Understanding and predicting wrinkle defect formation...• Laminates with an abrupt thickness or...
Transcript of Understanding and predicting wrinkle defect formation...• Laminates with an abrupt thickness or...
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Understanding and predicting wrinkle defect formation
Stephen HallettDmitry Ivanov, Ivana Partridge, Kevin Potter
Jonathan Belnoue, Ollie Nixon-Pearson, James Kratz, Tassos Mesogitis, Adam Thompson
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2Background
• Waviness in composites material is almost unavoidable in thick parts
• Can originate from a variety of sources• Waviness can have a very significant impact on
static and fatigue failure – through thickness strength reduced by >50%– tensile strength reduced by >30%– Compression strength reduced by >30%
• Modern FE techniques can capture the knockdown in strength caused by wrinkling
• Predicting the formation of wrinkle defects is less well advanced
• The focus of this work is the understanding, prediction and mitigation of wrinkle defects
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3Wrinkle Formation
• Consolidation of plies during layup, debulk and cure is one of the main generation mechanisms for wrinkles
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4Compaction Tests• Understanding compaction is
essential for wrinkle driving mechanisms
• Cruciform specimens designed and tested
• A range of configurations to challenge the models– Ply thickness changes– In-plane scaling
• Tested over a range of temperatures (30 – 90°C) and pressures, with time dependence– Suitable for AFP deposition and debulk
consolidation– Also applicable to broadgoods
• Two material systems – Interlayer toughened vs
no interlayer
1525
Cross Ply (CP)
Blocked Ply (BP)
3050
Baseline
Scaled-up
1.4
1.6
1.8
2.0
2.2
2.4
0 240 480 720 960
Thic
knes
s (m
m)
Time (s)
BP_30C
BP_60C
BP_90C
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5Hyper-viscoelastic Model• New constitutive model
formulated to model uncured pre-preg
• Accounts for shear flow andbleeding flow in the material
• Requires only 3 material parameters
• Able to model all experimental effects with a single set of parameters– Ply thickness– In-plane scaling– Temperature
• Applicable to both material systems tested
IM7-8552
1
1.5
2
2.5
20 40 60 80 100
Fina
l thi
ckne
ss (m
m)
T (°C)
Baseline specimen
CP (exp.)BP (exp.)CP (mod. pred.)BP (mod. pred.)
1
1.5
2
2.5
20 40 60 80 100
Fina
l thi
ckne
ss (m
m)
T (°C)
Scale-up specimen
CP (exp.)BP (exp.)CP (mod. pred.)BP (mod. pred.)
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• Numerical models run on previous good quality specimens to predict final geometry
• Matlab tools generate the models directly from a simple ply-book
• Good agreement achieved• New tooling for complex double
taper under manufacture• Will be able to deliberately form
wrinkles due to compaction for further study
Taper and Ply Drops
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7Surface Step Change
• Laminates with an abrupt thickness or step change on the surface can result in wrinkles
• Bag bridging gives reduced consolidation pressure at the base of step and allows a wrinkle to form
• Typical of stinger foot/skin interaction • Several cases of co-bonded and co-cured
investigated
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8Corner Radius
• Corner radii are typical of many components e.g. C or box section
• When laid onto a male tool the consolidation creates extra length of plies around the radius
• If constrained, either by geometry or inter-ply friction, then wrinkles can occur
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9Gaps and overlaps• Minor errors in Automated Fibre
Placement (AFP) can lead to ply movement during consolidation
• Specimens being made to deliberately induce waviness by positioning of gaps and overlaps
• Slightly artificial case, but is to provide validation for the models
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10Textile Composites
• Main focus is on pre-preg UD materials
• Wrinkle defects also occur in textile composites
• Mechanisms are more complex due to internal weave architecture
• Finite element modelling being undertaken to predict final geometry
• Initial focus is on unit cell, but modelling is also being extended to macro-scale capability
Vf91%
65%
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11Summary
• Consolidation is a major driver for wrinkle formation
• A range of experiments have been conducted to characterise consolidation behaviour of pre-preg systems
• New Finite Element material model developed to predict consolidation
• A range of industrially relevant cases showing wrinkle formation manufactured in controlled laboratory conditions
• These form the basis to study the mechanisms and develop predictive models
• Work has been extended to textile composites where weave pattern has a major influence on compaction behaviour
Understanding and predicting wrinkle defect formation BackgroundWrinkle FormationCompaction TestsHyper-viscoelastic ModelTaper and Ply DropsSurface Step ChangeCorner RadiusGaps and overlapsTextile CompositesSummary