characterization of impact damage in fibre reinforced composite plates using embedded fbg sensors j....
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Characterization of impact damage in fibre reinforced composite plates using
embedded FBG sensors
J. Frieden*, J. Cugnoni, J. Botsis, Th. Gmür
CompTest2011 5th International Conference on
Composites Testing and Model Identification14 Feb 2011 - 17 Feb 2011,
Ecole Polytechnique Fédérale de Lausanne, Switzerland
Swiss National Science Foundation, grant N° 116715
Objectives
Primary objective of this work:
• Impact localization and damage identification in CFRP plates with FBG sensors
Methods:
• Interpolation-based impact localization method using high rate FBG signals
• Inverse numerical-experimental damage identification method based on eigenfrequency changes and homogenized damage model
Objectives
Primary objective of this work:
• Impact localization and damage identification in CFRP plates with FBG sensors
Today’s focus:
• Influence of impact damage on the plate’s eigenfrequencies measured with FBG sensors
• Experimental characterisation of impact damage
• Finite element model of the plate with impact damage that reproduces the change of eigenfrequencies
Application
Introduction: Materials and specimen
CFRP cross-ply plate with 28 UD plies
[0°2, 90°2, 0°2, 90°2, 0°2, 90°2, 0°2]s
Embedded FBG sensors
Reference: Frieden J. et al, Composite Structures, 2010
Cross-section of plate
Sensitivity of eigenfrequencies to damage
Intact plate:Experimental modal analysis
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kintact
kdamagedk
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k
intactf
Damaged plate:Experimental modal analysis
kdamagedf
Impact1.7J – 6.7J
Experiment carried out on 8 plates using different impact energies.
Sensitivity of eigenfrequencies to damage
Relative frequency changes as a function of impact energy
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kintact
kdamagedk
f
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High resolution X-ray computed tomographySkyScan, model 1076 Aluminium filter : 1 mm thickness X-ray source voltage : 100 kV X-ray source power : 10 W Exposure time : 1750 ms
Experimental damage characterization
Damaged CFRP plate
Experimental damage characterization
Impact Location
Intralaminar cracks are rare and their occurrence is limited to a region located just beneath the impact point
Cross-section (Cut through plate thickness)Impact energy : 5.1 J
CT Resolution: 9 μm/pixelDistance between cross-section images: 9 μmTotal of 10 000 imagesConvert to black & white images
2 mm
Experimental damage characterization
Absorbed energy per unit of delamination area of 280 J/m2.
Detailed 3D delamination model
FE model in Abaqus 6.8-2:• Numerical modal analysis• 20-nodes brick elements with reduced
stiffness matrix integration• Mesh interfaces without node
connection between plies• Element size : 2 mm x 2 mm
Discrete delamination model
FE model in Abaqus 6.8-2:• Numerical modal analysis• 20-nodes brick elements with reduced
stiffness matrix integration• Mesh interfaces without node
connection between plies• Element size : 2 mm x 2 mm
Eigenfrequency changes are mainly due to delamination type damage
Homogenized damage model
Projected damage shape:•Rhombic area
Diagonal damage tensor D:Affected:•Transverse shear moduli
Not affected:•Longitudinal, transverse and through-the-thickness Young’s moduli•In-plane shear modulus•Poisson’s ratio
Incident energy [J] 3.37 5.06 6.75 6.75
Projected area [cm2] 10.7 16.0 20.7 20.6
Length [mm] 56.7 72.0 87.4 81.6
Width [mm] 37.8 47.2 47.3 47.3
232323
131313
1ˆ
1ˆ
GDG
GDG
Material properties:•Through-the-thickness homogenized material properties
Homogenized damage model
Diagonal damage tensor D:Affected:•Transverse shear moduli
Not affected:•Longitudinal, transverse and through-the-thickness Young’s moduli•In-plane shear modulus•Poisson’s ratio
Values of D13 and D23 identified through least square optimization:Minimize error between experimentally measured frequency change and numerically calculated frequency change
Projected damage shape:•Rhombic area
Material properties:•Through-the-thickness homogenized material properties
232323
131313
1ˆ
1ˆ
GDG
GDG
Homogenized damage model
Diagonal damage tensor D:Affected:•Transverse shear moduli
Not affected:•Longitudinal, transverse and through-the-thickness Young’s moduli•In-plane shear modulus•Poisson’s ratio
Values of D13 and D23 identified through least square optimization:Minimize error between experimentally measured frequency change and numerically calculated frequency change
Incident energy [J] 3.37 5.06 6.75 6.75
D13 [%] 84.4 86.1 88.4 91.9
D23 [%] 85.5 90.2 92.5 93.9
Projected damage shape:•Rhombic area
Material properties:•Through-the-thickness homogenized material properties
232323
131313
1ˆ
1ˆ
GDG
GDG
Prediction of eigenfrequency changeExperimentally measured damage size
Using the previously determined values of D13 and D23
Damage identification procedure
Values of D13 and D23 are fixed to 94 %
Parameters to identify:
• Damage position
• Damage surface
• Damage aspect ratio
Reduce discrepancy between experimentally measured eigenfrequency changes and numerically calculated eigenfrequency changes
Iterative minimization algorithm: Levenberg-Maquardt
Application example
Impact energy: 3.4 J
Predict the impact location
Identify damage size and position
Application
Reference measurements before impact:
• Arrival time delays for interpolation-based localization method
• Eigenfrequencies of intact plate
Application: Reference data
• Non-destructive hammer impacts• Grid of 3 x 3 reference points• Acquisition rate of FBG sensors : 1 GHz• Arrival time delays obtained by threshold method
Application: Reference data
• Non-destructive hammer excitation• Grid of 3 x 3 reference points• Acquisition rate of FBG sensors : 100 kHz• Eigenfrequencies obtained by modal curve fitting
FRF
Application: Impact
• Impact with energy of 3.4 J• Acquisition rate of FBG sensors : 1 GHz
Application: Impact
• Impact with energy of 3.4 J• Acquisition rate of FBG sensors : 1 GHz
Application: Identification of damage
Experimental data
Application: Identification of damage
Parameters to identify:
• Damage position
• Damage surface
• Damage aspect ratio
Initial guess for the damage identification:
• Predicted impact location
• Damage surface = 1 cm2
• Damage aspect ratio = 1
Experimental data
Application: Identification of damage
Convergence graph
Identification resultsPredicted eigenfrequency changes
compared to experimental eigenfrequency changes
Conclusion
• Embedded FBG sensors provide very accurate strain data for modal analysis and acoustic wave sensing.
• The eigenfrequency changes can be mainly attributed to delamination type damage.
• The simple homogenized damage model allows to reproduce the eigenfrequency changes.
• The damage size can be identified by a numerical-experimental optimization method based on eigenfrequency changes.
Thank you
Introduction: Fast FBG interrogation
FBG sensors for modal analysis
1st mode
3rd mode
2nd mode
4th mode