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Conference on Damage in Composite Materials, Stuttgart, 18.-19.09.2006
Jan Teßmer, Luise Kärger, Uwe Pfeiffer, Anja Wetzel
Damage Tolerance of Sandwich Structures- Modelling, Analysis and Testing -
Institute of Composite Structures and Adaptive Systems 2
High Performance Composites in Aerospace Structures
ROSETTA
Formel 1ARIANEAntenna
A380
ARIANE
Institute of Composite Structures and Adaptive Systems 3
High Performance Composites in Aerospace Structures
MonolithicComposites
Sandwich Structures withComposite Face sheets
Textile Composites
Institute of Composite Structures and Adaptive Systems 4
1 Double shell structures (Sandwiches)
2 Impact analysis
3 Residual strength analysis
ExperimentsSimulation tool CODACDamage ModellingSimulation results
4 Non-destructive Testing
CAI-TestsGeometrical non-linear simulation using ABAQUSSimulation results
Air-coupled Ultrasonic Testing for SandwichesSignal Processing for monolithic CFRP (skins)
Institute of Composite Structures and Adaptive Systems 5
1 Double shell structures
core
outer skin
inner skin
Ref.:Element S89: Kärger, Wetzel, Rolfes, Rohwer. Computers & Structures 84. 2006.Element S815: Wetzel, Kärger, Rolfes, Rohwer. Computers & Structures 83. 2005.
Development of two new shell elements
Modelling requirementsby accounting for the specific deformation behaviour:- fast for being used in the design process- sufficiently accurate
Institute of Composite Structures and Adaptive Systems 6
0
0 2
0
00
⎡ ⎤⎡ ⎤ ⎡ ⎤ ⎡ ⎤⎢ ⎥⎢ ⎥ ⎢ ⎥ ⎢ ⎥= + +⎢ ⎥⎢ ⎥ ⎢ ⎥ ⎢ ⎥⎢ ⎥⎢ ⎥ ⎢ ⎥ ⎢ ⎥⎣ ⎦ ⎣ ⎦ ⎣ ⎦⎣ ⎦
L L xL
L L L yL L
L L zL zL
u uv v z zw w
ψψψ ϕ
15 dof per node
Element S815: 3-layered shell element with 3-D stress analysis(Sandwich element with 8 nodes and 15 dof per node)
Kinematics of layer L :
Layer-wise full 3D-material law:
L L L=σ C ε
Stress computation: • in-plane stresses: material law• transv. shear stresses: equilibrium approach by Rolfes & Rohwer• transv. normal stress: material law and equilibrium approach
3xψ
0 01u u=
03uz3
x
z
z2z1
02u
02w
03w
2xψ
1xψ
h3
h2
h1/2h1/2
core
outerskin
0 01w w=
initial state
deformed state
innerskin
1 Double shell structures
Institute of Composite Structures and Adaptive Systems 7
1 Double shell structures
2 Impact analysis
3 Residual strength analysis
ExperimentsSimulation tool CODACDamage ModellingSimulation results
4 Non-destructive Testing
CAI-TestsGeometrical non-linear simulation using ABAQUSSimulation results
Air-coupled Ultrasonic Testing for SandwichesSignal Processing for monolithic CFRP (skins)
Institute of Composite Structures and Adaptive Systems 8
2 Impact analysis: ExperimentsTests conducted at ILR, TU Dresden
Force-time histories:
completely supported panel
top skin core top skin core
1 Joule damage: 4 Joule damage:
Institute of Composite Structures and Adaptive Systems 9
2 Impact analysis: Simulation tool CODAC
• CODAC = Composite Damage Tolerance Analysis Code
• fast evaluation of impact damage and residual strength of composite structures
• Finite Element Method
Impactor (Ø25.4 mm):• point mass • parabolically distributed surface load
Transient impact analysis:• dynamic FEA with Newmark time integration• application of Hertzian contact law
Institute of Composite Structures and Adaptive Systems 10
2 Impact analysis: Modelling of core damage
Degradation:
11 12 13
22 23
33
44
*11 12 13
*22 23
*33
55
*44
*55
*6 666
sym.
