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FOS&S Instruction manual Strain Gauge Kit – version 1.1 page 1/25

Fibre Optic Sensors & Sensing Systems

Instruction manual

Strain Gauge Kit

Version 1.1

FOS&S Fibre Optic Sensors and Sensing Systems B.V.B.A.

Cipalstraat 14, B-2440 Geel Belgium

Tel: +32 14 581 191 Fax: +32 14 591 514

website: www.fos-s.com



LIST OF CONTENTS

1 The Fibre Optic Strain Gage ................................................................................................................. 3

1.1 Basics ............................................................................................................................................. 3

1.2 Production Process ........................................................................................................................ 5

1.3 Strain and temperature sensitivity .................................................................................................. 6

1.3.1 Temperature compensation with the TC-Probe ...................................................................... 7

1.3.2 Temperature compensation with the TC-Plate ....................................................................... 8

1.3.3 Strain ranges and sensor configurations ................................................................................ 9

1.3.4 Strain induced by bending ...................................................................................................... 9

2 Specifications ...................................................................................................................................... 10

3 Installation tools ................................................................................................................................... 11

3.1 Optical Strain Gages SG-01 ......................................................................................................... 11

3.2 Strain Gage Installation kit ........................................................................................................... 11

3.3 UV-radiation source (optional) .................................................................................................... 14

4 Installation ........................................................................................................................................... 14

4.1 Choice of the adhesive ................................................................................................................. 14

4.2 Installation conditions ................................................................................................................... 16

4.3 Installation procedure ................................................................................................................... 16



1 THE FIBRE OPTIC STRAIN GAGE

1.1 Basics

Experimental strain and stress analysis is conventionally done by means of electrical strain gages. Basically, an electrical strain gage or foil gage is a metallic wire on a thin foil, as depicted in Figure 1. The gage is fixed to a surface and resistance of the metallic grid changes when it is being stretched.

Figure 1: An electrical foil gage.

Fibre Bragg Grating sensors (FBGs) have recently gained increasing attention in this field since they form the optical equivalent of the electrical foil gage technology. Their principle of operation is explained in Figure 2 and Figure 3. An FBG is in principle a local periodic modulation of the refractive index in an

optical fibre. The modulation period Λ is typically 480 nm and so of the order of the wavelength of UV-light whereas the typical modulation length (the FBG-length) is around 8 mm. This means that a single FBG is composed of several thousands of grating periods.

Figure 2: The reflection and transmission properties of a Fibre Bragg Grating.

A specific property of an FBG is that it reflects a single wavelength called the Bragg wavelength λB. The physics behind this is interference between the light rays that are reflected at each ridge of the grating.

Constructive interference happens only on the condition that the wavelength λB exactly matches with the

difference in optical path length between two successive rays (2nΛ), yielding the so-called Bragg

condition: λB = 2nΛ. When broadband light is guided in the fibre and incidents on the FBG, a single reflection peak can be observed in the reflection spectrum and correspondingly a dip at the same wavelength can be seen in the transmission spectrum (Figure 2). The principle of strain detection with an FBG is illustrated in Figure 3. By straining the fibre, the grating period is increased and this shifts the

λ

P

8 mm

FBG

Incident spectrum

Reflected spectrum

Transmitted spectrum

λ

P λ

P

Λ



Bragg peak to higher wavelengths. By analyzing the FBG-wavelength, the strain in the fibre can thus be traced back.

Figure 3: Strain sensing principle of an FBG.

The FBG-sensors thus can detect strain changes and therefore they can form an alternative for the electrical foil gages in some cases. In this context, we will refer to them as fibre optic (FO) strain gages. The FO strain gages provide some superior qualities with respect to electrical gages, making them very suitable for some particular applications. A comparison between the electrical and the FO gages is presented in the table below.

Features Fibre Optic Electric

EM-radiation Immune EM-sensitive

Lightning / electric discharge

Lightning and discharge proof May cause damage or complete failure

Electric conductivity

Non-conductive and hence no special precautions needed for outdoor use or usage under water

All wiring needs to be hermetically sealed for outdoor use or usage under water

Explosion Safety Spark-free and hence safe in potentially explosive atmospheres

Hazardous in explosive atmospheres

Measurement distance

Up to tens of kilometers Limited in range without any additional amplification

Multiplexing

Possible to multiplex, i.e. multiple sensors can be allocated within the same optical fibre

No multiplexing capability in series configuration

Weight Low in weight: fibres are lightweight and number of cables can be limited

1

Becomes heavy for large numbers of sensors because of the copper wiring

Installation and Sensor Read-out

Fast (< 20 ‘ per sensor) Easy (can be done by non-experts; no extra wiring required)

