An integral composite aircraft cabin had been instrumented with two different technologies: distributed fiber optic sensors (DFOSs) and Fiber Bragg Gratings (FBGs). This structural test aims at strain field monitoring in any part of structure when cabin pressurization is applied in order to simulate flight conditions. The Distributed fiber optic network used is based on Rayleigh scatter ing using an Optical Backscatter Reflectometer (OBR). The OBR provides a large number of strain sensor s with high spatial and strain accuracy with a plain optical fiber. DFOSs prove to be the most suitable technology for this test due to their capability to cover large areas with an important amount of strain data. Moreover, the technology of Fiber Bragg Grating sensors has been also applied for the monitoring of the cabin structural integrity. This type of sensors enable a continuous monitoring during the pressurization tests. 24 FBG sensors in four fiber optic circuits were installed on cabin surface . This test allows to correlate distributed and discrete fiber optic sensors and evaluate their damage detection capabilities. Several static pressure tests were conducted in order to detec t structural damage and static loads at the cabin structure

Díaz-Maroto, Patricia

Fernández López, Antonio

Fernández, Rosario

Gutierrez, Nicolás

Güemes Gordo, Jesús Alfredo

Lasagni, Fernando

Archivo Digital UPM

Strain monitoring on a composite aircraft cabin with fiber optic sensors

Servicio de Coordinación de Bibliotecas de la Universidad Politécnica de Madrid

 STRAIN MONITORING ON A COMPOSITE AIRCRAFT CABIN WITH 
FIBER OPTIC SENSORS 
 
Patricia F. DÍAZ-MAROTO1*, Antonio FERNÁNDEZ1, Rosario FERNÁNDEZ2,  
Nicolás GUTIÉRREZ2, Fernando LASAGNI2, J. Alfredo GÜEMES1 
 
1Laboratory of Composite Materials and Smart Structures, Dpt. Aeronautics, UPM  
Plaza Cardenal Cisneros, 3 - Madrid 28040 SPAIN. p.fernandezdm@aero.upm.es 
 
2
 Materials & Processes department, Center for Advanced Aerospace Technologies (CATEC) 
C/Wilbur y Orville Wright 17-19-21, E-41309 La Rinconada, Seville, SPAIN. 
 
Key words: Fiber Optic, FBG, Distributed sensing, Aerospace SHM applications.  
 
 
Abstract 
An integral composite aircraft cabin had been instrumented with two different technologies: 
distributed fiber optic sensors (DFOSs) and Fiber Bragg Gratings (FBGs). This structural 
test aims at strain field monitoring in any part of structure when cabin pressurization is 
applied in order to simulate flight conditions.  
The Distributed fiber optic network used is based on Rayleigh scattering using an Optical 
Backscatter Reflectometer (OBR). The OBR provides a large number of strain sensors with 
high spatial and strain accuracy with a plain optical fiber. DFOSs prove to be the most 
suitable technology for this test due to their capability to cover large areas with an important 
amount of strain data.  
Moreover, the technology of Fiber Bragg Grating sensors has been also applied for the 
monitoring of the cabin structural integrity. This type of sensors enable a continuous 
monitoring during the pressurization tests. 24 FBG sensors in four fiber optic circuits were 
installed on cabin surface. 
This test allows to correlate distributed and discrete fiber optic sensors and evaluate their 
damage detection capabilities. Several static pressure tests were conducted in order to detect 
structural damage and static loads at the cabin structure.   
 
1 INTRODUCTION 
 
Composite materials have a high structural integration capability that allows to 
manufacture complex structures without rivets or additional joining elements. In the 
aeronautic industry this advantage is one of the most important requirements since it 
represents important weight and manufacturing costs savings. For that reason, an integral 
composite aircraft cabin have been developed not only to compare with traditional methods 
of manufacturing but also to verify the structural integrity with one of the most powerful 
technology that exits for testing nowadays, fiber optic sensors [1-3]. Optical sensors have 
been proved as an alternative in structural test as they provide a more specific approach of 
strain field than conventional sensors such as electrical strain gauges. 
 
