GB2129936A

GB2129936A - Method of and apparatus for detecting an internal flaw in an article

Info

Publication number: GB2129936A
Application number: GB08312696A
Authority: GB
Inventors: Horst-Artur Crostack
Original assignee: GEWERTEC GmbH
Current assignee: GEWERTEC GmbH
Priority date: 1982-05-10
Filing date: 1983-05-09
Publication date: 1984-05-23
Also published as: GB8312696D0; FR2526546A1; DE3217530C2; FR2526546B1; GB2129936B; DE3217530A1

Abstract

An arrangement for detecting an internal flaw in an article, comprises a loading unit (2) for applying a variable load to a test piece (1) and an ultrasonic transmitter (3) for directing ultrasonic pulses into the test piece (1) via a test head (6). Ultrasonic pulses leaving the test piece (1) are superimposed on reference pulses in a computer-controlled unit (7) to produce a hologram. Ultrasonic pulses are input to the test piece under at least two different loads and the holograms produced under the different loads are superimposed to produce an interferogram using a computer unit (10). <IMAGE>

Description

SPECIFICATION A method of and apparatus for detecting an internal flaw in an article.

This invention relates to a method of and apparatus for detecting an internal flaw in an article.

Acoustic holography is extremely important in non-destructive testing, particularly for describing flaws inside structural components, because it allows the area in question to be represented three-dimensionally.

For this purpose ultrasonic signals are input to the component under test. The ultrasonic signals passing out of the component under test will have been effected by any flaw area and are superimposed on a reference beam which produces an interference field which is amplitude and phase dependent. The interference field is retained in the form of a halogram and produces the flaw picture in a subsequent mathematical or optical reconstruction.

Previously proposed method and apparatus used in acoustic holography have the following disadvantages: It is not possible to test thin components because, due to the shape of the ultrasonic pulses, which are usually cyclic sinusoidal oscillations, the pulse length is too large for the coherence length required.

Further, it is not possible to test materials which scatter sound waves, because the pulse length results in a relatively wide-band spectrum and there is accordingly dispersion in soundattenuating materials which changes the composition of the pulses and hence destroys coherence.

Also, the resolution is limited by the wavelength even under favourable conditions.

Difficulties arise even with easily tested materials because the resolution for the flaw contour drops with increasing aperture length. Because of the pulse length, the distance is relatively considerable, however, and the aperture length is small with the scanning operation usually carried out. Such a procedure is however timeconsuming and entails difficulties in respect of coupling, particularly in the case of components having complicated geometry and technically rough surfaces.

Another considerable disadvantage with this type of holography due not least to this reason is that only the absolute image of the flaw can be sensitively measured and not any changes or deformations. Although it is possible in this way to obtain a description of the flaw, it is not possible to detect its criticality. Analysis of the flaw risk requires not only the flaw geometry but also information regarding the toughness or state of deformation of the material at the flaw site which cannot be obtained with the previously proposed apparatus and methods.

It is an object of this invention to provide a method and apparatus for detecting an interior flaw in an article which overcomes or at least mitigates the disadvantages of the previously proposed methods and apparatus.

Accordingly to one aspect of the present invention, there is provided a method of detecting an internal flaw in an article, comprising: directing ultrasonic pulses into the article under first given conditions and super-imposing reference ultrasonic pulses on ultrasonic pulses leaving the article to produce a first hologram; directing ultrasonic pulses into the article under second given conditions and superimposing reference ultrasonic pulses on ultrasonic pulses leaving the article to produce a second halogram; and superimposing the first and second hologram to produce an interferogram. Conveniently, a load is applied to the article under both the first and second given conditions.

According to a second aspect of the present invention, there is provided a method of detecting an internal flaw in an article, comprising: directing ultrasonic pulses into the article under first given conditions in which a first load is applied to the article and superimposing reference ultrasonic pulses ultrasonic pulses leaving the article to produce a first hologram; directing ultrasonic pulses into the article under second given conditions in which a second different load is applied to the article and superimposing reference ultrasonic pulses on ultrasonic pulses leaving the article to produce a second hologram and superimposing the first and second holograms to form an inteferogram.

Preferably, the load applied to the article varies cyclically between the first load and the second load. Also the first load may be zero.

In a preferred arrangement, under the first conditions ultrasonic pulses directed into the article have 9 first frequency and under the second conditions the ultrasonic pulses directed into the article have a second different frequency.

Also, preferably, under the first given conditions the pulses are directed into the article from a first position and under the second given conditions the pulses are directed into the article from a second different position.

Generally, the phase difference between the ultrasonic pulses directed into the article and the reference pulses under the first given conditions is different from the phase difference between the ultrasonic pulses directed into the article and the reference pulses under the second given conditions. Conveniently, the method further comprises superimposing interferograms of different phases.

