www.bris.ac.uk/composites

Understanding and predicting wrinkle defect formation

Stephen HallettDmitry Ivanov, Ivana Partridge, Kevin Potter

Jonathan Belnoue, Ollie Nixon-Pearson, James Kratz, Tassos Mesogitis, Adam Thompson

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

3Wrinkle Formation

• Consolidation of plies during layup, debulk and cure is one of the main generation mechanisms for wrinkles

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

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.)

6

• 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

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

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

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

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%

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