Understanding and predicting wrinkle defect formation

Transcription

1 Understanding and predicting wrinkle defect formation Stephen Hallett Dmitry Ivanov, Ivana Partridge, Kevin Potter Jonathan Belnoue, Ollie Nixon-Pearson, James Kratz, Tassos Mesogitis, Adam Thompson

2 Background 2 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

3 Wrinkle Formation 3 Consolidation of plies during layup, debulk and cure is one of the main generation mechanisms for wrinkles

4 Compaction Tests 4 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 Baseline Cross Ply (CP) Thickness (mm) BP_30C BP_60C BP_90C Scaled-up Blocked Ply (BP) Time (s)

5 Hyper-viscoelastic Model 5 New constitutive model formulated to model uncured pre-preg Accounts for shear flow and bleeding 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 Final thickness (mm) Baseline specimen 1.5 CP (exp.) BP (exp.) CP (mod. pred.) BP (mod. pred.) T ( C) IM Final thickness (mm) Scale-up specimen 1.5 CP (exp.) BP (exp.) CP (mod. pred.) BP (mod. pred.) T ( C)

6 Taper and Ply Drops 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

7 Surface Step Change 7 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

8 Corner Radius 8 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

9 Gaps and overlaps 9 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

10 Textile Composites 10 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 Vf 91 % 65 % Initial focus is on unit cell, but modelling is also being extended to macro-scale capability

11 Summary 11 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