GB2124378A

GB2124378A - Non-destructive ultrasonic testing

Info

Publication number: GB2124378A
Application number: GB08319309A
Authority: GB
Inventors: Dr Ulrich Opara
Original assignee: Krautkraemer GmbH and Co; Krautkraemer GmbH
Current assignee: Krautkraemer GmbH and Co; Krautkraemer GmbH
Priority date: 1982-07-23
Filing date: 1983-07-18
Publication date: 1984-02-15
Also published as: GB8319309D0; FR2530817A1; DE3227521A1; JPS5934148A; DE3227521C2

Abstract

Echoes received from a test head are amplified by means of an amplifier, the gain factor of which depends upon a depth compensation signal which allows for attenuation of the ultrasound at any time. Synchronization of the individual units of the apparatus is effected by the first echo signal reflected by the surface of the test piece. In order to allow for the influence of the coupling liquid on the sonic field distribution in the test piece, a depth compensation signal A(t) is fed to the controllable amplifier after reception of the first surface echo. The starting value A(to) of the depth compensation signal is equal to the value T(to) of a depth compensation function T(t) derived at the time when the surface echo occurs, the depth compensation function T(t) starting with the transmission pulse (13) and allowing for attenuation of the ultrasound in the coupling liquid. <IMAGE>

Description

SPECIFICATION Improvements in ultrasonic testing It is conventional in non-destructive ultrasonic testing for a test piece to be immersed in a coupling liquid through which pulses of ultrasonic energy are transmitted to the test piece from a transducer. Echo pulses reflected from the test piece are received by the same (or another) transducer, and converted into electrical signals which are amplified by an amplifier controlled in dependence on a depth compensation signal which allows for the sonic field distribution of the test head used at any time. The transducer will receive a number of individual echo pulses, some of which will be produced by the surfaces of the test piece, and others of which will be produced by flaws within the test piece.

Various methods and devices operating in accordance with these principles are known from the literature. For example, Federal German Offenlegungsschrift No. 26 23 522 describes a method of non-destructive material testing in which the main amplifier amplifying the ultrasonic signals is controlled in dependence on time. The functional curve of this control signal takes into account both the sonic field distribution typical of the test head and also the attenuation of the ultrasonic signal due to the testpiece. The main disadvantage of this method is that the control signal does not take into account the effect of the coupling medium on the distribution of the sonic field in the test material.

In German Auslegeschrift No. 11 57 412 it is proposed to use the first of the echos reflected by the surface of the test piece to trigger the ultrasonic device. Again, the effect of the initial zone on the sonic field distribution in the test material is not taken into account in the control of the reception amplifier.

The method and device proposed herein allow the influence of the coupling liquid to be take into account in the depth compensation, so taking account of the specific sonic field distribution of the test head.

In the drawings: Figure 1 is a block schematic diagram of an ultrasonic testing device, Figures 2a and 3a each depict an echo diagram obtained from an arrangement as shown in Figure 1 with a different distance between the test head and the test piece, and Figures 2b and 3b are the depth compensation curves associated with Figures 2a and 3a.

Referring to Figure 1, a test piece 3 having flaws 20 is disposed at the bottom of a bath 1 containing water 2 as a coupling medium for coupling the test piece to an ultrasonic test head 4. The test head incorporates a transducer for converting electrical pulses into ultrasonic energy, and vice-versa. The electrical pulses are produced by a trigger 5 driving a transmitter 6 connected to the test head. Also connected to the test head is a controllable amplifier 7 for amplifying the signals received back from the test head, and transmitting the amplified signals to a cathoderay tube 8 for display. The time base for this cathode ray tube is produced by means of a sawtooth generator 9. A first control input 71 for the amplifier 7 is connected to a second amplifier 10 for amplification and pulse-shaping of the surface echo.The output from the amplifier 10 is also used for synchronizing signals to the sawtooth generator 9, amplifier 7 and a depth compensation function generator 11.

The depth compensation function includes a digital/analog (D/A) converter 111, the output of which is connected to the control input 72 of the amplifier 7 for the purpose of varying the gain of the amplifier. The input of the D/A converter 111 is connected to a (programmable) constant store 112, which stores the depth characteristic, the values of which are read out by means of a controllable clock generator 1 13. The generator 1 13 is connected to two coding elements 1 15 and 1 16 through an electronic switch 114. The coding elements 1 15, 1 16 may be, for example, variable resistors connected to a voltage supply 1 17 and are used to adjust the clock frequency.

The electronic switch 1 14 is driven from a flipflop 1 18, the setting input S of which is connected to the trigger 5 and the resetting input R to the amplifier 10.

The operation of the device shown in Figure 1 will now be described in detail with reference to Figures 2 and 3.

The pulses periodically generated by trigger 5 cause the transmitter 6 to generate electrical transmission pulses which in turn cause the test head 4 to generate corresponding sound pulses.

Each sound pulse passes through the coupling liquid and impinges upon the test piece. An echo is reflected from the top surface of the test piece 3. A subsequent echo is reflected from the flaw 30. Both echos are received by the test head and converted into respective electrical pulses.

An echo pulse train as shown diagrammatically in Figures 2a and 3a is thus provided at the inputs to the amplifiers 7 and 10, the shape of this pulse train depending inter alia upon the distance between the test head and the test piece as depicted in those Figures. The transmission pulse 13, which approximately coincides in time with the trigger pulse reaches the amplifier inputs first, followed by the surface echo 14, 1 6 and the fault echo 15, 17.

