This work was supported by the UK Engineering and Physical Sciences Research Council (EPSRC) HOME-Offshore: Holistic Operation and Maintenance for Energy from Offshore Wind Farms Consortium under grant EP/P009743/1.

1. Fiber optic thermal sensor design
<ol>
	<li>First identify the sensor design and specifications based on the target coil structure and the interrogation system features. The test coil used in this work has an oval geometry typical of electric machine coils (as illustrated in Figure 1A. Before individual sensing locations are determined, make design decisions to ensure that the optical sensing fiber remains operative in the mechanical and thermal environment typical of embedded wound coil application.</li>
	<li>Use a standard bend-insensitive polyimide-coated single mode fiber that is generally known to be able to operate in temperatures up to approximately 300 &#176;C; this fiber is thus suitable for application in wound coils used in conventional electric machines. 
	NOTE: The chosen optical fiber ensures sensor functionality in the thermal environment of a typical random wound coil operating in electric machines such as used in this work (class F and H with rated temperature of 155 and 180 &#176;C10, respectively. Bend-insensitive fiber is preferred for this application since it is designed to allow a small bending radius and to have a lower bending loss. This enables the sensor to be effectively conformed to the desired coil structure and sensing location(s) with minimal detrimental effect to sensing functionality.</li>
	<li>Set fiber length to 1.5 m. 
	NOTE: Fiber length is set according to the geometry of the target wound coil to be instrumented and the desired distance to the interrogation unit. The test coil circumferential length (shown in Figure 1A) is 0.3 meters and the chosen fiber length to interrogator from coil is 1.2 meters giving a total length of 1.5 m &#8211; this allows for sufficient fiber length to be looped within the test coil to ensure desired sensing locations are appropriately established and there is suitable distance between the test coil and the interrogator: Figure 3A illustrates the general length design approach. 
	NOTE: FBGSs can be located several kilometers from the interrogation unit. This is because an optic fiber is an efficient single carrier.</li>
	<li>Design the FBG array to consist of four FBG heads (5 mm) to enable distributed sensing within the coil structure so that two sensing locations are positioned in the coil sides and two are in the coil ends. 
	NOTE: Thermal sensing locations are identified based on relevant thermal monitoring standards for electric machines (i.e., 2 FBGS for slot sections and 2 for end-winding sections)10. The commercial interrogator design used in this work can enable simultaneous interrogation of up to 16 FBG sensing points down a single optical fiber.</li>
	<li>Use an FBG sensing head length of 5 mm; this is deemed sufficient to enable localized hot spot monitoring in current carrying random wound coils. 
	NOTE: Alternative commercial values of FBG head length (3 mm, 5 mm or 10 mm) can also be used in case a different sensing point dimension is required by the sensing application.</li>
	<li>Specify individual FBG heads to be grated with different wavelengths spaced in a bandwidth of 1529&#8211;60 nm to match the used commercial interrogator rating; this ensures prevention of FBG shifted wavelengths interference. 
	NOTE: The FBG heads wavelength, their expected wavelength shift bandwidth, and the application temperature variation need to be within the interrogation unit broadband light bandwidth to ensure that the sensing system can properly operate.</li>
	<li>Use an FC/APC fiber probe connector type, which is consistent with the interrogator unit. 
	NOTE: FC/APC is generally the preferred choice for FBG sensing due to low return losses.</li>
	<li>Provide the design and specifications of the sensor to a commercial FBG manufacturer - Figure 3B shows a final sketch of the FBG array design used in this work.</li>
</ol>
2. Interrogation system and sensor configuration
<ol>
	<li>Check and configure the designed and manufactured FBG array sensor to operate with the commercial interrogation system.</li>
	<li>Remove the protective cap from the FC/APC connector ferrule.</li>
	<li>Clean the connector end-face by wiping it gently with an optical connector cleaner. 
	NOTE: It is highly recommended to perform this step each time the sensor is connected to the interrogator. A Cletop-s commercial series optical cleaner was used in this work.</li>
	<li>Plug-in the cleaned FBG probe connector to the interrogator channel connector. 
	NOTE: Ensure that the keyway is correctly aligned when mating the connectors.</li>
	<li>Turn on the interrogator. 
	NOTE: The interrogator is connected to the PC via an RJ45 connector and internet cable.</li>
	<li>Run the configuration software. 
	NOTE: The interrogator software is a proprietary LabVIEW based software package provided by the interrogator manufacturer designed to enable the interrogator hardware unit operation.</li>
	<li>On the instrument set-up tab observe the reflected wavelength spectrums from the FBG array probe (for the FBG array design used in this work four peaks should be observed in the related channel spectrum). 
	NOTE: The reflected light intensity depends on FBG characteristics (above 50% is accepted).</li>
	<li>Set the sampling frequency to 10 Hz. This directly determines the number of temperature readings provided in a given 1 s period. 
	NOTE: The used interrogation system can operate at sampling frequencies up to 2.5 kHz; however, for thermal dynamics of current carrying coils monitored in this work 10 Hz is deemed a sufficient acquisition rate.</li>
	<li>In the measurements setting, name the FBG heads as FBG1, FBG2, FBG3 and FBG4. Choose wavelength as type of quantity to be presented graphically at this stage. The FBG array is configured and ready for the calibration step.</li>
</ol>
3. Packaging preparation
<ol>
	<li>Appropriately package the areas where FBG heads are imprinted (i.e., grated) in the array fiber to ensure sensing head isolation from mechanical excitation and thus yield exclusively thermal excitation responsive sensor. In addition, the fiber structure is fragile and it is not desirable to directly embedded it within the coil conductors: it requires adequate mechanical protection to retain integrity. In this work, the sensing area containing the four FBG heads that is embedded within the coil structure is packaged with polyetheretherketone (PEEK) and the remainder of the fiber is protected by Teflon &#8211; this is illustrated in Figure 3C.</li>
	<li>Design the packaging in the form of a narrow round capillary tube so the sensing fiber can be routed through and thus protected by the capillary. 
	NOTE: The capillary dimensions and thermal properties are particularly important where packaging of the area containing FBG sensing heads is concerned. It is generally desirable to ensure a relatively narrow wall thickness and use material that is not electrically conductive but provides a reasonable degree of thermal conductivity. The outer diameter of the PEEK capillary used in this work was 0.8 mm and its wall thickness is 0.1 mm.</li>
	<li>Prepare the PEEK capillary by cutting the adequate length of commercial PEEK tubing (length of the target coil structure with a few extra centimeters to allow for fiber insertion and Teflon to PEEK capillary joint preparation). 