xxxx
yyyy
zzzz
yzyz
xzxz
xyxy
C C CC C
CC
D D
C
DD D
DD
DD C
εσεσεσγτγτγτ
⎡ ⎤ ⎡ ⎤⎡ ⎤⎢ ⎥ ⎢ ⎥⎢ ⎥⎢ ⎥ ⎢ ⎥⎢ ⎥⎢ ⎥ ⎢ ⎥⎢ ⎥
=⎢ ⎥ ⎢ ⎥⎢ ⎥⎢ ⎥ ⎢ ⎥⎢ ⎥⎢ ⎥ ⎢ ⎥⎢ ⎥⎢ ⎥ ⎢ ⎥⎢ ⎥
⎢ ⎥⎢ ⎥ ⎢ ⎥⎣ ⎦⎣ ⎦⎣ ⎦
absorbed energy
Dij > 0
ε
σ
crushσuσ
elastic-plastic behaviour
Failure criterion:
Ref.: Petras, Sutcliffe (2000)
1⎛ ⎞ ⎛ ⎞ ⎛ ⎞
+ + ≥⎜ ⎟ ⎜ ⎟ ⎜ ⎟⎝ ⎠ ⎝ ⎠ ⎝ ⎠
n n nyzzz xz
cc cls cts
τσ τσ τ τ
- transverse shear and compression failure of honeycomb core- criterion by Besant et al.:
Ref.: Kärger, Baaran, Teßmer. Composite Structures. 2006
Institute of Composite Structures and Adaptive Systems 11
2 Impact analysis: Modelling of skin damage
Kra
ft
Weg
phase 2
fibre breakage in a few laminas:- degradation of membran stiffness
by degradation factor D11- further stiffness components without
considerable influence on impact response
( ) ( )( ) ( ) ( )( ) ( ) ( )
( )( )
( )
11 0 00 0 00 0 0
00
0
D⎡ ⎤⎢ ⎥⎢ ⎥⎢ ⎥⎢ ⎥⎢ ⎥⎢ ⎥⎢ ⎥⎢ ⎥⎣ ⎦
3-point bending test: phase 3
bending failure
fibre breakage in all laminas:- skin tears- simulation stops
(fracture mechanical modelsor energy based damagemechanical models withcontinuous degradation are needed)
phase 1
all laminas intact:linear-elastic material behaviour
forc
e
displacement
Institute of Composite Structures and Adaptive Systems 12
2 Impact analysis: Simulation results4J Impact
0
0.5
1
1.5
2
2.5
0 1 2 3 4 5 6t / ms
F / k
N
no degradation
experiments
with core degradation
with core & skin degradation
Core damage:
no degradation
computed core damage
measured core damage
core degradation(upper bound)
core & skin degradation
(lower bound)
Force time history:
Institute of Composite Structures and Adaptive Systems 13
1 Double shell structures
2 Impact analysis
3 Residual strength analysis
ExperimentsSimulation tool CODACDamage ModellingSimulation results
4 Non-destructive Testing
CAI-TestsGeometrical non-linear simulation using ABAQUSSimulation results
Air-coupled Ultrasonic Testing for SandwichesSignal Processing for monolithic CFRP (skins)
Institute of Composite Structures and Adaptive Systems 14
3 Residual strength analysis: CAI-Tests
CAI-Test-Equipment:
Failure phenomenon:• impacted face sheet: dent propagation transverse to the loading direction
• failure of the impacted face sheet: buckling across the whole specimen width sudden load decrease
• further load increase• failure of second face sheet
Source: ILR TU Dresden
Institute of Composite Structures and Adaptive Systems 15
0
20
40
60
80
100
120
0 100 200 300 400 500 600
Zeit
Kraf
t
0J1J2J4J6J10J15J
0°
1 2 43
6 7
8
9 10 11 12 13
Test data:• Force vs. time • Strain vs. time at specified location
Strain gage positions on front and back side
Force vs. strain for 4 Joule specimen: higher compression on the front side compared to the back side (bending)
Forc
e
Time
-125
-105
-85
-65
-45
-25
-5-12000 -10000 -8000 -6000 -4000 -2000 0
Dehnung
Kra
ft
DMS 03 DMS 04DMS 08 DMS 09DMS 10 DMS 11DMS 12 DMS 13
3 Residual strength analysis: CAI-Tests
Institute of Composite Structures and Adaptive Systems 16
3 Residual strength analysis: Non-linear simulation using ABAQUS
Non-linear FE analysis of impacted face sheet:• uniaxial in-plane loading (displacement-driven, ∆u)• face sheet supported by springs representing the core• including initial dent, face sheet and core damage due to impact• including core damage growth• using automatic stabilization because of local instabilities
initial dent∆u
u = v = w = 0
w = 0
w = 0
v = w = 0
face sheet damage:soft inclusion (stiffness reduction)
core damage:stiffness reduction
0 < Dij < 1
ε
σ
crushσultσ
elastic-plastic behaviour
face sheet supportedby springs:
Institute of Composite Structures and Adaptive Systems 17
3 Residual strength analysis: Simulation results
Dent growth with increasing in-plane loading:
Institute of Composite Structures and Adaptive Systems 18
0
10
20
30
40
50
60
