GB2269672A

GB2269672A - Detecting discontinuities in composite components

Info

Publication number: GB2269672A
Application number: GB9217277A
Authority: GB
Inventors: David Paul Rowse; Colin David Sillence; Christopher Budd
Original assignee: British Aerospace PLC
Current assignee: BAE Systems PLC
Priority date: 1992-08-14
Filing date: 1992-08-14
Publication date: 1994-02-16
Anticipated expiration: 2012-08-14
Also published as: GB2269672B; GB9217277D0

Abstract

Apparatus 14 detects the presence, presence and location, or absence of points of discontinuity within the composite component 12. The component comprises an insulating layer 28 sandwiched between two conductive layers 22 of the composite component, which may be a carbon fibre reinforced material. A transmitter 16 launches on electromagnetic wave into the insulating layer and means 18 receives the electromagnetic wave from the layer 28, the received wave being composed at 20 with a known wave so as to detect any difference in the two signals due to partial reflection at any point of discontinuity in the layers. The signal is launched into the insulating layer by connected equal and opposite currents WT+, WT- to connectors 30, 32 on the conducting layers, the combination of layers functioning as a waveguide. The composite component may be an aircraft wing. <IMAGE>

Description

A DETECTING APPARATUS The present invention relates to a detecting apparatus for detecting the presence, or presence and location of, or absence of points of discontinuity within a component and relates particularly, but not exclusively, to such a apparatus for use with a composite component.

When a component such as a carbon fibre composite (C.F.C.) panel is loaded, internal stresses can be generated which may cause a number of failure mechanisms to occur. These stresses may cause individual fibres, which are under tension, to fail.

This process may be repeated, resulting in the generation of transverse or longitudinal cracks, or even delamination within the panel. The likelihood of damage is increased under cyclical loading as experienced in, for example, an aircraft wing.

C.F.C. panels are manufactured by sandwiching layers ('plies') of roughly parallel carbon fibres (each of which is a good electrical conductor) in a matrix of insulating resin. In a typical aircraft application each ply is 125pm thick, has fibres running at 45 degrees to those in each of its neighbouring layers, and is in electrical contact with each neighbouring layer.

Unlike conventional materials, CFC panels tend to give little or no advanced warning of structural failure. Sudden structural failure of a CFC aircraft wing member could result in the loss of the aircraft and the loss of the lives of any passengers and crew on board.

It is an object of the present invention to provide a detecting apparatus for detecting the presence or presence and location of, or absence of points of discontinuity within a composite component thereby to give advanced warning of possible structure failure.

Accordingly the present invention provides a detecting apparatus for detecting the presence, or presence and location of, or absence of points of discontinuity within a component having a plurality of first electrically conductive layers each of which are in electrical contact with a next adjacent layer, the detector comprising a second non-conductive layer sandwiched between two of said conductive layers, a transmitter for transmitting an electromagnetic wave into said second layer, a receiver for receiving said electromagnetic wave from said second layer and a comparison means for comparing said received electromagnetic wave with a pre known wave so as to detect the presence, or presence and location of, or absence of points of discontinuity within the component.

It will be appreciated that such a detecting apparatus could be used as an in life monitor of structurally important aircraft components in order to anticipate structural failure and hence enable repairs to be effected before failure takes place.

Advantageously, the transmitter is configured to transmit a pulsed transverse electromagnetic wave.

Advantageously, the detecting apparatus includes a current generating means for generating equal and opposite currents for inserting into said second layer so as to generate an electrical field in said second layer.

Preferably the transmitter comprises one of said conductive layers between which said insulating second layer is sandwiched.

Preferably, the detecting apparatus comprises one or more ohmic, capacitative or conductive electrical contacts at an edge of the component, said one or more contacts being electrically connected to one or more of said conductive layers for enabling external electrical connections to be made to individual conductive layers.

Advantageously, said detecting apparatus is configured to compare said received electromagnetic wave with a previously received electromagnetic wave, a predicted electromagnetic wave or a mathematically modelled electromagnetic wave.

Alternatively, the detecting apparatus may be configured to compare the received electromagnetic wave with the transmitted wave.

Carbon fibres may be used in the conductive layers.

In an alternative embodiment of the present invention there is provided a method of monitoring the structural integrity of a composite component of the type having a plurality of first conductive layers each of which are in electrical contact with a next adjacent layer and at least one second insulating layer sandwiched between two of said first layers, the method comprising the steps of launching an electromagnetic wave into said second layer receiving a rebounded wave from said second layer and comparing said received electromagnetic wave with a pre known wave so as to detect the presence or, presence and location of, or absence of points of discontinuity within said component.

The present invention will now be more particularly described by way of example only with reference to the following drawings in which Figure 1 is a pictorial representation of an aircraft having CFC panels incorporating a detector according to the present invention.

Figure 2 illustrates in more detail the detector system and CFC panel shown in Figure 1.

Referring now to Figure 1, an aircraft shown generating at 10 includes at least one composite component such as, for example wing 12 and a detecting apparatus shown schematically at 14. The detecting apparatus, best seen in Figure 2, comprises a transmitter 16 for transmitting an electromagnetic wave WT, a receiver 18 for receiving an electromagnetic wave WR, and a comparator 20 for comparing said received wave WR with a pre-known wave WK so as to detect the presence or presence and location of or absence of points of discontinuity within said component in a manner to be described in detail below.

