Peculiarities of damage behaviour of NCF carbon/epoxy ... · 0.3% strain for fiber direction loading, 1.3% for bias loading. These values are twice as low as the damage initiation

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Peculiarities of damage behaviourof NCF carbon/epoxy laminates

under tension

Stepan V. Lomov, Dmitry S. Ivanov, Katleen Vallons, Ignaas Verpoest, Thanh Chi Truong

Department MTM, Katholieke Universiteit Leuven


Contents

1. Introduction. Features of the damage in NCF composites

2. Premature fibre damage, as discovered by Mattsson et al – “Swedish effect”

3. “Is it a bug or is it a feature?” – new experimental observations

4. Challenge for the modelling work


1. Introduction

• Non-crimp fabrics (NCF)

• Internal structure on NCF composites

• Damage in NCF composites





Non-crimp fabrics = Multi-axial multi-ply fabrics

Source: www.liba.de

Weft insertion Warp knitting unit


Applications

Floor pan TECABS project

Rear pressure bulkhead, AIRBUS

Sources: www.tecabs.org; AIRBUS


Examples of NCF

B1 B2 Q


Internal structure of NCF composite

90q

90q

0q

Cross-ply laminate

NCF

Stitching yarn Gap

Inner opening

Surface channel


Studying the damage initiation and development in tension

Acoustic emission sensor

Transverse extensometer

Longitudinal extensometer

Longitudinal strain gauge

Q2-MD2

0

1000

2000

3000

4000

5000

6000

7000

8000

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

Strain, %

Eve

nt

0

100

200

300

400

500

600

Str

ees,

MP

a

event

stress-strain

Transition point H1: strain at

which initial damage is expected

Impregnation of specimens

before tensile loading

Tensile loading on specimens

up to H1 H2 H3

X-ray inspection on specimens before tensile

loading

X-ray inspection on specimens after tensile

loading

Comparison of X-ray images for damage evaluation

H1 H2 H3


Damage development –loading in fibre direction


General features of damage in NCF composites –tension

1. Mechanical properties of carbon/epoxy NCF composites are close to the mechanical properties of UD laminates.

2. Damage in carbon/epoxy NCF composites is initiated very early: 0.3% strain for fiber direction loading, 1.3% for bias loading. These values are twice as low as the damage initiation thresholdfor UD laminates. Damage sites coincide with the stitching positions.

3. For loading in fibre direction the damage develops by multiplication of transversal cracks in 90°plies. 0°plies stay intackand the stiffness of the sample does not change.

4. Breackage of the longitudinal fibres happens immediately before the sample failure


1. Introduction




David Mattsson, Roberts Joffe and Janis Varna, Damage in NCF composites under tension, Engineering Fracture Mechanics, 2007, in print


Two stacking sequences

NCF

NCFNCFNCF

strain, %

stress

0.8 1.6

B

A

Vf=60%


Damage in lay-up Type B

stress B

1.6

strain, %

0.91%


Damage in lay-up type A

strain, %

stress

0.81.6

A

0.66%

0.87%


Explanation by Mattsson et al

“The main reason is the larger number of inside 0°-bundle breaks in laminates where imperfect bundles of this orientation are supported by 90°-bundles compared to cases where two 0°-bundles from different layers are together. The mechanics is not clear at present but it may be related to larger bending resistance of two imperfect bundles as compared with one. The difference is larger if these bundles would be delaminated which may be initiated by transverse cracks in 90°-bundles.

Another possible reason for larger stiffness reduction in [0/90/0/90]S composite is, according to FE calculations, directly related to the larger effect of each surface 0°-bundle break on the composite stiffness: due to less constraint from the surrounding material the opening of surface bundle breaks is much larger”

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1. Introduction



4. Possible explanations



Old results: 0°/90°, Vf = 45%

Truong Chi, T., M. Vettori, S.V. Lomov and I. Verpoest Carbon composites based on multiaxialmultiply stitched preforms. Part 4: Mechanical properties of composites and damage observationComposites part A 36 2005 1207-1221

16Plies in the laminate

45Vf, %

(0/90,90/0,0/90,90/0)s(90/0,0/90,90/0,0/90)s

Stacking sequence MDCD

8Fabric layers in the laminate

329Fabric areal density, g/sq.m


No “Swedish effect”…

40.8r0.90.05r0.011.59r0.04654r3243.6r1.9CD

39.5r1.30.06r0.011.55r0.05659r3645.5r1.1MDB2 0°/90°

Vf(%)

Poisson ratio

Ult.strain(%)

Strength(MPa)

Modulus(GPa)

Test directionMaterial

CD = A ?