 
8th European Workshop On Structural Health Monitoring (EWSHM 2016), 5-8 July 2016, Spain, Bilbao
www.ndt.net/app.EWSHM2016
M
or
e 
in
fo
 a
bo
ut
 th
is 
ar
tic
le
: h
ttp
://
w
w
w
.n
dt
.n
et
/?
id
=2
00
72
2 
 
This project is focused on a structural test monitoring by means of an optical sensor network 
based on FBGs and distributed sensing using Rayleigh scattering. 
 
2 DISTRIBUTED FIBER OPTIC SENSING 
The distributed sensing network was carried out using an Optical Backscatter 
Reflectometer (OBR) equipment to measure the Rayleigh scattering. The OBR is an optical 
frequency domain reflectometer (OFDR) that uses a swept-wavelength interferometry by 
means of a tunable laser to interrogate the device under test (DUT) with very high spatial 
resolution [4, 5]. This powerful technique allows to obtain a strain measurement each 5 mm 
with a high resolution of 10µɛ. An OBR schematic is shown in Figure 1. 
 
 
Figure 1: Schematic of Optical Backscatter Reflectometer. 
Rayleigh scattering is caused by random fluctuations in the refractive index when fiber optic 
is manufactured. The OBR is an optical frequency domain reflectometer (OFDR) that uses a 
swept-wavelength interferometry to obtain the total reflected spectrum with high spatial 
resolution. When a segment of fiber optic is strained or heated then reflected spectrum 
experiences a shift. A cross-correlation is performed in order to determine the spectral shift 
between two different DUT conditions. One of them considered as a reference state and other 
recorded as a strained state. The amount of spectral shift is linearly dependent on temperature 
and strain changes, see Figure 2. 
 
 
  KTKT00       (1) 
-100 -75 -50 -25 0 25 50 75 100
-0.1
-0.05
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
Shifts
Sa
m
pl
e
 
Cr
o
ss
 
Co
rr
e
la
tio
n
Sample Cross Correlation Function
----- 2 kN
 
Figure 2: Frequency shift calculated for a strained sensor under 2kN tensile force by means of a cross 
correlation function. The equation is the relationship between strain or temperature changes and the amount of 
spectral shifts. 
3 
 
All shifts are calculated in a sensing range by lots of cross-correlation for each gauge length 
segment along the DUT, with the purpose to obtain a distributed strain measurement.  
 
3 FIBER BRAGG -GRATING 
A Fiber Bragg Grating sensor is a short segment of fiber optic in whose core multiple 
transverse stripes have been engraved periodically. These stripes cause that the core 
refraction index changes along the longitudinal direction [6, 7]. The FBG sensor performance 
is similar to an optical filter: one specific wavelength is reflected and the rest of the incident 
spectrum is transmitted. This wavelength is defined by the equation (2): 
λB=2Λn0 (2) 
Where λB is the wavelength, Λ is the modulation period and n0 is the mean refraction 
index. If the sensor is exposed to mechanical or thermal deformation, the characteristic 
wavelength changes proportionally to the applied deformation. Measuring the variation 
experimented by the reflected wavelength enables to calculate the deformation [8-11]. 
The optical sensing interrogator Micron Optics SM130-700 has been used for the monitoring 
and measuring of strain state with FBG sensors. It is built upon the X30 optical interrogator 
core, featuring a high power and high speed swept wavelength laser, which allows a large 
data acquisition coming from multiple sensors. 
 
4 EXPERIMENTAL SET-UP 
Both technologies of fiber optics sensors, OBR and FBGs, were installed to monitor the 
full scale pressurization test on a composite aircraft. The evolution of the strain status has 
been registered by means of this dense network spread out on the element surface. Optical 
sensors were placed in the most interesting regions, where maximum stress values are 
reached, e.g. high stress areas due to the influence of doors or windows corners as well as 
riveted zones. Furthermore, these points are coincident with strain gauge locations, which 
provide a reference value for optical network measurements. 
4.1 Distributed sensing network 
Distributed sensing network were performed in 5 paths. Four of them covered all cabin 
surface both sides. Moreover, this configuration was chosen since the most critical areas were 
close to these paths. A schematic representation is shown in Figure 3. 
 