	NOTE: The in situ instrumentation of the FBG array requires installation of the packaging first which is then inserted with the sensing fiber. Care must be taken to ensure smooth and cleaned capillary end openings.</li>
	<li>Take careful measurements of the FBG array and the PEEK capillary to accurately identify sensing locations on the outer surface of the PEEK capillary. This enables positioning of FBG sensing heads in target locations within the motorette test coil.</li>
	<li>Prepare the Teflon capillary by cutting an adequate length of commercial Teflon tubing to ensure that the fiber section outside the test coil geometry is protected and contained. 
	NOTE: The external packaging material of the non-sensing array section needs to have sufficient rigidity to provide adequate mechanical protection but also be flexible to allow for a practical connection to the interrogator; it is also desirable for this material to be EMI immune in this application. Teflon is found to provide satisfactory performance in this study however alternative materials can be applied.</li>
	<li>Prepare the appropriate shrink tube length to make the joint between the PEEK and the Teflon capillaries.</li>
</ol>
4. Free thermal calibration
<ol>
	<li>Calibrate the packaged FBG array sensor by inserting it into the thermal chamber to extract its discrete temperature versus wavelength points. 
	NOTE: Preferably the sensing area is shaped to match that of the target coil structure to provide calibration under strain levels similar to those when the package is embedded within the test coil.</li>
	<li>Connect the grated optical fiber to the interrogator and launch the pre-configured interrogator software routine.</li>
	<li>Set the thermal chamber oven to operate in a sequence of thermal steady state points &#8211; these are in a range of ambient to 170 &#176;C and in steps of every 10 degrees in this work. Create a table from the measured reflected wavelengths of each individual FBG in the array for every constant temperature emulated in the chamber. 
	NOTE: Sufficient time has to be allowed during calibration tests for thermal equilibrium to be reached at every examined steady state thermal point.</li>
	<li>Use the recorded shifted wavelength versus temperature measurements in 10 &#176;C steps to determine the optimal temperature-wavelength shift fit curves and their coefficients for each FBG. Figure 4 and Table 1 show the recorded calibration data measurements and the calculated fit curve, respectively. 
	NOTE: The relationship between the wavelength shift and the temperature variation of the FBG heads in the array is analyzed by polynomial quadratic regression in this work as this was found to deliver optimal characterization. From this analysis the polynomial quadratic regression fit curve coefficients are calculated11.</li>
	<li>Input the calculated coefficients in the relevant setting of the interrogator software to enable on-line temperature measurements from the FBG array.</li>
</ol>
5. Test coil build and FBG instrumentation
<ol>
	<li>First build and instrument the motorette random wound coil.
	<ol>
		<li>Design a winding bobbin to fit onto the winder device. 
		NOTE: The bobbin geometry is designed to match the desired turn geometry of the coil and ensure that the desired wound coil dimensions. The bobbin is designed to be easily dismantled so to facilitate straightforward removal of the wound coil without damaging its insulation.</li>
		<li>Place the selected enameled copper wire reel in the winder device and pull the copper wire through the winder rollers and the tension controller. 
		NOTE: Class F enameled copper wire is used in this work.</li>
		<li>Set the winder device turn number counter to zero.</li>
		<li>Set the winder to operate at low speed and control the desired wire tension.</li>
		<li>Wind half of the coil turns.</li>
		<li>Fit the prepared PEEK capillary in the center of the coil using Kapton tape. 
		NOTE: Care must be taken to ensure that the indexes on the PEEK capillary are positioned in the target places.</li>
		<li>Wind the rest of the coil turns.</li>
		<li>Remove the bobbin from the winder machine and disassemble to free the wound coil embedded with a PEEK capillary.</li>
		<li>Place the coil in the motorette frame. 
		NOTE: The motorette coil insulation system (slot insulation and slot wedges) must be appropriately installed with the coil.</li>
		<li>Prepare coil terminals and connect them to motorette terminals.</li>
		<li>Varnish the motorette using a winding varnish and place in an oven at appropriate temperature (150 &#176;C) to cure.</li>
	</ol>
	</li>
	<li>FBG array instrumentation:
	<ol>
		<li>First connect the FBG array to the interrogator; launch the interrogator software to monitor the FBG reflected wavelength while installing.</li>
		<li>Pull the fiber through the prepared shrinking tube.</li>
		<li>Carefully insert the fiber (sensing area) into the PEEK capillary until the end openings of Teflon and PEEK capillaries are in contact.</li>
		<li>Move the shrink tube to cover the capillaries ends and appropriately heat it until the desired fit is achieved.</li>
	</ol>
	</li>
</ol>
6. In situ calibration and evaluation
<ol>
	<li>Validate the obtained thermal calibration in step 4 after embedment and correct if required. The test also enables evaluating the FBG array performance in controlled static thermal condition.</li>
	<li>Place the motorette embedded with the FBG thermal array in the thermal oven. 
	NOTE: Conventional thermal sensor can be used for performance comparison purposes. Here thermocouples installed on the motorette coil surface are used.</li>
	<li>Repeat steps 4.3 and 4.4.</li>
	<li>Repeat step 4.5 including the temperature measured by FBG heads based on calibrated fit in step 4.</li>
	<li>Evaluate and compare the FBG array temperature measurements with the reference temperature. If measurement error is high, the recorded measurement in step 6.4 can be used to update the calibration.</li>
	<li>Take the motorette out of the thermal oven; it is ready for testing.</li>
</ol>
7. Testing
<ol>
	<li>Perform a static thermal condition test.
	<ol>
		<li>Connect the motorette to the DC power supply.</li>
		<li>Connect the FBG array to the interrogator; monitor and record its FBG temperature measurements.</li>
		<li>Control the DC power supply to inject the motorette with a DC current. 
		NOTE: The chosen DC current level must ensure that the T-rise in coil internal thermal hotspots is less than the permissible insulation temperature; this allows for non-destructive testing on the prototype coil.</li>
		<li>Stop recording measurements when the motorette coil thermal equilibrium is reached.</li>
	</ol>
	</li>
	<li>Perform a non-uniform thermal condition test.
	<ol>
		<li>Wind the external coil containing 20 turns around a selected test coil section.</li>
		<li>Connect the external coil to a separate DC power supply.</li>
		<li>Energize the motorette with the DC current applied in 7.1.3.</li>
		<li>Start recording thermal measurements once the thermal equilibrium is reached.</li>
		<li>Energize the external coil with a DC current to provide non-uniform thermal conditions by delivering localized thermal excitation on the test coil.</li>
		<li>Stop recording measurements once thermal equilibrium is reached.</li>
	</ol>
	</li>
</ol>