In a further aspect, the present invention provides apparatus for detecting an internal flaw in an article, comprising: means for directing ultrasonic pulses into the article; means for superimposing reference ultrasonic pulses on ultrasonic pulses leaving the article to produce a hologram; means for altering the conditions under which ultrasonic pulses are directed into the article from first given conditions to second given conditions; and means for superimposing a first hologram produced under the first given conditions on a second hologram produced under the second given conditions to produce an interferogram.

Conveniently, means are provided for applying a load to the article under both the first and second given conditions.

In another aspect, the present invention provides apparatus for detecting an internal flaw in an article, comprising: means for directing ultrasonic pulses into the article; means for superimposing reference ultrasonic pulses on ultrasonic pulses leaving the article to produce a hologram; means for applying a load to the article; means for altering the conditions under which ultrasonic pulses are directed into the article so that, in use, ultrasonic pulses are directed into the article under first conditions in which a first load is applied to the article and under second conditions in which a second different load is applied to the article; and means for superimposing a first hologram produced under the first conditions on a second hologram produced under the second given conditions.

Conveniently, control means are provided for actuating the ultrasonic directing means to direct ultrasonic pulses into the article when the load applying means applies the first or second load to the article and usually means are provided for delaying actuation of the ultrasonic pulse directing means by the control means by a preselectable time. Generally, the control means comprises adjustment means having a counter for counting the number of occurences of the first and second loads; comparator means for comparing the count produced by the counter with a preselected value and for delivering a pulse for actuating the ultrasonic pulse directing means when the count is equal to the preselected value.

Preferably, adjustment means are provided for adjusting the phase shift of the ultrasonic pulses directed into the article relative to the reference pulses.

In a preferred arrangement the condition altering means comprises frequency selector means so that, in use, the first given conditions are defined by ultrasonic pulses of a first frequency and the second given conditions are defined by pulses of a second different frequency.

Conveniently, the conditions altering means comprise position selector means so that, in use, the first given conditions are defined by a first position for the ultrasonic pulse directing means and the second given conditions are defined by a second different position for the ultrasonic pulse directing means.

For a better understanding of the present invention and to show how the same may be put into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a block diagram of apparatus embodying the invention; and Figure 2 illustrates graphically the principle of a method in accordance with the invention.

The method is based on the idea of determining the deformation properties of the material, and particularly the elastic-plastic transitions and any enlargement in the area of the flaw by detecting the deformation behaviour of the flaw under at test load means of acoustic interferometry.

Referring now the Figure 1, a component or test piece having a flaw in its interior is denoted by reference numeral 1. A load unit 2 is arranged to apply a test load either mechanically, electro magneticaily or thermally in the testpiece 1 either in the form of a single load or cyclically. Figure 2a of Figure 2 is a graph illustrating the load applied by a varying test load of period T.

An ultrasonic transmitter 3 is provided to generate ultrasonic pulses to be input to the test piece for application to the flaw area, which pulses are hereinafter referred to as flaw pulses IF, and reference pulses 1R which are to be superimposed on the flaw pulses output from the test piece in order to form holograms. The flaw pulses lf generated by the transmiter 3 are of short duration compared with the duration or period of the load applied to the testpiece and are optimized in respect of the test head and testpiece properties. The phase difference between each flaw pulse IF and the corresponding reference pulse 1R is controlled from one flaw pulse to the next to be a preselected amount, for example 7d or a different amount.Also, if required, a new interferogram having an offset phase can be superimposed. In this way the measurement takes account of acoustic noise due to the flaw surface, the material structure and the testpiece surface, and the noise is also reduced. The sensitivity is such that deformations of dimensions far below the wavelength of the ultrasonic signal used, for example amounting to one-thousandth thereof, can be detected. The wavelength of the ultrasonic signal is generally in the millimetre range. Deformations or extensions of these dimensions are not likely in the case of small flaws.

To provide the phase shift between two flaw signals, the load unit 2 controls a phase shift unit 4 in synchronism with the loading. The phase shift unit 4 is adjustable by an adjustment means 5 in respect of the amount of the phase shift. The phase shift unit 4 alters the phase of the flaw pulse IF provided with the ultrasonic transmitter 3 before the flaw pulses is introduced via a testhead 6 into the testpiece 1 under investigation. The phase shift and hence the relative phase between the flaw pulse IF and the reference pulse 1R is shown in Figure 2c. The amount of phase shift is adapted to the acoustic noise.

