Fibre-reinforced composites with polymeric based ......Fibre-reinforced composites with...
Transcript of Fibre-reinforced composites with polymeric based ......Fibre-reinforced composites with...
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O. Kahle1, M. Wegener², C. Uhlig1, K. Klauke1, O. Seidel², C. Dreyer1
1 Fraunhofer Institute for Applied Polymer Research IAP – Research Division Polymeric Materials and Composites PYCO, Kantstr. 55, D-14513 Teltow
² Fraunhofer Institute for Applied Polymer Research IAP – Department Sensors and Actuators, Geiselbergstr. 69, D-14476 Potsdam-Golm
Fibre-reinforced composites with polymeric based piezoelectric sensors for impact detection and presentation of an advanced fracture toughness testing
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Research Division: Polymeric Materials and Composites
Alternative core materials Alternative curing methods Microwave, e-beam, UV
New monomers, Resin modifications Formulation of resins (adhesives, matrix materials, foams, optic polymers, ...)
Recycling, Repair
Process engineering
Thermosets Composites (CFK, GFK, ...) Components
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Research Division: Polymeric Materials and Composites
Processing and Characterization techniques Chemical labs
Chemical and thermophysical characterization GPC, HPLC, GC-MS, FTIR, UV DSC, DMA, Rheology, TGA, TMA Ellipsometry, refractive index, microscopy
Processing Prepreg (vertical, horizontal), autoclave, RTM, injection
molding for resins, microwave curing (oven, continuous), presses, ovens
Testing Universal testing machines 5 N to 250 N (tension, pressure,
shear, peel, bending, fatigue), fracture toughness, climate, FST (cone calorimetry)
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Department Sensors and Actuators
(Thermoplastic) Materials (for transducers) Electrets Piezoelectric, pyroelectric and ferroelectric polymers and
composites Dielectric elastomers Composites polymer / magnetic or metallic particles Electrode materials: metals, composites (polymer / CNT,…),
conductive polymers Preparation processes and characterization methods
Processing of layers and 3D-structures by means of spin-coating, doctor-blade, solvent casting, inkjet printing, melt pressing, air-brush
Poling/charging of polymers, determination of break down field strength and surface potentials
Characterization of mechanical, electrical and dielectric, electro-mechanical as well as electro-optical properties
Functional elements Piezoelectric / pyroelectric / electrostrictive sensors and
actuators Sensors for detecting pressure, magnetic fields and humidity
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InnoTesting 2018 Fibre-reinforced composites with piezoelectric sensors
Fibre Reinforced Plastics (Thermosets)
Advanced composites comprising thermoset resin systems and high-performance fibres have become the material of choice for structural applications in numerous sectors.
Reinforcements are commonly available as two-dimension non-crimp fabrics (uni- and multi-axial), woven fabrics and braids.
Hybrid 2D Weave GF Köper CF UD CF +/-45°
Foto: Dreyer, privat https://goo.gl/SaNv62
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Fibre Reinforced Thermosets
Conventional laminated composites consist of stacked individual plies of reinforcement. Fibres may be oriented preferentially at the 2-dimensional level and thus in-plane
mechanical properties are easily tailored to end-use requirements.
The lack of through-thickness reinforcement results in poor out-of-plane mechanical performance.
2D-fibre reinforced composites are vulnerable to damage if impact events occurs.
Lay-up of conventional multi-layered reinforcement in the xy-plane
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Fibre Reinforced Thermosets and Impact Sensitivity
Impact damage (e.g. fibre fracture, matrix cracking, surface buckling and delamination) can cause serious deterioration in load-carrying capabilities.
Composites in aerospace (or wind power) are exposed to impact risks (e.g. hailstones, bird strike, tool drop during MRO). Depending on the nature of the impact, the damage state may not be easily detectable; some degree of internal damage can persist.
Research interest Mechanisms for mitigating impact damage (e.g. 3D fabric architectures, toughening of
the (brittle) matrix materials, enhance fibre-matrix-adhesion) Detection of impact events to get a signal about possible impacts and their strength
Damage mechanisms associated with low to medium energy impact
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material with intrinsic polarization
change of polarization
example: origin of piezoelectricity in ferroelectric polymers: change of dipole density
Piezoelectricity conversion of a mechanical excitation into an electrical signal
⇒ reversible polarisation change upon application of a mechanical stress
Piezoelectric coefficient Q: charge on electrodes F: force caused by mechanical stress y: sample geometry V: voltage between electrodes Sensor Actuator
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Ferroelectric Polymers
o PVDF (Polyvinylidenfluoride)
P(VDF-TrFE), P(VDF-HFP), P(VDF-TeFE)
o (odd) Polyamides, Nylon 11
Cellulose electro-active paper actuators (EAPap)
o F&E mainly in Korean institutes
o Special processing (crystal structure, stretching, polarization) needed
Examples for polymeric based piezoelectric materials
Piezoelectric L-Polylactide (PLLA)
o F&E by an institute and an enterprise
o Piezoelectric shear effect o Product transfer
Foamed Piezoelectrics (Ferroelectrets / Piezoelectrets)
o foamed foils of PP, PET, PEN, COC o „Regular“ (artificial / man-made) foamed systems
e.g. made of PTFE, FEP
Composites o Ceramic-Polymer-Composites
o Nanoparticles in piezoelectric polymers (Variation in mechanical properties, phases, formation of plasmons)
o etc.
