GB1589731A - Method for determining the thickness of a material between two boundary surfaces thereof - Google Patents

Description

(54) METHOD FOR DETERMINING THE THICKNESS OF A MATERIAL BETWEEN TWO BOUNDARY SURFACES THEREOF (71) We, YEDA RESEARCH AND DEVELOPMENT CO. LTD., an Israel Company of P.O. Box 95, Rehovot, Israel, do hereby declare the invention, for which we pray that a patent may be granted to us and the method by which it is to be performed to be particularly described in and by the following statement: This invention relates to a method for determining the thickness of a material between two walls thereof.

An important factor in the quality control of the building and construction industry is the type of materials that are used. Various methods have been devised and devices produced which are useful in detecting certain types of flaws and defects in pipe walls. One such method has been the utilization of ultrasonic waves to determine the composition of these pipe walls. Ultrasonic imaging is particularly well suited for this determination since it is quite sensitive to composition, density, elasticity and shape. However, prior known methods can determine these values only if certain parameters such as wall thicknesses, and the propagation velocity through the material to be tested, are already known.

In accordance with the present invention, there is provided a method for determining the thickness of a material between two boundary surfaces thereof, said method comprising the steps of: providing a reflecting surface in contact with a fluid medium having a known velocity of propagation; recording the position in time of the reflecting surface utilizing a transducer to create ultrasonic waves and echoes reflected off said reflecting surface; placing the material in the fluid between the transducer and the reflecting surface with the first of the boundary surfaces facing the transducer and the second of the boundary surfaces facing the reflecting surface; recording the positions in time of the echoes corresponding with the first and the second of the boundary surfaces and the position in time of the echo of said reflecting surface as modified by the presence of said material; and calculating the said thickness from the recorded position in time of said echoes.

For some purposes the said reflecting surface may be provided by a wall of a container for the fluid medium. When the material is piping, the first boundary surface is the outer surface of the piping, the second boundary surface is the inner surface of the piping and the reflecting surface is arranged to be positioned within the piping, the method of the invention may be applied to the determination of the wall thickness of the piping, eg. to detect local defects.

It is not necessary for the velocity of propagation within the material whose thickness is to be measured to be known. Such velocity, as well as the thickness of the material, can be calculated from the recorded positions aforesaid.

The present method may be applied without prior knowledge of the nature of the material. Thus the material can be a complicated alloy or a laminated material of arbitrary composition.

The following description in which reference is made to the accompanying drawing is given in order to illustrate the invention. In the drawling: Figure 1 is a block schematic diagram illustrating an apparatus for use in the method of the invention Figure 2 shows a sample output produced by the apparatus of Figure 1; and Figure 3 is a diagrammatic view of an alternative apparatus.

Figure 1 illustrates a device for measuring the velocity of propagation in, and the thickness of, a sample material 22 having a velocity of propagation V and thickness , placed in a vessel containing a fluid 30 having a velocity of propagation V0 between a transducer 10 and a back plate 24 which is a wall of the vessel and serves as a reflecting surface. The transducer 10 is electrically connected to a generating means 12 such as a pulse generator which produces a high voltage electrical pulse. This pulse is then converted into ultrasonic energy by the transducer 10 which transmits this energy through the fluid medium in the holder, and receives echo pulses when it strikes the back plate 24 and the inner and outer walls of the sample 22.

The transducer 10 converts the ultrasonic echo pulses into electrical pulses and displays them according to their positions in time upon a display means 14 such as an Automation Industry's Reflectoscope after they have been decoded by a processor 11. The reflectoscope itself may embody the processor 11 and the generator 12. An oscilloscope 16, such as a Tectronix monitor oscilloscope, is slaved to the reflectoscope 14 thus enabling the wave form to be photographed using a still or movie camera 18.

