GB2136119A

GB2136119A - Crack Monitor

Info

Publication number: GB2136119A
Application number: GB08405725A
Authority: GB
Inventors: Michael Anthony Halliday; Kenneth Frank Hale; Henry Bryant Boyle; Garth William Pearce
Original assignee: NMI Ltd
Current assignee: NMI Ltd
Priority date: 1983-03-08
Filing date: 1984-03-05
Publication date: 1984-09-12
Also published as: GB8405725D0

Abstract

A crack monitor 1 comprises a translucent body 2 having a lower surface 2a defining a plurality of grooves 3. Optical fibres 4 are disposed in the grooves 3. When the body 2 is attached to a structure 5 to be monitored, with the fibres in close proximity to the structure, and with light transmitted through the fibres, light interruption caused by a crack in the structure, which propagates into the monitor, can be used to indicate the presence of a crack. Light provided by a source 22, such as a light emissive diode or injection laser, is directed into one end of each fibre 4, the light transmitted therethrough being received by a detector 24. The output of which is connected to a level sensing device 25 which generates an alarm signal at a device 26 if ever the received level abruptly diminishes by an amount exceeding a predetermined threshold. The light source can be pulsed. <IMAGE>

Description

SPECIFICATION Improvements in or Relating to Crack Monitors Background of the Invention This invention relates to crack monitors and is concerned with crack monitors which make use of fibre optics. A light source is provided at one end of a fibre and a light detector at the opposite end thereof. When a structure to which the fibre is attached is stressed sufficiently to fracture the fibre, the transmission of light is diminished. This loss of light can be used to generate an alarm signal.

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 the invention, a crack monitor, (as defined herein), comprises a body having a surface defining at least one groove, and optical fibre(s) disposed in the groove(s), so that when the body is attached to a structure to be monitored, with the fibre(s) in close proximity with the structure, and with light transmitted through the fibre(s), light interruption caused by a crack in the structure can be used to indicate the presence of a crack.

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 the crack monitor, Figures 2, 3 and 4 are underneath, side and end views thereof, Figures 5 and 6 are sections taken on the lines V-Vand VI--VI of Figure 3, Figure 7 is a view in perspective of the monitor (in an inverted position), and Figure 8 is a side view, in longitudinal section, of the monitor, attached to a structure.

Detailed Description of the Preferred Embodiment With reference to the figures, a crack (and strain) monitor 1 comprises a body 2 having a (lower) surface 2a defining a plurality of grooves 3, and optical fibres 4, (Figure 8 only), disposed in the grooves, so that when the body 2 is attached to a structure 5 to be monitored, with the fibres in close proximity to the structure, and with light transmitted through the fibres, light interruption caused by a crack in the structure, which propagates into the monitor, can be used to indicate the presence of a crack.

The body 2 comprises an epoxy resin or other translucent premould, using a casting material which suits the working environment, and which provides some flexibility. A suitable resin is of the epoxy group, using an amine based catalyst, for example, "M/BOND AE-10", a two-part epoxy adhesive manufactured by the Micro Measurements Division, Measurements Group Inc., P.O. Box 27777, Raleigh, North Carolina, U.S.A., obtainable from Welwyn Strain Measurements Limited, Basingstoke, Hampshire, England. Alternatively, "Permabond (R.T.M.) E30", obtainable from Permabond Limited of Eastleigh, Hampshire, England, or "Silverlock (R.T.M.) 1401, 1701 or 1901", obtainable from BTR Development Limited, Burton-on-Trent, England. Choice of resin will depend on operating conditions.

The body 2 could be made by injection moulding.

Alternatively, the body 2 comprises a resinimpregnated paper, such as "Nomex" (Registered Trade Mark) material, used for insulation purposes in the manufacture of electrical transformers, or a "pre-preg" (i.e. a component previously impregnated with resin), comprising glass of carbon fibre reinforcement incorporated into a mass of heat curabie epoxy resin, for example "Fibredux" (R.T.M.), which is obtainable from Cieba-Geigy, Duxford, Cambridgeshire, England.

The body 2 of the present example incorporates a fibreglass scrim 10, but this can sometimes be dispensed with.

The body surface 2a defines, (in this example), three laterally-spaced grooves 3. The grooves 3 extend in substantially parallel array, into slots 11 of much larger cross-section, which have tapering walls, as best shown in Figures 4 and 5.

The body 2 of the example is provided with an outwardly extending skirt portion 12 which extends around the whole of the periphery of the body 2, except for the ends of the slots 11.