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Piezoelectric transducer examples made of ferroelectric PVDF and PVDF copolymers
in coop. with Fraunhofer ENAS
in coop. with Fraunhofer FIRST
Piezoelectric polymers deposited on circuit boards
Ultrasonic transducer with piezoelectric polymer layers
Piezoelectric polymer sensor cables
Energy harvesters with embedded piezoelectric polymers in structures with vibrating masses
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1. Formulation PVDF copolymer / solvent → different solvent systems (DMSO/Acetone, DMF/MEK, NMP, …)
2. Layer processing → thickness: 0.4 – 5 µm inkjet-printing, spraying, spin-coating → thickness: 5 – 150 µm solvent casting, doctor blade, spraying → thickness: above 150 µm doctor-blade, melt-pressing
3. Solvent evaporation / annealing → 80°C – 160°C
4. Deposition of electrode → metallization: Cr/Au, Au, Cr, Al, Ag → printing / spraying: Ag, CNTs, CB, Ag-nanowire
5. Electrical poling
6. Characterization
Processing of PVDF and PVDF copolymer layers
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Compatibility of Sensor Material with Prepreg-System?
Questions Is the performance of polymeric based sensors affected by the processing
condition occurring during prepreg curing process (high temperature (>=120°C) and pressure)?
How are the mechanical properties of the laminate influenced by the embedding this type of thin sensor?
Requirements for prepreg-system Using ONE system for all investigations Representative resin system for commercial systems for light weight structures Wide curing temperature (60°C – 150°C) and pressure (2 bar and above) range Working temperature up to 180°C, high surface smoothness
Krempel: GGBX 2808 Köper (twill) 2/2, sheet thickness 0.22 mm
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Fabrication and Characterization of Laminates
Fabrication of laminates using hand-lever press Variation of curing regime (65°C - 150°C, 2 - 5 bar)
Investigations for state of curing via measuring the glass transition temperature Tg using DMA
(Tg: max of tan delta-curve) Determination of optimal curing time for each curing temperature via
maximum Tg
Determination of mechanical properties (stiffness, strength) at RT No influence of degree of curing resulting from curing at 60°C to 150°C
8090
100110120130140150160170180
0 5 10 15
Tg /
°C
Duration / h
65°C
80°C
100°C
120°C
130°C
140°C
150°C
0,01
0,1
1
1E+06
1E+07
1E+08
1E+09
1E+10
-100 0 100 200
tan
delta
G' /
Pa
T / °C
80°C 4h100°C 1.5h150°C 1h
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Sensor design, processing and mechanical characterization Sensors Sheets processed via doctor-blading, 35µm Material: P(VDF-TrFE) Area: 3cm × 3cm, Al electrode: 2cm × 2cm
Manufacture of laminates with sensors Embedding of sensors between 4 – 8 layers Variation of curing temperature (65°C to 150°C) Electrical contacting via metal wires
Mechanical characterization Peel test, (3pb, ILSS) Adhesion between Al electrode and piezoelectric
material weakest point, would be improved by adhesion promoters
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Electrical poling of laminated sensors Embedded sensor materials must be polarized in order to render them piezoelectric
Electrical poling was performed on the laminated sensor materials applying an alternating electric field
Poling with voltages up to 2.1kV leads to a somehow saturated polarization of about 35mC/m²
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Piezoelectric characterization of laminated sensors
Applied: Dynamical mechanical excitation at a frequency of 2 Hz and a force amplitude of 2 N
Measured: electrical response of the sample after amplification
Calculated: d33 coefficient from the applied force and the resulting electrical signal
Result: piezoelectric activity d33 ≈ 20pC/N (for all processing temperatures below TM)
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Additional piezo-electric characterization in progress – Impact and endurance tests Endurance test
in-situ measurement of sensor signals
under continuous large deflection
Impact test in-situ measurement of sensor
signals after short time excitation directly in areas near and away from sensor
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A polymeric ferroelectric material was successfully embedded as a sensor inside laminates
Representative processing conditions (temperature, pressure) are compatible with piezoelectric material based on PVDF if sensor is polarized after laminate processing
Laminated sensors were polarized successfully Piezoelectric properties of the devices consisting of piezoelectric sensors
laminated between sheets were demonstrated Further piezoelectric and mechanical characterization as well as influence of
storage testing (climate under high humidity, high temperature 85°C, temperature cycle tests) are in progress
Results and next steps
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Part 2: Optical Crack Tracing (OCT) — A Method for the Automatic Determination of Fracture Toughness for Crack Initiation and Propagation
Why did we develop “Optical Crack Tracing”?
Fracture toughness is one key property for polymeric structural and functional materials (adhesives, composites, electronic materials) These materials are in most cases thermosets and are developed by chemists
All physical techniques that are in wide-spread use among chemists are fully automated (NMR, FTIR, DMA, DSC, HPLC, GPC, ...)
