peculiarities of damage behaviour of ncf carbon/epoxy ... · 0.3% strain for fiber direction...
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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
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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
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1. Introduction
• Non-crimp fabrics (NCF)
• Internal structure on NCF composites
• 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
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Non-crimp fabrics = Multi-axial multi-ply fabrics
Source: www.liba.de
Weft insertion Warp knitting unit
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Applications
Floor pan TECABS project
Rear pressure bulkhead, AIRBUS
Sources: www.tecabs.org; AIRBUS
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Examples of NCF
B1 B2 Q
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Internal structure of NCF composite
90q
90q
0q
Cross-ply laminate
NCF
Stitching yarn Gap
Inner opening
Surface channel
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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
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Damage development –loading in fibre direction
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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
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1. Introduction
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
David Mattsson, Roberts Joffe and Janis Varna, Damage in NCF composites under tension, Engineering Fracture Mechanics, 2007, in print
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Two stacking sequences
NCF
NCFNCFNCF
strain, %
stress
0.8 1.6
B
A
Vf=60%
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Damage in lay-up Type B
stress B
1.6
strain, %
0.91%
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Damage in lay-up type A
strain, %
stress
0.81.6
A
0.66%
0.87%
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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
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. Possible explanations
5. Challenge for the modelling work
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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
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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 ?
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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
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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-
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Splitting of the outer plies in BD+ samples
BD +BD-
Images taken just before final failure of the samples
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Damage development
BD +
+
BD-
-
0.3% 0.5% 0.9% 1.3%
BD+
BD-No difference in damage development;
No major delaminations / fibre breakage
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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
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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…
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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-
-
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…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 +
+
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1. Introduction
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
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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.
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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.
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