<ol>
	<li>Stone, G. C., Boulter, E. A., Culbert, I., Dhirani, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Electrical+insulation+for+rotating+machines-design,+evaluation,+aging,+testing,+and+repair-Book+Review.">Electrical insulation for rotating machines-design, evaluation, aging, testing, and repair-Book Review.</a> IEEE Electrical Insulation Magazine. 20 (3), 65-65 (2004).</li><li>Mohammed, A., Djurovi&#263;, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Stator+Winding+Internal+Thermal+Monitoring+and+Analysis+Using+In+Situ+FBG+Sensing+Technology.">Stator Winding Internal Thermal Monitoring and Analysis Using In Situ FBG Sensing Technology.</a> IEEE Transactions on Energy Conversion. 33 (3), 1508-1518 (2018).</li><li>Zhang, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Online+thermal+monitoring+models+for+induction+machines.">Online thermal monitoring models for induction machines.</a> IEEE Transactions on Energy Conversion. 30 (4), 1279-1287 (2015).</li><li>Mohammed, A., Djurovi&#263;, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=FBG+array+sensor+use+for+distributed+internal+thermal+monitoring+in+low+voltage+random+wound+coils.">FBG array sensor use for distributed internal thermal monitoring in low voltage random wound coils.</a> 2017 6th Mediterranean Conference on Embedded Computing (MECO). , 1-4 (2017).</li><li>Rao, Y. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=In-fibre+Bragg+grating+sensors.">In-fibre Bragg grating sensors.</a> Measurement Science and Technology. 8 (4), 355 (1997).</li><li>Mohammed, A., Djurovi&#263;, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+study+of+distributed+embedded+thermal+monitoring+in+electric+coils+based+on+FBG+sensor+multiplexing.">A study of distributed embedded thermal monitoring in electric coils based on FBG sensor multiplexing.</a> Microprocessors and Microsystems. 62, 102-109 (2018).</li><li>Lu, P., Men, L., Chen, Q. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Resolving+cross+sensitivity+of+fiber+Bragg+gratings+with+different+polymeric+coatings.">Resolving cross sensitivity of fiber Bragg gratings with different polymeric coatings.</a> Applied Physics Letters. 92 (17), 171112 (2008).</li><li>Mohammed, A., Djurovi&#263;, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=FBG+Thermal+Sensing+Features+for+Hot+Spot+Monitoring+in+Random+Wound+Electric+Machine+Coils.">FBG Thermal Sensing Features for Hot Spot Monitoring in Random Wound Electric Machine Coils.</a> IEEE Sensors Journal. 17 (10), 3058-3067 (2017).</li><li>IEEE. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=IEEE+standard+test+procedure+for+thermal+evaluation+of+systems+of+insulating+materials+for+random-wound+AC+electric+machinery.">IEEE standard test procedure for thermal evaluation of systems of insulating materials for random-wound AC electric machinery.</a> IEEE Std 117-2015. , 1-34 (2016).</li><li>IEC. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Rotating+electrical+machines+-+Part+1:+Rating+and+performance.">Rotating electrical machines - Part 1: Rating and performance.</a> IEC. , (2010).</li><li>Mohammed, A., Djurovi&#263;, S., Smith, A. C., Tshiloz, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=FBG+sensing+for+hot+spot+thermal+monitoring+in+electric+machinery+random+wound+components.">FBG sensing for hot spot thermal monitoring in electric machinery random wound components.</a> 2016 XXII International Conference on Electrical Machines (ICEM). , 2266-2272 (2016).</li></ol>

The authors have nothing to disclose.

The paper has demonstrated the procedure required to design, calibrate and test in situ FBG thermal sensors in low voltage wound coils. These sensors offer a number of advantages for in situ sensing applications within current carrying wound coil structures: they are fully EMI immune, are flexible and can conform to an arbitrary desired geometry to deliver arbitrary desired sensing point locations with high accuracy, and can provide a large number of sensing points on a single sensor. While thermal sensing within wound coils can be achieved with conventional thermal monitoring techniques employing thermocouple or resistance temperature detectors, the application of FBGs is shown to provide a number of attractive functional advantages.
Appropriate packaging of the FBG array sensor is key to its effective utilization. It is important that individual sensing heads or the entire sensing area of the fiber be appropriately packaged to ensure isolation of FBG heads from mechanical excitation in a rigid yet flexible thermally conductive capillary. It is desirable for the capillary to be designed of non-electrically conductive material as this ensures optimal performance in the EMI rich environment characteristic of current carrying coils.
Care needs to be taken during the process of packaging capillary installation into the coil to accurately position the package segments in their corresponding sensing locations. It is also essential to optimize the capillary geometry in case highly dynamic thermal conditions are to be observed.
It is vital to ensure accurate characterization of the coil embedded sensor. This is best done by performing free packaged sensor calibration before its installation within the wound coil geometry. While a high degree of protection from mechanical excitation is provided by the in situ packaging, the installation process can result in wavelength shift due to strain sensitivity. If performed carefully this can be negligible; however, it is good practice for this to be ascertained in in situ calibration tests where possible.
This application of FBGs within wound coils is relatively new and opens a number of opportunities for improved design, utilization, monitoring and health diagnosis of electrical machines. Further work is needed to reduce cost of these and make them a credibly viable option for large scale application in electric machinery.