The reference pulse 1R may also be introduced by a test head 6' into the testpiece 1, wherein it is superimposed on the flaw pulse IF to form an interference field. A receiver unit 8, which may be an ultrasonic testhead or an interferometry unit, is provided to receive, in this case, the interference field resulting from superimposition of the flaw pulse IF and the reference pulse 1R in the testpiece 1.

The output from the receiver unit is input to a computer-controlled unit which forms the hologram. Alternatively, the reference pulse may be a purely electrical pulse in which case the reference pulse is input directly to the computer- controlled unit 7 for comparison with flaw pulses output from the testpiece and detected by the receiver unit 8 to produce the hologram.

Figure 2d of Figure 2 illustrates graphically the time of the optical sonic field reception. In Figure 2d, At2 denotes the transit time of an ultrasonic flaw pulse from the flaw to the testpiece surface and U the period of the ultrasonic oscillation. T is the interval of time between two load values governing the ultrasonic pulse application times, either between the extreme values of the load or between average or intermediate load values.

The ultrasonic pulse is triggered by means of the loading unit 2 via a delay unit 11 which delays the control signal from the loading unit by a predetermined amount Atl, which is equal to the interval of time during which the load has advanced from the surface of the testpiece to the location of the flaw. The load signal delayed in this way is fed to an adjustment unit 1 2 for determining the time interval T. The adjustment unit 12 selectively records either the extreme values of the loading or intermediate values thereof and feeds the same to a counter 1 3 which records the number of time intervals T elapsing.

The output of counter 13 is connected to one input of a comparator 1 5. An adjustment unit 14 set to output the number n of time intervals T required to give the interval of time between two ultrasonic pulse applications provides a signal to the other input of the other input of the comparator 1 5. When the number of intervals recorded by the counter 13 agrees with the number n preset by the unit 14, the comparator 1 5 delivers a pulse which triggers the ultrasonic transmitter 3 to produce a sonic pulse. The time sequence described above is illustrated in Figure 2b of Figure 2 for T=T/2 (T=period of oscillation of the loading) and n=1,3, 5;7. Simultaneously with the ultrasonic pulse triggering the counter 13 is re-zeroed and reset.

The respective holograms produced by the superimposition of reference ultrasonic pulses on the ultrasonic pulses leaving the article when the article is under a first given load and the ultrasonic pulses leaving the article when the article is under a second given load are optically reconstructed and superimposed in a reconstruction unit 9 either mathematically or after conversion (reduction based on wavelength differences). The resulting interferogram or interference pattern is evaluated in a computer unit 10 in respect of its streak configuration (streak curve, streak distances etc) and the resulting information is converted to movements or deformations of the flaw.

To ensure that the flaw location has the coherence required for holography in materials having a highly attenuating effect on sound, or thin-walled components, the transmission pulse for the investigation of such components or materials is so devised that the transmission properties of the material are allowed for so that, for example, it is possible to use pulses with similarly criteria or image functions. This is effected by analyzing a test pulse, the variation of which is measured and superimposed as a correction on the transmission pulse, which already has the required coherence.

The apparatus and method provide improved resolution and description of the flaw configuration and also eriable the deformation behaviour of flaws to be detected. Moreover, the apparatus and method can also be applied to thin materials and those which scatter sound. Thus, the apparatus and method allow the detection of even small movements and deformations of flaws, and their first enlargement when still not critical for the testpiece. In these conditions the sensitivity can be so increased as to detect movements which are just a fraction of a wavelength. In order further to reduce noise, it is also possible subsequently to carry out negative superimposition of two interferograms of the same object which are offset in phase.

If the loading applied to the test piece remains unchanged and instead the location at which the ultrasonic pulses are applied or the test frequency is changed between the pulses, the contour of the flaw can be distinguished in the form of altitude lines. The result is an interferogram which reacts sensitively to the frequency change or angle change of the pulses and which in addition to lateral extension also allows direct viewing of the depth extension of the flaw.

Using apparatus and/or a method embodying the invention not only the flaw itself, but also its movement and deformation are detected. In this way it is possible to estimate the flaw critically.

Moreover, the flaw deformation is determined directly at the flaw and hence independently of flaw depth. Any propagation of a crack can immediately be detected both in respect of direction and dimensions and areas of different deformability are detected according to the type of excitation.

It is also possible to carry out the test with guided waves, since the pulse length is not important in interferometry.

With optical reception of the interference field the accuracy of reproduction of the deformation picture is increased as a result of the large aperture and more accurate reception.

The influence of acoustic noise due to, for example, the shape of the flaw and the test piece surface, can be reduced by phase shifting, thus giving increased sensitivity. The high increase in sensitivity enables extremely small movements to be detected so that it is possible to assess flaw areas in materials having a high modulus of elasticity.