 
 
Figure 3: Distributed fiber optic sensors placed on cabin surface organized in 4 paths. 
4 
 
Another path was placed in the torsion box. This structure was manufactured in aluminum 
alloy and the behavior obtained under the pressurization load have been different. Fiber optic 
path cover both box faces by means of several sensing lines. 
 
 
 
 
 
4.2 Fiber Bragg Grating Network 
The FBG network have four measurement channels, as this the maximum number of fiber 
optics that can be connected at the same time. Based on this, the final FBG distribution on 
cockpit surface is shown in Figure 5, where sensor positions are indicated with blue dots over 
the lines which represent the fiber optic paths. Strain gauges locations are represented as 
well. 
 
 
Figure 5: Fiber optic circuits: (a) line 1, (b) line 2, (c) line 3 and (d) line 4. Strain gauges positions are 
represented by rectangles. 
Fiber optic lines were manufactured ad-hoc for this application. As can be observed in Figure 
5, fiber distribution spans the entire cockpit surface except the ceiling area, due to the lack of 
access to this region. 
Figure 4: Path 5 of distributed sensors placed in the torsion box. 
5 
 
5 RESULTS 
A pressurization test was carried out over the cockpit in order to evaluate its structural 
integrity. The maximum inner pressure applied was 3 psi. This value was reached through 
eight pressure steps: 1, 1.5, 1.75, 2, 2.25, 2.5, 2.75 and 3 psi. The structural integrity of the 
component was checked and verified in each load increment. After that, pressure was 
released up to 0.29 psi, by means of the valves opening. A second pressurization test was 
performed, reaching a maximum value of 1.5 psi with the subsequent complete 
depressurization.  
5.1 Distributed sensing Network 
Measurements were acquired through Optical Backscatter Reflectometer equipment. 
OBR’s software saves data in a binary format which records lots of information. For 
distributed fiber optic technology the structural test was stopped at each loading step since 
measurement acquisition needs few seconds to carry out. 
Figure 6 shows the strain level all along the first fiber optic path. The highest level was 
measured around the door, where the strains are significantly higher than on surface skin. 
Furthermore, reinforcement’s zones are noticeable in the sensor signal since strain level is 
decreased.  
 
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10
-200
-150
-100
-50
0
50
100
150
200
250
300
350
400
450
500
550
Fiber optic length (m)
St
ra
in
 
(     
)
 
 
Path 1
1 psi 1.5 psi 1.75 psi 2 psi 2.5 psi 2.75 psi 3 psi
DoorRight side Front side Left side
 
Figure 6: Strain profile all along of fiber optic path 1. 
 
The most remarkable behaviour during the pressurization test was registered in the torsion 
box. Thanks to distributed strain measurement a local buckling has been detected during 
pressurization test. In contrast to punctual sensors which cannot provide the full information 
of the strain field, DFOS allows to detect buckling and high loading areas. In order to carry 
out a visual buckling representation on 2D view a signal post-processing has been performed 
as a strain map of the measured information. 
 
6 
 
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 4 4.2 4.4 4.6 4.8 5 5.2 5.4 5.6
-220
-180
-140
-100
-60
-20
0
20
60
100
140
180
220
Path 5
Fiber Length (m)
St
ra
in
 
(     
)
 
 
1 psi 1.5 psi 1.75 psi 2 psi 2.5 psi 2.75 psi 3 psi
Section 1 Section 2 Section 3 Section 4
debonding
 
Figure 7: Strain field of fiber path 5 along torsion box face. 
 
 
 
Figure 8: Strain color-map representation for buckling detection inside the torsion box. 
 
 
5.2 Fiber Bragg Grating Network 
FBG sensor measurements are represented in Figure 9, where each graphic corresponds to 
one fiber circuit. The inner pressure value has been overlapped, allowing relating the load 
state with the strain distribution. 
 