The body 2 could be shaped so as to conform with the shape of structure being monitored. For example, it could be shaped so as to be disposed across the line of a weld, in contiguous relationship with the weld.

The ends of the optical fibres 4 are enclosed in sleeves 1 5 of polyvinylchloride (P.V.C.), Nylon or other suitable polymer, with epoxy buffers 1 6.

The sleeves 1 5 are a push fit into the slots 11, the tapering walls thereof holding the sleeves securely. The ends of the fibres 4 are made to curve slightly, within the slots 11, so as to accommodate any bending of the body 2. An index matching compound 1 7 is used here to secure the ends of the fibres 4 to the buffers 1 6 and to minimise light loss at the discontinuity, but it may be possible to dispense with this. A suitable matching compound is a single component polymer having a low refractive index, for example, "EPO-TEK" (y=1.394), manufactured by Epoxy Technology Incorporated, U.S.A.

The optical fibres 4 are carefully "clipped" in place within the grooves 3, using a minimal amount of adhesive to ensure precise location.

The "package" provided by the body 2, fibres 4 etc., is applied to the structure 5 under controlled pressure, using adhesive 20. For application to curved surfaces some pre-heat may be required.

The active region of the fibres 4, i.e. the region of the contiguous relationship between the fibres 4 and the structure 5, is indicated at 21.

Greater sensitivity is obtained when the body 2 is placed obliquely across the anticipated crack.

With further reference to Figure 8, in use, light from a source 22, such as a light emissive diode or injection laser, is directed into one end of each fibre 4 and the light transmitted therethrough is received by a detector 24. The output of the detector 24 is connected to a level sensing device 25 operable to generate an alarm signal at a device 26 if ever the received level abruptly diminishes by an amount exceeding a predetermined threshole. The light source does not need to be operated continuously, but can be pulsed.

When the light path is broken or otherwise interrupted, arising from when a crack initiates, or strain occurs, either on the surface of the structure 5 or within the bulk thereof, not only does attenuation of the light signal cause an alarm signal to be triggered, but light caused to scatter by fracture of a fibre 4 provides a visual indication of where the crack (or strain) exists.

The above-described system makes use of visible light at all times. In a preferred modification, source 22 provides alternatives of infra-red and visible light, and detector 24 is sensitive only to infra-red light.

In use, infra-red light is initially employed.

However, once alarm device 26 is brought into operation, source 22 automatically is caused to emit visible light whereby a visual indication of a fracture fibre is immediately provided.

Use of infra-red instead of visible light for crack and strain monitoring purposes has distinct advantages, including the ready availability (and relative cheapness) of infra-red sources and detectors.

It will be appreciated that the monitor 1 may be modified so as to make use of any desired number of optical fibres. Typically, bundles of three, seven or nineteen fibres may be used to achieve generally circular grouping of fibres within optical fibre cabling.

The grooves 3 need not be linear.

A sandwich-like "package" may be constructed, by laying down a thin (e.g. 10 micron) skin of heat-curable "pre-preg" (see above) material, followed by one or more optical fibres, and then another thin (e.g. 10 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.

This method of manufacture is 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 vunerable to attack by water in the atmosphere. The method of manufacture just described encapsulates the bared fibres and thus protects them.

Although, in use, the 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.

Claims

1. A crack monitor (as defined herein) comprising a body having a surface defining at least one groove, and optical fibre(s) disposed in the groove(s), so that when the body is attached to a structure to be monitored, with the fibre(s) in close proximity with the structure, and with light transmitted through the fibre(s), light interruption caused by a crack in the structure can be used to indicate the presence of the crack.

2. A crack monitor as claimed in Claim 1, wherein the body is translucent.

3. A crack monitor as claimed in Claim 1 or 2, wherein the body is of a casting material.

4. A crack monitor as claimed in Claim 1 or 2, wherein the body comprises a resin-impregnated paper.

5. A crack monitor as claimed in Claim 1 or 2, wherein the body comprises glass or carbon reinforcement incorporated into a mass of plastics material.

6. A crack monitor as claimed in any one of Claims 1 to 5, wherein said groove(s) is/are linear.

7. A crack monitor as claimed in any one of Claims 1 to 5, wherein a plurality of grooves are provided, said grooves extending in substantially parallel array.

8. The combination of a crack monitor as claimed in any one of Claims 1 to 7, light source means at one end of the fibre(s), light detector means at the other end of the fibre(s) and light sensing means operable to generate an alarm signal if the received level of light abruptly diminishes by an amount exceeding a predetermined threshold.

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