GB2046909A

GB2046909A - Ultrasonic inspection of welds

Info

Publication number: GB2046909A
Application number: GB8009408A
Authority: GB
Original assignee: Balteau Sonatest Ltd
Current assignee: Balteau Sonatest Ltd
Priority date: 1979-03-20
Filing date: 1980-03-20
Publication date: 1980-11-19
Also published as: GB2046909B

Abstract

Welds in workpieces are inspected ultrasonically by using an angle probe which generates transverse waves in the workpiece. Automatic adjustment of the A scan of a cathode ray tube (43) which displays the results is effected in order to zero the probe to compensate for the acoustic delay resulting from the stand-off material of the probe. The time base of the display is delayed automatically after calibration of the apparatus by using the probe on a test block of known pulse transit time. Zeroing can be effected by operation of a pushbutton (27). <IMAGE>

Description

SPECIFICATION Ultrasonic inspection of welds This invention relates generally to the inspection of welds by ultrasonic techniques.

Our co-pending UK patent application 8000383 is concerned with the measurement of the thickness of materials by the use of ultrasonic techniques. In making a measurement of the thickness of a piece of material, because the ultrasonic pulses have to traverse both the stand-off on which the crystal is mounted and also the material under test, it is necessary to subtract from the time interval between the excitation of the transmitter crystal and the reception of the first echo from the far side of the material under test a time period which is equal to the transit time of the ultrasonic pulses through the material of the stand-off. This subtraction results in a time period which is representative of the actual thickness of the material under test.

The method of thickness measurement described in the aforesaid application is designed for push-button control. In one mode of operation, the probe is placed on a "zero block" which has a known acoustic transit time, this transit time is duplicated electronically inside the measuring instrument, and the subsequent depression of a zero push-button by the operator causes the instrument automatically to adjust electronically for the acoustic delay resulting from the stand-off material.

It is an object of the present invention to make use of a similar technique in a different sphere of activity, namely in the ultrasonic inspection of welds. In the inspection of welds for flaws, one is not concerned with an absolute thickness measurement but with determining the size and especially the position of any flaws in the weld. It is consequently an object of the present invention to effect automatic adjustment of the scan of a cathode ray tube in order to zero the probe which is producing the scan pattern which determines the position of any weld flaws. By this means the zeroed CRT scan enables one to see immedately the true depth of a flaw relative to the weld surface, without the need to apply any correction factor or adjust the CRT controls.

In accordance with the present invention there is provided a method of inspecting a weld in a workpiece using an ultrasonic probe comprising ultrasonic pulse transmitting and receiving means mounted on an acoustic stand-off, the method comprising placing the probe on a test block which has a determined pulse transit time, duplicating said transit time as a first time-value signal, detecting the first return echo received by the pulse receiving means from the remote face of the block, setting up a second time-value signal representative of the time period from the transmission of a pulse by said pulse transmitting means to the receipt of the first such return echo arising from said pulse, using said first and second time-value signals to adjust the scan of a cathode ray tube to compensate for the acoustic delay time of the standoff material, and then placing the probe on the workpiece adjacent to the weld and displaying the zeroed time scan on the cathode ray tube.

Also in accordance with the present invention there is provided apparatus for inspecting welds in workpieces comprising an ultrasonic probe comprising ultrasonic pulse transmitting and receiving means mounted on an acoustic stand-off, presettable means to duplicate the transit time of such pulses through a test block on which the probe may be placed and to produce a first time-value signal representative thereof, means to generate a second timevalue signal representative of the time period from the transmission of a pulse by said pulse transmitting means to the receipt of the first return echo from the remote face of said test block, a cathode ray tube connected to receive the output of said pulse receiving means, and control means operative to use said time-value signals to adjust the scan of the cathode ray tube to compensate for the acoustic delay time of the stand-off material.

In order to describe fully the present invention and the disadvantages of the conventional methods of inspecting welds using ultrasonic probes, reference is now made to the following description and the accompanying drawings, in which: Figure 1 illustrates the use of an angle probe for inspecting welds; Figure 2 illustrates a modified technique, using the angle probe, for inspecting the upper portion of a weld; Figure 3 shows the type of CRT display conventionally obtained using the angle probe of Figs.1 and 2; Figure 4 shows a test block as used for calibrating the apparatus of the present invention; Figure 5 is a schematic timing diagram illustrating the method of the present invention; and, Figure 6 is a block schematic diagram of weld inspection apparatus in accordance with the present invention.

