GB2145517A - Crack or strain monitors - Google Patents

Abstract

A crack or strain monitor system 1 employs a crack or strain monitor 2 comprising a plurality of laterally-spaced optical fibre light paths, formed by e.g. four optical fibres 3 disposed in close proximity to a structure 4 to be monitored. The fibres 3 are disposed so that they traverse the line 8 of the expected crack. A light source device 5 is disposed at one end of each fibre 3 and a light detector 6 at the other end thereof. When light is transmitted through the fibres 3, light interruption caused by a crack in the structure 4 is used to mark the presence of the crack. Light interruption in successive light paths, i.e. successive fibres 3, provides an indication of crack propagation. <IMAGE>

Description

SPECIFICATION Improvements in or relating to crack monitors BACKGROUND TO THE INVENTION This invention relates to crack monitors and is concerned with crack monitors which make use of optical fibres wherein a light is passed along a fibre to be received by a light detector. When a structure to which the fibre is attached (or made to form part of) is strained sufficiently to fracture the fibre, the transmission of light is diminished. This loss of light can be used to generate an alarm signal.

An object of the invention is to provide a crack monitor whereby crack propagation may be monitored.

The invention can also be used to monitor strain. Accordingly, as used herein, the term 'crack monitor" is intended to include monitors for measuring strain as well as cracks.

SUMMARY OF THE INVENTION According to one aspect of the invention, a structure to be monitored for cracks is provided with a crack monitor comprising a plurality of laterally-spaced optical fibre light paths disposed so that, when light is transmitted along the light paths, light interruption caused by a crack in the structure is used to mark the presence of the crack, and means whereby light interruption in successive light paths provides an indication of crack propagation.

According to another aspect of the invention, a crack monitor comprises a plurality of laterally-spaced optical fibre light paths disposed in close proximity to a structure to be monitored, whereby, when light is transmitted along the light paths, light interruption caused by a crack in the structure is used to mark the presence of the crack, and means whereby light interruption in successive light paths provides an indication of crack propagation.

According to yet another aspect of the invention, a crack monitor comprises a plurality of laterally-spaced optical fibre light paths disposed within a structure to be monitored, whereby, when light is transmitted along the light paths, light interruption caused by a crack in the structure is used to mark the presence of the crack, and means whereby light interruption in successive light paths provides an indication of crack propagation.

The light paths may be provided by separate optical fibres.

Alternatively, the light paths may be provided by a single optical fibre. For example, an optical fibre wound in the form of a spiral.

BRIEF DESCRIPTION OF THE DRAWINGS An embodiment of the invention will now be described by way of example only, with reference to the accompanying drawings, wherein: Figure 1 is a plan view of a crack monitor system employing a crack monitor, Figure 2 is a similar view of a system employing one modified monitor, Figure 3 is a similar view of a system employing another modified monitor, and Figure 4 illustrates yet another modification.

In the figures, like reference numerals refer to like components.

DETAILED DESCRIPTION OF THE PRE FERRED EMBODIMENT With reference to Fig. 1, a crack monitor system 1 employs a crack monitor 2 comprising a pluraity of laterally-spaced optical fibre light paths, formed by (in this example), four optical fibres 3 disposed in close proximity to a structure 4 to be monitored. A light source device 5 is disposed at one end of each fibre 3 and a light detector 6 at the other end thereof. When light is transmitted along the fibres 3, light interruption caused by a crack in the structure 4 is used to mark the presence of a crack. As explained hereinafter, means are provided whereby light interruption in successive light paths, i.e. successive fibres 3, provides an indication of crank propagation.

The light source may comprise a light emissive diode or injection laser.

The fibres 3 are substantially equi-shaped, and disposed so that they traverse the line 8 of the expected crack.

The fibres 3 are disposed in grooves 10 formed in a locating body or "package" 11 secured to the structure 4 by adhesive. The arragement is such that the fibres 3 are contiguous with the surface of the structure 4.

