Methodology for Characterization of Internal Structure of ...comptest/proc/files/presentations/Joffe.pdf · Volume fraction of fibers in the bundle ( ) is obtained from image analysis

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COMPTEST2006, 10th-12th April, 2006Faculdade de Engenharia da Universidade do Porto

Methodology for Characterization of Internal Structure of NCF Composite and Its Influence on

Mechanical Properties

Roberts Joffe and David MattssonDivision of Polymer EngineeringLuleå University of Technology

S-971 87, Luleå, Sweden


Where is Luleå?


• Introduction

• Objectives

• Structure-properties relation

• Characterization of structure

CONTENTS

• Results

• Summary


INTRODUCTION: What is NCF?

Non-crimp fabrics - NCF

a

bVariable parameters:

a) Gauge stitching

b) Stitch length

c) Stitch pattern


INTRODUCTION: applications of NCF composites

NCF composites are used in large ship structures (72m long corvette Visby in SWEDEN)

Glass fiber (and lately also carbon fiber) NCF is used in wind generator blade and recreation boats

Lately there is interest from aircraft manufacturers to use NCF

However, due to complicated internal structure performance in connection with internal geometry must be studied


INTRODUCTION: NCF - multi-scale material

V fφ

NCF composite - inherently multi-scale material.

On the microscale each bundle with orientation φ is a UD composite with a certain fiber content ( ) and the homogenized bundle properties may be calculated using micromechanics expressions for long fiber composites.

On the mesoscale the bundle is considered as homogeneous transversely isotropic material surrounded by matrix and other bundles of the same or different orientation. A mesoscalecharacteristic of the composite is bundle content ( ) in the composite. V b

φ

The geometrical shape of bundles (cross-section and axial alignment) is complex and depends on bundle orientation in the blanket, surface compression during production, resin pockets etc. The described mesoscale configuration determines the NCF composite properties on macroscale and is typically used in simulations of macro behaviour.


INTRODUCTION: NCF - multi-scale material


OBJECTIVES

Main objective is to identify and characterize critical parameters of internal structure of NCF composites which are important for certain in-plane mechanical properties

This work is focused on structural parameters and, therefore, the role of fiber and matrix properties is not analyzed, although it is certainly of great importance.

The stitching effect on the material performance is not discussed

Quasi-isotropic [(45/0/-45/90)2, (90/45/0/-45)2] and cross-ply [0/90/0/90]S laminates are studied


Structure-Properties Relation: stiffness

The stiffness of a multidirectional NCF composite is strongly dependent on the contribution of fibers.

The main parameter characterizing fiber contribution is the average fiber content of a certain orientation φ in the composite - V fa

φ

Detailed mesoscale description: volume content of bundles with a certain orientation –fiber volume fraction inside a bundle of each orientation –shape of the cross-section of bundles

V bφ

V fφ

However, it has been shown that the effect of mesoscale details on the NCF laminate stiffness may be neglected and a “smearing out“ routine leads to high accuracy of the determined stiffness matrix.


Structure-Properties Relation: stiffness

Waviness of the bundles will have strong effect on the stiffness of the NCF composites, therefore such mesoscale parameters as wavelength and amplitudeof imperfection have to be characterized.

x

z

Gap Wavelength, λ

90° bundle width, w


Structure-Properties Relation: damage, tensile loadingDamage in tensile loading can be analyzed as 2-step process: 1) intrabundle cracks in layers with off-axis orientation with

respect to the load; 2) failure of longitudinal bundles.

Intrabundle cracks: bundle cross-section (height to width ratio), the ratio of the bundle width and the matrix channel size, elastic modulus of the matrix and the longitudinal and transverse modulus of the bundle.

Failure of longitudinal bundles : out-of plane waviness


Structure-Properties Relation: compressive failure

Two failure mechanisms are considered: 1) interbundle splitting; 2) kink-band formation in the longitudinal bundle.

