Experimental characterization and unit cell modeling of
structurally stitched NCF laminates
M. Magin, N. Motsch, H. Schmidt, H. Heß
Institut für Verbundwerkstoffe GmbH
Erwin-Schrödinger-Str. 58
67663 Kaiserslautern
Germany
www.ivw.uni-kl.de
FACC Technical Colloquium, July 5th, 2012
© Institut für Verbundwerkstoffe GmbH, Magin, Motsch, Schmidt, Heß
1
Content
•
•
•
•
•
•
•
•
•
•
Introduction
History of investigation of structurally stitched C-NCF laminates at
IVW
FE based 2D unit cell model
In-plane stiffness and strength
Compression after impact (CAI) strength
3D enhancement of unit cell model
Out-of-plane stiffness and strength
Mode-I behavior
Qualitative effect of structural stitching
Conclusion
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2
Effects of structural stitching on
characteristics of C-NCF laminates
Fabrication
x
z
z
z
y
z
x, y
x, y
x, y
Void characterization
Modeling
In-plane tension,
compression, shear
y
void due to
unit-cell model
Parameters:
structural stitching (FEA)
• stitching pattern
yarn
• diameter of yarn
y
• stitching direction
NCF void
• test direction
x
fiber
Results:
Results:
misalignment
Reduction of in-plane stiffness
Void geometry
and strength properties for the
UD-layer
most part
x
Identification of stitching configurations with a maximum increase of out-ofplane and a minimum decrease of in-plane properties of structurally stitched
C-NCF laminates
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History of investigation of structurally
stitched C-NCF laminates at IVW
LuFo II, 1.7.99 – 31.12.02
•
in-plane stiffness (tension, compression) of unstitched and
structurally stitched C-NCF laminates
LuFo III, 1.1.03 – 30.6.07
•
in-plane strength (tension, compression) of unstitched and
structurally stitched C-NCF laminates
DFG, Schubbelastete MAG, 1.5.06 – 30.06.10
•
in-plane shear stiffness and strength of unstitched and
structurally stitched C-NCF laminates
LuFo IV, first Call - HIT, 1.1.07 – 31.12.10
•
out-of-plane stiffness and in-plane stress/strain characteristics
of unstitched and structurally stitched C-NCF laminates
LuFo IV, second Call - HIGHER, 1.1.09 – 31.12.12
•
out-of-plane strength of unstitched and structurally stitched CNCF laminates
•
simulation of the mode-I behavior
•
simulation of the behavior of unstitched and structurally
stitched T-joints under T-pull load
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4
LuFo II: Stitching parameters
• linear yarn density
upper thread: 68 tex or 136 tex E-glass yarn
lower: 68 tex E-glass yarn
• loading and stitching direction
parallel x (0° to fiber direction)
parallel y (90° to fiber direction)
upper yarn
• stitching patterns
spacing si and pitch length pi
stitching direction x
y
© Institut für Verbundwerkstoffe GmbH, Magin, Motsch, Schmidt, Heß
10 mm
4.00
6.06
9.18
6.06
reinforcement density in cm-2
minimum and maximum
reinforcement densities:
10 mm
10 mm
s1 = 5.0 mm s3 = 3.3 mm
p1= 5.0 mm
p3= 3.3 mm
lower yarn
x=0°
s1; p1
s3; p3
5
LuFo II: Characterization of voids due to
stitching yarn
Micrograph through a -45°-layer of a
structurally stitched [A1-B-A2] CFRP
laminate parallel to laminate plane
y
structural stitching yarn (E-glass, 68 tex)
global fiber
orientation in single
layer
void width (w)
void crosssection (A)
Results
• area of void > area of NCF void
• area of void >> area of yarn
• area, width and length of voids
decrease from the surface
towards the center of the laminate
• area, width and length of voids
increase with an increasing yarn
thickness
x
NCF void
void length (l)
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LuFo II: FE based unit cell model
Model definition
Parameters taken into account
y
z
x
• thickness, fiber orientation and number
of layers
stitching direction y
pitch length p
• stitching direction (x; y)
suppressed
single layers
© Institut für Verbundwerkstoffe GmbH, Magin, Motsch, Schmidt, Heß
• spacing and pitch length
(5.0 mm; 3.3 mm)
• linear yarn density (68 tex; 136 tex)
• loading direction (x; y)
voids due to
stitching yarn
7
LuFo II: FE based unit cell model
Meshed 90°-layer
Unit cell model
fiber orientation
layer 1
fiber orientation
90°-layer
y
z
y
x
x
undisturbed
fiber orientation
(20-node SOLID186
element)
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local region of fiber
dislocation
8
LuFo II: Determination of elastic
constants
y
calculation of compliance
matrix [ABD]-1
unit cell generation
y
unit load case calculations
• normal strain ex°
• normal strain ey°
• shearing strain gxy°
• curvature kx
• curvature ky
• torsional kxy
B 1  A* B* 

   *T
*
D B B 
x
z
y
z
estimation of
stiffness matrix
[ABD]
[ ABD] 1
x
z
A

B 
elastic constants (Ex, Ey, etc.)