In order to determine the velocity of propagation or the wall thickness, a scan is first taken of the holder without the sample and then a scan is taken with the sample 22 inserted into the fluid medium 30 between the transducer 10 and the back plate 24. A sample display is shown in Figure 2 with ATt denoting the displacement between the inner and outer walls of the material 22, and AT2 denoting the position of the back plate with the sample 22 in place (reflected pulse 32) and the position of the back plate without the sample 22 in place (reflected pulse 34). If the backplate echo is displaced to the left when the sample is inserted, AT2 is negative; if the backplate echo is displaced to the right when the sample is inserted, AT2 is positive.

Referring to Figure 1, L is the distance between the transducer surface and the reflecting backplate. VO is the ultrasonic propagation velocity in the fluid 30. V is the ultrasonic propagation velocity in the sample and 6 is the thickness of the sample.

We define the time intervals AT, and AT, as follows; AT1 is the round-trip propagation time through the sample. as would be displayed on a Reflectoscope or other such similar device calibrated in suitable units such as microseconds per division. AT2 is defined as the round-trip propagation time for the transducer to the backplate with the sample, minus the round-trip propagation time from the transducer to the backplate without the sample in place.

From the above considerations, the round trip propagation time ATl through sample of thickness (' and propagation velocity V is simply AT1 2 (1) =V6 where the factor 2 arises because the sound makes a round trip through the sample.

Similarly, the factor AT can be derived. Without the sample between the transducer and backplate, the round-trip propagation is simply 2L/V0. With the sample in place, the ultrasound propagates through two media, the liquid medium and the sample. The round-trip distance traveled through the liquid medium is 2(L-#), at a propagation velocity Vo, and the roud trip distance traveled through the sample is 2#, with a propagation velocity V. The overall round-trip propagation time between the transducer and backplate with the sample in place is therefore 2(L-6) + 26 V,, V From the last two results. we found that the difference in round-trip propagation time between the transducer and backplate, with and with and without the sample in place, is 2(L-#) 2# 2L #T2 = | + | - = 2# (1/v-1/Vo ) (2) Vo V Vo Having found ATj and AT, in terms of the unknown quantities f and V and in terms of the known propagation velocity V" through the lost medium, we can calculate 6 and V from the measured values of AT, and AT.

From (1), 2t = VAT, . Substituting this into (2) gives #T2 = V#T1(1/V-1/Vo) = AT1 - AT1 V/VO Rearranging gives #T1 - #T2 #T1 = 1 - #T2/#T1 (3) V T1 we now obtain the value of sample thickness . From (1), # = 2 Substituting this in (3) gives l = Vo#T1(1-#T2/#T1) (4) 2 we now define #o = Vo#T1/2 (5) The physical meaning of (5) is that #o is the apparent thickness of the sample, as if the sample had a propagation velocity Vo instead of V.

The value of #o can be read directly from a Reflectoscope calibrated for a propagation velocity V". From (4) and (5) we obtain t = Co (1 - T2/T1) (6) This, then completes the derivation of equations 1-5.

The following equations then apply: #T1 = 2 /V# (1) 2(L-#) 2# 2L #T2 = + - (2) Vo V Vo = 2# (1/V-1/Vo) From equations (1) and (2) V/Vo = 1-#T2/#T1 (3) and t = C (1-#T2/#T1) (4) where C = VoATl/2 (5) For a linear display, the quantity AT2/ATl is the ratio of the displayed backplate echo displacement to the displayed sample thickness. If, in addition, the reflectoscope 14 is calibrated properly for the particular fluid medium 30, the term tO is read directly from the display (i.e., equation (5) is then performed by the reflectoscope itself).To avoid problems with multiple echoes, the transducer-sample distance should be larger than the sample-toback plate distance. On the other hand, the sample-back plate distance should be large enough so that multiple echoes originating within the sample do not overlap the displaced echo of the back plate 24.