 
 
 
 
 
 
 
7 
 
0 500 1000 1500 2000 2500 3000 3500
-50
50
150
250
350
450
550
650
Time [s]
St
ra
in
 [ ]
Fiber Optic Nº2
 
 
0
0.5
1
1.5
2
2.5
3
3.5
Pr
es
su
re
 [p
si]
FBG 1
FBG 2
FBG 3
Pressure
 
Figure 9: FBG sensor measurements during pressurization tests up to 3 psi. 
 
The analysis of FBG sensor records enables to identify the load increments. Maximum 
positive strain values was detected in FBG Nº3 of fiber optic Nº2 (553 µƐ), corresponding to 
the sensor located closest to the cockpit ceiling. The minimum value was registered by sensor 
Nº2 of fiber optic circuit Nº3 (-132 µƐ). This sensor was situated near to the landing gear 
hole. Both values were measured when the inner pressure was 3 psi. 
Table 1 shows the comparison between FBG measurements and strain gauges records, for 
an applied pressure of 3 psi.  
 
Circuit Sensor 
type 
Nº 1 Nº 2 Nº 3 Nº 4 Nº 5 Nº 6 Nº 7 
  [µƐ] [µƐ] [µƐ] [µƐ] [µƐ] [µƐ] [µƐ] 
1 FBG 47 143 -17 -10 227 389 253 
 SG 23 168 -124 -159 -26 397 171 
2 FBG 370 167 553 - - - - 
 SG 707 195 401 - - - - 
3 FBG 389 -132 105 361 -21 381 -13 
 SG 378 -90 -207 -215 -35 349 13 
4 FBG -56 145 -20 212 262 210 136 
 SG -36 376 -29 150 120 188 169 
Table 1: Strain measurements with FBG and strain gauge sensors at 3 psi. 
0 500 1000 1500 2000 2500 3000 3500
-100
0
100
200
300
400
500
600
Time [s]
St
ra
in
 [ ]
Fiber Optic Nº1
 
 
0
0.5
1
1.5
2
2.5
3
3.5
Pr
es
su
re
 [p
si]
FBG 1
FBG 2
FBG 3
FBG 4
FBG 5
FBG 6
FBG 7
Pressure
0 500 1000 1500 2000 2500 3000 3500 4000
-200
-100
0
100
200
300
400
500
Time [s]
St
ra
in
 [ ]
Fiber Optic Nº3
 
 
0
0.5
1
1.5
2
2.5
3
3.5
Pr
es
su
re
 [p
si]
FBG 1
FBG 2
FBG 3
FBG 4
FBG 5
FBG 6
FBG 7
Pressure
0 500 1000 1500 2000 2500 3000 3500 4000
-100
-50
0
50
100
150
200
250
300
Time [s]
St
ra
in
 [ ]
Fiber Optic Nº4
 
 
-0.5
0
0.5
1
1.5
2
2.5
3
3.5
Pr
es
su
re
 [p
si]
FBG 1
FBG 2
FBG 3
FBG 4
FBG 5
FBG 6
FBG 7
Pressure
(a) (b) 
(c) (d) 
8 
 
Significant differences between both values are detected in some cases, even registering 
strain values with opposite sign. In most of them, these discrepancies are promoted by the 
distance between both types of sensors. For example, for FBG Nº4 of fiber optic circuit Nº3, 
this distance exceeds 15 cm.  
In fiber lines Nº2 and Nº3, the maximum differences are registered by the couple of 
sensors identified by numbers 1 and 2, respectively. In both cases, the measuring devices are 
located in a region under the effect of a stress concentrator (window and door corners). 
Although FBG sensors were positioned as proximate as possible to strain gauges, the 
presence of stress concentrators can cause large strain changes in short distance, as can be 
distinguished in Table 1. 
Finally, great differences are observed in sensors Nº 5 (fiber optic circuit Nº1). In this 
case, strain gauge was fixed in a riveted region, where a frame is joined with the cockpit skin. 
In spite of distance between both devices was lower than 2 cm, the fiber sensor was attached 
out of the riveted area, having as a consequence large differences between the strain records.  
 