Time consuming (extra wiring) Requires skilled and experienced workers

Fatigue resistance Excellent fatigue resistance: negligible effects for at least 2 million cycles for +/- 0.24 %

Similar performance not possible with electrical gages

λ

P ''2' Λ= nBraggλ

λ

P Λ= nBragg 2λ Λ

Λ'

Strained FBG

Unstrained FBG



straining

Price (sensor + read-out)

Becomes more cost-effective for more sensors

1

Break even point already for 20 sensors

Price scales linearly with the number of sensors since every sensor needs a separate read-out channel

Temperature sensitivity

Temperature compensation necessary when temperature varies since read-out is intrinsically temperature dependent.

Other Temperature Induced Effects

None

Resistive heat generation can occur for gages installed on materials with low thermal conductivity, resulting into measurement errors.

Spurious voltage read-out by changing temperature due to thermocouple effects at the junction of two dissimilar metals.

1 Because of the multiplexing capability: multiple sensors can be read-out by the same optical line.

1.2 Production Process

In general, FBGs can be ‘written’ into an optical fibre by locally illuminating the fiber with UV-light of which the power varies spatially along the length of the fiber. The period of the power variation then

corresponds to the grating period Λ and thus determines the Bragg wavelength of the FBG. Usually, the power modulation is obtained via an interferometric setup. The interference pattern can be made in different ways. The most straightforward way is by using a phase mask but this has the disadvantage that the Bragg wavelength is fixed. In order to write FBGs with variable wavelengths, a Talbot interferometer can be used, see Figure 4. The wavelength of the FBG can be tuned by turning the mirrors and thus changing the angle of incidence of the laser beams.

Figure 4: Operational principle of the Talbot interferometer (source: IPHT Jena).

It needs to be noted that the FBG-writing process needs to happen on uncoated fibre. The standard procedure for writing FBGs is to remove the coating from standard telecom fibre over a few centimetre, to expose it to the UV-interference pattern as long as needed and then to recoat it. The UV-power and the exposure time determine the reflectivity of the FBG. The stripping process needs to be done very carefully and even then this process often introduces damage in the fibre which drastically reduces its strength. In order to avoid these problems, a different method for FBG-inscription was developed in which the FBGs are written into the fiber while it is being produced (drawn) and before the coating layer is being applied. These are the so-called Draw Tower Gratings

1 (DTGs), which exhibit high strength simply

because the stripping and recoating phases of the FBG-production process are omitted. The main disadvantage of this technique is that the UV-exposure period needs to be very short because the fiber is

1 see www.fbgs-technologies.com



moving (it is being pulled). The exposure times are typically in the nanosecond range and this compromises on the reflectivity of the FBGs. For this reason, one normally uses a fiber with an increased sensitivity for UV-radiation. The UV-photo-sensitivity is usually increased by doping the core with germanium. With the increased photo-sensitivity, reflectivity values in the range of 10 to 20 % for single shot FBGs can be reached. The optical strain gages SG-01 are essentially the DTGs as discussed above. These sensors have high strength compared to standard FBGs (fiber breakage at 5% strain compared to typically 1% for standard FBGs). This makes that the sensors can be used up to high strain levels and that they will exhibit excellent fatigue resistance. The used coating is ORMOCER, which has ideal bonding with the fibre glass and which is excellent for transferring strain because of its relatively high Young Modulus (2 GPa). These properties make that the coating does not need to be removed prior to installation of the strain gage to a surface.

1.3 Strain and temperature sensitivity

An FBG is sensitive to both strain (ε) and temperature (T) changes. In general, one can write:

),,(.ln 21,

0

TSSgk fmech ∆+=

ε

λ

λ (1)

with 0TTT −=∆

where λ is the current wavelength of the strain gage, λ0 the wavelength of the strain gage at the measurement start, T the current temperature, T0 the temperature at the measurement start, k the sensor

gage factor, εmech,f the mechanical strain on the fibre (relative to its mechanical strain at the measurement start), g(S1,S2,∆T) an FBG intrinsic temperature sensitivity function defined by two temperature sensitivity parameters S1 and S2. In case the FBG is fixed to a structure, there are two different contributions to the mechanical strain on

the fibre εmech,f :

• the mechanical strain on the structure εmech,s to which the fibre is attached

• an additional mechanical fibre strain induced by the thermal expansion of the structure to which the fibre is attached

The latter is often referred to as the thermally induced strain or simply the thermal strain. In this case, equation (1) transforms into:

( )[ ] ),,(,.ln 21,

0

TSSgTfk ssmech ∆+∆+=

αε

λ

λ (2)

where αs is the thermal expansion coefficient of the structure. From equation (2), the mechanical strain on the structure can be deduced, provided that the temperature variation can be monitored with a separate temperature compensating sensor:

( ) ( )TfTSSgk

ssmech ∆−

∆−

= ,,,ln.