Although the main field of application of the above described method and apparatus is in the testing and monitoring of structural components and machinery, apparatus and the like for damage, and any progressive development of such faulty areas, they can also be applied to any other field in which the movement or deformation of an area within a body is to be determined by non-destructive testing.

Claims

1. A method of detecting an internal flaw in an article, comprising: directing ultrasonic pulses into the article under first given conditions and superimposing reference ultrasonic pulses on ultrasonic pulses leaving the article to produce a first hologram; directing ultrasonic pulses into the article under second given conditions and superimposing reference ultrasonic pulses on ultrasonic pulses leaving the article to produce a second halogram; and superimposing the first and second holograms to produce an interferogram.

2. A method according to Claim 1, wherein a load is applied to the article under both the first and second given conditions.

3. A method of detecting an internal flaw in an article, comprising: directing ultrasonic pulses into the article under first given conditions in which a first load is applied to the article and superimposing reference ultrasonic pulses on ultrasonic pulses leaving the article to produce a first hologram; directing ultrasonic pulses into the article under second given conditions in which a second different load is applied to the article and superimposing reference ultrasonic pulses on ultrasonic pulses leaving the article to produce a second hologram; and superimposing the first and second holograms to form an interferogram.

4. A method according to Claim 3, wherein the load applied to the article varies cyclically between the first load and the second load.

5. A method according to Claim 3 or 4, wherein the first load is zero.

6. A method according to any preceding claim, wherein under the first conditions ultrasonic pulses directed into the article have a first frequency and under the second conditions the ultrasonic pulses directed into the article have a second different frequency.

7. A method according to any preceding claim, wherein under the first given conditions the pulses are directed into the article from a first position and under the second given conditions the pulses are directed into the article from a second different position.

8. A method according to any preceding claim, wherein the phase difference between the ultrasonic pulses directed into the article and the reference pulses under the first given conditions is different from the phase difference between the ultrasonic pulses directed into the article and the reference pulses under the second given conditions.

9. A method according to any preceding claim, further comprising superimposing interferograms of different phases.

10. Apparatus for detecting an internal flaw in an article, comprising: means for directing ultrasonic pulses into the article; means for superimposing reference ultrasonic pulses on ultrasonic pulses leaving the article to produce a hologram; means for altering the conditions under which ultrasonic pulses are directed into the article from first given conditions to second given conditions; and means for superimposing a first hologram produced under the first given conditions on a second hologram produced under the second given conditions to produce an interferogram.

11. Apparatus according to Claim 10, wherein means are provided for applying a load to the article under both the first and second given conditions.

12. Apparatus for detecting an internal flaw in an article, comprising: means for directing ultrasonic pulses into the article; means for superimposing reference ultrasonic pulses on ultrasonic pulses leaving the article to produce a hologram; means for applying a load to the article; means for altering the conditions under which ultrasonic pulses are directed into the article so that, in use, ultrasonic pulses are directed into the article under first conditions in which a first load is applied to the article and under second conditions in which a second different load is applied to the article, and means for superimposing a first hologram produced under the first given conditions on a second hologram produced under the second given conditions to produce an interferogram.

13. Apparatus according to Claim 12, further comprising control means for actuating the ultrasonic directing means to direct ultrasonic pulses into the article when the load applying means applies the first or second load to the article.

14. Apparatus according to Claim 1 3 wherein means are provided for delaying actuation of the ultrasonic pulses directing means by the control means by a preselectable time.

1 5. Apparatus according to Claims 1 2 or 13, wherein the control means comprises adjustment means having a counter for counting the number of occurences of the first and second loads; comparator means for comparing the count produced by the counter with a preselected value and for delivering a pulse for actuating the ultrasonic pulse directing means when the count is equal to the preselected value.

1 6. Apparatus according to any one of Claims 9 to 15, wherein adjustment means are provided for adjusting the phase shift of the ultrasonic pulses directed into the article relative to the reference pulses.

17. Apparatus according to any one of Claims 9 to 16, wherein the condition altering means comprises frequency selector means so that, in use, the first given conditions are defined by ultrasonic pulses of a first frequency and the second given conditions are defined by pulses of a second different frequency.

1 8. Apparatus according to any one of Claims 9 to 17, wherein the condition altering means comprise position selector means so that, in use, the first given conditions are defined by a first position of the ultrasonic pulse directing means and the second given conditions are defined by a second different position for the ultrasonic pulse directing means.

1 9. A method of detecting an internal flaw in an article substantially as hereinbefore described with reference to the accompanying drawings.

20. Apparatus for detecting an internal flaw in an article substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings,

21. Any novel feature or combination of features described herein.