6 CONCLUSIONS 
A total of 5 distributed sensing lines and 24 FBG sensors were installed on the composite 
cabin surface obtaining a dense optical sensing network around the structure. The strain field 
was successfully registered with the fiber optic sensors during pressurization test, being 
possible to identify the applied load steps and high local strain concentrations. Moreover, it 
was verified that structural damage was not detected after the test since sensors signal 
showed a normal behaviour without residual strain. Optical sensor network has been proved 
to be as an alternative to conventional strain sensors such as strain gauges. In comparison 
with them, fiber optic sensors afford a significant reduction in terms of integration and 
installation time, in addition to eliminate the electric wires required for the operation and it 
also offers significant weight savings.  
In conclusion, fiber optic sensors is a powerful technology for structural tests that may be 
monitored with major improvements in terms of spatial and strain resolution, detection of 
high local strained areas and the ease of network installation. 
REFERENCES 
[1] Criado, A., Riezu, M., Fernandez, A. and Oizm, A. 2009. “Evaluation of OBR for 
strain measurements in blade testing”. Proceedings of European Wind Energy Conference, 
Marseille, France. 
 
[2] Güemes, J A., Menendez, J M., Frovel, M., Fernandez, I., Pintado, J M. 2001 
“Experimental  analysis of buckling in aircraft skin panels by fibre optic sensors” Smart 
Materials and  Structures Vol. 10, no. 3, pp. 490-496. 
 
[3] F. Diaz-Maroto, P., Fernandez-Lopez, A., Güemes, J A., “Distributed fibre optic 
sensors for stiffened skin panels strain monitoring and failure mode detection” Proceedings 
of the 10th International Workshop on Structural Health Monitoring 2015, Stanford, 
California 
 
 
 
9 
 
[4] Soller, B J., Wolfe, M., and Froggatt, M. E. 2005 “Polarization resolved measurement 
of Rayleigh backscatter in fiber-optic components.” Optical Fiber Communication 
Conference and Exposition and the National Fiber Optic Engineers Conference Technical 
Digest. Anaheim, California.  
 
[5] Güemes, J A, Fernandez-Lopez, A., Soller B. 2010 “Optical Fiber Distributed 
Sensing. Physical Principles and Applications” J. Structural Health Monitoring, Vol. 9, No. 
3, 233-245. 
 
[6] Luyckx, G., Voet, E., Lammens, N., Degrieck, J., Strain Measurements of Composite 
Laminates with Embedded Fibre Bragg Gratings: Criticism and Opportunities for Research. 
Sensors, 11, 384-408,2011. 
 
[7] Hill, K.O., Meltz, G., Fiber Bragg Grating Technology Fundamentals and Overview. 
Journal Lightwave Technology, 15, 1263–1276, 1997. 
 
[8] Mawatari, T., Nelson, D., A Multi-Parameter Bragg Grating Fiber Optic Sensor and 
Triaxial Strain Measurement. Smart Materials and Structures, 17, 035033, 2008. 
 
[9] Tanaka, N., Okabe. Y., Takeda, N., Temperature-Compensated Strain Measurement 
using Fiber Bragg Grating Sensors Embedded in Composite Laminates. Smart Materials and 
Structures, 12, 940-946, 2003. 
 
[10] Guemes, J A., Menendez, J M., Response of Bragg Grating Fiber-Optic Sensors when 
Embedded in Composite Laminates. Composite Science Technology, 62, 959-966, 2002. 
 
[11] Surrea, F., Scott, R H., Banerji, P., Basheer, P.A.M., T. Sun, T., Grattan, K. T.V., 
Study of Reliability of Fibre Bragg Grating Fibre Optic Strain Sensors for Field-Test 
Applications. Sensors ans Actuators A, 185, 8-16, 2012. 
 


http://oa.upm.es/48192/1/INVE_MEM_2016_262805.pdf

Strain monitoring on a composite aircraft cabin with fiber optic sensors

Abstract

Similar works

Full text

Available Versions

Archivo Digital UPM

Servicio de Coordinación de Bibliotecas de la Universidad Politécnica de Madrid