121

0

, αλ

λε (3)

Formula (3) is the generic formula for fibre optic strain gages attached to a structure and hence it forms the basis for every temperature compensation formalism.



NOTE The strain gage parameters k, S1 and S2 are determined on a batch level and can be found in the calibration sheets delivered together with the strain gages. If the temperature does not vary during a measurement (∆T=0), equation (3) simplifies to:

=

0

, ln.1

λ

λε

ksmech (4)

If the temperature varies during a measurement, one certainly needs to correct for it. Compensation for temperature effects can be done by:

• the temperature compensating probe method: temperature compensation is performed by means of a temperature compensating probe (TC-probe), which measures the temperature variation ∆T.

• the temperature compensating plate method: Temperature compensation is performed by means of a temperature compensating plate (TC-plate), which is made of the same material as the structure under test but which remains unstrained.

1.3.1 Temperature compensation with the TC-Probe

The temperature compensating probe is basically a strain gage which is isolated in a mechanical housing so that it will stay all the time isolated from the mechanical strain (k = 0). The TC-probe is depicted in Figure 5. The probe should be mounted in the vicinity of the strain gages so that it is exposed to the same temperature.

Figure 5: Picture of the TC-probe used for temperature compensation.

Because the FBG is isolated from the mechanical strain (k = 0), it measures only temperature variations via the intrinsic temperature sensitivity:

),,(ln'

2

'

1'

'

ref

ref

TSSg ∆=

λ

λ (5)

with refref TTT −=∆

where λ’ is the current wavelength of the TC-probe, λref‘ the wavelength of the TC-probe at a pre-defined fixed reference temperature Tref, T the current temperature and S’1, S’2 the temperature sensitivity

parameters of the TC-probe. The TC-probe parameters λref’, S1‘ and S2‘ can be found in the calibration sheets, delivered together with the TC-probes.



From equation (5), the temperature at the measurement start, relative to the reference temperature Tref as well as the temperature at any moment during the measurement, relative to the same reference temperature, can be derived. With these known temperature differences, the generic equation (3) can be

solved for the mechanical strain on the structure εmech,s.

It is important to notice that a good estimation of the thermal expansion of the structure αs is required. This parameter is however not always accurately known and might be temperature dependent as well:

αs(T).

1.3.2 Temperature compensation with the TC-Plate

The most straightforward way to perform temperature compensation is by means of a temperature compensating plate (TC-plate). Figure 6 shows a picture of a typical TC-plate together with the technical drawings.

Figure 6: Picture and technical drawings of the TC-plate used for full temperature compensation.

The TC-plate is designed for mounting a compensating strain gage which records only the thermal expansion of the structure. This is done by means of the cantilever design: the strain acting on the host material will not be transferred to the floating part of the plate and hence the compensating gage will register only temperature changes and variations in the thermal strain of the structure. The TC-plate should be mounted next to the actual strain gage (same temperature) and it should be constructed from the same material as that of the structure under test (same thermal strain). The gage mounted on the TC-

plate has εmech,s = 0 so that equation (2) can be written as:

( ) ),,(,.ln'

2

'

1

'

'

0

'

TSSgTfk s ∆+∆=

α

λ

λ (6)

with 0TTT −=∆

where again λ’ is the current wavelength of the strain gage which is attached to the TC-plate (=TC-plate

strain gage), λ0‘ the wavelength of the TC-plate strain gage at the measurement start, T the current temperature, T0 the temperature at the measurement start, k’ the gage factor of the TC-plate strain gage and S’1, S’2 the temperature sensitivity parameters of the TC-plate strain gage. The sensor parameters k’, S1‘ and S2‘ can be found in the calibration sheets of the TC-plate strain gage.

In case the strain gage attached to the structure (with wavelength λ) and its compensating TC-plate strain

gage (with wavelength λ’) are from the same production batch (i.e. k = k’, S1 = S1’ and S2 = S2‘), the



mechanical strain on the structure can be calculated by substitution of equation (6) into equation (2) and

solving it to εmech,s:

−

=

'

0

'

0

, lnln1

λ

λ

λ

λε

ksmech (7)

Note that this formula contains only the gage factor k. All other parameters such as S1, S2, αs, αf and ∆T

have no influence on the final result.