x
N  A B  ε 0 
 
 
B
D  κ 
M  
  
ABD
© Institut für Verbundwerkstoffe GmbH, Magin, Motsch, Schmidt, Heß
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LuFo II: Analysis of unit load cases
Nodal displacements
| USUM |  u x  u y  u z
e x° = 1 %
e y° = 1 %
y
x
A11
A12
A16
B11
B12
B16
2
2
y
x
gxy° = 1 %
A12
A22
A26
B12
B22
B26
ky = 1 %
kx = 1 %
z
x
B11
B12
B16
D11
D12
D16
© Institut für Verbundwerkstoffe GmbH, Magin, Motsch, Schmidt, Heß
2
y
x
A16
A26
A66
B16
B26
B66
kxy = 1 %
z
y
B12
B22
B26
D12
D22
D26
z
x
B16
B26
y B
66
D16
D26
D66
10
LuFo II: NCF lay-up of CFRP test
laminates (HTA/RTM6)
NCF CFRP laminate
tension test
NCF CFRP laminate
compression test
x
x
x
A1
A1
y
[45°/0°/-45°]
[45°/0°/-45°]
y
y
20 mm
(B/2)s
[0°/90°/90°/0°]
x
A2
[-45°/0°/45°]
B
[0°/90°/0°]
y
A1
[45°/0°/-45°]
x
A2
[-45°/0°/45°]
z
© Institut für Verbundwerkstoffe GmbH, Magin, Motsch, Schmidt, Heß
(B/2)s
[0°/90°/90°/0°]
y
A2
[-45°/0°/45°]
z
11
Through the thickness stitching effect on in-plane
tensile modulus of [A1-B-A2] NCF CFRP laminates
relative to unstitched laminate [%]
LuFo II: Experimentally and theoretically
determined in-plane tensile modulus
loading direction x
140
130
experiment
stitching direction x
analysis (FE-unit cell)
stitching direction y
120
68-tex-yarn
68-tex-yarn
136-tex-yarn
• through-thickness
stitching generally
reduces in-plane
110 modulus of CFRP laminates
tensile
71.5 GPa
• stitching
configurations
with slight modulus increase exist
100
(loading direction y)
90
• maximum
reduction of in-plane tensile modulus amounting
to 23 % compared to unstitched laminate (29 % loading
80
direction
y)
• tensile stiffness is most influenced by thickness of stitching
70
yarn
• averaged discrepancy of the calculated results from
60
experimental data = 5 %
0
136-tex-yarn
s [mm] un5.0 3.3 5.0 3.3 5.0 3.3 5.0 3.3 5.0 3.3 5.0 3.3 5.0 3.3 5.0 3.3
p [mm] stitched 5.0 5.0 3.3 3.3 5.0 5.0 3.3 3.3 5.0 5.0 3.3 3.3 5.0 5.0 3.3 3.3
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LuFo III: Strength prediction
Iterative, 3D layer-by-layer failure analysis
Fracture criteria for composite
• Inter fiber failure (IFF): Puck‘s 3D IFF actionplane criterion
stress states
- Mode A, A*, B uncritical
- Modus C critical for
stress exposure ≥ 1.25 and
fracture plane angle ≤ - 30 ° or ≥ 30 °
Fracture criterion for resin
(void)
Maximum principal stress
criterion
• Fiber failure (FF): Maximum stress criterion
(tension and compression)
stiffness degradation
based on fracture modus and fracture plane
angle analysis
© Institut für Verbundwerkstoffe GmbH, Magin, Motsch, Schmidt, Heß
stiffness degradation
13
Through the thickness stitching effect on in-plane
compressive strength of [A1-(B/2)S-A2]2 NCF CFRP
laminates relative to unstitched laminate [%]
LuFo III: Experimentally and theoretically
determined in-plane compressive strength
•
•
•
•
•
loading direction x
140
experiment
analysis (FE-unit cell)
130
785 MPa
120
110
100
90
80
through-thickness
stitching generally reduces in-plane
compressive
strength of CFRP laminates
70
stitching
60 configurations with slight strength decrease exist
maximum
reduction of in-plane compressive strength amounting
50
to 31 % compared to unstitched laminate
40
compressive strength is most influenced by thickness of
30 yarn
stitching
20 discrepancy of calculated results from experimental
averaged
data10
=8%
0
s [mm] un5.0 3.3 5.0 3.3 5.0 3.3 5.0 3.3 5.0 3.3 5.0 3.3 5.0 3.3 5.0 3.3
p [mm] stitched 5.0 5.0 3.3 3.3 5.0 5.0 3.3 3.3 5.0 5.0 3.3 3.3 5.0 5.0 3.3 3.3
© Institut für Verbundwerkstoffe GmbH, Nicole Motsch, Henrik Schmidt
14
DFG-Schub: Three-rail shear test and
sample geometry
In-plane shear testing of unstitched and structurally stitched laminates with the three
rail shear method
loading direction x or
y
CFRP
specimen
CFRP
specimen
rail
connection
to test
machine
© Institut für Verbundwerkstoffe GmbH, Magin, Motsch, Schmidt, Heß
tab
15
