FIBRE OPTIC DISTRIBUTED SENSING:
A POWERFUL TOOL FOR
STRUCTURAL TESTS
Patricia Fernández Díaz-Maroto
Antonio Fernández López
Alfredo Güemes Gordo
Department of Aerospace Materials and Manufacturing, ETSI Aeronáuticos,
Polytechnic University of Madrid, Spain.
7th International Conference on Composites Testing and Model Identification
Madrid, Spain, 8 - 10 April
Index
1. Fiber Optic Sensors.
2. Distributed measurement theory.
3. Experimental structural tests:
3.1. Wind turbine blade: bending test.
3.2. Stiffened skin panel: tensile and compressive test.
4.
Conclusions and Challenges.
DIAPOSITIVA 1
1. Fiber Optic Sensors.
What is an optical fiber?
A glass optical fiber made of fused silica (SiO2) and used for transmitting light
over large distances with very small losses. It has central core with a refractive
index n1, embedded in a cladding with a refractive index n2 < n1
Single mode fiber
CLADDING
n1
Ø=125 µm
Ø=9 µm
n2
What’s a fiber optic sensor?
Any fibre optic device (intrinsic or extrinsic) that interferes with the surrounding
media modifying the characteristics of the transmitted/reflected light can be
used as a sensor.
Short gauge sensors
Extrinsic Fabry-Perot
Interferometry (EFPI)
{ε, T}
Fiber Bragg-Grating (FBG)
{ε, T}
CORE
External coating,
typically acrylate or
polyamide, is applied to
fibres for mechanical and
chemical protection.
Distributed sensors
FIBER
OPTIC
SENSORS
(FOS)
Brillouin scattering
{ε, T}
Raman scattering{T}
Rayleigh scattering
{ε, T}
DIAPOSITIVA 2
Fiber Bragg Gratings
1. Fiber Optic Sensors.
Single or Multiple point sensor?
Sensor
interrogation
system
Single Point Sensing
Distributed sensing techniques
Temperature
Strain
Pressure
…
{ε,T }
Sensor
interrogation
system
L
Multiple point - Distributed Sensing
DIAPOSITIVA 3
1. Fiber Optic Sensors.
What are the main advantages?
•
Small size.
•
High multiplexing capability.
•
Light weight of the sensing network.
•
Immunity to electromagnetic interferences.
•
Fast response.
•
Linear response to strain and temperature.
•
Mechanical (>20000 µε) and high-
•
Almost inert in aggressive environments:
moisture, fire, radiations…
temperature resistance (>900ºC).
•
Could be embeddable.
•
Long term stability.
Disadvantages?
•
Equipment and fiber expenses.
•
Fiber brittleness.
•
Ingress/Egress fiber installation.
DIAPOSITIVA 4
2. Distributed Measurement theory
Optical Backscatter reflectometer (OBR)
{ε ,T }
wavelength
DUT
S
Tunable Laser
90%
10%
= Coupler 3 dB
= Beam splitter
Measure
interferometer
Polarization
controller
P
Eref
time
•
•
•
E*m
Em
τf
Reference
interferometer
Spatial resolution ≈ 5 mm
Strain accuracy ≈ 1 με
Measure range ≈ 70 m
[1] Güemes, JA, Fernandez-Lopez, A, Soller B “Optical Fiber Distributed Sensing. Physical
Principles and Applications” J. Structural Health Monitoring , Vol. 9, No. 3, 233-245 (2010)
Instrumentation of upper cap panels
3.1- Wind Turbine Blade testing.
Blade transversal view and fiber optic position
Strain gauge on the surface
Fiber optic
Switch
Optical
Backscatter
Reflectometer
Static tests were performed in
three principal directions of the
blade:
- Flapwise negative.
- Edgewise positive.
- Edgewise negative.
Resulting fiber measurements
were compared to design and
Strain Gauge values.
DIAPOSITIVA 6
Upper Cap at 90% Test Loads
3.1- Wind Turbine Blade testing.
Upper Cap at 90% Test Loads (Mflap-ve Test)
C
L
A
M
P
C
L
A
M
P
C
L
A
M
P
C
L
A
M
P
Strain (µε)
Excellent correlation
with the values
obtained in strain
gauges
C
L
A
M
P
C
L
A
M
P
Upper Cap at 60% Test Loads
Upper Cap at 50% Test Loads (Mflap-ve Test)
Radius (m)
C
L
A
M
P
C
L
A
M
P
C
L
A
M
P
C
L
A
M
P
C
L
A
M
P
OBR
Strain Gauges
Strain (µε)
C
L
A
M
P
R(m)
FO
DIAPOSITIVA 7
3.1- Wind Turbine Blade testing.
Buckling
Initiation at 65%
load
DIAPOSITIVA 8
3.2- Stiffened skin panel test
•
Configuration 1, stiffened skin panel.