Although longitudinal probes may be used for inspecting welds, it is more usual to use what are known as angle probes which entail transverse wave inspection of the weld. One such angle probe is indicated generally at 10 in Fig. 1 and comprises a piezoelectric crystal 1 2 mounted at an angle on one plane surface of a wedge 14 of plastics material. The crystal 1 2 in fact consists of a transmitter crystal and a receiver crystal mounted side-by-side. In use, the probe 10 is placed on the surface of a workpiece 1 6 at a distance from the weld 18, and a beam of ultrasonic pulses is fired at the weld from the side at an angle as illustrated in Fig. 1.When the piezoelectric crystal 1 2 is excited it generates a longitudinal wave beam L in the wedge material which acts as an acoustic stand-off. At the boundary between the plastics wedge 14 and the workpiece 16 there is both reflection and refraction of the wave. The incident longitudinal wave L is reflected inside the wedge 14 and can be absorbed by providing acoustically absorbent material 20 on the outer surface of the wedge. The absorption of the reflected longitudinal wave can also be aided by using a wedge 14 having an appropriate geometrical shape.

A mode change also takes place at the wedge/workpiece interface, producing a refracted transverse wave T in the workpiece material. The wedge design and geometry are preferably chosen so that the transverse wave T occurs in the workpiece at an angle of between 20 and 45 to the surface of the workpiece. By moving the probe 10 backwards and forwards, away from and towards the weld 18, the transverse wave beam T will scan the depth of the weld and thus inspect for faults in the weld material.

Fig. 2 illustrates how the upper portion of the weld 18 is usually inspected. The probe 10 is moved further away from the weld 1 8 and the transverse wave T in the workpiece is bounced off the bottom surface of the workpiece and thence into the upper portion of the weld.

Fig. 3 shows the type of display which is conventionally obtained as an A scan on a cathode ray tube resulting from such a transverse wave inspection of the weld 1 8. The transmitter pulse Tx which excites the probe crystal 1 2 is indicated at the left-hand end of the scan, and a period of time base scan elapses while the longitudinal wave L traverses the plastics wedge 14. The time when the wave enters the workpiece 1 6 is indicated by the vertical broken line. The subsequent time base scan period represents the transverse wave travelling at an angle in the material of the workpiece. It should be noted that the velocity of the longitudinal waves in the plastics material differs by of the order of 20% from the velocity of the transverse waves in the material of the workpiece.

It will be appreciated that it is only the time period represented by the transverse wave T, i.e. the time subsequent to the time when the transverse wave enters the workpiece, which represents depth in the workpiece material, i.e. the information needed to determine the position and size of any flaws.

In carrying out the inspection of a weld, if one can zero the display then one achieves the important advantage of being able to avoid showing on the display the period when the ultrasonic waves are travelling through the plastics wedge. This is normally done by delaying the occurrence of the time base on the display, the adjustment conventionally being made manually. However, this is timeconsuming, requires a relatively skilled operator, and necessitates repeated operator adjustments if one is working under conditions where the pole characteristics are changing, as for example with high temperature workpieces.

The method of inspection according to the present invention utilises a test block such as is shown at 22 in Fig. 4. The test block 22 is an international standard block, for example of steel, which has to be manufactured to close dimensional tolerances. The material of the test block 22 is also closely controlled in order to ensure that the velocity of both transverse and longitudinally ultrasonic waves in the block is known to a high degree of accuracy. The test block 22 should have a reasonable surface for coupling to the probe 1 0. The radius of the one end of the block and the ultrasonic velocity in the material of the block must be interrelated in order to provide a known pulse transit time through the block which can be duplicated electronically.One end of the test block is shaped to a 100 mm radius, and in use the probe 10 is placed so that the exit point of the ultrasonic beam from the probe is at the centre of curvature of the arcuate surface. At the centre of curvature there is a small vertical slot 24 in the test block 22 to provide a corner reflector, so that once a transverse wave is excited in the test block 22 it is able to bounce back and forth between the corner reflector and the arcuate surface of the block to provide a pattern of multiple echoes at regularly spaced time intervals.

The A scan pattern which arises from placing the probe 10 on a test block 22 of this type is shown in Fig. 5. As Fig. 5 shows, after a time period t, following the excitation Tx of the transmitter crystal 1 2 an ultrasonic pulse leaves the face of the probe, i.e. of the standoff wedge 14, and enters the test block 1 5.

After a further time period t2 the transverse wave will have traversed the test block and the first return echo will re-enter the stand-off 14. After a further time period t3 the receiver crystal 1 2 picks up the first return echo E, from the remote curved face of the test block.

Successive return echoes E2, Es etc. of decreasing amplitude follow at equally spaced intervals of time t2.

The depression of a zero push-button can be used to activate an electrical circuit, and when the push-button is released the following process takes place. The circuitry incorporates an approximation circuit, as will be described in more detail later in relation to Fig.