A suitable bonding adhesive is "M/BOND AE-10", a two-part epoxy adhesive manufactured by the MICRO-MEASUREMENTS DIVI SION, Measurements Group Inc., P. O Box 27777, Raleigh, North Carolina, U.S.A.

This adhesive is available in the U.K. from Welwyn Strain Measurements Limited, Basingstoke, Hampshire.

The body 11 may comprise a resin-impregnated glass mat or resin-impregnated "NO MEX" (R.T.M.) paper, as used for electrical transformer insulation. The body 11 is thin, having a preferred mean thickness of about 0.5 mm. The body 11 is also translucent.

The body 11 is sufficiently flexible to allow it to conform to curved structures being monitored.

"NOMEX" material is available from H.D.

Symons and Co. Ltd., Kingston-upon-Thames, Surrey.

Alternatively, a "pre-preg" (i.e. a component previously impregnated with resin), comprising glass or carbon fibre reinforcement incorporated into a mass of heat-curable epoxy resin may be used. A suitable resin is "Fibredux" material, obtainable from CIBA GEIGY, Duxford, Cambridgeshire, England.

A sandwich-like "package" may be constructed, by laying down a thin (e.g. 100 micron) skin of heat-curable "pre-preg" (see above) material, followed by one or more optical fibres, and then another thin (e.g. 100 micron or more) heat-curable "pre-preg" skin.

The whole is then subjected to pressure and heat. The application of pressure causes the fibres to form their own grooves in the surrounding body of "pre-preg" material, and the heat causes the epoxy resin to flow around the fibres.

In a modified method of manufacture, the first-mentioned skin is dispensed with, so that a non-sandwich "package" is formed.

These "pre-preg" methods of manufacture are particularly suited to large-scale production.

The optical fibres used are originally provided with primary and secondary coatings. In order to provide a really sensitive monitor it is desirable to remove these coatings. However, once this is done, the fibres immediately become vulnerable to attack by water in the atmosphere. The method of "sandwich" manufacture just described encapsulates the bared fibres and thus protects them.

Although, in use, the "sandwiched" optical fibres of the package are no longer in contact with a surface being monitored, they remain in close proximity thereto, being separated only by the first-mentioned skin of "pre-preg" material.

In use, the output of each detector 6 is connected to a level sensing device 1 5 which generates an alarm signal by way of an alarm device 16, should strain cause the received level abruptly to diminish by an amount exceeding a predetermined threshold. Only one set of devices 15 and 16 is shown in Fig. 1.

The light sources 5 need not be operated continuously, but can be pulsed, if means are provided whereby the responses are integrated sufficiently to smooth out the pulses.

Making the fibre-locating body 11 translucent, (or transparent), provides it with means whereby light interruption in successive fibres 3 provides an indication of crack propagation.

As each fibre 3 fails, a "light spot" is generated which can easily be seen. Light spots 20, 21 are shown on successive fibres 3 in Fig. 1, which provide a visual indication of crack propagation, including the direction in-which the crack is progressing.

In the modification of Fig. 2, equi-spaced fibres 3a are disposed substantially concentrically within a circular body 11 a secured to a structure 4a being monitored. This arrangement is particularly useful in the monitoring of radially extending cracks, particularly when the direction of the crack cannot be predicted with any real degree of certainty.

In the modification illustrated by Fig. 3, a plurality of light paths is provided by a single, spirally wound, fibre 3b. This fibre 3b is disposed in a "hair-pin"-like manner within a circular body 11 b secured to a structure 4b being monitored.

In this modification only a single set of devices 5. 6, 1 5 and 1 6 is required, device 1 6 being re-settable. Successive alarm signals emitted by the device 1 6, (repeated fractures of the fibre 3b will not necessarily cause a total absence of light at detector 6), give an indication of crack propagation. "Light spots" at fibre fracture points provide a visual indication of crack propagation.