Random wound coils are a key design element of most electric apparatus in modern industrial systems and are commonly used in low voltage electric machinery. A major barrier to improved usage of wound coils in these applications is their sensitivity to in-service electro-thermal stress. Thermal overloads are particularly pertinent in this regard as these can cause insulation coil insulation system breakdown and ultimately its total failure1; this can arise due to excessive coil current levels, or other causes such as a coil electrical fault or a cooling system malfunction, where localized hot spots are induced in the coil structure leading to insulation breakdown. Enabling operative in situ distributed thermal monitoring of an in-service coil&#8217;s internal structure allows for development of improved utilization and condition based maintenance routines; it would allow for advanced understanding and identification of the coils&#8217; operating status and any degradation process, and thus condition based corrective action to maintain the operating status and prevent or slow down further damage2,3.
The presented method is aimed at enabling in situ monitoring of electric coil structure embedded thermal conditions through use of flexible and electromagnetic interference immune (EMI) fiber Bragg grated optical thermal sensors. The method offers a number of functional advantages over existing thermal monitoring techniques used in electric coils: these almost invariably rely on usage of thermocouple (TCs) or resistance temperature detectors (RTDs) that are not EMI immune; they are made of conductive materials; and they are generally reasonably bulky hence not ideally suited for sensing applications within the structure of wound electric coils. The usage of robust and flexible fiber optic FBG thermal sensors provides a number of considerable improvements in this respect, not solely due to sensor EMI immunity but also its small size, multiplexing ability and its flexibility, which enables them to be embedded into and conform to an arbitrary wound coil architecture to achieve thermal sensing with pinpoint accuracy in desired structural locations4. These features are especially attractive in electrical machine (EM) applications where device thermal limits are defined by electric coil thermal conditions and are particularly pertinent in the light of the expected considerable growth in EM usage with proliferation of electric transport.
This paper presents the methodology of instrumenting a typical low voltage random wound coil structure with thermal FBG sensors to enable on-line monitoring of internal hotspots. A detailed protocol of the FBG sensor choice, design, packaging, instrumentation, calibration and usage is reported. This is presented on an IEEE standard random wound coil motorette system. The paper also reports the obtained in situ thermal measurements under static and non-uniform thermal operating condition of the examined test coil.
FBGs are formed by the process of &#8216;grating&#8217; the optical fiber core to create periodic longitudinal imprints (usually referred to as sensing heads in FBG sensing applications); when the fiber containing FBGs is exposed to ultraviolet light each existing FBG head will cause its refractive index to be periodically modulated5. The sensing head reflected wavelengths will be affected by the thermal and mechanical conditions that the fiber is exposed to, and thus enable the grated fiber to be applied as a thermal or mechanical sensor assuming adequate design and application.
The FBG technology is especially attractive for distributed sensing applications: it allows for a single optical fiber to be grated to contain multiple FBG sensing heads, where each head is coded with a distinct Bragg wavelength and acts as a distinct sensing point. This type of FBG based sensing device is known as an FBG array sensor6 and its operating concept is illustrated in Figure 1. Broadband light is used to excite the array resulting in distinct reflected wavelengths from each contained FBG head; here, each head reflects a defined wavelength (i.e., Bragg wavelength) that matches its grating design and is also dependent on the prevailing thermal and mechanical conditions at the head (i.e., sensing) location. An interrogator device is needed to enable array fiber excitation with light and the inspection of the reflected spectra for distinct Bragg wavelengths containing information on localized thermal and/or mechanical conditions.
A particularly important aspect of FBG thermal sensor implementation is the mitigation of thermo-mechanical cross sensitivity effects to obtain as close as possible to exclusively thermal readings7. The FBG inherent feature of thermo-mechanical cross-sensitivity requires careful design of FBG sensors aimed at thermal only or mechanical only sensing applications. Where thermal sensing is concerned an effective method of mitigation FBG mechanical excitation sensitivity is to isolate the sensing head with a packaging capillary made of material suitable for a given application; in the coil embedded thermal sensing application examined in this work this not only reduces cross-sensitivity problems but also serves to protect the fragile sensing fiber structure from underside and potentially destructive mechanical stress8.
Figure 2A shows the random wound electric coil test specimen used as a demonstration vehicle in this paper. The coil is designed according to IEEE standards9 for thermal evaluation procedures of random wound coils&#8217; insulation system; the resulting test system shown in Figure 2B is known as a motorette system and is representative of a winding and its insulation system in a low voltage electrical machine. In the presented case study, the motorette will be instrumented with an FBG array thermal sensor consisting of four thermal sensing points, to emulate typical thermal sensing hot spots of interest in practical machine applications that tend to be localized in coil end winding and slot sections. For calibration and performance evaluation, the FBG embedded motorette will be thermally excited using a commercial thermal chamber and a DC power supply.

