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Effects of mesostructure
on the in-plane properties
of tufted carbon fabric
composites
CompTest 2011, 14th February 2011
Johannes W G Treiber
Denis D R Cartié
Ivana K Partridge
[email protected]
Tufting process
- Modified one-sided stitching process
- Automated insertion of carbon, glass or aramid thread
0.5% carbon tufted NCF
- For dry composite preforms
Top
Automated KSL KL150 tufting head
Hollow
needle
Presser
foot
Dry
fabric
Tuft thread
loop
2/14
Tufting process
Exp. database
Bottom
Support
foam
Meso-structure
FE-Models
Introduction
Main purpose: Through-the-thickness reinforcement technique
Tuft bridging
+ 460%/+60% mode I/II delamination toughness
for only 0.5% areal tuft density (Cranfield)
Delamination crack
Drawback: Potential reduction of in-plane properties
DCB of 0.5% carbon tufted NCF
Stitching: - considerable database
Stiffness: -15% to +10%
Tufting:
Tensile strength: -25% to +25%
- to date only 3 experimental studies (KU Leuven, Cranfield)
Tensile strengths: -14% to +10%
no agreement
Need for detailed testing database on
wider range of tufted materials
3/14
Tufting process
Exp. database
Meso-structure
FE-Models
Materials
- Carbon Preforms:
Uni-weave
- [0°]7 , [0°]10
balanced NCF - [(0°/90°)s]2
- Tufted with 2k HTA carbon thread in at sx = sy = 5.6 mm (0.5%) /2.8 mm (2%)
Square arrangement
- Cross-over
Free loop height:
- Pattern shift
3.5 – 5 mm
- RTM injection of epoxy resin (ACG MVR 444) for dimensional control
4/14
Tufting process
Exp. database
Meso-structure
FE-Models
In-plane tension behaviour
1.5
1.1
UD^
1.4
1.3
1.2
1.1
UD||
1
NCF||
Normalised strength (-)
Normalised elastic modulus (-)
Tensile tests (BS EN ISO 527-4:1997):
NCF ‘threadless‘
1
0.9
0.8
NCF||
UD||
0.7
UD^
0.6
0.5
0.9
0.0
0.5
1.0
1.5
2.0
0.0
2.5
0.5
1.0
1.5
2.0
Areal tuft density (%)
Areal tuft density (%)
Property changes depend on fabric and tuft morphology
5/14
Tufting process
Exp. database
Meso-structure
FE-Models
2.5
Meso-structure
Thread
Tuft
Loop
In-plane disturbance (x-y):
z
y
x
UD
Triangular
4°
w ,φ
Resin
pocket
Tuft
Fabric
deviation
0.5%
w
UD: 6°
NCF: 10°
2.0%
Square
Square
Thermal crack
w,
φ
2φ
UD: 3°
NCF: 4°
7°
NCF
6/14
Tufting process
Exp. database
Meso-structure
FE-Models
Meso-structure
Thread
Tuft
Loop
Out-of-plane disturbance (x-z/y-z):
z
y
Local fibre volume fraction (%)
x
80
75
z
70
x
65
60
Vf = f(wi,tloop, tthread)
Vf,2D
55
Thread layer tth
50
0
0.5
1
1.5
2
2.5
Distance between tuft rows (mm)
- Surface seam causes local fabric crimp
- Resin rich layers and pockets affect global and local Vf
7/14
Surface
crimp
Tufting process
Exp. database
Meso-structure
Loop layer tl
z
y
FE-Models
Thread
seam
Numerical Unit Cell model
φ = cosine
fct.
Loop
Vf = f(tl,tth,wi)
wdev
Vf (%)
67.5
w
Tuft
y
Resin
channel
‘Smeared’ UD
z
y
x
¼ UC
y
63.0
Thread
x
wi
x
Parametric 3D Unit Cell model (Marc): UD, NCF, square and triangular arrangement
Isotropic, linear elastic material + ‘Rule of mixtures’ (Chamis)
Failure and degradation:
Ply:
Puck (FF + 3 modes of IFF)
Resin: Maximum strength
8/14
Tufting process
Exp. database
0.03* / 1.0  En 
 En 

G   
 nt  deg  0.67  Gnt  0
Edeg  0.1  E0
Meso-structure
FE-Models
*
n  0
Knops, Comp.
Sci. Tech. 2006
Failure prediction – NCF
0° Ply
900
¼ UC
A
Transverse
Shear
tension
failure
0.5
Longitudinal
Longitudinal
splitting
splitting
0.0
Experiment
Tensile stress σ|| (MPa)
Stress exposure IFF σn>0
1.0
750
FE Model
A
600
90° ply
cracking
450
300
150
0.5% NCF||
0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Tensile strain ε|| (%)
Cracks
- Accurate modulus and strength, also for 2%
density (error < 2/4%)
- Fabric straightening leads to transverse
tension failure in fabric and longitudinal
splitting of resin pocket
Longitudinal
splitting
9/14
Tufting process
Exp. database
Meso-structure
FE-Models
Failure prediction – NCF
0° Ply
0.8
B
Fibre
failure
Initiation
of
fibre
failure
initiation
0.6
B
900
¼ UC
Experiment
Tensile stress σ|| (MPa)
Stress exposure FF
1.0
750
FE Model
600
450
300
150
0.5% NCF||
0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Tensile strain ε|| (%)
Rupture
Tuft
0°
10/14
Tufting process
- Ultimate fabric fibre failure in close vicinity of tuft
Exp. database
Meso-structure
FE-Models
Vf distribution
0.5% NCF||
Normalised axial properties (-)
1.20
NCF - Modulus
NCF - Strength
1.10
1.00
0.90
0.80
ΔV
ΔV
f=f(wi)
f=f(Arp)
ΔVf=f(wi)
ΔVf=f(Ar.p.)
Vf distribution models
ΔV
f=0
ΔVf=0
- Local fabric fibre distribution affects both stiffness and strength prediction
- Gradient Vf model agrees best with true morphology and tension results
11/14
Tufting process
Exp. database
Meso-structure
FE-Models
Fibre misalignment φ
0.5% square
Normalised properties (-)
1.05
1.00
0.95
0°
¼ UC
0.90
UD
Modulus
0.85
NCF
Strength
wmin
0.80
0
2
4
6
8
10
Max. Fibre deviation φdev (°)
- Fabric fibre deviation critical on tensile strength, effect on modulus negligible
- UD strength more sensitive to fabric deviation
12/14
Tufting process
Exp. database
Meso-structure
FE-Models
Tuft arrangement
UD||
Tensile strength S|| (-)
2500
2000
94%
Square
1500
80%
Triangula
r
1000
95%
68%
Model
Experiment
500
0.0
0.5
1.0
1.5
2.0
Tensile strain e|| (%)
- Upper and lower strength bounds defined by square and triangular pattern
- Triangular pattern causes most critical strength reduction
13/14
Tufting process
Exp. database
Meso-structure
FE-Models
Conclusions
• Tuft introduces structural complexity into Z-reinforced composite
• Critical meso-structural tuft defects: resin rich pockets, fibre deviation,
matrix cracking and local fibre compaction
• Tufting has no effect on longitudinal tensile stiffness of UD and biaxial
NCF composite, but surface loops increase matrix dominated
transverse stiffness
• Reduction in longitudinal tensile strength most prominent for UD (<-19%)
• Fibre undulation most critical contributing factor, fibre breakage limited
effect on tensile strength
• High quality experimental morphology data allows accurate tensile
stiffness and strength prediction of tufted composites
14/14