GB2145515A - Crack or strain monitor systems - Google Patents

Crack or strain monitor systems Download PDF

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Publication number
GB2145515A
GB2145515A GB08419110A GB8419110A GB2145515A GB 2145515 A GB2145515 A GB 2145515A GB 08419110 A GB08419110 A GB 08419110A GB 8419110 A GB8419110 A GB 8419110A GB 2145515 A GB2145515 A GB 2145515A
Authority
GB
United Kingdom
Prior art keywords
fibre
light
crack
disposed
fibres
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
GB08419110A
Other versions
GB8419110D0 (en
Inventor
Ian Miles Allison
Henry Bryant Boyle
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
NMI Ltd
Original Assignee
NMI Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by NMI Ltd filed Critical NMI Ltd
Publication of GB8419110D0 publication Critical patent/GB8419110D0/en
Publication of GB2145515A publication Critical patent/GB2145515A/en
Withdrawn legal-status Critical Current

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
    • G01M11/00—Testing of optical apparatus; Testing structures by optical methods not otherwise provided for
    • G01M11/08—Testing mechanical properties
    • G01M11/083—Testing mechanical properties by using an optical fiber in contact with the device under test [DUT]
    • G01M11/085—Testing mechanical properties by using an optical fiber in contact with the device under test [DUT] the optical fiber being on or near the surface of the DUT
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B11/00—Measuring arrangements characterised by the use of optical techniques
    • G01B11/16—Measuring arrangements characterised by the use of optical techniques for measuring the deformation in a solid, e.g. optical strain gauge
    • G—PHYSICS
    • G08—SIGNALLING
    • G08B—SIGNALLING OR CALLING SYSTEMS; ORDER TELEGRAPHS; ALARM SYSTEMS
    • G08B13/00—Burglar, theft or intruder alarms
    • G08B13/02—Mechanical actuation
    • G08B13/12—Mechanical actuation by the breaking or disturbance of stretched cords or wires
    • G08B13/126—Mechanical actuation by the breaking or disturbance of stretched cords or wires for a housing, e.g. a box, a safe, or a room
    • G—PHYSICS
    • G08—SIGNALLING
    • G08B—SIGNALLING OR CALLING SYSTEMS; ORDER TELEGRAPHS; ALARM SYSTEMS
    • G08B21/00—Alarms responsive to a single specified undesired or abnormal condition and not otherwise provided for
    • G08B21/18—Status alarms

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Chemical & Material Sciences (AREA)
  • Analytical Chemistry (AREA)
  • Business, Economics & Management (AREA)
  • Emergency Management (AREA)
  • Length Measuring Devices By Optical Means (AREA)

Abstract

A crack or strain monitor system 1 for monitoring a structure 8 comprises a pair of optical fibres 2, 3 with light sources at one end of the fibres 2, 3 and light detectors at the other end thereof. The optical fibre 2 is disposed in closer proximity to the structure 8 than the optical fibre 3. Fig. 2 is disposed in a locating body 11 secured to the structure 8 by adhesive. The arrangement is such that the fibre 2 is contiguous with the surface of the structure 8, and is a "crack-sensing" fibre. The fibre 3 is mounted on the exterior of the body 11, and is an "integrity" fibre. In operation, should the "crack- sensing" fibre 2 fail whereby a signal is initiated by an alarm device, the likely cause is overstraining of the structure 8. On the other hand, should the "integrity" fibre 3 fail whilst the fibre 2 remains intact, the likely cause is not overstraining of the structure 8, but instead a spurious fault in the system. The invention has particular applications where operating conditions make it difficult or even impossible to carry out a visual check. <IMAGE>