<table border="0" cellpadding="0" cellspacing="0" style="width:665px;" width="665">
	<colgroup>
		<col />
		<col />
		<col />
		<col />
	</colgroup>
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Cletop-S</td>
			<td style="width:121px;">Fujikura</td>
			<td align="right" style="width:161px;">14110601</td>
			<td style="width:167px;">Commercial optic connector cleaner</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Copper wire AWG24</td>
			<td style="width:121px;">RS</td>
			<td style="width:161px;">357-744</td>
			<td>Commercial insulated copper wire</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">DC power supply</td>
			<td style="width:121px;">TTi</td>
			<td style="width:161px;">CPX400SP</td>
			<td>Commercial 420W DC power supply</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">FBG sensors</td>
			<td style="width:121px;">ATGratings</td>
			<td style="width:161px;">NA</td>
			<td>Commerically manufactured FBG array to design spec</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Heat Shrink Tubing</td>
			<td style="width:121px;">RS</td>
			<td style="width:161px;">700-4532</td>
			<td>Heat Shrink Tubing 3mm Sleeve Dia. x 10m</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Kapton masking tape</td>
			<td style="width:121px;">RS</td>
			<td>436-2762</td>
			<td>Orange Masking Tape Tesa 51408</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">PEEK tubing</td>
			<td style="width:121px;">Polyflon</td>
			<td align="right" style="width:161px;">4901000060</td>
			<td>Commercial PEEK tubing</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">SmartScan04</td>
			<td style="width:121px;">Smartfibres UK</td>
			<td style="width:161px;">S-Scan-04-F-60-U-UK</td>
			<td>Commercial interrogator system</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Thermal Oven</td>
			<td style="width:121px;">Lenton</td>
			<td style="width:161px;">WHT6/30</td>
			<td>Commercial thermal oven</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Winder machine</td>
			<td style="width:121px;">RS</td>
			<td style="width:161px;">244-2636</td>
			<td>Commercial winder machine</td>
		</tr>
	</tbody>
</table>

design, instrumentation and usage protocols for distributed in situ thermal hot spots monitoring in electric coils using fbg sensor multiplexing

Random wound coils are a key operational element of most electric apparatus in modern industrial systems, including low voltage electric machines. One of the major current bottlenecks in improved exploitation of electrical devices is the high sensitivity of their wound components to in-service thermal stress. The application of conventional thermal sensing methods (e.g., thermocouples, resistance temperature detectors) for thermal condition monitoring of current carrying random wound coils can impose considerable operational limitations due to sensor size, EMI sensitivity and the existence of electrically conductive material in their construction. Another substantial limitation exists in distributed sensing applications and is caused by what is often a considerable length and volume of conventional sensor wiring leads.
This paper reports the design of a fiber optic FBG&#160;sensing system&#160;intended for enabling real-time distributed internal thermal&#160;condition monitoring within random wound coils. The procedure of random wound coil instrumentation with the FBG sensing system is reported in a case study on an IEEE standard wound coil representative of those utilized in electrical machines. The reported work also presents and discusses important practical and technical aspects of FBG sensing system implementation and application, including the FBG array geometry design, sensing head and fiber packaging, the sensor array installation and calibration procedure and the use of a commercial interrogation system for obtaining thermal measurements. Finally, the in situ multiplexed FBG sensing system thermal monitoring performance is demonstrated in representative static and dynamic thermal conditions.&#160;

Figure 5 presents the temperatures measured by the array sensor in the static thermal test. The four internal temperature readings, taken by respective array FBG heads in corresponding coil locations, are observed to be closely similar as is generally expected for the examined test conditions; there is a slight variation between the reported individual measurement of less that &#8776;1.5 &#176;C between the observed average hotspot temperatures of &#8776;75.5 &#176;C.
Figure 6 reports the array sensor measurements obtained in the non-uniform thermal condition test. These are shown first for the period where there is no excitation in the external coil (first &#8776;75s) indicating closely uniform measured thermal levels, as would be expected. The external coil is then excited resulting in additional localized thermal excitation: this results in a clear change in the observed measurements, with the sensing point in closest proximity to the external coil (i.e., FBG4) measuring the highest thermal level (&#8776;128.6 &#176;C) and that furthest away the lowest (&#8776;117.6 &#176;C); the FBG temperature sensors located between these report intermediate and closely similar temperature levels (&#8776;122.7 and &#8776;121.6 &#176;C). The observed readings clearly relate to individual sensing head distribution in the examined test coil geometry. Furthermore, the results clearly demonstrate the functional capability of the coil embedded array sensor for monitoring and identification of internal distributed thermal hotspot distribution in random wound coils.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/59923/59923fig1.jpg" /> 
Figure 1. The FBG array sensor operating concept. This figure has been modified from a previous publication4. <a href="https://www.jove.com/files/ftp_upload/59923/59923fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/59923/59923fig2.jpg" /> 
Figure 2. IEEE standard motorette coil assembly. (A) Random wound electric coil; see IEEE standards9. (B) Assembled and varnished IEEE standard motorette. <a href="https://www.jove.com/files/ftp_upload/59923/59923fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/59923/59923fig3.jpg" /> 
Figure 3. FBG thermal sensor array design. (A) FBG array fiber length, (B) FBG head locations in the array structure, (C) FBG array packaging design. <a href="https://www.jove.com/files/ftp_upload/59923/59923fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/59923/59923fig4.jpg" /> 
Figure 4. The packaged array sensor FBG heads calibration characteristics. The characteristics are derived from the data obtained in the array free thermal calibration tests. This figure has been modified from a previous publication4. <a href="https://www.jove.com/files/ftp_upload/59923/59923fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/59923/59923fig5.jpg" /> 
Figure 5. FBG array thermal measurements obtained in steady state thermal condition test. The individual head thermal measurements reported by the FBG array sensor are shown with an inset detail steady-state measurement view. This figure has been modified from a previous publication4. <a href="https://www.jove.com/files/ftp_upload/59923/59923fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" class="xfigimg" src="/files/ftp_upload/59923/59923fig6.jpg" /> 
Figure 6. Thermal measurements in the non-uniform thermal condition test. This figure has been modified from a previous publication4. <a href="https://www.jove.com/files/ftp_upload/59923/59923fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<table border="1" fo:keep-together.within-page="1">
	<tbody>
		<tr>
			<td></td>
			<td colspan="2">Intercept</td>
			<td colspan="2">B1</td>
			<td colspan="2">B2</td>
			<td>Statistics</td>
		</tr>
		<tr>
			<td></td>
			<td>Value</td>
			<td>Standard Error</td>
			<td>Value</td>
			<td>Standard Error</td>
			<td>Value</td>
			<td>Standard Error</td>
			<td>Adj. R-Square</td>
		</tr>
		<tr>
			<td>FBG1</td>
			<td>1555.771</td>
			<td>0.0137</td>
			<td>0.00855</td>
			<td>2.85E-04</td>
			<td>1.50E-05</td>
			<td>1.34E-06</td>
			<td>0.99978</td>
		</tr>
		<tr>
			<td>FBG2</td>
			<td>1547.669</td>
			<td>0.0112</td>
			<td>0.00851</td>
			<td>2.34E-04</td>
			<td>1.41E-05</td>
			<td>1.10E-06</td>
			<td>0.99985</td>
		</tr>
		<tr>
			<td>FBG3</td>
			<td>1539.852</td>
			<td>0.0101</td>
			<td>0.00871</td>
			<td>2.11E-04</td>
			<td>1.30E-05</td>
			<td>9.90E-07</td>
			<td>0.99988</td>
		</tr>
		<tr>
			<td>FBG4</td>
			<td>1531.768</td>
			<td>0.0131</td>
			<td>0.00808</td>
			<td>2.72E-04</td>
			<td>1.67E-05</td>
			<td>1.28E-06</td>
			<td>0.9998</td>
		</tr>
	</tbody>
</table>
Table 1: Calculated polynomial qudratic fit curve parameters. The calculated parameters standard error and individual head correction coefficientsare included; good linearity and a coorection factor coefficient in excess of 0.999 was observed for the four tested FBG heads. This table has been modified from a previous publication4.