Parameters affecting compressive failure:

fiber distribution in the composite

in-plane and out-of-plane waviness


Property V fa

φ V b

φ V f

φ Shape of the cross-section

Waviness

Stiffness 5 1 1 1 5 Failure in tension 1 3 3 3 5 Failure in compression 2 3 3 1 5

Influence of mesostructural parameters on different mechanical properties of the NCF composite

Structure-Properties Relation: summary


Classification of bundles by their shape

Characterization of structure: bundle shape


Classification of “unusual” bundles

Characterization of structure: bundle shape

Shape of bundle is assumed to be “half-ellipse”


Characterization of structure: volume fraction

A

AAAV l

rli

heik

ektottotb

∑+∑+∑==

φφφφφ v/v

Volume fraction of a bundle with 90º, ±45º or 0ºorientation in the composite

90450bbb

totb VVVV ++= ±The total volume fraction of

bundles in the composite

Ellipse Rectangle

We

He

Wr

Hr

The volume fraction of bundles is defined as the cross section area fraction of bundles in each layer

4

eee

WHA π=4

WHAhehe

heπ= rr

r WHA =


Characterization of structure: volume fraction

Volume fraction of fibers in the bundle ( ) is obtained from image analysis as ratio between the area were measurements are performed and area occupied by fibers

φfV

The average volume fraction of fibers with the same orientation φ in the composite

φφφfbfa VVV =

90450fafafa

lamf VVVV ++= ±

The total average volume fraction of fibersin the composite, where

0faV 45±

faV 90faV, and are average volume

fractions of fibers (in the laminate) with corresponding orientations


Characterization of structure: resin channels

LCH

Matrixarea

Bundle Bundle

Matrixarea

Bundle Bundle

Matrixarea

Bundle Bundle

Matrixarea

BundleBundle

LCH

Bundle

Bundle

Matrix area

1)

2)

3)

4)

5)

6)

Matrixarea

Bundle Bundle

channelsofNr

WWL

totbundle

tot

CH−

=

The average linear size of the resin channel between the bundles in a layer

Where is the total width of the bundles in the layer in the window of observation and is the total width of the window

totbundleW

totW


Characterization of structure: waviness

x

z

Gap Wavelength, λ

90° bundle width, w

In ideal case waviness can be assumed of periodic nature and described by sinus function. In this case wavelength and amplitude are sufficient characteristics of the misalignment of the 0°-bundles. The amplitude of the wave in the 0°-bundles can be determined from the measured standard deviation with respect to the theoretical horizontal direction of the bundles (and it is called SDO).

However, If the waviness is not periodic, another parameter of imperfection should be used instead of wavelength and amplitude, for example, the maximum inclination angle.



Quasi-isotropic laminate


Stitches inducing small scale waviness.


Cross-ply laminate


Size and shape of the bundles with indicated standard deviation for NCF quasi-isotropic composites produced with base configuration and high stitch tension fabrics.

a) Bundle width b) Bundle thickness

0

1

2

3

4

5

6

0° 90° ±45° 0° 90° ±45°

Base configuration High stitch tension

Bun

dle

wid

th (m

m)

Half ellipsEllipsRectangle

(a)

0,0

0,2

0,4

0,6

0,8

1,0

1,2

0° 90° ±45° 0° 90° ±45°


Bun

dle

thic

knes

s (m

m) Half ellips

EllipsRectangle

(b)

RESULTS: size and shape of bundles


Size and shape of the bundles with indicated standard deviation for NCF quasi-isotropic composites produced with base configuration and high stitch tension fabrics.

c) Bundle area d) Frequency of particular bundle shape

0,00,20,40,60,81,01,21,41,6

0° 90° ±45° 0° 90° ±45°


Bun

dle

area

(mm

^2)

Half ellipsEllipsRectangle

(c)

0

20

40

60

80

100

0° 90° ±45° 0° 90° ±45°

Base configuration High stich tension

Bun

dle

shap

e (%

)

Half ellipseEllipsRectangle

(d)

RESULTS: size and shape of bundles


totbV

Volume fractions in quasi-isotropic composites produced with base configuration and high stitch tension; a) Volume fraction of bundles with certain orientation in the laminate, b) Volume fraction of fibers within the bundles with certain orientation.

