GB2194051A

GB2194051A - Ultrasonic non-destructive testing apparatus

Info

Publication number: GB2194051A
Application number: GB08718811A
Authority: GB
Inventors: Richard Thomas Wilson; David Leslie Cotton
Original assignee: British Aerospace PLC
Current assignee: BAE Systems PLC
Priority date: 1986-08-15
Filing date: 1987-08-07
Publication date: 1988-02-24
Anticipated expiration: 2007-08-07
Also published as: GB8619912D0; GB2194051B; GB8718811D0

Abstract

Ultrasonic non-destructive test apparatus includes a B-SCAN probe 10 comprising a transducer element or a co-linear array of transducer elements each having an active face for transmitting ultrasonic signals in a direction normal to the plane of said face and towards a specimen 20 under test, a hollow probe housing comprising an elongated nozzle having a slot therethrough of a length substantially equal to the length of said transducer element or array and sealably secured to the perimeter of said element or array, and means 14 communicating with said slot for connecting said nozzle to a supply of fluid having a specific acoustic impedance matched to that of the material of the transducer element(s) and the material of the specimen 20 so as to optimise the transmission coefficient therebetween and for providing a continous fluid path for the ultrasonic signal between the transducer element(s) and the specimen 20 in the form of at least one fluid jet 19. In automated scanning apparatus with a pair of probe/nozzle assemblies 10, one head transmits and one receives ultrasonic energy. <IMAGE>

Description

SPECIFICATION Ultrasonic non-destructive testing apparatus This invention relates to ultrasonic non-destructive testing apparatus for detecting flaws in articles such as semi-finished products, raw materials and manufactured components and in particular to B-SCAN systems.

A B-SCAN ultrasonic non-destructive testing apparatus comprises a linear array of ultrasonic transducers, such as piezoelectric or quartz elements for converting eiectrical signals into acoustic ultrasonic signals, which is moved by hand over a specimen under test.

A multi-channel display or plotter, each channel of which is associated with a particular one of the transducer elements, displays or plots signals returned to each element from the specimen. The horizontal position of a signal return on the display or plot indicates the transducer element receiving it and the vertical position of the signal return on the display or plot indicates the depth with respect to the surface of the specimen, conventionally the top of the display or plot of the source of that return.A typical B-SCAN display of a parallel sided specimen with no defects will thus comprise two parallei horizontal lines, the upper of which represents the top surface of the specimen, which reflects a proportion of the ultrasonic energy transmitted to it, and the lower of which represents the bottom surface of the specimen, which reflects further proportion of the ultrasonic energy. A defect within the specimen will be indicated by a spot or line, depending on the shape and orientation of the defect, somewhere between the lines indicating top and bottom surfaces of the specimen, the horizontal position of which indicates which of the transducer elements in the array has received the defect return. The brightness of display, or the colour/intensity of the plot, is controlled by the amplitude of the return signals from various depths within the specimen.

In such known B-SCAN arrangements it is usual to use an acoustic coupling medium between the array of transducer- elements and the specimen. The purpose of the coupling medium is to optimise the ratio of the ultrasonic energy passing into the specimen to the incident energy from the array, and vice-versa for the return signals. This ratio, known as the transmission coefficient, is dependent on the specific acoustic impedances of the transducer material and the specimen material, respectively. If the values of these respective specific acoustic impedances are very close to one another, then the transmission coefficient approaches its maximum value of unity and the two materials are said to be well matched and the acoustic coupling between them is said to be good.

Where, for example, a quartz crystal transducer element transmits ultrasonic energy into a steel specimen then, unless the crystal and the steel surfaces maintain perfect contact with one another, which is extremely unlikely in practice, air forms the coupling medium.

The transmission coefficients for sound passing from quartz to air and from air to steel are of the order of 0.004% and 0.01% respectively. Consequently of the order of only 0.00004% of the original sound energy would be transmitted into steel. Such an enormous loss of energy is overcome by introducing a suitable coupling medium having a specific acoustic impedance more closely matched to that of the quartz and the steel. Typical coupling media are films of lubricating oil, water or glycerine. With water coupling the transmision coefficients are 40% and 15% for the quartz-water and water-steel transitions, respectively, so that of the order of 8% of the original energy is transmitted into the steel.