NOTE The use of TC-plates needs no specific knowledge of the material parameters and therefore forms a straightforward way for measuring the mechanical strain under fluctuating temperature conditions. However, it is important that the compensating plate is manufactured from the same material as that of the host material. If not, the temperature compensation is possibly inaccurate due to differences in thermal expansion of the used materials.

1.3.3 Strain ranges and sensor configurations

The Strain Gage Kit contains two identical sets of 10 strain gages and two TC-probes for temperature compensation. Each set of strain gages has wavelengths as listed in the table below (left). Each gage has a reference number and letter (A or B). The two TC-probes have also different wavelengths as indicated in the right table below. When installing the sensors in series to an interrogation system, it is important to connect them from low to high wavelengths in order to obtain optimal signal quality. The TC-probes are always located at the end of a sensor line (single ended and so no serial configuration possible) and hence were attributed with the highest wavelengths.

SG-01 Wavelength [nm]

1A 1527

1B 1530.5

2A 1534

2B 1537.5

3A 1541

3B 1544.5

4A 1548

4B 1551.5

5A 1555

5B 1558.5

TC-probe Wavelength [nm]

TA 1562.5

TB 1564

The choice of wavelengths has been made so that all strain gages of a set can be placed in series together with a TC-probe (TA or TB) at the end. When all sensors are used simultaneously (A and B), the

strain difference between two succeeding gratings can be up to 2000 µε. For example, sensor 1A can

have a strain of +1000 µε and at the same time sensor 1B can have a strain of -1000 µε without the risk of peak overlaps. If more extended strain ranges are required, the user needs to select only the gages and TC-probe from the A- or B- series. The strain difference between two succeeding gratings is in this

case extended up to 5000 µε. For example, sensor 1A can have a strain of +2500 µε and at the same

time sensor 2A can have a strain of -2500 µε without the risk of peak overlaps.

1.3.4 Strain induced by bending

In case the strain gages are subjected to bending of a body, one needs to keep in mind that the strain gages measure the strain at the level of the fibre core and this does not correspond to the strain at the level of the surface of the structure. For bending, the strain increases or decreases when proceeding further away from the centre of the body that is being bend. If one is interested in the strain at the surface of the structure, one needs to introduce a correction factor which takes into account the distance of the fibre core above the surface. For the used fibre, this distance amounts 95 µm.



2 SPECIFICATIONS The Installation Kit contains two different adhesives for the installation of the optical gages: a UV-curable adhesive and a cyano-acrylate based rapid curable adhesive with brand name Z70. The latter is a well-known adhesive for the installation of conventional electrical gages. Installation with the UV-curable adhesive is the preferred method as it is highly controllable. This method requires an extra UV source which is sold as a separate accessory. Installation with the cyano-acrylate based adhesive is a second option, but the method is less controllable as the curing depends on the ambient conditions (humidity, temperature, applied pressure, …). This alternative method is only advised when a UV-lamp is not available or difficult to use on the site of installation. The characteristics of the strain gages SG-01 when installed with the Installation Kit according to the prescribed procedure are presented in the table below, depending on the used adhesive. The installation was validated on the following materials: steel, stainless steel, aluminium, inconel, titanium, glass composite and carbon composite.

Parameter Unit UV Z70

Gage factor (k) - 0.777 (typical)

Relative statistical error on gage factor % 0.5

Transverse sensitivity1 - < 2.1 10

-3

Temperature coefficient of gage factor2 °C

-1 2.7 10

-4

Strain range (tension / compression) % 0.5

Fatigue shift3

µε / 106

cycles ≤ 4

S1 (linear temperature sensitivity) 10-6

°C-1

6.30 (typical)

S2 (quadratic temperature sensitivity) 10-9

°C-2

8.02 (typical)

Active gage length (FBG length) mm 8

Overall gage length (free fiber length) mm 28

Coating material - ORMOCER®

Fibre Diameter (coated) µm 195

Operating temperature range °C -45 to +110 -30 to +90

Tubing material - FEP

Tubing diameter µm 900

Tubing length (left and right from strain gage) cm 45

Connector type - FC/APC 1

According to ASTM E 251-92. The transverse strain sensitivity is the ratio of the gage factor of a strain gage mounted perpendicular to a uniaxial strain field (transverse gage) to the gage factor of a similar gage mounted parallel to the same strain field (longitudinal gage). 2 The temperature coefficient of the gage factor k expresses the relative variation of k per degree Celsius.

3 The bonding during fatigue cycling was tested by mounting gages on unidirectional glass composite material that was

strained from -0.24 % to +0.24 % up to 2 million cycles.

The Installation Kit contains also two Temperature Compensating probes. Their specifications are listed in the table below.