(a) (b)

RESULTS: volume fraction


totbV

Volume fractions in quasi-isotropic composites produced with base configuration and high stitch tension; c) Average volume fraction of fiber in layers with a certain orientation, d) Total volume fraction of bundle and total average volume fraction of fibers in the composite .

totbV

lamfV

(c) (d)

RESULTS: volume fraction


0,0

0,1

0,2

0,3

0,4

0,5

0,6

0° 90° ±45°

Dis

tanc

e be

twee

n bu

ndle

s (m

m)

Base configurationHigh stitch tension

Average distance between bundles (mm)

RESULTS: resin channels


Result from the measurements of waviness in 0°-bundles. The normalization of the coordinates is performed with respect to the thickness and observation length of the composite; a) Cross-ply laminate with base configuration; b) Quasi-isotropic laminate with base configuration.

RESULTS: wavinessB-C [0, 90, 0, 90]s

0,0

0,2

0,4

0,6

0,8

1,0

0,0 0,2 0,4 0,6 0,8 1,0Normalized x-coordinate

Nor

mal

ized

z-c

oord

inat

e

Upper 0º-bundle

Second 0º-bundle

Third 0º-bundle

Lower 0º-bundle

(a)

B-C [ (45, 0, -45, 90)2, (90, 45, 0, -45)2 ]

0,0

0,2

0,4

0,6

0,8

1,0

0 0,2 0,4 0,6 0,8 1Normalized x-coordinate

Nor

mal

ized

z-c

oord

inat

e Upper 0º-bundle

Second 0º-bundle

Third 0º-bundle

Lower 0º-bundle

(b)


Result from the measurements of waviness in 0°-bundles. The normalization of the coordinates is performed with respect to the thickness and observation length of the composite; c) Cross-ply laminate with high stitch tension; d) Quasi-isotropic laminate with high stitch tension.

RESULTS: wavinessH-S [0, 90, 0, 90]s

0,0

0,2

0,4

0,6

0,8

1,0

0,0 0,2 0,4 0,6 0,8 1,0Normalized x-coordinate

Nor

mal

ized

z-c

oord

inat

e

Upper 0º-bundle

Second 0º-bundle

Third 0º-bundle

Lower 0º-bundle

(c)

H-S [ (45, 0, -45, 90)2, (90, 45, 0, -45)2 ]

0,0

0,2

0,4

0,6

0,8

1,0

0 0,2 0,4 0,6 0,8 1Normalized x-coordinate

Nor

mal

ized

z-c

oord

inat

e Upper 0º-bundle

Second 0º-bundle

Third 0º-bundle

Lower 0º-bundle

(d)


Cross-ply Laminate SDO (µm) Base configuration 27 High stitch tension 22

Quasi-isotropic Laminate Base configuration 71 High stitch tension 86

RESULTS: waviness


“Global wave”“Local wave”

RESULTS: wavinessIt can be seen that the waviness of the 0°-bundles is rather disordered, which complicates the definition of a characteristic wavelength.

It seems that waviness of bundles is on two scales: “global” wave with large wavelength and amplitude and “local” waviness (small wavelength and amplitude) most probably related to the mesoscale characteristic dimension which is superposed to the global wave and “hidden” in it.

However, “local” waviness is of such small amplitude that effect of it might be negligible. Unfortunately none of the waves (either local or global) has an obvious periodicity.


SUMMARY

• Mechanical performance of NCF composites in contrast to prepreg tape composite laminates is not uniquely determined by fiber volume fraction in layers and by volume fraction of layers of different orientation. In addition to above parameters the mesoscale internal structure is as important.

• The main mesoscale parameter is related to imperfect bundle out-of-plane orientation which affects the compressive strength, the longitudinal bundle breaks in tension and composite stiffness.

• The structural parameters characterizing composite layer in the cross-section transverse to the bundle axis are of major importance analyzing matrix dominated damage mechanisms like intrabundle cracks. Intrabundle cracks and the imperfect mesoscale bundle structure are responsible for the most of secondary damage mechanisms serving as initiators for more severe damage modes.

• Consistent methodology is suggested to determine the identified basic parameters governing performance of NCF composites by optical microscopy investigation.


Acknowledgements

Authors would like to thank Mr Clement Touron for his help with the microscopy.

Part of this study has been carried out with financial support from the Commission of the EC granted to

the Project G4RD-CT-2001-00604 "FALCOM". The study does not necessarily reflect the views of the

European Commission.


Thank you very much for your attention!

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Methodology for Characterization of Internal Structure of ...comptest/proc/files/presentations/Joffe.pdf · Volume fraction of fibers in the bundle ( ) is obtained from image analysis