With suitable amplification in the receiver circuitry this provides sufficient energy available for accurate measurements.

Hitherto the requirement to use a film of suitable coupling medium in this way has made the B-SCAN system impractical to use in automated scanning systems. Nevertheless, because ultrasonic B-SCAN system is potentially capable of assessing quite large areas in a single sweep there are obvious advantages to be gained by automating the system.

It is an object of the present invention to provide an acoustic coupling arrangement suitabie for an automatic B-SCAN ultrasonic nondestructive test system.

According to the present invention, ultrasonic non-destructive test apparatus including a B-SCAN probe comprising a transducer element or a collinear array of transducer elements, the or each element having an active face for transmitting ultrasonic signals in a direction normal to the plane of said face and towards a specimen under test, a hollow probe housing comprising a nozzle having a highly elongated slot therethrough of a length substantially equal to the length of said transducer element or array and of a width which is less than that of said element or array, said nozzle being sealedly secured to the perimeter of said element or array, and means communicating with said slot for connecting said nozzle to a supply of fluid having a specific acoustic impedance matched to that of the material of the transducer element(s) and the material of the specimen so as to optimise the transmission coefficient therebetween and for providing a continuous fluid path for the ultrasonic signal between the transducer element(s) and the specimen in the form of at least one fluid jet.

Preferably, said slot length is an order of magnitude greater than said slot width, e.g.

10-30 times greater.

The nozzle may be made of a flexible or resilient material.

Advantageously, the probe and probe housing may be carried by support means and so as to provide a gap between a fluid outlet or said slot and the specimen.

Expediently, the support means may be moved automatically by, for example, electric motors so as to cause the B-SCAN probe automatically to scan a continuous section of the specimen.

Preferably, the nozzle and slot are designed to accept water as the coupling fluid. There may be two inlets for fluid located substantially on either side of the transducer element or array in the nozzle assembly.

The invention will now be described by way of example only and with reference to the accompanying drawings in which: Figure 1 is a perspective fragmentary view of a pair of ultrasonic B-SCAN probe and nozzle assemblies according to the invention in use; Figure 2a is a diagrammatic top plan view of one assembly according to Fig. 1; Figure 2b is a side view of one assembly according to Fig. 1 in the plane of a transducer array of the probe, and Figure 2c is a side view in a plane normal to the plane of the drawing of Fig. 2b; and Figure 3 is a schematic diagram of a further embodiment of a probe and nozzle assembly in use and testing a carbon fibre composite panel for defects.

Referring to the drawings, a B-SCAN probe head 10 includes a multi-element linear array transducer bank 12 of a length of e.g. 75 millimetres accommodated in a casing 3 of e.g. rubber.

The head 10 has a pair of water inlet pipes 14 communicating with a slot 15 within the head 10. The slot 15 extends from the transducer bank 2 to an outlet orifice 16. Thus the head 10 constitutes a nozzle. The slot 15 communicates with the transducer bank 12, and is of substantially the same length as the bank but its depth tapers from the depth of the transducer bank 12 at an initial section 1 7 of the slot 15 adjacent to the transducer bank 12 and the pipes 14 to a constant depth section 18, of e.g. 3 millimetre depth, leading to the outlet orifice 16. Electrical connections to the transducer bank 12 to carry both electrical transmission pulses and electrically received signals are made via multicore cable 22.The width of the slot 15 is arranged to be less than the width of the transducer array, typically 6.0 mm, so tht a collimating effect is achieved, producing a long, narrow beam of relatively uniform sound intensity, resulting in high sensitivity over e.g. a 0-40 mm thickness range of carbon fibre composite material.