Through the thickness stitching effect on inplane shear strength relative to unstitched
laminate [%]
DFG-Schub: Experimentally and theoretically
determined in-plane shear strength
[45°/0°/-45°/0°/90°/90°/0°/-45°/0°/45°]-HTS-laminate
140
loading direction x
120
222 MPa
experiment
analysis (FE-unit cell)
100
80
• through-thickness stitching generally reduces in-plane shear
60 of CFRP laminates
strength
• maximum reduction of in-plane shear strength amounting to
68-tex-yarn
136-tex-yarn
68-tex-yarn
22 %
40compared to unstitched laminate
• in-plane shear strength is most influenced by thickness of
stitching yarn
20
• averaged discrepancy of calculated results from
experimental data = 15 %
0
136-tex-yarn
s [mm] un5.0 5.0 3.3 3.3 5.0 5.0 3.3 3.3 5.0 5.0 3.3 3.3 5.0 5.0 3.3 3.3
p [mm] stitched 5.0 3.3 5.0 3.3 5.0 3.3 5.0 3.3 5.0 3.3 5.0 3.3 5.0 3.3 5.0 3.3
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Experimentally determined compression
after impact (CAI) strength
[45°/0°/-45°/0°/90°/90°/0°/-45°/0°/45°]-HTS laminate
Impact energy 30 J, ca. 12 J/mm
damage area
delamination
at impact surface
length
diameter of impact
area
compressive
failure
at impact area
damage area at
laminate opposite to
impact surface
delamination
width
m-CT photographs after impact
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17
Normalized compression after impact (30J)
strength of structurally stitched [A1-(B/2)s-A2]HTS laminates [%]
Normalized compression strength after
30 J impact
[45°/0°/-45°/0°/90°/90°/0°/-45°/0°/45°]-HTS laminate
loading direction y
loading direction x
• through-thickness stitching generally raises CAI strength of
CFRP laminates
• stitch
maximum
enhancement of CAI strength amounting to
density
48 % compared to unstitched laminate
• reduction of damage area at laminate opposite to impact
yarn
surface up to 70 %
yarn
Reinforcement density
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LuFo IV-I (HIT): NCF lay-up of CFRP test
laminates (HTS/RTM6)
[(+45/-45/0/90)S]2 laminates
+45°
-45°
0°
90°
z
Saertex NCF-HTS
• 0°/90°, (283/267) g/m²
• +45°/-45°, (267/267) g/m²
• -45°/45°, (267/267) g/m²
90°
0°
-45°
+45°
RTM6 resin
stitching yarns
• Amann Group Serafil 200/2
polyester yarn
• Culimeta E-glass yarn
• 34x2 tex
• 68x2 tex
© Institut für Verbundwerkstoffe GmbH, Magin, Motsch, Schmidt, Heß
+45°
-45°
y
0°
90°
90°
0°
-45°
+45°
[Premium Aerotec]
x
19
LuFo IV-I (HIT): Parameter configurations
loading
directions
stitching
direction
linear yarn
density
in tex
spacing
in mm
pitch
length
in mm
K1
x
x
68
3.3
3.3
K2
x
x
68
5.0
5.0
K3
x
x
136
3.3
3.3
K4
x
x
136
5.0
5.0
K5
x
y
136
3.3
3.3
K6
y
x
136
3.3
3.3
parameter
configuration
unstitched
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LuFo IV-I (HIT): Enhancement of unit-cell
model
•
generation of six additional load cases
→ 12 unit load cases for 3D stiffness prediction
normal load cases
bending load cases
shear load cases
twisting load cases
© Institut für Verbundwerkstoffe GmbH, Magin, Motsch, Schmidt, Heß
21
LuFo IV-I (HIT): Enhancement of unit cell
model
• Generation of load case
for strength prediction
in z-direction
p
displacements uz in
mm
0
fracture plane angle in °
-90
s
• Estimation of IFF failure
by means of Puck 3D
action plane criterion
y
z
0,0014
x
90
IFF effort NCF
relation between von-Mises stress
and allowable stress
0,96
in yarn and pure resin
0,230
1,00
© Institut für Verbundwerkstoffe GmbH, Magin, Motsch, Schmidt, Heß
0,5
22
LuFo IV-I (HIT): Out-of-plane tensile tests
x- axis
rotation
z
x
y
connection to
testing machine
clamping
device
y-axis
rotation
specimen
stitch
camera 1 camera 2
optical 3D
displacement
measurement
© Institut für Verbundwerkstoffe GmbH, Magin, Motsch, Schmidt, Heß
23
120
120
110
110
to unstitched reference [%]
Ezt modulus of unstitched and structurally
stitched [(+45/-45/0/90)S]2 laminates compared
LuFo IV-I (HIT): Out-of-plane modulus Ezt:
comparison experiment and FE unit cell model