An exemplary assembly of the present device is shown in Figure 3 for measuring pipe thickness and propagation velocity. This assembly shows the transducer 10 which is partly immersed in a holder 28 containing water as the fluid medium 30 which has a velocity of propagation of 1.48 x 102 m/sec. A polyvinyl chloride back plate 24 is used. Although many types of transducers can be used, it has been found that a 5 mHz flat transducer having a 1 cm beam width performed with quite satisfactory results.

The A-scan apparatus is activated and a measurement is taken of the back plate echo without the pipe 26 being placed between the transducer 10 and the back plate 24. The position of this echo is displayed on the reflectoscope 14 and is brought to a convenient location on its display screen. The partiocular reflectoscope which was used in this experiment was calibrated in terms of one division equalling x2 mm.

The pipe 26 is then slid into position between the back plate 24 and the transducer 10 and the new position of the back plate echo 32 and the position of the echo corresponding to the outer and inner walls of the pipe 26 are recorded. Sample readings are shown in Figure 2.

From these four echo locations, the velocity of the propagation ratio V/VO is ascertained.

Both ends of the pipe 26 having a wall thickness 6 of approximately 5 mm was used in this experiment with the results shown in Table I.

TABLE I First end of pipe 26 Measurement V/VI, f 1. 2.23 4.9 mm 2. 2.27 5.0 3. 2.23 4.9 4. 2.25 4.5 5. 2.24 4.7 Second end of pipe 26 Meaqsurement V/Vo 1. 2.13 5.1 2. 2.17 5.2 3. 2.17 50 4. 2.23 4.9 5. 2.()8 5.() 6. 2.13 5.1 This procedure was repeated for a number of points at each of the two ends of the pipe and the average velocity of propagation of the pipe and standard deviation were then calculated for cach of these two ends.

For end No.1, the average velocity ratio was V/Vo = 2.24 with a standard deviation #1 = 0.017. In absolute terms, since Vo = 1.48 = x 103 m/sec.. V1 = 3,321 m/sec. and # = 25 m/sec. Making the same calculations for end No. 2 of the pipe 26, V2/Vo = 2.15 (#2 = 0.051) and therefore V2 = 3,185 m/sec. (# = 75 m/sec.). Since V1 - V2 = 2#2, the difference in propagation velocity is statistically significant. Therefore it is shown that the velocity of propagation through inhomogeneous or heterogeneous pipe can be readily measured and the difference in propagation velocity of only a few percentages can be detected.

Accordingly, this type of velocity gaging can be useful in detecting certain types of flaws and defects in the pipe wall.

For flat samples. the far end of the sample can be placed against the backplate 24. In this case, the second sample echo and displaced backplate echo coincide. This may simplify the measurement procedure.

The present invention has numerous advantages. The apparatus employed in the method of the invention is simple, using for the most part conventional ultrasonic equipment.

Measurements of propagation velocity and thickness are accurate to 1 X" with a laboratory prototype and with an automated digital readout model measurements even more accurate.

Claims (4)

WHAT WE CLAIM IS:

1. A method for determining the thickness of a material between two boundary surfaces thereof, said method comprising the steps of: providing a reflecting surface in contact with a fluid medium having a known velocity of propagation; recording the position in time of the reflecting surface utilizing a transducer to create ultrasonic waves and echoes reflected off said reflecting surface; placing the material in the fluid between the transducer and the reflecting surface with the first of the boundary surfaces facing the transducer and the second of the boundary surfaces facing the reflecting surface; recording the positions in time of the echoes corresponding with the first and the second of the boundary surfaces and the position in time of the echo of said reflecting surface as modified by the presence of said material; and calculating the said thickness from the recorded positions in time of said echoes.

2. A method according to Claim 1 in which the reflecting surface is provided by a wall of a container for the fluid medium.

3. A method according to Claim 1 in which the material is piping, the first surface is the outer surface of the piping, the second boundary surface is the inner surface of the piping and the reflecting surface is arranged to be positioned within the piping.

4. A method according to Claim 1 for determining the thickness of a material between two boundary surfaces thereof, substantially as hereinbefore described and illustrated by reference to the accompanying drawing.