Description

SPECIFICATION Improvements in or relating to crack monitor systems BACKGROUND TO THE INVENTION This invention relates to crack monitor systems, and is concerned with crack monitor systems which make use of optical fibres wherein light is passed along a fibre to be received by a light detector. When a structure to which the fibre is attached 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.
It can happen that the fibre is fractured not be straining of the structure being monitored but by some extraneous force. For example, an accidental hammer blow.
The matter may be of particular concern when conditions are such that a visual check is difficult or impossible.
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 present invention, a crack monitor system comprises at least one pair of optical fibres with light source means for transmitting light along each fibre, and light detector means for reducing the light so transmitted, wherein one fibre is disposed in closer proximity to the structure being monitored than the other fibre.
According to another aspect of the present invention, a crack monitor system comprises at least one pair of optical fibres with light source means at one end of each fibre and light detector means at the other end of each fibre, one fibre being disposed in closer proximity to the structure being monitored than the other fibre.
It will be appreciated that one fibre, (the sensing fibre), is likely to be subjected to more strain than the other fibre, (the integrity fibre). If the integrity fibre should fail before the sensing fibre, this suggests that a spurious fault has developed in the system.
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 sectional view of the crack monitor system, Figure 2 is a semi-diagrammatic plan view thereof, with some components displaced from each other, some components removed and some components added, for reasons of clarity, and Figures 3 and 4 are, respectively, sectional and plan views which illustrate modifications.
DETAILED DESCRIPTION OF THE PRE FERRED EMBODIMENT With reference to Figs. 1 and 2, a crack monitor system 1 comprises a pair of optical fibres 2, 3 disposed in substantially parallel array with light source means 4, 5 at one common end of the fibres 2, 3 and light detector means 6, 7 at the other common end thereof. As shown in Fig. 1 (only) the optical fibre 2 is disposed below and in closer proximity to the structure 8 being monitored than the other fibre. Fig. 1 illustrates the true relative (super-incumbent) positions of fibres 2 and 3 of this embodiment. Fig. 2 does not.
The fibre 2 is disposed within a groove 10 formed in a locating body or "package" 11 having outwardly extending flange portions 1 2 so as to overlay the fibre. The flange portions 1 2 are secured to the structure 8 by adhesive. The fibre 2 is bonded to the body 11 before the body is secured to the structure 8. The arrangement is such that the fibre 2 is contiguous with the upper surface of the structure 8.
The body 11 may comprise a resin-impregnated glass mat or resin-impregnated "NO MEX" (R.T.M.) paper as used for transformer insulation.
"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 disposed 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 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.
The body 11 is sufficiently flexible to allow it to conform to curved structures being monitored.
The mean thickness of the body 11 is preferably about 0.5 mm.
The fibre 3 is mounted on the exterior of the body 11, so that the longitudinal axes of the fibres 2, 3 are disposed in substantially the same plane. The fibre 3 is enclosed in a sleeve 13 of polymer, e.g. "NYLON" (R.T.M.) which provides it with some protection. The sleeve 1 3 is secured to the body 11 by bonding adhesive 14.
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.
Fibre 2 can be considered to be a cracksensing fibre, and fibre 3 an "integrity" fibre.
Fibre 2 being closer to the structure 8 than fibre 3, is likely to be subjected to more strain than fibre 3.
In use, light from light sources 4, 5, each of which may comprise a light emissive diode or injection laser, is directed into the adjacent ends of the fibres 2, 3 and the light transmitted therethrough is received by light detectors 6, 7. The outputs of the detectors 6, 7 are connected to level sensing devices 20, 21 which generate alarm signals by way of alarm devices 22, 23 if ever strain causes the received levels abruptly to diminish by amounts exceeding a predetermined threshold. The light sources 4, 5 need not be operated continuously, but can be pulsed if somewhere in the system the responses are integrated sufficiently to smooth out the pulses.
The fibres 2, 3 are disposed so that they traverse the line 25 of the likely crack (Fig. 2) should the structure 8 be strained unduly.
Should the "crack-sensing" fibre 2 fail, (as illustrated by Fig. 2), whereby the alarm signal is initiated by device 22, the likely cause is overstraining of the structure 8. On the other hand, should the "integrity" fibre 3 fail whilst the fibre 2 remains intact, the reason for this is unlikely to be overstraining of the structure 8, but a spurious fault in the system.
By making the body 11 translucent, a visual indication of a featured fibre is provided.
The invention has particular applications where operating conditions make it difficult or even impossible to carry out a visual check.
For example, the monitoring system 1 may be disposed on the leg of an off-shore oil rig, well below the water level. or within the narrow confines of the rear end of an aircraft undergoing flight tests.
The "integrity" fibre 3 need not be disposed directly above the "crack-sensing" fibre 2, but it must be disposed away from it by a distance sufficient to render it less vulnerable to strain than fibre 2.
More than one pair of fibres 2, 3 may be employed. One such arrangement is illustrated in Fig. 3 where a fibre-locating body 11 a of corrugated form is used by a monitor system 1 a. Here the fibres 2 and 3 are disposed in the elongate concavities of the body 11 a, fibres 2 being disposed in the "tunnels" defined by the concavities of the undulating body 11 a and the structure 8 being monitored, and the fibres 3 between the "crests" formed by the concavities above.
With further reference to Fig. 2, it will be appreciated that some of the components illustrated therein may be replaced by single components. For example, light sources 4, 5 may be replaced by a single light source.
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 b makes use of a relatively short length of optical fibre 2b so as to carry light signals back to light detector 6 as well as out from light source 4.
To enable this two-way light signal traffic to take place, the end of the optical fibre 2b remote from light source 4 is provided with light reflector means in the form of a coating or cap 30 of metal, preferably gold. Furthermore, the opposite end of the fibre 2b is provided with a "Y" connection 31 connecting the fibre with both the light source 4 and the light detector 6.
The systems described above make use of visible light. Alternatively, or in addition, infrared light may be used with appropriate substitution of components.

