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 CartiéIvana K Partridge

[email protected]

Effects of mesostructure on the in-plane properties of tufted carbon fabric composites

2/14

Tufting process

- Modified one-sided stitching process

- Automated insertion of carbon, glass or aramid thread

- For dry composite preforms

Hollow needlePresser

foot

Tuft thread loop

Automated KSL KL150 tufting head

Dry fabric

Support foam

0.5% carbon tufted NCF

Bottom

Top

Tufting process

Exp. database Meso-structure FE-Models

3/14

Need for detailed testing database on wider range of tufted materials

Tufting process Exp. database Meso-structure FE-Models

Introduction

Main purpose: Through-the-thickness reinforcement technique

+ 460%/+60% mode I/II delamination toughness

for only 0.5% areal tuft density (Cranfield)

Tufting: - to date only 3 experimental studies (KU Leuven, Cranfield)

Tensile strengths: -14% to +10%

no agreement

Drawback: Potential reduction of in-plane properties

Stitching: - considerable database

Stiffness: -15% to +10% Tensile strength: -25% to +25%

Delamination crack

Tuft bridging

DCB of 0.5% carbon tufted NCF

4/14 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%)

- Cross-over

- Pattern shift

Free loop height:

3.5 – 5 mm

- RTM injection of epoxy resin (ACG MVR 444) for dimensional control

Square arrangement

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0.5

0.6

0.7

0.8

0.9

1

1.1

0.0 0.5 1.0 1.5 2.0 2.5

Areal tuft density (%)N

orm

alis

ed s

tren

gth

(-)

UD||

NCF||

UD^

NCF ‘threadless‘

0.9

1

1.1

1.2

1.3

1.4

1.5

0.0 0.5 1.0 1.5 2.0 2.5

Areal tuft density (%)

UD||

Nor

mal

ised

ela

stic

mod

ulus

(-)

NCF||

UD^


In-plane tension behaviour

Tensile tests (BS EN ISO 527-4:1997):

Property changes depend on fabric and tuft morphology

6/14

Meso-structure

In-plane disturbance (x-y):


Thread

Loop

Tuft

z

x

y

Resin pocket Tu

ft

Thermal crack

Fabric deviati

on0.5%

UD

NCF

Square

2.0%

Triangular

Square

2φ

w

w,φ

w ,φUD: 6°NCF: 10°

4°

7°

UD: 3°NCF: 4°

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50

55

60

65

70

75

80

0 0.5 1 1.5 2 2.5Distance between tuft rows (mm)

Loca

l fib

re v

olum

e fr

actio

n (%

)

Vf,2D

Meso-structure


- Surface seam causes local fabric crimp

- Resin rich layers and pockets affect global and local Vf

Out-of-plane disturbance (x-z/y-z):

Surface

crimp

Thread

seam

Loop layer tl

Thread

Loop

Tuft

z

x

y

y

z

x

z

Thread layer tth

Vf = f(wi,tloop, tthread)

8/14

67.5

63.0

Vf (

%)

wde

v

w xy

z


Numerical Unit Cell model

Parametric 3D Unit Cell model (Marc): UD, NCF, square and triangular arrangement

wi

Vf = f(tl,tth,wi)

¼ UC

Isotropic, linear elastic material + ‘Rule of mixtures’ (Chamis)

Failure and degradation:

0

*

deg67.0

0.1/03.0

nt

n

nt

n

G

E

G

E0* n

Knops, Comp. Sci. Tech. 2006

φ = cosine fct.

Ply: Puck (FF + 3 modes of IFF)

Resin: Maximum strength

Tuft

Resin chann

el‘Smeared’

UD

Loop

Thread

0deg 1.0 EE

x

y x

y

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0

150

300

450

600

750

900

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Experiment

FE Model

Tensile strain ε|| (%)T

ensi

le s

tres

s σ

|| (M

Pa)

90° ply cracking

1.0

0.0

0.5

Shear

Longitudinal splitting

Str

ess

exp

osur

e IF

F σ

n>0

0° Ply

A


A

- 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

Failure prediction – NCF

Transverse

tension failure


Cracks


0.5% NCF||

¼ UC

10/14

1.0

0.6

0.8

Str

ess

expo

sure

FF

Initiation of fibre failure


- Ultimate fabric fibre failure in close vicinity of tuft

B

Failure prediction – NCF

0°

Fibre failure

initiation

Rupture

0

150

300

450

600

750

900

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Experiment

FE Model

Tensile strain ε|| (%)T

ensi

le s

tres

s σ

|| (M

Pa)

B

0° Ply

0.5% NCF||

Tuft

¼ UC

11/14 Tufting process Exp. database Meso-structure FE-Models

Vf distribution

0.5% NCF||

0.80

0.90

1.00

1.10

1.20

ΔVf=f(wi) ΔVf=f(Ar.p.) ΔVf=0

NCF - Modulus

NCF - Strength

Vf distribution models

ΔVf=f(wi) ΔVf=f(Arp) ΔVf=0

Nor

mal

ised

axi

al p

rope

rtie

s (-

)

- Local fabric fibre distribution affects both stiffness and strength prediction

- Gradient Vf model agrees best with true morphology and

tension results

12/14

0.80

0.85

0.90

0.95

1.00

1.05

0 2 4 6 8 10

Modulus

StrengthNor

mal

ised

pro

pert

ies

(-)

Max. Fibre deviation φdev (°)

UDNCF


Fibre misalignment φ

0.5% square

- Fabric fibre deviation critical on tensile strength, effect on modulus negligible

- UD strength more sensitive to fabric deviation

0°

¼ UC

wmin

13/14

500

1000

1500

2000

2500

0.0 0.5 1.0 1.5 2.0

Model

Experiment

Tensile strain e|| (%)

Ten

sile

str

engt

h S

|| (-

)

Square

Triangular 68%

95%

80%

94%


Tuft arrangement

UD||

- Upper and lower strength bounds defined by square and triangular pattern

- Triangular pattern causes most critical strength reduction

14/14

• 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

Conclusions

effects of mesostructure on the in-plane properties of tufted carbon fabric composites

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