100
100
90
90
•
80
80
70
70
•
•
experiment
•
experiments show stiffness
increase for all configurations
maximum enhancement of 7 %
altogether good correlation
between experiment and unit-cell model
conservative prediction
FE unit-cell model
0
60
unstitched
unstitched
K1
K1
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K2
K2
K3
K3
K4
K4
K5
K5
24
120
120
compared to unstitched reference [%]
Out-of-plane strength Rzt of unstitched and
structurally stitched [(+45/-45/0/90)S]2 laminates
LuFo IV-I (HIT): Out-of-plane strength Rzt
•
115
•
110
110
experiments show slight strength increase
for K3 and K5
significant strength reduction for K4
(largest resin areas)
105
100
100
95
90
85
80
75
70
0
unstitched
1
© Institut für Verbundwerkstoffe GmbH, Magin, Motsch, Schmidt, Heß
K1
2
K2
3
K3
4
K4
5
K5
6
25
Mode I interlaminar fracture behavior:
TDCB test
Tapered
double
cantilever
beam test
(TDCB test)
load introduction
element
aluminum tabs
FEP insert
z
CFRP specimen (width 25 mm)
x
first stitching row
fiber bridging
activated stitching
yarns
FEP insert
broken stitching yarn
© Institut für Verbundwerkstoffe GmbH, Magin, Motsch, Schmidt, Heß
matrix and interface
failure
26
Normalized Mode I energy release rate G1R of
structurally stitched [A1-(B/2)s-A2]-HTS
laminates [%]
Normalized Mode-I energy release rate
[45°/0°/-45°/0°/90°/90°/0°/-45°/0°/45°]-HTS laminate
(modified beam theory)
(compliance calibration method)
direction of crack
propagation y
stitch density
• through-thickness stitching generally raises energy release
rate of CFRP
laminatesder Restdruckfestigkeit bei strukturell vernähtem
• Steigerung
• maximum enhancement
of energy48%
release rate amounting to
Laminat um maximal
factor of 5.4 compared to unstitched laminate
direction of crack
• reduction of the differences between energy release rates
of
propagation
x
different directions of crack propagation (unstitched 42 %;
structurally stitched 7 %)
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Reinforcement density
27
Qualitative effect of structural stitching
on in-plane and out-of plane properties
Stitching
direction
Yarn
diameter
Spacing
Pitch
Property
x
y
Tensile modulus
Tensile strength
Compression modulus
Compression strength
Shear modulus
Shear strength
0
0
0
0
-
0
0
0
+
0
+
--0
--
0
0
+
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
CAI-strength
Mode I energy release rate 0
0
0
0
++
-
0
-
+
+
- (--)
0
+ (++)
increasing increasing increasing
Stitch
density
increasing
negative (strongly negative)
negligible
positive (strongly positive)
© Institut für Verbundwerkstoffe GmbH, Magin, Motsch, Schmidt, Heß
28
Conclusion
Experimental and theoretical work on structurally stitched C-NCF laminates
•
•
•
•
development of a 2D FE based unit cell model
enhancement of unit cell model for out-of plane investigations (3D)
development of G1R model for investigation of Mode I behavior
modulus and strength changes due to structural stitching compared to unstitched
reference
• in-plane tension modulus => max. reduction 29 %
• in-plane tension strength => max. reduction 36 %
• in-plane compression modulus => max. reduction 11, max. increase 22 %
• in-plane compression strength => max. reduction 31 %
• in-plane shear modulus => max. reduction 15 %
• in-plane shear strength => max. reduction 22 %
• out-of-plane tension modulus => max. increase 7 %
• out-of-plane tension strength => max. increase 4 %
• CAI strength => max. increase 48 %
• damage area after impact loading => max. reduction 70 %
• mode I energy release rate => max. increase factor of 5.4
© Institut für Verbundwerkstoffe GmbH, Magin, Motsch, Schmidt, Heß
29
Thanks to
Airbus Deutschland GmbH
Bundesministerium für Wirtschaft und Technologie (BMWi)
Deutsche Forschungsgemeinschaft (DFG)
for their financial support of the projects
and to you for your attention.
© Institut für Verbundwerkstoffe GmbH, Magin, Motsch, Schmidt, Heß
30