Watch this Scientific Journal Video about Design, Instrumentation and Usage Protocols for Distributed In Situ Thermal Hot Spots Monitoring in Electric Coils using FBG Sensor Multiplexing at JoVE.com

Design, Instrumentation and Usage Protocols for Distributed In Situ Thermal Hot Spots Monitoring in Electric Coils using FBG Sensor Multiplexing

This paper presents a protocol that enables instrumentation of random wound electric coils with fiber Bragg grating (FBG) thermal sensors for the purpose of distributed condition monitoring of internal thermal hot spots.

Random wound coils are a key operational element of most electric apparatus in modern industrial systems, including low voltage electric machines. One of ...

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5.7K Views.  University of Manchester. This paper presents a protocol that enables instrumentation of random wound electric coils with fiber Bragg grating (FBG) thermal sensors for the purpose of distributed condition monitoring of internal thermal hot spots.

Video: Design, Instrumentation and Usage Protocols for Distributed In Situ Thermal Hot Spots Monitoring in Electric Coils using FBG Sensor Multiplexing

In Situ Neutron Powder Diffraction Using Custom-made Lithium-ion Batteries

Li-ion batteries are widely used in portable electronic devices and are considered as promising candidates for higher-energy applications such as electric vehicles.1,2 However, many challenges, such as energy density and battery lifetimes, need to be overcome before this particular battery technology can be widely implemented in such applications.3 This research is challenging, and we outline a method to address these challenges using in situ NPD to probe the crystal structure of electrodes undergoing electrochemical cycling (charge/discharge) in a battery. NPD data help determine the underlying structural mechanism responsible for a range of electrode properties, and this information can direct the development of better electrodes and batteries.
We briefly review six types of battery designs custom-made for NPD experiments and detail the method to construct the &#8216;roll-over&#8217; cell that we have successfully used on the high-intensity NPD instrument, WOMBAT, at the Australian Nuclear Science and Technology Organisation (ANSTO). The design considerations and materials used for cell construction are discussed in conjunction with aspects of the actual in situ NPD experiment and initial directions are presented on how to analyze such complex in situ data.

In Situ Neutron Powder Diffraction Using Custom-made Lithium-ion Batteries

We describe the design and construction of an electrochemical cell for the examination of electrode materials using in situ neutron powder diffraction (NPD). We briefly comment on alternate in situ NPD cell designs and discuss methods for the analysis of the corresponding in situ NPD data produced using this cell.

Li-ion batteries are widely used in portable electronic devices and are considered as promising candidates for higher-energy applications such as electric ...

Detection and Recovery of Palladium, Gold and Cobalt Metals from the Urban Mine Using Novel Sensors/Adsorbents Designated with Nanoscale Wagon-wheel-shaped Pores

Developing low-cost, efficient processes for recovering and recycling palladium, gold and cobalt metals from urban mine remains a significant challenge in industrialized countries. Here, the development of optical mesosensors/adsorbents (MSAs) for efficient recognition and selective recovery of Pd(II), Au(III), and Co(II) from urban mine was achieved. A simple, general method for preparing MSAs based on using high-order mesoporous monolithic scaffolds was described. Hierarchical cubic Ia3d wagon-wheel-shaped MSAs were fabricated by anchoring chelating agents (colorants) into three-dimensional pores and micrometric particle surfaces of the mesoporous monolithic scaffolds. Findings show, for the first time, evidence of controlled optical recognition of Pd(II), Au(III), and Co(II) ions and a highly selective system for recovery of Pd(II) ions (up to ~95%) in ores and industrial wastes. Furthermore, the controlled assessment processes described herein involve evaluation of intrinsic properties (e.g., visual signal change, long-term stability, adsorption efficiency, extraordinary sensitivity, selectivity, and reusability); thus, expensive, sophisticated instruments are not required. Results show evidence that MSAs will attract worldwide attention as a promising technological means of recovering and recycling palladium, gold and cobalt metals.

Because of the importance and extensive use of palladium, gold and cobalt metals in high-technology equipment, their recovery and recycling constitute an important industrial challenge. The metal recovery system described herein is a simple, low-cost means for the effective detection, removal, and recovery of these metals from the urban mine.

Developing low-cost, efficient processes for recovering and recycling palladium, gold and cobalt metals from urban mine remains a significant challenge in ...