The composite component 12 comprises a plurality of first electrically conducting layer 22 of, for example carbon fibre strands 24 in an insulating matrix of resin 26, and at least one second, insulating layer 28 sandwiched between two of said electrically conducting layers 22.

A plurality of electrical contacts 30, 32 provided at the edged of the two sandwiching electrically conducting layers 22 in order to enable electrical contact to be made thereto. The electrical connnections to the two carbon fibre first layers 22 may be made simply by embedding wires or metal foils in the plies to make a direct DC connection ("Ohmic" Connection). Alternatively, one might embed wires or foils in the same place but rely upon the inevitable capacitance between wire/foil and carbon fibres to provide a route for the flow of current from the voltage pulse source to the carbon fibre plies (capacitive connections).

The transmitter is preferably configured to transmit a pulsed transverse electromagnetic wave and includes one of said electrically conductive layers 22 between which the insulating layer is sandwiched. The detecting apparatus 14 further includes a current generating means 34 for generating equal and opposite currents for inserting into the transmitter.

Preferably the transmitter is configured to transmit a pulsed transverse electromagnetic wave.

In operation, a pulsed transverse electromagnetic wave is launched into the insulating layer 28 by applying an equal and opposite current to the electrical contacts 30, 32 in the same manner as transverse electromagnetic waves can be launched into a parallel plate transmission line. As the wave travels between the carbon plies 22 it will undergo partial reflection at any discontinuity eg crack, delamination, bridging fibre, or edge of/in either of the carbon plies involved or in the intervening insulating glass ply. This provides the basis of a time domain reflectometry technique in which internal cracks. in the CFC panel can be instantly and non-destructively detected.

Electromagnetic 'sounding pulses' may be injected at regular intervals and discontinuities detected by comparing each return (rebounded) signal with records of previously returned signals thereby to enable continuous monitoring of the structural integrity of the panel to be achieved. The frequency of injection and signal strength may be varied to suit the particular material component. By injecting the 'sounding pulses' at a plurality of positions along two sides of the panel it will be possible to analyse the rebounded pulses in a manner well known in the art and therefore not described herein so as to produce a 'contour map' or discontinuity profile of the component.

It will be appreciated that by adopting the above mentioned apparatus and method it will be possible to test composite panels after production as well as continually or periodically during their operational life. Advanced warning of structural failure by detecting the propagation and/or development of points of discontinuity within a component will enable the component to be withdrawn from service prior to complete and failure, thereby avoiding possible castastrophe.

It should be noted that the electric field and magnetic field components of the wave will be transverse to its direction of travel (actually, normal to the two carbon fibre plies). Such a wave will travel at the speed of light in the glass fibre ply second layer (which will be something like half the speed of light in free space) and will be partially reflected at any discontinuity as defined above.

Claims

1. A detecting apparatus for detecting the presence, or presence and location of, or absence of points of discontinuity within a component having a plurality of first electrically conductive layers each of which are in electrical contact with a next adjacent layer, the detector comprising a second non-conductive layer sandwiched between two of said conductive layers, a transmitter for transmitting an electromagnetic wave into said second layer, a receiver for receiving said electromagnetic wave from said second layer and a comparison means for comparing said received electromagnetic wave with a pre known wave so as to detect the presence, or presence and location of, or absence of points of discontinuity within the component.

2. A detecting apparatus as claimed in claim 1 or claim 2 in which the transmitter is configured to transmit a pulsed transverse electromagnetic wave.

3. A detecting apparatus as claimed in any one of claims 1 or 2 including current generating means for generating equal and opposite currents for inserting into said second layer so as to generate an electrical field in the insulating layer.

4. A detecting apparatus as claimed in any one of claims 1 to 3 in which the transmitter comprises one of said conductive layers between which said insulating layer is sandwiched.

5. A detecting apparatus as claimed in any one of claims 1 to 4 including one or more ohmic, capacitative or conductive electrical contacts at an edge of the component, said one or more contacts being electrically connected to one or more of said conductive layers for enabling external electrical connections to be made to individual conductive layers.

6. A detecting apparatus as claimed in any one of claims 1 to 5 in which said comparison system is configured to compare said received electromagnetic wave with a previously received electromagnetic wave or a predicted electromagnetic wave or a mathematically modelled electromagnetic wave.

7. A detecting apparatus claimed in any one of claims 1 to 6 in which said comparator is configured to compare said received electromagnetic wave with the transmitted wave.

8. A detecting apparatus as claimed in any one of claims to 7 in which the conductive layers comprise carbon fibres.

9. A detecting apparatus substantially as described herein with reference to and as illustrated in Figures 1 or 2 of the accompanying drawings.

10. A method of monitoring the structural integrity of a composite component of the type having a plurality of first conductive layers each of which is in electrical contact with a next adjacent layer and a second insulating layer sandwiched between two of said first layers, the method comprising the steps of launching an electromagnetic wave into said second layer receiving a reflected wave from the edge said second layer or discontinuity within said second layer and comparing said received electromagnetic wave with a pre known wave so as to detect the presence or, presence and location or, absence of points of discontinuity within said component.

11. A method of monitoring the structural integrity of a composite component substantially as hereinbefore described with reference to and as illustrated in Figures 1 or 2 of the accompanying drawings.