6, which makes successive approximations of the position in time of the first return echo (E,) received from the far side of the test block 22. The approximation circuit is triggered from the rear edge of a preset monostable which duplicates the test block transit time, and the electronic circuitry thus produces the time period t, + t2 + t3 which is equal to the transit time t2 through the test block plus the acoustic delay time t, + t3 in the standoff 1 4. The approximation circuit approximates this time period t, + t2 + t3 to within about 3 nanoseconds. The test block transit time t2 which is known from the block dimensions and the ultrasonic velocity in the block material is duplicated electronically by the aforesaid present monostable.When the approximation process for the period t, + t2 + t3 is completed, the preset monostable which duplicates the test block transit time t2 is bypassed in order effectively to subtract this time period t2 from the total time period t, t2 + t3, thereby setting an internal time period t, + t3 which is equal to the acoustic delay time through the stand-off material 14, i.e.

only the approximated stand-off delay time remains in circuit. This time period signal is then applied to the control circuitry of a cathode ray tube to zero the scan display. The timing circuitry is triggered in coincidence with the pulse applied to the transmitter crystal 12, so that the internal time datum which is produced corresponds to the instant at which the pulse leaves the probe and enters the workpiece 1 6 under test, i.e. it corresponds with the probe zero (Pz). The approximation circuit is digitally controlled so that the stand-off delay time t, + t3 may be reproduced, i.e. "remembered", after the probe is removed from the test block.

The push-button probe zeroing process has a number of important advantages. It is speedy, and the results are not dependent upon the skill of an operator in adjusting a control mechanism. The zeroing process covers a wide range, thereby enabling the use of a large number of different probe sizes. In the case of a high temperature measurement, the probe has less time to cool down while the zero adjustment is being made, and because the adjustment is quick and easy the operator can make repeated zero adjustments in between carrying out actual measurements, the repeated adjustment meaning that more accurate results can be achieved.

Reference is now made to Fig. 6 which shows one example of test apparatus used with the probe in accordance with the invention. The timing circuitry is essentially based upon the provision of a probe zero monostable 1 5 and successive approximation circuits 1 7 which comprise a series of registers. The probe zero monostable 1 5 receives on its input line 1 9 an input pulse which is arranged to be coincident in time with the pulse Tx applied to the transmitter crystal 1 2 of the probe (Fig. 1). The probe zero monostable 15 is preset as referred to above to a value equal to the ultrasonic transmit time t2 through the test block.The pulse output from the monostable, shown as a rectangular pulse, equal in length to the aforesaid transit time, is provided on two output lines 23a and 23b, so that the triggering of subsequent circuitry can be initiated by the rear or the front edge of the pulse respectively. At the output side of the monostable 1 5 is a two-position timing switch 21 which also receives an input on line 25 from a control logic circuit 26. This control logic circuit 26 incorporates a zero pushbutton 27. Connection is made between the control logic circuit 26 and the start and reset terminals of the approximation circuits 1 7 on lines 29 and 30 respectively.The control logic circuit 26 is triggered by an end-ofconversion signal (EOC) rom the approximation circuits 1 7. A clock 31 provides clock pulses on line 32 to the approximation circuits 17.

Associated with the approximation circuits 1 7 is a coarse range adjustment monostable 33 and a fine range adjustment monostable 34. The coarse range monostable 33 is controlled by seven bits from the approximation registers and the fine range monostable 34 is controlled by nine bits from the approximation registers. The output from the timing switch 24 is also fed through to the coarse range monostable 33. The output from the fine range adjustment monostable 34 is fed to the clock input of a time comparison latch circuit 35 which also receives a data input on line 36 from an RF amplifier 37 which amplifies the echo signals E,, E2, ... picked up by the probe and produces a digital output representative thereof.The output of the time comparison latch circuit 35 is fed back on line 38 to the data input of the approximation circuits 1 7.

The output from the fine range monostable 34 is also taken on line 39 to a range memory latch circuit 40 whose output (a time signal representing an approximation to the stand-off delay time Pz) is fed to an output circuit 41 which also receives an input on line 42 from the RF amplifier 37 in the form of a signal coincident in time with the first echo signal E,. The output of the output circuit 41 is taken to the A scan circuit of a CRT 43.

The range memory latch circuit 40 enables the aforesaid approximation time signal to be repeatedly fed to the instrument output 41 during the test procedure after the probe has been removed from the test block.