Parameter Unit Value

Relative Temperature accuracy1 °C 1

Temperature sensitivity nm/°C 0.010 (typical)

Temperature range2 °C -45 to +110

FBG wavelength nm 1510 to 1590

Sensor length mm 58

Housing diameter mm 5

Housing material - SS316

Pigtail diameter mm 0.9

Pigtail length m 1

Pigtail material - FEP

Connector type - FC/APC 1 The sensor accuracy does not contain error sources originating from the read-out equipment.

2 Only for the sensor, not for the connector. Splicing is recommended for the extreme temperature values.



3 INSTALLATION TOOLS The following items are required for installation:

• Optical Strain Gages SG-01 (contained in the Kit)

• Strain Gage Installation Kit

• UV radiation source (optional)

3.1 Optical Strain Gages SG-01

The optical strain gages SG-01 are delivered as depicted in Figure 7. The fibre is pigtailed at both ends with an FC/APC connector (1) and protected in a 900 µm FEP-tubing (2). At the position of the DTG, the tubing is omitted over a length of 28 mm (4). This is done so that the fibre can be fixed directly onto the surface which it should measure. This zone thus forms the active part of the fibre optic strain gage and the DTG is located in the middle of this zone. A rubber sleeve (5) is placed over the free fibre part for protective purposes and needs to be removed before installation. The standard package of these sensors is depicted in Figure 8.

Figure 7: Schematic drawing of the standard optical strain gages.

Figure 8: Standard package containing 5 optical strain gages.

3.2 Strain Gage Installation kit

The Strain Gage Installation Kit SGK-01 contains all the necessary accessories needed for installation of the gages. A picture is shown in Figure 9.



Figure 9: Picture of the Strain Gage Installation Kit SGK-01.

The Installation Kit contains the following items:

Item Quantity

Instruction manual 1

Data sheets and safety forms 3

Sensor pads: L =45 mm, W= 8 mm (re-usable) 2

UV-curable adhesive (1 oz. bottle) 1

Z70 cyano-acrylate based rapid adhesive (10 ml bottle) 1

Dosing nozzle for Z70 2

Teflon band 1

Abrasive paper (1 m ribbon) 1

Box with cleaning tissues (90 pieces) 1

Bottle of cleaning agent (85 g) 2

Teflon patches 20

Rectangular glass piece 1

Rapid Adhesive component A 1

Rapid Adhesive component B 2

Mixing cups 9

Spoons 2

Wooden stirring sticks 25

Tweezers 1

Scalpel 1

Mechanical protection (90 ml tube) 1

Fibre Optic Strain Gages SG-01* 20

Temperature Compensating probe (TC-probe)* 2

Fibre optic patchcord (6 m) 2

Fibre optic connector adapter 12 * With wavelengths as specified in section 1.3.3.

NOTE When not being used, the adhesives (both UV as well as Z70) are best stored in a cool dark place. Therefore, when the kit is not in continuous use, it is advised to store the adhesives in a fridge. If refrigerated, allow the adhesives to come to room temperature prior to use. Under these conditions, the shelf life is at least 4 months. We refer to the data sheets for further storage instructions. The optical Strain Gage is mounted to the structure by means of a patented methodology which makes use of a specially designed mounting tool called a ‘sensor pad’. It is depicted in Figure 10.



Figure 10: Pictures of the sensor pad holding the DTG-fiber (left) and during the installation process. The glass piece is used to apply pressure on the pad during curing.

The design of the sensor pad is fine tuned in order to allow fast, easy and reproducible installation of the sensor. It is schematically depicted in Figure 11. The UV-transparent pad has dimensions of 45 x 8 mm. At both ends of the pad, there are slits that can be clipped onto the 900 µm buffer. This way, the fiber is secured to the pad. In between these clamping areas, there is a zone of 30 x 8 mm where the adhesive layer will be applied. In this zone, the fiber will be free (no 900 µm buffer) so that it can make direct contact with the surface to which it will be attached.

Figure 11: Schematic drawing of the sensor pad.

Fixation of the fiber to the structure goes roughly as follows. After proper preparation of the surface, the adhesive is put on the surface and the sensor pad with the fiber is put on place. The pad is kept under slight pressure in order to have an equal distribution of the adhesive layer and in order to keep the fiber under slight tension. The pressure is applied on the pad by means of a specially designed rectangular glass piece, which fits on the top of the pad as shown in Figure 10. After curing of the adhesive, the sensor pad can be removed. This is done so that it does not interfere with the strain measurement during operation of the sensor. Finally, the fiber cables are secured and a protection layer can be applied over the sensor for mechanical and / or chemical protection. The complete installation procedure is outlined in detail in section 4. The sensor pad thus combines the following features:

Free fiber with FBG



• Easy fixation / removal of the pad to the fiber cable.