As shown in Fig. 3, adjacent fibre portions of each winding are shown slightly spaced apart for illustrative purposes only. In practice.

the juxtaposed fibre portions defining the light transmitting and light receiving paths are contiguous.

In order to reduce the amount of optical fibre material employed by a crack monitor system, a shorter length of optical fibre can be used to convey light signals in two directions.

Fig. 4 illustrates such a modification. Here a crack monitor system 1 c makes use of a relatively short length of optical fibre 3c so as to carry light signals back to light detector 6 as well as out from light source 5.

To enable this two-way light signal traffic to take place, the end of the optical fibre 2c remote from light source 5 is provided with light reflector means in the form of a coating or cap 25 of metal, preferably gold. Furthermore, the opposite end of the fibre 2c is provided with a "Y" connection 26 connecting the fibre with both the light source 5 and the light detector 6.

The light paths provided by Figs. 3, 3a, 3b, 3cueed not be equi-spaced.

The light paths may be disposed within a structure to be monitored. For example, the structure may comprise a component for a fixed wing or rotary blade aircraft, manufactured from composite materials, with one or more light-transmitting optical fibres embedded within the component.

Identification of each alarm device 1 6 as it emits an alarm signal, will, alternatively, or in addition, provide an indication of crack propagation. The alarm device 1 6 may be coupled to suitable electronic monitoring and/or recording devices for this purpose.

The above-mentioned systems make use of visible light. Alternatively, or in addition, infrared light may be employed, with appropriate substitution of components.

Claims (16)

1. A structure to be monitored for cracks provided with a crack monitor comprising a plurality of laterally-spaced optical fibre light paths disposed so that, when light is transmitted along the light paths, light interruption caused by a crack in the structure is used to mark the presence of the crack, and means whereby light interruption in successive light paths provides an indication of crack propagation.

2. A crack monitor comprising a plurality of laterally-spaced optical fibre light paths disposed in close proximity to a structure to be monitored, whereby, when light is transmitted aong the light paths, light interruption caused by a crack in the structure is used to mark the presence of the crack, and means whereby light interruption in successive light paths provides an indication of crack propagation.

3. A crack monitor comprising a plurality of laterally-spaced optical fibre light paths disposed within a structure to be monitored, whereby, when light is transmitted along the light paths, light interruption caused by a crack in the structure is used to mark the presence of the crack, and means whereby light interruption in successive light paths provides an indication of crack propagation.

4. A crack monitor as claimed in Claim 1, 2, or 3, wherein the light paths are provided by separate optical fibres.

5. A crack monitor as claimed in Claim 1, 2, 3 or 4, wherein the light paths are provided by a single optical fibre.

6. A crack monitor as claimed in Claim 5, wherein the optical fibre is wound in the form of a spiral.

7. A crack monitor system comprising a crack monitor as claimed in any one of Claims 1 to 6, provided with light source means for transmitting light along the light paths and light detector means for receiving light so transmitted.

8. A crack monitor system comprising a crack monitor as claimed in any one of Claims 1 to 6, provided with light source means and light detector means at a common end of the optical fibre, the opposite end of the fibre being provided with light reflector means.

9. A crack monitor substantially as hereinbefore described with reference to Fig. 1 of the accompanying drawings.

1 0. A crack monitor substantially as herein before described with reference to Fig. 2 of the accompanying drawings.

11. A crack monitor substantially as hereinbefore described with reference to Fig. 3 of the accompanying drawings.

1 2. A crack monitor substantially as hereinbefore described with referene to Fig. 4 of the accompanying drawings.

1 3. A crack monitor system substantially as hereinbefore described with reference to Fig. 1 of the accompanying drawings.

1 4. A crack monitor system substantially as hereinbefore described with reference to Fig. 2 of the accompanying drawings.

1 5. A crack monitor system substantially as herein before described with reference to Fig. 3 of the accompanying drawings.

16. A crack monitor system substantially as hereinbefore described with reference to Fig. 4 of the accompanying drawings.

1 7. Every novel feature and every novel combination of features disclosed herein.