Three Panels are tested.
- 2 for configuration 1: tensile test.
- 1 for configuration 2: compressive and tensile test.
Fiber Optic Instrumentation
Configuration 2, panel with manhole
UPPER
Load
Panel 1
Panel 2
Compressive
Compressive
MIDDLE
Design
values
BOTTOM
Test
A B
Panel 3
Compressive
Tensile
Nominal
-170 kN
- 113 kN
340 kN
Ultimate
-255 kN
- 170 kN
510 kN
50 %
-350 kN
x
442 kN
75 %
-550 kN
x
650 kN
x
900 kN
100 %
-740 kN
-730 kN
Loads registered at the structural test.
DIAPOSITIVA 9
3.2- Stiffened skin panel test – Panel 1, compressive test
Fiber detach
Stringer 1. Bottom fiber
0
-500
Strain (microstrain)
-1000
-1500
-2000
-2500
-3000
-3500
-4000
-4500
100
SG5
SG4
SG3
SG2
800
SG1
Fiber length (mm)
Fn=170kN
Fu=255kN
F50%=350kN
F75%=550kN
F100%=720kN
UPPER
Buckling initiation
660 kN
MIDDLE
BOTTOM
A B
DIAPOSITIVA 10
3.2- Stiffened skin panel test – Panel 1, compressive test
Stringer 1. Middle fiber
Good correlation between both
stringers, same strain field and
behaviour.
0
Strain (microstrain)
-500
-1000
-1500
-2000
-2500
Buckling initiation
660 kN
-3000
-3500
-4000
100
SG5
SG4
SG3
SG2
800
SG1
Fiber length (mm)
Stringer 3. Middle fiber
0
Strain (microstrain)
-500
-1000
-1500
UPPER
-2000
-2500
MIDDLE
-3000
-3500
-4000
SG6
100
SG5
SG4
SG3
SG2
800
SG1
Fiber length (mm)
Fn=170kN
Fu=255kN
F50%=350kN
F75%=550kN
F100%=720kN
BOTTOM
A B
DIAPOSITIVA 11
3.2- Stiffened skin panel test – Panel 2, compressive test
When ultimate load is applied, global buckling starts at the upper fiber before the
others. Strain level is constant at each load.
Fn = -113 kN
Fu = -170 kN
Stringer 1. Bottom fiber (B)
-400
-600
-800
-1000
0
0
-200
-200
Strain (microstrain)
Strain (microstrain)
Strain (microstrain)
-200
-400
-600
-800
100
SG5
SG4
SG3
SG2
Fiber length (mm)
Fn=-113kN
Fu=-170kN
800
SG1
-1200
SG6
-400
-600
-800
-1000
-1000
-1200
-1400
SG6
Stringer 1. Upper fiber (B)
Stringer 1. Middle fiber (B)
0
100
SG2
SG3
SG4
SG5
Fiber length (mm)
Fn=-113kN
Fu=-170kN
800
SG1
-1200
SG6
100
Fn=-113kN
UPPER
UPPER
MIDDLE
MIDDLE
MIDDLE
SG1
Fu=-170kN
BOTTOM
BOTTOM
A B
800
Fiber length (mm)
UPPER
BOTTOM
SG2
SG3
SG4
SG5
A B
A B
DIAPOSITIVA 12
3.2- Stiffened skin panel test – Panel 2, tensile test
Stringer 1. Bottom fiber
7000
Strain (microstrain)
6000
5000
4000
3000
2000
1000
SG6
100
SG5
SG4
SG3
SG2
800
Fiber length (mm)
Fn=340kN
Fu=510kN
F50%=442kN
F75%=650kN
Failure initiated
at stringer face B
F100%=900kN
Stringer 1. Bottom fiber (B)
Signal losses
8000
Strain (microstrain)
7000
6000
5000
4000
3000
2000
UPPER
1000
MIDDLE
0
SG6
100
SG5
SG4
SG3
SG2
800
SG1
Fiber length (mm)
Fn=340kN
Fu=510kN
F50%=442kN
BOTTOM
A B
F75%=650kN
F100%=900kN
DIAPOSITIVA 13
4. Conclusions and Challenges
•
OBR technology was proven to be useful for detailed strain measurement in blade static tests
and stiffened skin panels.
•
Strain data recorded with distributed measurements enormously exceeded the information
collected by strain gauges.
•
Integration with OBR fibres was fast and simple. Installation times were below those of
Strain Gauges.
•
A distributed sensor is the most suitable technique to detect catastrophic failure.
•
Damage detection only occurs when it is produced very close to the sensor location.
•
Fiber ingress/egress need to be improved for the application and handling in structural test.
DIAPOSITIVA 14
Thank you for your
attention!
Any questions?
Patricia Fernández Díaz-Maroto
[email protected]