• Transparency for UV-light (when UV-curing is required).

• No bonding with the adhesives so that it can be removed after curing.

• Flexible enough to bend around the fiber (in transverse direction) when put under pressure. This is needed to have the adhesive layer thin enough around the cylindrical fiber shape.

• Hard enough to hold the buffered fiber ends without slippage. This is needed to keep the fiber under slight tension during installation.

Because the sensor pad can be removed from the sensor after installation, it can be reused for installing other sensors on the condition that it is properly cleaned with a cleaning tissue wetted with the cleaning agent. Typically, one can install 10 sensors before replacement of the sensor pad. When the Z70 cyano-acrylate based adhesive is used, the bottom part of the pad should be covered with a Teflon tape, provided in the kit as well, in order to prevent bonding of the pad to the structure. It is advised to replace the Teflon tape for every new installation.

3.3 UV-radiation source (optional)

When the UV-curable adhesive is used for installation, a UV-source is required. The following source is recommended:

• Omnicure 1000 spot curing system from EXFO: bench top model containing a 100 W mercury lamp, an optical light guide and a timer.

4 INSTALLATION

4.1 Choice of the adhesive

The Installation Kit contains two different adhesives for the installation of the optical gages: a UV-curable adhesive and the Z70 cyano-acrylate based rapid curable adhesive. Installation with the UV-curable adhesive is recommended as it is highly controllable. Installation with the cyano-acrylate based adhesive is only advised when a UV-lamp is not available or difficult to use on the site of installation. Below, some specific properties of both types of adhesives are described in more detail.

• Curing Z70 cyano-acrylate: The curing of the Z70 adhesive is initiated by the humidity in the air and by the mechanical pressure that is being applied. This makes that the curing process is less controllable since it depends on:



o The humidity: Relative humidity between 40 % and 70 % provide the best curing conditions. Below 30 % relative humidity, the curing is noticeably delayed and in extreme cases may not occur at all; above 80 % relative humidity impact curing will result.

o The temperature: since the relative humidity is strongly temperature dependent, the ambient temperature plays an important role in the curing process as well. At temperatures below room temperature, the curing time should be extended and full curing will take longer.

o Adhesive thickness: thick layers of adhesive will not cure completely or even not at all. o The surface roughness: since this determines the adhesive thickness. o The applied pressure: since the pressure is applied manually, it can not really be

quantified and this makes the process less controllable. o The chemical composition of the surface: basic surfaces will accelerate the curing

process whereas acidic surfaces will delay or even prevent curing. In the latter case, the surface should be treated with a primer that accelerates the curing. The primer is available separately. The table below shows the recommended values for the curing time as a function of the surface material at 20°C and at 65% relative humidity.

Material Curing time [s]

Steel 80 to 120

Aluminum 50 to 100

Plastics 10 to 60

Ones this time has expired, the adhesive has bonded to such an extent that the installation can be finalized. Final hardness will be reached only after 24 hours but measurements can already be taken after the times as stated in the table below.

Type

Bonding Temperature

+5°C +20°C

Minimum curing time [min]

Dynamic 90 10

Static 120 15

• Curing UV-adhesive: The curing of the UV-adhesive on the other hand only requires UV-light of the right wavelength for the curing. The ambient conditions, adhesive thickness and applied pressure play no critical role. This makes that the installation with the UV-curable adhesive is much more controllable: the glue only cures when the users wants it to and the curing times are always similar for the same UV-source. This makes the installation with the UV-curable adhesive the preferred method. Only when the gage is bonded to glass, it has not reached its optimal adhesion after the final UV-cure. This will come with aging over a period of about 1 week in which a chemical bond will form between the glass and the adhesive. This optimum adhesion with glass can also be obtained by aging the sample at 50°C for 12 hours.

• Sensor performance: the performance for both types of adhesives was investigated and the deduced specifications are listed in section 2. For most characteristics, the performance was found to be nearly identical except for the temperature range. The gages fixed with the cyano-acrylate adhesive start to show failures of the bonding at temperatures below -30°C and above 90°C whereas the bonding for the UV-curable adhesive stays intact from at least -45°C up to at least 110°C.

The table below summarized the advantages and disadvantages of both adhesives.

