Guemes et al 2018a - Revision history

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Scipediacontent: Scipediacontent moved page Draft Content 603023624 to Guemes et al 2018a

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Scipediacontent moved page Draft Content 603023624 to Guemes et al 2018a

Scipediacontent: Created page with "== Abstract == Fiber optic sensors, both FBG point sensors and distributed sensors based on OFDR, are accepted and validated as temperature and strain sensors, enjoy a high T..."

2022-03-31T15:44:09Z

Created page with "== Abstract == Fiber optic sensors, both FBG point sensors and distributed sensors based on OFDR, are accepted and validated as temperature and strain sensors, enjoy a high T..."

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== Abstract ==

Fiber optic sensors, both FBG point sensors and distributed sensors based on OFDR, are accepted and validated as temperature and strain sensors, enjoy a high TRL and are already in use in many industrial applications.
The determination of occurrence of damage from strain measures is a technique still not mature, its TRL can be 2-3. A local crack can greatly weaken a structure, it will be the point of initiation of the catastrophic failure, but until the failure has occurred, the crack only causes very small changes in the overall behavior of the structure (stiffness, natural frequencies, strain field, etc.). Therefore, and unless the position of the crack coincides with the predetermined position of the sensors, it is not easy to detect the existence of damage from strain measurements.
In this work two parallel techniques, experimentally validated, are presented. The first one is to use distributed measurement, with an optical fiber placed in the area where the damage is expected. It is verified that a delamination causes residual strains in the delaminated area, of  significant value, and that will be detected by the fiber, if it crosses the delamination zone.
The second technique is to compare the strains measurements obtained at different points, when the structure subjected to external loads. Although the changes are going to be very small, local damage causes a redistribution of the rigidity of the structure, and of the load paths, and therefore the strains readings, may be slightly different. Results of the application of the model are presented to actual composite elements, such as an isogrid structure, and a small wind turbine blade.

== Full document ==
<pdf>Media:Draft_Content_603023624native-articles-118.pdf</pdf>

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 As before, the structure was submitted to loads of increasing levels, and strains were recorded, first without any damage, to obtain the baseline, and next after artificial damages.
-==[insert Figure 12]==
+[insert Figure 12]<div class="center" style="width: auto; margin-left: auto; margin-right: auto;">
-−
 <div class="center" style="width: auto; margin-left: auto; margin-right: auto;">
 <span style="text-align: center; font-size: 75%;">'''Figure 12. '''PCA algorithm.</span></div>

