuit 21 being indicative of the 
beginning of a test cycle and of the launching of a new test pulse by the 
transmitter 23. The timing signal provided by circuit 85 to the AND gate 
83 is an enabling signal of a particular duration which is indicated by 
reference numeral 15 in FIGS. 2 and 4. It is a timing signal which begins 
at an instant immediately succeeding the end of the detection period 8. 
Period 8 covers the portion of the plate stock defined by the front surface 
through which ultrasonic test pulses are launched and the level 
representing the minimum plate stock thickness (level 5 in FIG. 1). The 
period 15 is metered by the circuit 85 for use in the and gate 83; it 
begins when period 8 ends, and ends for example, at an instant measured 
from the time of triggering (circuit 21) and elapsing after a rear wall 
echo will occur for a maximum plate stock thickness (see FIG. 2). The 
period 15 thus covers, in terms of a transit time range equivalent, the 
variation or differential between minimum and maximum plate thickness. 
The immediate detection within the interval 8 is provided through a gate 88 
receiving a second timing signal from circuit 85 that begins shortly after 
the trigger pulse 21 or more precisely shortly after the front wall echo 
(7 in FIGS. 2, 3, and 4) has decayed and it ends when the period 15 
begins. Therefore, the And gate 88 is enabled during the period 8 (being 
in fact defined by this enabling state) and gate 88 with pass an output 
signal from the threshold device 81 which may occur as a result of a 
defect within the more frontal portion of the plate stock as defined. the 
gate 83 is enabled after gate 88 is disabled again and will pass 
subsequently occurring echo signals which, however, can be a rear wall 
echo or the echo from a defect such as 6 in FIG. 1. The output of AND gate 
88 is fed to one input of an OR gate 89 whose output is fed to the defect 
indicator 29. The output of AND gate 83 is processed otherwise. 
The trigger signal from circuit 21 is used in addition to reset a regular, 
set-reset type flip flop or latch 87. Any output signal from threshold 
circuit 81 that passes gate 83 is used to set the flip flop 87. The output 
of flip flop 87, particularly the set state or Q signal is used to trigger 
another timing circuit 86 which provides the supplemental time interval 10 
(see FIGS. 2, 3, and 4). The output of 86 is also in the form of a 
suitable gating signal for the AND gate 84. That particularly timing 
signal begins shortly after the flip flop 87 has been set by an echo pulse 
from gate 83 and in order to wait for the completion of that particular 
echo signal that caused circuit 81 to respond and to pass through the AND 
gate 83 and trigger or set flip flop 87. The time interval period 10 as 
provided by the circuit 86 may be of a like duration as the time period 8 
or, more appropriately, like the period 15. This timing and gating signal 
from circuit 86 is fed to the second input of the AND gate 84 whose first 
input receives the output of the comparator 821 as mentioned above. 
Therefore, the AND gate 84 constitutes a second input for the OR gate 89 
and passes its output on to the circuit 29 as another alternative way of 
signaling the presence of a defect. 
The circuit as described operates as follows during a particular test 
cycle. Circuit 21 provides a trigger pulse signal which begins a test 
cycle and that trigger signal gives rise to the launching of a ultrasonic 
test pulse by the transducer 23. In addition that trigger signal resets 
flip flop 87 and starts the timer 85. Shortly thereafter, the first timing 
signal is provided through the AND gate 88 and the detection period 8 thus 
begins. 
If a defect exists in a more frontal portion of the plate stock being 
tested an echo signal will appear during this period 8 and will pass 
though AND gate 88 and OR gate 89 tube registered in 29. As stated above, 
the period 8 is selected such that a rear wall echo cannot possibly occur 
within that detection period because the minimum wall thickness is such 
that a rear wall echo of the thinest portion still will arise a little 
after the period 8 has elapsed. Now, however, timing circuit 85 provides 
another enabling signal to the AND gate 83 for detecting later occurring 
echoes during the period 15. The echo signal that occurs during 15 in the 
situation depicted in FIGS. 2 and 3 is the rear wall echo. The rear wall 
echo when detected sets the flip flop 87 and the timer 86 begins to run 
beginning shortly after that rear wall has decayed and the gate 84 is 
enabled for the period 10. In the cases of test positions A and B (FIGS. 2 
and 3) no further pulses occur except a second rear wall echo at some 
later time, but that is delayed well beyond the expiration of supplemental 
detection period 10, therefore, no pulse passes gate 84 and a defect is 
not indicated. 
The situation is different in position C the timing being shown in FIG. 4. 
Again it is assumed that no defect exists in the frontal portion of the 
material so that the period 8 expires uneventfully. In the now ensuing 
period 15 during which gate 83 is open an echo from defect 6 appears and 
passes through gate 83 to set the flip flop 87, and timing circuit 86 
begins to furnish the detection interval 10 after this echo signal has 
decayed. the rear wall echo 9 will occur at some time thereafter. The rear 
wall echo 9 may pass also through the gate 83 which will still be open, 
but flip flop 87 is in the set state so that no further action occurs. The 
gate 88 is closed when the rear wall echo arrives so that the rear wall 
echo will not in fact go beyond the circuit 81-83. However, the rear wall 
echo is also fed to the comparator 821 of the threshold circuit 82. The 
threshold level for the comparator 821 may well be selected to be 
sufficiently high to suppress any other pulses except the relatively large 
rear wall echo. That output signal from the comparator 821 now passes the 
AND gate 84 and is fed through the OR gate 89 to the error detection 
circuit 29. The rear wall echo is used here as an indicator for the 
presence of a defect close to the rear wall of the plate stock to be 
tested. This way one does in fact detect the presence of defects in the 
zone which otherwise is excluded from the regular or normal detection 
carried out through the AND gate 88 as described. 
It is an important feature of the apparatus as described that in the 
conventional testing equipment the basic components are already included, 
such as a trigger circuit and the transmitter and launch circuit to 
operate the transducer, an amplifier for the detected signal as well as 
gating circuitry for receiving echo signals limited to a particular 
interval, such as interval period 8 as mentioned above. The inventive 
system (i.e., the realization of the inventive concept) can be carried out 
through supplementing circuitry which establishes additional time 
intervals for purposes of recognizing under certain circumstances the rear 
wall echo as a defect signal. 
The invention is not limited to the embodiments described above, but all 
changes and modifications thereof not constituting departures from the 
spirit and scope of the invention, are intended to be included.