How to Build a Vacuum Spring-transport Package for Spinning Rotor Gauges

The spinning rotor gauge (SRG) is a high-vacuum gauge often used as a secondary or transfer standard for vacuum pressures in the range of 1.0 x 10-4 Pa to 1.0 Pa. In this application, the SRGs are frequently transported to laboratories for calibration. Events can occur during transportation that change the rotor surface conditions, thus changing the calibration factor. To assure calibration stability, a spring-transport mechanism is often used to immobilize the rotor and keep it under vacuum during transport. It is also important to transport the spring-transport mechanism using packaging designed to minimize the risk of damage during shipping. In this manuscript, a detailed description is given on how to build a robust spring-transport mechanism and shipping container. Together these form a spring-transport package. The spring-transport package design was tested using drop-tests and the performance was found to be excellent. The present spring-transport mechanism design keeps the rotor immobilized when experiencing shocks of several hundred g (g = 9.8 m/sec2 and is the acceleration due to gravity), while the shipping container assures that the mechanism will not experience shocks greater than about 100 g during common shipping mishaps (as defined by industry standards).

Here we describe how to build a robust spring-transport mechanism for a spinning rotor gauge. This device securely immobilizes the rotor and keeps it under vacuum during transportation. We also describe packaging that minimizes the risk of damage during transport. Tests show our design works for typical shocks during transport.

The spinning rotor gauge (SRG) is a high-vacuum gauge often used as a secondary or transfer standard for vacuum pressures in the range of 1.0 x 10-4 Pa to ...

Plasmonic Trapping and Release of Nanoparticles in a Monitoring Environment

Plasmonic tweezers use surface plasmon polaritons to confine polarizable nanoscale objects. Among the various designs of plasmonic tweezers, only a few can observe immobilized particles. Moreover, a limited number of studies have experimentally measured the exertable forces on the particles. The designs can be classified as the protruding nanodisk type or the suppressed nanohole type. For the latter, microscopic observation is extremely challenging. In this paper, a new plasmonic tweezer system is introduced to monitor particles, both in directions parallel and orthogonal to the symmetric axis of a plasmonic nanohole structure. This feature enables us to observe the movement of each particle near the rim of the nanohole. Furthermore, we can quantitatively estimate the maximal trapping forces using a new fluidic channel.

A microchip fabrication process that incorporates plasmonic tweezers is presented here. The microchip enables the imaging of a trapped particle to measure maximal trapping forces.

Plasmonic tweezers use surface plasmon polaritons to confine polarizable nanoscale objects. Among the various designs of plasmonic tweezers, only a few ...

Micro/Nano-scale Strain Distribution Measurement from Sampling Moir&#233; Fringes

This work describes the measurement procedure and principles of a sampling moir&#233; technique for full-field micro/nano-scale deformation measurements. The developed technique can be performed in two ways: using the reconstructed multiplication moir&#233; method or the spatial phase-shifting sampling moir&#233; method. When the specimen grid pitch is around 2 pixels, 2-pixel sampling moir&#233; fringes are generated to reconstruct a multiplication moir&#233; pattern for a deformation measurement. Both the displacement and strain sensitivities are twice as high as in the traditional scanning moir&#233; method in the same wide field of view. When the specimen grid pitch is around or greater than 3 pixels, multi-pixel sampling moir&#233; fringes are generated, and a spatial phase-shifting technique is combined for a full-field deformation measurement. The strain measurement accuracy is significantly improved, and automatic batch measurement is easily achievable. Both methods can measure the two-dimensional (2D) strain distributions from a single-shot grid image without rotating the specimen or scanning lines, as in traditional moir&#233; techniques. As examples, the 2D displacement and strain distributions, including the shear strains of two carbon fiber-reinforced plastic specimens, were measured in three-point bending tests. The proposed technique is expected to play an important role in the non-destructive quantitative evaluations of mechanical properties, crack occurrences, and residual stresses of a variety of materials.

Micro/Nano-scale Strain Distribution Measurement from Sampling Moir&amp;#233; Fringes

A sampling moir&#233; technique featuring 2-pixel and multi-pixel sampling methods for high-accuracy strain distribution measurements at the micro/nano-scale is presented here.

This work describes the measurement procedure and principles of a sampling moir&#233; technique for full-field micro/nano-scale deformation measurements. ...

In Situ Monitoring of the Accelerated Performance Degradation of Solar Cells and Modules: A Case Study for Cu(In,Ga)Se2 Solar Cells

The levelized cost of electricity (LCOE) of photovoltaic (PV) systems is determined by, among other factors, the PV module reliability. Better prediction of degradation mechanisms and prevention of module field failure can consequently decrease investment risks as well as increase the electricity yield. An improved knowledge level can for these reasons significantly decrease the total costs of PV electricity. 
In order to better understand and minimize the degradation of PV modules, the occurring degradation mechanisms and conditions should be identified. This should preferably happen under combined stresses, since modules in the field are also simultaneously exposed to multiple stress factors. Therefore, two 'Combined Stress test with in situ measurement' setups have been designed and constructed. These setups allow the simultaneous use of humidity, temperature, illumination, and electrical biases as independently controlled stress factors on solar cells and minimodules. The setups also allow real-time monitoring of the electrical properties of these samples. This protocol presents these setups and describes the experimental possibilities. Moreover, results obtained with these setups are also presented: various examples about the influence of both deposition and degradation conditions on the stability of thin film Cu(In,Ga)Se2 (CIGS) as well as Cu2ZnSnSe4 (CZTS) solar cells are described. Results on the temperature dependency of CIGS solar cells are also presented.

In Situ Monitoring of the Accelerated Performance Degradation of Solar Cells and Modules: A Case Study for CuIn,GaSe2 Solar Cells

Two &#39;Combined Stress test with in situ measurement&#39; setups, which allow real-time monitoring of accelerated degradation of solar cells and modules, were designed and constructed. These setups allow the simultaneous use of humidity, temperature, electrical biases, and illumination as independently controlled stress factors. The setups and various experiments executed are presented.