MD = B ?


New results. Vf = 56%

ToughtenedMatrix

8Plies in the laminate

56Vf, %

[+45/-45,+45/-45,-45/+45;-45/+45]Stacking sequence MD

4Fabric layers in the laminate

540Fabric areal density, g/sq.m

Tests in BD+ and BD- directions

BD-BD+

BD +

BD-

= B

= A

Stacking sequence the same as with Mattsson et al


First series –some effect is present

0.36 f 0.0372 f 21.57 f 0.051115 f 55-45e (BD-)

0.40 f 0.0568 f 61.5 f 0.3840 f 77+45e (BD+)

Damage initiation strain (%)

E modulus (GPa)

Failure strain (%)

Tensile strength (MPa)Test direction

Six tests each direction

BD +

BD-


Splitting of the outer plies in BD+ samples

BD +BD-

Images taken just before final failure of the samples


Damage development

BD +

+

BD-

-

0.3% 0.5% 0.9% 1.3%

BD+

BD-No difference in damage development;

No major delaminations / fibre breakage


Acoustic emission: cumulative event energy

0

50000

100000

150000

200000

250000

300000

0 0.2 0.4 0.6 0.8 1 1.2

strain, %

cum

ulat

ive

AE

eve

nt e

nerg

y

BD +

+

BD-

-

AE sensors removed


Summary first series

1. Difference in the composite strength for two stacking sequences: same as in the “Swedish effect”, albeit less pronounced

2. Difference in the damage initiation strain: opposite

3. No evidence of the premature fibre damage

4. Splitting of the plies on the sample surface (not observed by Mattsson et al).

Probably, we deal here with different phenomena…


Second series: almost no effect…

BD+ Stiffness (GPa)

Strength (MPa)

Strain to failure

1 70 1037 0,0132 64 969 /3 65 994 0,0154 66 1013 0,0165 68 985 0,0156 65 998 0,0167 73 958 0,0138 69 979 0,015

average 68 992 0,015stdev 3 25 0,001

BD- Stiffness (GPa)

Strength (MPa)

Strain to failure

1 / 1010 /2 65 970 0,0153 67 1032 0,0154 67 1029 0,0155 67 1058 0,0166 65 1091 0,017

average 66 1036 0,016stdev 1 41 0,001

STRENGTH

0

300

600

900

1200

Stre

ngth

(MPa)

BD+ BD-

STRAIN TO FAILURE

0

0.005

0.01

0.015

0.02Stre

ngth

(MPa)

BD+ BD-

BD +

+

BD-

-


…also some disturbances for BD+ at the latter stage

0

200

400

600

800

1000

1200

0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80

BD-

-

BD +

+


1. Introduction





Summary

1. The effect observed by Mattsson et al in carbon NCF/epoxy 0/90 composites constitutes the premature fibre damage for a stacking sequence:

• with 90°plies concentrated in the centre of the laminate

• with 0°plies lying on the surface of the laminate

(two different interpretations of the same lay-ups)

2. Using same material system “carbon NCF/epoxy”, but different fabrics and different resin, we did not observe the influence of the stacking sequence for thicker plates with more plies, having more double 0°plies. Also fibre volume fraction for these tests is less (about 45%)

3. Using same material system “carbon NCF/epoxy”, but different fabrics and different resin, and the same fibre volume fraction and stacking sequences, we have observed the difference in strength for different types of lay-ups, less pronounced then in Sweedish tests, but with the same sign.

However, the mechanism of the change of strength is different and is associated with the splitting of the fibres in outer layers.


Challenge

The effects of premature damage, both “Swedish” and “Belgian”, could be qualitatively understood, but are not predicted by the current damage models of NCF composites.

Such a prediction would be of great value, as it would help to establish safer design boundaries for these materials, which are used in load-carrying automotive and aircraft parts.

Peculiarities of damage behaviour of NCF carbon/epoxy ... · 0.3% strain for fiber direction loading, 1.3% for bias loading. These values are twice as low as the damage initiation

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