In operation, when the zeroing push-button 27 is depressed and then released the successive approximation circuits 1 7 begin their conversion. The time between the excitation of the piezoelectric transmitter crystal Tx and the first return echo E, is approximated on the two monostable circuits 33, 34 controlled from the successive approximation registers 1 7. As the coarse range adjustment monostable 33 is controlled by seven bits and the fine range adjustment monostable 34 by a further nine bits, this enables the total time from Tx to E1, i.e. t, + t2 + t3 in Fig. 5, to be electronically duplicated to a sixteen bit approximation. A time change represented by the least significant bit of the fine range monostable 34 is about 3 nanoseconds, corresponding to a distance resolution of about 0.01 mm in steel.

Each of the sixteen bits is adjusted in sequence and the total time produced by the preset probe zero monostable 1 5 and by the coarse and fine range monostables 33 and 34 is compared in the time comparison latch circuit 35 with the arrival time of the digital waveform from the RF amplifier 37 which occurs coincident with the echo waveform E,.

The time comparison latch circuit 35 makes a decision based upon this comparison and provides an output on line 38 which is fed back as data to the approximation circuits 1 7 so that the approximation circuits can decide whether the bit under test should be retained or removed. The data on line 38 depends upon the value of the data input to the circuit 35 when the fine range monostable 34 resets.

When the zero adjustment push-button 27 is actuated the control logic 26 changes the triggering of the coarse range monostable 33 from the rear edge to the front edge of the pulse from the preset probe zero monostable 1 5 by means of the two-way timing switch 21 upon receipt of an end-of-conversion signal (EOC) from the approximation circuits 1 7. In other words the delay introduced by the probe zero monostable is removed and the monostable 1 5 is in effect by-passed. This automatically causes the A scan of the CRT to be zeroed to compensate for the delay introduced by the material of the probe wedge. Weld inspections can then be carried out using the probe, with the A scan display now being referred to the compensated datum. Re-zeroing of the probe, or of another probe, can be carried out as necessary.

Claims

1. A method of inspecting a weld in a workpiece using an ultrasonic probe comprising ultrasonic pulse transmitting and receiving means mounted on an acoustic stand-off, the method comprising placing the probe on a test block which has a determined pulse transit time, duplicating said transit time as a first time-value signal, detecting the first return echo received by the pulse receiving means from the remote face of the block, setting up a second time-value signal representative of the time period from the transmission of a pulse by said pulse transmitting means to the receipt of the first such return echo arising from said pulse, using said first and second time-value signals to adjust the scan of a cathode ray tube to compensate for the acoustic delay time of the stand-off material, and then placing the probe on the workpiece adjacent to the weld and displaying the zeroed time scan on the cathode ray tube.

2. A method as claimed in claim 1, in which a transverse wave is transmitted through the test block and workpiece and the display is on the A scan of the cathode ray tube.

3. A method as claimed in claim 2, which includes transmitting the ultrasonic pulses from an angle probe movable towards and away from the weld to scan the depth of the weld.

4. A method as claimed in any preceding claim, in which the first time-value signal is subtracted from the second time value signal upon operation of a push-button to adjust the CRT scan for the acoustic delay time of the stand-off material.

5. A method as claimed in any preceding claim, in which said second time-value signal is obtained by making successive approximation of the position in time of said first return echo.

6. Apparatus for inspecting welds in work.

pieces comprising an ultrasonic probe comprising ultrasonic pulse transmitting and receiving means mounted on an acoustic standoff presettable means to duplicate the transit time of such pulses through a test block on which the probe may be placed and to produce a first time-value signal representative thereof, means to generate a second timevalue signal representative of the time period from the transmission of a pulse by said pulse transmitting means to the receipt of the first return echo from the remote face of said test block, a cathode ray tube connected to receive the output of said pulse receiving means, and control means operative to use said time-value signals to adjust the scan of the cathode ray tube to compensate for the acoustic delay time of the stand-off material.

7. Apparatus as claimed in claim 6, in which the probe is an angle probe comprising pulse transmitting and receiving means mounted on a face of a workpiece-contacting wedge which lies at an inclined angle to the workpiece surface.

8. Apparatus as claimed in claim 7, in which the angle probe is such that it generates a transverse wave in the workpiece at an angle of 20 to 45 to the surface of the workpiece.

9. Apparatus as claimed in any of claims 6 to 8, in which the test block has a rounded portion of constant radius and the probe is placed on the test block with the pulse transmitting zone of the probe surface at the centre of curvature of said constant radius portion.

1 0. Apparatus as claimed in any of claims 6 to 9, in which said control means comprises a push-button, operation of which zeroes the probe for the acoustic delay time of the standoff material.

11. Apparatus as claimed in any of claims 6 to 10, in which said presettable means comprises a monostable circuit.

12. A method of inspecting a weld substantially as hereinbefore described with reference to the accompanying drawings.

1 3. Apparatus for inspecting welds substantially as hereinbefore described with reference to the accompanying drawings.