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 * Distributed sensor: Sensing is distributed along the length of the fiber, the optical fiber works simultaneously for transmitting the information and for sensing the local external variables (temperature, strain).
 [[File:Guemes_et_al_2018a-image5.png|centre|600x600px]]
-'''Figure 1.''' Topology of Fiber Optic Sensors.
+<div class="center" style="width: auto; margin-left: auto; margin-right: auto;">
 Fiber optic sensors (FOS) have reached a maturity as strain/temperature sensors. It can be said that its TRL (technology readiness level) is about 8-9, they have already been demonstrated in real aircrafts, and are routinely used in many other industrial applications, like monitoring oil wells. An excellent 50 pages review on FOS technologies has recently been done [3], including more than 170 references; so a long introduction describing FOS is not needed, readers are referred to it or to other similar documents [4].
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 To be effective, this technique requires of distributed sensing, meaning getting the strains all along the optical fibre. Several kinds of Fiber Optic Distributed sensing systems are available, in dependence of the wavelength they are working with [10].  Table 1 summarizes its performances. Rayleigh system working with OFDR (Optical Frequency Domain Reflectometry) is the only one to offer spatial resolution in the range of mm, as needed for aeronautic applications; for civil engineering applications, a long measurement range may be the preferred quality, which may drive to other choices. Performances are quickly evolving, so this table must be taken with caution. Distributed sensing has opened new possibilities for the instrumentation of structural tests, particularly for very large structures, like civil engineering structures [11-13], wind turbine blades [14]. Again, getting strains is not the same as getting damage information, even though at concrete structures, cracks are easily identified as the points with very high strains readouts.
 {| style="width: 100%;border-collapse: collapse;"
 |-
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-Table 1. Comparison of Distributed Fiber Optic sensor systems
 ==='''2.1''' Detection of delaminations caused by impacts===
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 [[File:Guemes_et_al_2018a-image7.png|centre|600x600px]]
-'''Figure 3.''': Detection of the delamination caused by an impact; Ultrasonic C-Scan of the impacted laminate; ('''c''') Strains measured along the white line of the upper figure by an O.F..
+<div class="center" style="width: auto; margin-left: auto; margin-right: auto;">
 ==='''2.2''' Detection of delaminations at laminate edges===
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 The laminates were submitted to impacts of controlled energy, by using a drop weight test machine, both perpendicular to the laminate and on-edge direction. The energy was gradually increased until a visible damage was produced (figure 5), and the residual strains were recorded after every impact (figure 6).
 [[File:Guemes_et_al_2018a-image8.png|centre|600x600px]]
-''  ''Figure 5. Delamination caused at a (04, 904)s CFRP laminate after a 5 J on-edge impact.
+<div class="center" style="width: auto; margin-left: auto; margin-right: auto;">
-[[File:Guemes_et_al_2018a-image9-c.png|centre|418x418px]]
+<span style="text-align: center; font-size: 75%;">'''Figure 5.''' Delamination caused at a (04, 904)s CFRP laminate after a 5 J on-edge impact.</span></div>
-'''Figure 6.''' Strains recorded by the optical fiber after successive impacts of increasing .energy, from 2 to 5 joules. The two identifiable peaks recorded at 100 and 400 mm are for the first and second loops of the optical fiber, respectively
+[[File:Guemes_et_al_2018a-image9-c.png|centre|600x600px]]
 Similar findings were obtained when impacts were done at the direction normal to the laminate. For these tests, an embedded O.F. was used, located at the second ply of the surface opposed to the impact. The energy needed to cause a BVID was slightly higher than in the former case. These results show the high reliability of the technique, as far damage happen on the optical fiber path, and a system is available to get the strains all along the fiber with adequate spatial resolution.
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 This technique only may afford an alert for damage occurrence; it does not seems a main limitation, once damage is known to happen, it may be located by checking which area has the largest strain changes.
-[[File:Guemes_et_al_2018a-image10.png|centre|393x393px]]
+[[File:Guemes_et_al_2018a-image10.png|centre|600x600px]]
-Figure 7. Schematics of the PCA algorithm. Firstly identify a new coordinate system which reduce the dimensionality of the data set. Secondly, identify if new data fits or not inside the former reference system.
+<div class="center" style="width: auto; margin-left: auto; margin-right: auto;">
 ==='''3.1''' Damage detection for a CFRP Isogrid structure===
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 The application of the PCA algorithm to the former data set is straightforward, with quite good results (Figure 10). Worthy to mention that there is no need to prepare the data, neither a FEM model. It is only needed to arrange the X matrix (36 X 800), there were 36 sensors, and 800 measurements were taken to generate the baseline. Next measurements, projected on this baseline, easily discriminate the load which produce the first failure, and after that, next measurements have a clearly detectable damage index.
 [[File:Guemes_et_al_2018a-image11.png|centre|600x600px]]
-''  '' '''Figure 8. '''Isogrid structure under compression test. Detail of broken bars after. exceeding the max. load
+<div class="center" style="width: auto; margin-left: auto; margin-right: auto;">
 [[File:Guemes_et_al_2018a-image12.png|centre|600x600px]]
-'''Figure 9. '''Isogrid structure under compression test. Detail of a broken bars after exceeding the first failure load
+<div class="center" style="width: auto; margin-left: auto; margin-right: auto;">
 [[File:Guemes_et_al_2018a-image13.png|centre|600x600px]]
-'''Figure 10. '''PCA algorithm applied to isogrid strain data. Blue lines are damage indexes calculated from strains acquired on the undamage structure, each experiment means a new data acquisition (load level); green line correspond to data taken under increasing loads, first failure happened at test 400.
+<div class="center" style="width: auto; margin-left: auto; margin-right: auto;">
 ==='''3.2'''  Damage detection for a wind turbine blade (WTB)===
 Compared to the former case, this experiment has two added difficulties: Firstly, the load cases are not as simple as it was for the isogrid structure, always uniformly distributed compressive loads of increasing values. As shown at Figure 11, different load cases, or distribution of weights were loads were applied, may be done; consequently, it will be found that there is not a single dominant Principal Component, the first three components play a similar role.
-[[File:Guemes_et_al_2018a-image14.png|centre|361x361px]]
+[[File:Guemes_et_al_2018a-image14.png|centre|600x600px]]
-'''Fig. 11. '''PCA algorithm applied to isogrid strain data.
+<div class="center" style="width: auto; margin-left: auto; margin-right: auto;">
 The second difficulty comes from the nature of the inflicted damage. To do a representative experiment, the typical damages that happen in WTB were reproduced, that is a partial debonding of the shells at the trailing edge The blade 13,5 mt long, manufactured in the conventional way as a long spar with two bonded shells, was instrumented with 4 optical fibres, with 6 FBGs each, regularly spaced. For a cantilever beam under flexural loads, a partial trailing edge debonding changes the torsional stiffness, but do not alter strongly the bending stiffness.
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 ==[insert Figure 12]==
-'''Figure 12. '''PCA algorithm
+<div class="center" style="width: auto; margin-left: auto; margin-right: auto;">
 The damage index was calculated, and it was found that when debonding was 100 mm long, it was clearly detectable. Results are shown at figure 13, green dots. Obviously when new damages were inflicted at the main spar, as load carrying member, they were more easily detectable.
-[[File:Guemes_et_al_2018a-image15.png|centre|253x253px]]
+[[File:Guemes_et_al_2018a-image15.png|centre|600x600px]]
-'''Figure 13. '''PCA algorithm applied to the Wind turbine blade
+<div class="center" style="width: auto; margin-left: auto; margin-right: auto;">
 ==4  Discussion==

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 A sixteen plies crossply CFRP laminate was built from UD prepreg material by OOA (Out of Autoclave) procedures. A poliimide coated optical fiber was embedded inside the laminate during layup. The laminate was impacted by a drop weight test, and a delamination was produced. The white line show the position of the optical fibre, and the lower image (figure 3) show the strains measured by the optical fiber along this line. It can be seen the appearance of residual strains at the delaminated area. Worthy to mention strains caused by damage are significant, with a peak of 300 microstrains, and the delaminated length is perfectly depicted, 25 mm. In fact, the delaminated area can nearly be plotted if the optical fiber follows a crooked path, with parallel fibers every 5 mm. The strain field map of the area can be obtained with relatively high accuracy.
+[[File:Captura de pantalla 2022-10-12 133755.png|centre|600x600px]]
-[[Image:Guemes_et_al_2018a-image6.tmp|600px]]
 [[File:Guemes_et_al_2018a-image7.png|centre|600x600px]]

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