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Electric-field Control of Electronic States in WS2 Nanodevices by Electrolyte Gating

A method of carrier number control by electrolyte gating is demonstrated. We have obtained WS2 thin flakes with atomically flat surface via scotch tape method or individual WS2 nanotubes by dispersing the suspension of WS2 nanotubes. The selected samples have been fabricated into devices by the use of the electron beam lithography and electrolyte is put on the devices. We have characterized the electronic properties of the devices under applying the gate voltage. In the small gate voltage region, ions in the electrolyte are accumulated on the surface of the samples which leads to the large electric potential drop and resultant electrostatic carrier doping at the interface. Ambipolar transfer curve has been observed in this electrostatic doping region. When the gate voltage is further increased, we met another drastic increase of source-drain current which implies that ions are intercalated into layers of WS2 and electrochemical carrier doping is realized. In such electrochemical doping region, superconductivity has been observed. The focused technique provides a powerful strategy for achieving the electric-filed-induced quantum phase transition.

Electric-field Control of Electronic States in WS2 Nanodevices by Electrolyte Gating

Here, we present a protocol to control the carrier number in solids by using the electrolyte.

A method of carrier number control by electrolyte gating is demonstrated. We have obtained WS2 thin flakes with atomically flat surface via scotch tape ...

Methods of Ex Situ and In Situ Investigations of Structural Transformations: The Case of Crystallization of Metallic Glasses

We demonstrate the use of two nuclear-based analytical methods that can follow the modifications of microstructural arrangement of iron-based metallic glasses (MGs). Despite their amorphous nature, the identification of hyperfine interactions unveils faint structural modifications. For this purpose, we have employed two techniques that utilize nuclear resonance among nuclear levels of a stable 57Fe isotope, namely M&#246;ssbauer spectrometry and nuclear forward scattering (NFS) of synchrotron radiation. The effects of heat treatment upon (Fe2.85Co1)77Mo8Cu1B14 MG are discussed using the results of ex situ and in situ experiments, respectively. As both methods are sensitive to hyperfine interactions, information on structural arrangement as well as on magnetic microstructure is readily available. M&#246;ssbauer spectrometry performed ex situ describes how the structural arrangement and magnetic microstructure appears at room temperature after the annealing under certain conditions (temperature, time), and thus this technique inspects steady states. On the other hand, NFS data are recorded in situ during dynamically changing temperature and NFS examines transient states. The use of both techniques provides complementary information. In general, they can be applied to any suitable system in which it is important to know its steady state but also transient states.

Methods of Ex Situ and In Situ Investigations of Structural Transformations: The Case of Crystallization of Metallic Glasses

Here, we present a protocol to describe ex situ and in situ investigations of structural transformations in metallic glasses. We employed nuclear-based analytical methods which inspect hyperfine interactions. We demonstrate the applicability of M&#246;ssbauer spectrometry and nuclear forward scattering of synchrotron radiation during temperature-driven experiments.

We demonstrate the use of two nuclear-based analytical methods that can follow the modifications of microstructural arrangement of iron-based metallic ...

Radio Frequency Magnetron Sputtering of GdBa2Cu3O7&#8722;&#948;/ La0.67Sr0.33MnO3 Quasi-bilayer Films on SrTiO3 (STO) Single-crystal Substrates

Here, we demonstrate a method of coating ferromagnetic La0.67Sr0.33MnO3 (LSMO) nanoparticles on (001) SrTiO3 (STO) single-crystal substrates by radio frequency (RF) magnetron sputtering. LSMO nanoparticles were deposited with diameters from 10 to 20 nm and heights between 20 and 50 nm. At the same time, (Gd) Ba2Cu3O7&#8722;&#948; ((Gd) BCO) films were fabricated on both undecorated and LSMO nanoparticle decorated STO substrates using RF magnetron sputtering. This report also describes the properties of GdBa2Cu3O7&#8722;&#948;/ La0.67Sr0.33MnO3 quasi-bilayer films structures (e.g., crystalline phase, morphology, chemical composition); magnetization, magneto-transport, and superconducting transport properties were also evaluated.

Radio Frequency Magnetron Sputtering of GdBa2Cu3O7ÃƒÂ¢Ã‹â€ &#039;ÃƒÅ½ ´/ La0.67Sr0.33MnO3 Quasi-bilayer Films on SrTiO3 STO Single-crystal Substrates

Here, we present a protocol to grow LSMO nanoparticles and (Gd) BCO films on (001) SrTiO3 (STO) single-crystal substrates by radio frequency (RF)-sputtering.

Here, we demonstrate a method of coating ferromagnetic La0.67Sr0.33MnO3 (LSMO) nanoparticles on (001) SrTiO3 (STO) single-crystal substrates by radio ...

Crack Monitoring in Resonance Fatigue Testing of Welded Specimens Using Digital Image Correlation

A procedure using digital image correlation (DIC) to detect cracks on welded specimens during fatigue tests on resonance testing machines is presented. It is intended as a practical and reproducible procedure to identify macroscopic cracks at an early stage and monitor crack propagation during fatigue tests. It consists of strain field measurements at the weld using DIC. Images are taken at fixed load cycle intervals. Cracks become visible in the computed strain field as elevated strains. This way, the whole width of a small-scale specimen can be monitored to detect where and when a crack initiates. Subsequently, it is possible to monitor the development of the crack length. Because the resulting images are saved, the results are verifiable and comparable. The procedure is limited to cracks initiating at the surface and is intended for fatigue tests under laboratory conditions. By visualizing the crack, the presented procedure allows direct observation of macrocracks from their formation until rupture of the specimen.

Digital image correlation is used in fatigue tests on a resonance testing machine to detect macroscopic cracks and monitor crack propagation in welded specimens. Cracks on the specimen surface become visible as increased strains.

A procedure using digital image correlation (DIC) to detect cracks on welded specimens during fatigue tests on resonance testing machines is presented. It ...