Adhesive Pros Cons

UV-curable • Highly controllable and reliable

• More extended temperature ranges • UV-lamp relatively expensive and less

practical to take into the field

Cyano-acrylate

• Applicable when UV-lamp not available • Curing less predictable and hence

installation less controllable

• Reduced temperature ranges



4.2 Installation conditions

Installation should be done in a dust-free environment. The presence of dust particles on the installation surface can have a negative influence on the fiber adhesion. There are no limitations on temperature and air humidity for the curing of the UV-adhesive. The adhesive only cures by direct exposure to UV-light. It is recommended that direct sunlight is avoided. Curing of the cyano-acrylate on the other hand is temperature and humidity dependent, as outlined in section 4.1. Optimal conditions are at room temperature and with the relative humidity between 40 and 70%. It is recommended that the curing time of the glue is evaluated prior to the real sensor installation process in order to reduce the risk for faulty installations.

4.3 Installation procedure

NOTE Installation of the optical strain gages needs to be done very carefully and according to the procedure outlined below. Only when the instructions from this manual are accurately followed, proper operation of the strain gages can be guaranteed.

NOTE In general, the installation procedure is independent on the choice of the used adhesive (UV-curable or Z70 cyano-acrylate) but some steps require some specific manipulations that are meant for a particular adhesive type only. If this is the case, this step is marked with one of the bullets as shown below. In the other cases, no mark is shown. UV-curable adhesive Cyano-acrylate based rapid adhesive

Z70 UV UV



� STEP 1: Surface preparation

1-1

Clean and degrease the surface with a cleaning tissue wetted with the cleaning agent. Repeat this operation until the surface is perfectly clean.

1-2

Striate the surface with abrasive paper with grain size of 120. First rub in the direction of the fiber and later on rub into the perpendicular direction.

1-3

Wipe the surface with the cleaning tissue wetted with the cleaning agent.

1-4

Repeat this operation until the treated surface is perfectly clean.

1-5

Mark out the glue zone with the window-shaped Teflon patch. Note that the Teflon patch consists of two separate pieces: the window-shaped pad and the rectangular inside of the window. This inside piece is needed for the installation procedure with the Z70 adhesive, as indicated further below. For the installation with the UV-curable adhesive, this second piece is not needed.



� STEP 2: Preparation of the sensor pads

2-1

Check the reference and batch numbers of the FBGs.

2-2

Remove the fiber from the box.

2-3

Clean the fiber and sensor pad with a cleaning tissue wetted with the cleaning agent.

2-4

Take the remaining rectangular Teflon sticker and stick it on the down side of the sensor pad (= the side where the fiber will be mounted on). This part of the pad will be exposed to the Z70 adhesive and the Teflon sticker is required since this adhesive sticks to the rubber. Without the sticker, the pad cannot be removed after installation. This Teflon sticker should not be re-used for the installation of other sensors.

Note

The Z70 adhesive sticks on the rubber from the sensor pad and therefore the zone of the pad which gets in contact with the adhesive should be covered with a Teflon sticker in order to be able to remove the pad after installation. If this step is omitted, the pad will be glued on the sensor and structure and cannot be recovered for other installations. For the UV-curable adhesive, this operation is not required since this adhesive does not stick to the rubber pad.

Z70

Z70



� STEP 3: Installation of the fiber into the sensor pad 3-1

Click the fiber in the sensor pad. Make sure the fiber is under slight tension. This is of particular importance to assure that the fiber is straight after being fixed. To ensure that the fiber has the right amount of tension, make sure that the sensor pad is slightly bend when the fiber is hanging free, as shown in the right part of the figure below. Another way that the tension can be adjusted is by monitoring the wavelength of the FBG during installation. The wavelength should increase ideally by a few hundred picometer above its nominal wavelength when it is being clicked into the sensor pad.

Note

The Teflon sticker makes the sensor pad more rigid and hence more difficult to bend. Therefore, the bending needed to ensure the right amount of pre-strain should be smaller as compared to the picture above for the pad without Teflon sticker.

Note

Applying the right tension on the fiber when being mounted in the sensor pad is crucial for the fiber to become straightly fixed to the surface and should not be overlooked. Please, follow the instructions as outlined above.

Z70



� STEP 4: Applying the adhesive 4-1

Apply a layer of the UV-curable adhesive. Distribute the adhesive evenly with a wooden stick. Make sure the adhesive covers the entire glue zone i.e. the entire window inside the Teflon patch. There is no risk of using too much of this adhesive as the excess of adhesive will flow on the Teflon patch while pressing the sensor pad later on.

4-2

Apply a few drops of the Z70 adhesive (1 to 2 droplets with the dosing nozzle mounted on the bottle). Distribute the adhesive with a piece of the Teflon band by moving the band back and forth over the surface within the window of the Teflon patch. Ensure that the layer is not too thick, as thick layers of this adhesive will not cure completely or even not at all.