Chemist: “How can I gain for my new material a maximum in relevant and accurate information about fracture behavior with minimum effort, minimum material required and all this independent of the operator?”
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Why Measuring Fracture Toughness and not Strength?
Fracture toughness is the only meaningful parameter to describe the mechanical performance (damage tolerance = resistance against crack growth) of thermosets
a u
Fc
Stress intensity factor KIc = KIc(Fc,a0,Y(a0))
For brittle materials strength is not an intrinsic material parameter but reflects only the distribution of flaws within the sample
Stress-strain-curves Fracture Mechanics
ε
σB
εΒ
σ ductile material
brittle material
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“Conventional Test Practice” (as recommended in Standards A.S.T.M. D5045 or ISO 13586)
Time-consuming manual effort to determine the initial crack length
Rather arbitrary recommendations for cases where there is a deviation from linearity in the load-displacement curve before the load drops
For the same material and for nearly the same initial crack length very different load-displacement curves (and peak loads) are found
020406080
100120140160180200
0 10 20 30 40 50
time / sfo
rce
/ N
155160
165170
175180
185190
21.5 22.5 23.5 24.5 25.5 26.5 27.5 28.5 29.5
time / s
forc
e / N
01020304050607080
0 5 10 15 20 25 30
time / s
forc
e / N
0102030405060708090
0 5 10 15 20 25 30
time / s
forc
e / N
5%
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Advantage of measuring the R–Curve data to be measured fracture mechanical analysis
KIc = f(a/w) P bw1/2
GIc ~ ∆U ∆a
0
50
100
150
200
250
10 20 30
a / mmG
Ic /
N/m
10
15
20
25
30
35
0 5 10 15 20 25 30
t / s
a / m
m ∆a
0
20
40
60
80
0 5 10 15 20 25 30
t / s
P(t)
/ N
∆U
0.5
0.6
0.7
0.8
15 20 25 30 35
a / mm
KI /
MN
m-3
/2Stress intensity factor
Energy release rate
GIc ~ 1-ν² Ε KIc
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by using change of electrical resistance of a conductive paste
by measuring the compliance
Conventional methods of measuring the R–Curve
u (COD)
Ω
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OCT - Automatic Fracture Toughness Measurement by Optical Monitoring of Crack Propagation
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Accuracy of Crack Length Determination by OCT Comparison to “true” crack length measured by optical microscopy
Difference to crack length at center
Difference to crack length at edge
a center
a edge
-0.800
-0.700
-0.600
-0.500
-0.400
-0.300
-0.200
-0.100
0.000
0.100
0.200
15 20 25 30 35
a Microscope / mm
Diff
eren
ce (a
Dav
is -
a M
icro
scop
e ce
nter
) / m
m
non-transparentsemi-transparenttransparent - dirty surfacetransparent
-0.200
-0.100
0.000
0.100
0.200
0.300
0.400
0.500
0.600
0.700
0.800
15 20 25 30 35
a Microscope / mm
Diff
eren
ce (a
Dav
is -
a M
icro
scop
e ed
ge) /
mm
non-transparentsemi-transparenttransparent - dirty surfacetransparent
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Increase of KIc for Crack Initiation caused by non-ideal pre-cracks
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 5 10 15 20 25 30 35a / mm
KIc
/ M
N/m
3/2
sample 1
sample 2
sample 3
non-ideal pre-crack (twisted crack front)
nearly ideal pre-crack
01020304050607080
0 5 10 15 20 25 30
time / s
forc
e / N
0102030405060708090
0 5 10 15 20 25 30
time / s
forc
e / N
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0
0.5
1
1.5
2
2.5
100 150 200 250 300 350Tg / °C
KIc /
MNm
-3/2
Neat Resin HMW-TPLMW-TP SiloxaneRubber Filled Resin
Mechanisms of Toughening
GIc = w1 GIc,1 + w2 GIc,2
1...Thermoset (brittle phase) 2...Thermoplast (tough phase)
Vp, high cross-link density
• Vp, low cross-link density • High ductility of matrix • Larger volume of energy
dissipation by multiple plastic shear yielding bends
Multiple Shear Yielding, induced by rubber particles Superposition
Glass transition temperature / °C
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OCT Fracture Mechanical Analysis
Advantages No additional sample preparation Automatic determination of KIc and GIc
No subsequent manual analysis of the broken specimen required High accuracy, needs very few samples High reliability (for transparent and non-transparent samples) Fast characterisation (testing, calculations, graphs, printouts) within 10 min Easy to use Determination of the true KIc, no artefacts due to non plain pre-cracks
“non-ideal” pre-crack
ideal pre-crack
0.5
0.6
0.7
0.8
15 20 25 30 35
crack length / mmK
I / M
Nm
-3/2
0.5
0.6
0.7
0.8
15 20 25 30 35
crack length / mm
KI /
MN
m-3
/2
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Thank you for your attention
Kontakt: Dr. Olaf Kahle, [email protected], 03328 330 276 Dr. Michael Wegener, [email protected], 0331 568 1209
Part of this work was supported as Fraunhofer High Performance Center for Functional lntegration in Materials