0 0.1 0.2 0.3 0.4 0.5(-1) Displacement / mm
(-1) F
orce
/ kN
Experimental failure load Experimental onset of non-linearitySlopeABAQUS
m/m-10
0
10
20
30
40
50
60
-20000 -16000 -12000 -8000 -4000 0
Strain in
Forc
e in
kN
DMS10_ABAQUS
DMS10_Test
DMS11_ABAQUS
DMS11_Test
DMS12_ABAQUS
DMS12_Test
3 Residual strength analysis: Simulation results
4 Joule specimen: force vs. displacement ∆u 4 Joule specimen: force vs. strain
For 4 Joule damage, Dij=0.7, kcore=Ezz/h, Ezz and σult according to data sheet, σplat/ σult = 0.3:
• very good correlation between experimental failure load and maximum load of ABAQUS simulation
• good correlation between experiment and ABAQUS simulation for strains at strain gage locations
Strain in µm/m
Institute of Composite Structures and Adaptive Systems 19
1 Double shell structures
2 Impact analysis
3 Residual strength analysis
ExperimentsSimulation tool CODACDamage ModellingSimulation results
4 Non-destructive Testing
CAI-TestsGeometrical non-linear simulation using ABAQUSSimulation results
Air-coupled Ultrasonic Testing for SandwichesSignal Processing for monolithic CFRP (skins)
Institute of Composite Structures and Adaptive Systems 20
Sandwich inspection with water coupling
Optimal frequency forSandwiches: <500kHz=> Bad Focussing,low spatial resolution
Institute of Composite Structures and Adaptive Systems 21
Air-coupled Ultrasonics
Benefits:Contact free, without coupling mediaConstant and reproducible couplingNo incoming water<500kHz and focussed
Challenges:Bad acoustic matchingMore than 160 dB amplitude loss
ApproachTransducers with optimised matching layersOptimised transmitter and receiver electronics
Our resultsSignal-to-noise ratio in transmission: 30 dBNarrow band, strongly focussing transducersBest choice for sandwich testing
Sound pressure
0dB
-50
-100
-150
-200
Test spcimenz.B. CFRP ReceiverTransmitter
Water coupling(for comparison)
Air coupling
-21 dB
-93 dB
-24 dB
-162 dB -156 dB
-18 dB
Wat
erco
uplin
gA
ir co
uplin
g
Institute of Composite Structures and Adaptive Systems 22
CFRP: High and frequency-specific sound attenuation
Transmitter ReceiverdB dB
Arrow thickness ~ wavelengthArrow length ~ amplitude
= signified microstructure
f = 3 MHz
f = 2 MHz
f = 1 MHz
f = 3 MHz
f = 2 MHz
f = 1 MHz
Loss by scattering is dominating at high frequencies
The HF fraction is attenuated superproportional
delivered signal:E.g. same amplitude at 1, 2 and 3 MHz
Transmission:Spectrum shifted to low frequencies
Institute of Composite Structures and Adaptive Systems 23Depth [mm]15 30 45
100
50
0
-50
-100
Processed A-Scan
Depth [mm]15 30 45
Frequency
[MHz]
4
3
2
1
0
5Processed Spectrogram
Depth [mm]15 30 45
4
3
2
1
0
5Spectrogram
STFT
Inve
rse
STFT
Frequency
[MHz]
Depth [mm]15 300 45
4
3
2
1
0
5Correction Matrix Γ(d,f)
Depth [mm]15 30 45
Amplitude [%]
100
50
0
-50
-100
A-Scan (+DAC)
Frequency
[MHz]
Spectral Distance Amplitude Correction „SDAC“
-80 -60 -40 -20 0Amplitude spectrum [dB]
-20 -10 0 10 20Amplification [dB]
Institute of Composite Structures and Adaptive Systems 24
Stringer Specimen: Analysis of Layer Echoes
Gate 1:0.2 – 0.8μs„shallow“ Gate 2:
0.8 – 1.4μs„middle“
Gate 3:1.4 – 2.0μs„deep“
Gate 0:Interfacetrigger
Institute of Composite Structures and Adaptive Systems 25
Flaw Detection in various depths(Time-of-Flight D-Scans)
shallow
middle
deep
unprocessed SDAC processedSDAC: Spectral Distance Amplitude Correction
Institute of Composite Structures and Adaptive Systems 26
Impact
NDT:Detection and Characterization
Residual Strength
Simulation
Impact
DAMAGE
High Performance Composites in Aerospace Structures
Institute of Composite Structures and Adaptive Systems 27
Impact
NDT:Detection and Characterization
Residual Strength
Simulation
Impact
DAMAGE
High Performance Composites in Aerospace Structures
Thank you for your Attention!Contact: [email protected]