Note

Make sure that the used adhesive is not expired. The expiry date of the adhesive is mentioned on the bottle. The shelf life of the adhesives is at least 4 months when being stored in a cool dark place, see the data sheet for further information on storage conditions.

UV UV

Z70



� STEP 5: Installation of the strain sensor:

5-1

Put the sensor pad in place and apply manual pressure on the pad by means of the rectangular piece of glass. The applied force should be in the range of 10 to 15 N (1 to 1.5 kg). The pressure ensures a uniform distribution of the adhesive and at the same time makes sure that the fiber is slightly stretched. The excess of adhesive will flow on the Teflon patch due to the pressure and can be removed later on.

5-2

While pressing, perform the pre-cure with the Omnicure 1000 by exposing the sensor pad during 30 seconds to the UV-light. Move the light guide back and forth in order to make sure that each part of the sensor pad receives an equal dose of UV-light. The light guide should be as close as possible to the glass surface. Make sure to wear the protective eyewear to protect your eyes from the UV-radiation.

Note

UV-radiation may be harmful for the eyes and skin and protective eyewear (mandatory) and clothing (advised for frequent usage) should be used. Please follow the safety instructions as explained in the Users Guide of the UV-radiation source.

5-3

Put the sensor pad in place and apply manual pressure for at least 2 to 3 minutes on the pad by means of the rectangular piece of glass. The applied force should be in the range of 50 to 100 N (5 to 10 kg). This larger pressure is needed for the curing of the adhesive.

UV

UV UV

UV UV

Z70



Note

The advised 2 to 3 minutes curing time is only under optimal curing conditions. Keep in mind that the curing time of the Z70 adhesive depends on the ambient conditions and on the applied pressure. It is therefore advised to evaluate the curing time prior to the actual installation in order to reduce the risk of faulty installations.

� STEP 6: Removal of pad and Teflon sticker 6-1

Temporarily secure the fiber cables left and right from the sensor pad with pieces of tape in order to avoid the fiber from being pulled out of the adhesive accidentally. Forces acting on the fiber cable in perpendicular direction can very easily pull out the fiber from the adhesive.

Note

Forces acting on the fiber cable in perpendicular direction with respect to the surface can very easily pull the glued fiber out of the adhesive. Make sure not to pull at the cables at this stage of the installation process.

6-2

Carefully remove the sensor pad while retaining the fiber cable with your finger, as shown in the picture below. Avoid pulling on the fiber cables at any time.

Z70



6-3

Carefully cut away the drops of adhesive that have spilled on the Teflon patch by means of the scalpel or a knife.

6-4

Carefully remove the Teflon patch by making 2 incisions in the patch at the positions of the fiber ends.

.

UV UV



� STEP 7: Post-curing 7-1

Perform the final cure by exposing the strain gage during 5 to 10 minutes to the UV-light from the Omnicure 1000. Make sure the beam spot covers the entire glue zone but avoid making the beam spot larger than necessary. A stage can be used for mounting the UV-light guide above the sensor.

Note

The final cure makes sure that the adhesive is fully cured and this is a decisive factor for the final sensor performance. When not executed correctly, the sensor may not reach the specified characteristics. The required exposure time depends on the intensity of the used UV-source. The above description is made under the assumption that the Omnicure 1000 unit is being used at full power.

� STEP 8: Secure the fiber cabling

8-1

Secure the buffered fiber at both ends of the sensor with rapid adhesive. Mix a spoon of the component A with a few drops of the component B by means of the mixing cup. Stir the mixture with a wooden stick. Apply a few droplets of the adhesive by means of the stick at both ends of the glue zone, as indicated in the figure below. Make sure the rapid adhesive covers only the 900 µm buffered fiber cable and not the free fiber part. Curing of the adhesive happens already after a few minutes. It is fully cured after 10 minutes.

UV UV

UV UV



� STEP 9: Mechanical protection (optionally): In case the sensors are going to be used in an environment where a mechanical protection is needed, a layer of the protective sealant can be used to cover the entire assembly.

9-1

Clean and dry the surface

9-2

Apply a layer of a few millimeter thickness over the entire sensor. Use a wooden stick to distribute the gel-like substance more evenly.

9-3

The sealant begins to cure when it gets in contact with moisture in air. The relative air humidity should be at least 30 %.

9-4

Leave to cure for at least 2 hours. The sealant is fully cured after 72 hours.

Contact FOS&S for further information: tel. +32(0)14 58.11.91 fax +32(0)14 59.15.14

email: [email protected]

This product has been developed in the framework of a joint collaboration between the Belgian Science Policy and the Federal Public Service of Economy, SMEs, Independent Professions and Energy of Belgium.

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