Fig. 1 shows a pair of B-SCAN probe and nozzle assemblies in operation, disposed on opposite sides of a specimen under test, e.g.

a carbon fibre composite panel 20. Each head 10 is essentially similar but one is arranged for transmission of ultrasonic energy only while the other is arranged for reception of ultrasonic energy only. In this way, via two water jets or 'curtains' 19 serving as acousting couplants a continuous acoustic path for ultrasonic radiation is provided through the specimen 20. Each head 10 is carried by supporting arm 21 adapted to be driven by electro-mechanical means (not shown) so as to move the head 10 over the test specimen 20 automatically. The specimen 20 is thus scanned in e.g. 75 millimetre wide sections and the electro-mechanical means is arranged to ensure that successive scans are contiguous so that the whole test specimen 20 is tested non-destructively.

Fig. 3 differs from Fig. 1 only insofar as only one head 10 is provided, for testing the specimen 20 from one side only. The transducer bank 12 is adapted both for transmission of ultrasonic energy into the specimen 20 via the water jet 19 and for reception of the reflected energy from the specimen.

The head 10 is not in contact with the specimen panel 20 but is maintained by the supporting arm 21 at a constant gap of e.g. 25 millimetres from it.The transducer bank 12 is acoustically coupled to the specimen by the water 'curtain' or jet 19 fed from water inlets 14 by an automated water jet system (not shown). In use, a continuous acoustic water path between the transducer bank 12 and the test panel 20 is provided. The water jet 19 emerging from the outlet orifice 16 impinges on the panel 20 and acoustically couples ultrasonic energy transmitted by the transducer bank 12 into the test panel 20 and vice-versa.

The water jet 19 from the siotted head 10 provides a consistent ultrasonic coupling with good repeatability of results. Using the improved coupling large areas of specimen may be automatically inspected easily and rapidly.

Electrical output signals corresponding to transmitted or reflected ultrasonic energy from the surfaces of the specimen 20 and from its internal defects may be conveyed by cable 22 to a conventional B-SCAN display or plotter (not shown) or alternatively they may be stored for future analysis/display, for example on a video tape recorder not shown.

The invention also includes a non-illustrated embodiment wherein the linear array is replaced by a single transducer element. Nevertheless, a linear array has the advantage that each element or bank of elements can be pulsed sequentially to eliminated 'cross-talk' interference.

Claims

1. Ultrasonic non-destructive test apparatus including a B-SCAN probe comprising a transducer element or a collinear array of transducer elements, the or each element having an active face for transmitting ultrasonic signals in a direction normal to the plane of said face and towards a specimen under test, a hollow probe housing comprising a nozzle having a highly elongated slot therethrough of a length substantially equal to the length of said transducer element or array and of a width which is less than that of said element or array, said nozzle being sealedly secured to the perimeter of said element or array, and means communicating with said slot for connecting said nozzle to a supply of fluid having a specific acoustic impedance matched to that of the material of the transducer element(s) and the material of the specimen so as to optimise the transmission coefficient therebetween and for providing a continuous fluid path for the ultrasonic signal between the transducer element(s) and the specimen in the form of at least one fluid jet.

2. Apparatus according to claim 1, wherein said slot length is an order of magnitude greater than said slot width, e.g. 10-30 times greater.

3. Apparatus according to claim 1 or claim 2, wherein the nozzle is made of a flexible or resilient material.

4. Apparatus according to any preceding claim, wherein the probe and probe housing are carried by support means and so as to provide a gap between a fluid outlet of said slot and the specimen.

5. Apparatus according to claim 4, wherein means are provided for moving the support means automatically so as to cause the B SCAN probe automatically to scan a continuous section of the specimen.

6. Apparatus according to any preceding claim wherein there are two inlets for fluid located substantially on either side of the transducer element array in the nozzle assembly.

7. Apparatus according to any preceding claim, wherein a said probe is provided on each side of a generally planar specimen, the transducer elements of one probe being adapted for ultrasonic energy transmission only while the other probe is adapted for ultrasonic energy reception only.

8. Apparatus according to claim 1, sub stantialiy as herein described with reference to and as shown in Figs. 1 and 2 or Fig. 3 of the accompanying drawings.