Claims (11)

1. A crack monitor system comprising at least one pair of optical fibres with light source means for transmitting light along each fibre, and light detector means for receiving the light so transmitted, wherein one fibre is disposed in closer proximity to the structure being monitored than the other fibre.
2. A crack monitor system comprising at least one pair of optical fibres with light source means at one end of each fibre and light detector means at the other end of each fibre, one fibre being disposed in closer proximity to the structure being monitored than the other fibre.
3. A system as claimed in Claim 1 or 2, wherein the said one fibre is disposed in a body secured to the structure being monitored.
4. A system as claimed in Claim 1, 2 or 3, wherein said other fibre is superincumbent on the said one fibre.
5. A system as claimed in Claim 3, wherein said body is of corrugated form, the fibres being disposed within the concavities of the body.
6. A system as claimed in any one of Claims 1 to 5, wherein each fibre is provided with its own light source means.
7. A system as claimed in Claim 1, wherein the light source means and the light receiving means are disposed at a common end of an optical fibre, the opposite end of the fibre being provided with light reflecting means.
8. A crack monitor system substantially as hereinbefore described with reference to Figs.
1 and 2 of the accompanying drawings.
9. A crack monitor system substantially as hereinbefore described with reference to Figs.
1 and 2 of the accompanying drawings, modified substantially as hereinbefore described with reference to Fig. 3 of said drawings.
10. A crack monitor system substantially as hereinbefore described with reference to Figs. 1 and 2 of the accompanying drawings, modified substantially as hereinbefore described with reference to Fig. 4 of said drawings.
11. Every novel feature and every novel combination of features disclosed herein.
GB08419110A 1983-08-20 1984-07-26 Crack or strain monitor systems Withdrawn GB2145515A (en)

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
GB838322485A GB8322485D0 (en) 1983-08-20 1983-08-20 Crack monitor systems

Publications (2)

Publication Number Publication Date
GB8419110D0 GB8419110D0 (en) 1984-08-30
GB2145515A true GB2145515A (en) 1985-03-27

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GB08419110A Withdrawn GB2145515A (en) 1983-08-20 1984-07-26 Crack or strain monitor systems