On receiving the surface echo 14, 16 the amplifier 10 generates a pulse which is fed through conductor 12 both to the sawtooth generator 9 and to the first control input 71 of amplifier 7, which it opens. All of the following pulses are amplified in accordance with the depth compensation signal at the control input 72 and are displayed on the screen of the tube 8.

The depth compensation signals at the input 72 of the amplifier 7 are generated as follows: The pulse from trigger 5 causes the flip-flop 1 18 to set the switch 1 14 to connect the clock generator 113 to the coding switch 1 15 which is set to the speed of sound in the coupling liquid (usually water). The clock generator 1 13 then delivers a specific pulse train, the frequency of which is proportional to the square of the speed of sound to which the switch 1 15 is set.

The depth characteristic values stored in the constant store 112 are read out successively in time with the pulses from the clock generator 1 13 and, after D/A conversion, are applied as analog values to amplifier 7.

When the surface echo signal 14, 16 is fed to amplifier 10, the output of this amplifier causes the flip-flop 118 to reset the switch 114 to the element 1 16, the setting of which corresponds to the speed of sound in the test material. The clock frequency of generator 11 3 increases. since the frequency is in turn proportional in square law relationship to the speed of sound for the test material and this is greater than the speed of sound in the coupling liquid. The depth characteristic compensation values are therefore read out of the constant store 112 at a correspondingly faster rate and in turn pass to the' input 72 of amplifier 7 through the D/A converter 111.

If, for example, the coupling liquid used is water (CH2o=1 500 m/sec) and the clock frequency fH2o is 0.24 MHz when the switch 114 is connected to the coding element 11 5, the clock frequency fst for a test piece 3 of steel (Cst=5 900 m/sec) must be:

Figures 2b and 3b show two examples for the curve of the depth compensation voltage at the amplifier 7 against time. The curves I, I' each show the functional course of the depth characteristic compensation curve T(t) for the coupling liquid. The curve II illustrates the depth characteristic compensation curve A(t) for the test material.The starting point of curve A(t) coincides with the curve T(t) at the time to i.e., at the time when the surface echo 14, 1 6 occurs.

The depth compensation curve stored in the constant store 112 can be calculated mathematically as already known, or be measured by means of appropriate standard reflectors, e.g. circular disc reflectors. In these conditions a function F (S) is usually obtained, where F denotes the measured echo amplitude and S the distance between the test head and the standard reflector. If this curve is related to the short-range field length N of the test head used at any time, where D2 D2f N= = 4n 4c D=diameter of test head oscillator, and c=speed of sound in the material and f the test head frequency and because the amplifier 7 is controlled in dependence upon time, and not location then, taking S=c. t, we have:

The material dependency is thus included in the depth compensation function in proportion to the square of the speed of sound, so that a correct depth compensation must take this factor into account in the time scale in the event of a change of the speed of sound.

Claims

1. A method of non-destructive material testing by the ultrasonic pulse reflection method, in which a liquid is used to couple the test piece to a test head, and in which the echo signals are amplified by means of an amplifier, the gain of which is controlled in dependence on a depth compensation signal A(t) which takes into account the sonic field distribution at any time, and is fed to the amplifier after reception of the first surface echo, the starting value A (to) of said depth compensation signal A (t) being equal to the value T (to) of a depth compensation function T (t) at the time when the surface echo occurs, said depth compensation function T (t) starting with the transmission pulse and allowing for the sonic field distribution of the test head in the coupling liquid.

2. A method according to claim 1, wherein: (a) the depth compensation function T (t) for the liquid zone and the corresponding function A (t) for the test piece are derived from a common depth compensation function stored in a constant store and standardized to the close-range length of the test head, (b) on occurrence of the transmission pulse and until reception of the surface echo the function values stored in the constant store are read out at a frequency proportional to the square of the speed of sound in the coupling liquid, (c) on reception of the surface echo, the other function values stored in the constant store are read out at a frequency proportional to the square of the speed of sound in the test piece, and (d) the digital function values each read out of the constant store are fed to the controllable amplifier via a D/A converter.

3. A method according to claim 1, in which the controllable amplifier receives the depth compensation signal from a D/A converter, the input of which is connected to a constant store containing the depth characteristic function, and in which the stored function values are each read out according to the pulses of a clock generator connected to the constant store, the clock generator being controllable in respect of the pulse train generated, setting elements being connected in series with the clock generator in order to control the clock frequency, said elements being used to set the speeds of sound in the test piece and the coupling liquid, only one of the setting elements being connected to the clock generator at any time through the agency of a switch; and the change-over of the switch being operated by the transmission pulse and the surface echo.

4. A depth compensation generator for ultrasonic testing apparatus comprising a digitalanalogue converter for controlling an amplifier of the testing apparatus, the input of the converter being connected to a store containing a depth characteristic function to be read out as the store is pulsed by a clock generator connected to the store; the clock generator being controlled from setting elements presettable with the speed of sound in coupling medium and the material of a sample under test, and means for connecting the clock generator selectively to the setting elements in response to pulses from the apparatus.

5. A method of testing substantially as described with reference to the drawings.

6. Apparatus as described with reference to and as illustrated in Figure 1.