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Cited By (11)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0278143A1 (en) * 1987-02-13 1988-08-17 G2 SYSTEMS CORPORATION (a California corporation) Structural monitoring system using fiber optics
DE3716168A1 (en) * 1987-05-14 1988-11-24 Bayerische Motoren Werke Ag Sensor for a safety device for motor vehicles
GB2214771A (en) * 1988-01-06 1989-09-13 Unisearch Ltd Optical fibre pressure or weight transducer
US4927232A (en) * 1985-03-18 1990-05-22 G2 Systems Corporation Structural monitoring system using fiber optics
US5026141A (en) * 1981-08-24 1991-06-25 G2 Systems Corporation Structural monitoring system using fiber optics
US5038618A (en) * 1986-11-11 1991-08-13 British Aerospace Public Limited Company Measurement of distortion
EP0562891A1 (en) * 1992-03-26 1993-09-29 Opto Industrie Optical fiber support for pressure detector
EP0640824A1 (en) * 1993-08-24 1995-03-01 British Aerospace Public Limited Company Fibre optic damage detection system
DE102007048817A1 (en) * 2007-10-10 2009-04-16 Eads Deutschland Gmbh Sensor for measuring voltages in a substrate
EP2112374A1 (en) 2008-04-21 2009-10-28 Siemens Aktiengesellschaft Crack detection system
US10495608B2 (en) 2016-08-21 2019-12-03 Elbit Systems Ltd. System and method for detecting weakening of the adhesion strength between structural elements

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN106840869B (en) * 2016-12-15 2019-08-23 北京航空航天大学 A kind of hole-edge crack diagnostic method based on fiber grating spectral image analysis under two kinds of cloth patch modes

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0025344A1 (en) * 1979-09-05 1981-03-18 Maxwell Ag A fluid transport hose
GB2101378A (en) * 1981-07-09 1983-01-12 Gordon James Pill Improvements in and relating to underground installation protection systems

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0025344A1 (en) * 1979-09-05 1981-03-18 Maxwell Ag A fluid transport hose
GB2101378A (en) * 1981-07-09 1983-01-12 Gordon James Pill Improvements in and relating to underground installation protection systems

Cited By (19)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5026141A (en) * 1981-08-24 1991-06-25 G2 Systems Corporation Structural monitoring system using fiber optics
US4927232A (en) * 1985-03-18 1990-05-22 G2 Systems Corporation Structural monitoring system using fiber optics
US5038618A (en) * 1986-11-11 1991-08-13 British Aerospace Public Limited Company Measurement of distortion
EP0278143A1 (en) * 1987-02-13 1988-08-17 G2 SYSTEMS CORPORATION (a California corporation) Structural monitoring system using fiber optics
DE3716168A1 (en) * 1987-05-14 1988-11-24 Bayerische Motoren Werke Ag Sensor for a safety device for motor vehicles
GB2214771A (en) * 1988-01-06 1989-09-13 Unisearch Ltd Optical fibre pressure or weight transducer
US5293039A (en) * 1992-03-26 1994-03-08 Opto Industrie Sandwiched fiber-optic pressure detector
FR2689234A1 (en) * 1992-03-26 1993-10-01 Opto Ind ADVANCED FIBER OPTIC PRESSURE DETECTOR.
EP0562891A1 (en) * 1992-03-26 1993-09-29 Opto Industrie Optical fiber support for pressure detector
EP0640824A1 (en) * 1993-08-24 1995-03-01 British Aerospace Public Limited Company Fibre optic damage detection system
US5723857A (en) * 1993-08-24 1998-03-03 British Aerospace Public Limited Company Method and apparatus for detecting cracks and strains on structures using optical fibers and Bragg gratings
DE102007048817A1 (en) * 2007-10-10 2009-04-16 Eads Deutschland Gmbh Sensor for measuring voltages in a substrate
DE102007048817B4 (en) * 2007-10-10 2016-06-30 Airbus Defence and Space GmbH Sensor for measuring voltages in a substrate
EP2112374A1 (en) 2008-04-21 2009-10-28 Siemens Aktiengesellschaft Crack detection system
US8149394B2 (en) 2008-04-21 2012-04-03 Siemens Aktiengesellschaft Crack detection system
CN101571494B (en) * 2008-04-21 2012-12-05 西门子公司 Crack detection system
EP2112374B1 (en) 2008-04-21 2015-11-18 Siemens Aktiengesellschaft Crack detection system
EP2112374B2 (en) † 2008-04-21 2018-10-17 Siemens Aktiengesellschaft Crack detection system
US10495608B2 (en) 2016-08-21 2019-12-03 Elbit Systems Ltd. System and method for detecting weakening of the adhesion strength between structural elements

Also Published As

Publication number Publication date
GB8322485D0 (en) 1983-09-21
GB8419110D0 (en) 1984-08-30

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