P eculiarities of damage behaviour of N C F carbon/epoxy - PowerPoint PPT Presentation

P eculiarities of damage behaviour of N C F carbon/epoxy laminates under tension Stepan V. Lomov, Dmitry S. Ivanov, Katleen Vallons, Ignaas Verpoest, Thanh Chi Truong Department MTM, Katholieke Universiteit Leuven 1 downloaded from: http://www.mtm.kuleuven.ac.be/Research/C2/poly/index.htm

C ontents 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 2 downloaded from: http://www.mtm.kuleuven.ac.be/Research/C2/poly/index.htm

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 3 downloaded from: http://www.mtm.kuleuven.ac.be/Research/C2/poly/index.htm

N on-crimp fabrics = Multi-axial multi-ply fabrics Warp knitting unit Weft insertion Source: www.liba.de 4 downloaded from: http://www.mtm.kuleuven.ac.be/Research/C2/poly/index.htm

A pplications Floor pan TECABS project Rear pressure bulkhead, AIRBUS Sources: www.tecabs.org; AIRBUS 5 downloaded from: http://www.mtm.kuleuven.ac.be/Research/C2/poly/index.htm

E xamples of N C F B2 Q B1 6 downloaded from: http://www.mtm.kuleuven.ac.be/Research/C2/poly/index.htm

Internal structure of N C F composite 90 � 0 � Surface channel NCF 90 � Inner opening Cross-ply laminate Stitching yarn Gap 7 downloaded from: http://www.mtm.kuleuven.ac.be/Research/C2/poly/index.htm

S tudying the damage initiation and development in tension Q 2 -M D 2 8 0 0 0 6 0 0 Acoustic emission sensor 7 0 0 0 Transition 5 0 0 Impregnation 6 0 0 0 point � 1 : s tre s s -s tra in Longitudinal strain gauge Strees, MPa of specimens 4 0 0 5 0 0 0 strain at Event before 4 0 0 0 3 0 0 which initial tensile 3 0 0 0 Transverse extensometer 2 0 0 damage is 2 0 0 0 e v e n t loading 1 0 0 expected 1 0 0 0 Longitudinal extensometer 0 0 0 .0 0 .2 0 .4 0 .6 0 .8 1 .0 1 .2 1 .4 1 .6 1 .8 2 .0 S tra in , % � 1 � 2 � 3 X-ray inspection Comparison of X-ray inspection Tensile on specimens X-ray images on specimens loading on for damage after tensile specimens before tensile evaluation loading loading up to � 1 � 2 � 3 8 downloaded from: http://www.mtm.kuleuven.ac.be/Research/C2/poly/index.htm

D amage development –loading in fibre direction 9 downloaded from: http://www.mtm.kuleuven.ac.be/Research/C2/poly/index.htm

G eneral features of damage in N C F 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 threshold for 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 intack and the stiffness of the sample does not change. 4. Breackage of the longitudinal fibres happens immediately before the sample failure 10 downloaded from: http://www.mtm.kuleuven.ac.be/Research/C2/poly/index.htm

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 11 downloaded from: http://www.mtm.kuleuven.ac.be/Research/C2/poly/index.htm

T wo stacking sequences NCF NCF NCF NCF Vf=60% stress B A strain, % 0.8 1.6 12 downloaded from: http://www.mtm.kuleuven.ac.be/Research/C2/poly/index.htm

D amage in lay-up T ype B stress B strain, % 1.6 0.91% 13 downloaded from: http://www.mtm.kuleuven.ac.be/Research/C2/poly/index.htm

D amage in lay-up type A 0.66% stress A 0.87% 0.8 strain, % 1.6 14 downloaded from: http://www.mtm.kuleuven.ac.be/Research/C2/poly/index.htm

E xplanation by Mattssonet 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” 15 downloaded from: http://www.mtm.kuleuven.ac.be/Research/C2/poly/index.htm

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 16 downloaded from: http://www.mtm.kuleuven.ac.be/Research/C2/poly/index.htm

O ld results: 0° /90° , Vf = 45% Fabric areal density, g/sq.m 329 Fabric layers in the laminate 8 Plies in the laminate 16 Stacking sequence MD (0/90,90/0,0/90,90/0)s CD (90/0,0/90,90/0,0/90)s Vf, % 45 Truong Chi, T., M. Vettori, S.V. Lomov and I. Verpoest Carbon composites based on multiaxial multiply stitched preforms. Part 4: Mechanical properties of composites and damage observation Composites part A 36 2005 1207-1221 17 downloaded from: http://www.mtm.kuleuven.ac.be/Research/C2/poly/index.htm

N o “S wedish effect”… Test Modulus Strength Ult.strain Poisson V f Material direction (GPa) (MPa) (%) ratio (%) 45.5 � 1.1 659 � 36 1.55 � 0.05 0.06 � 0.01 39.5 � 1.3 MD B2 0°/90° 43.6 � 1.9 654 � 32 1.59 � 0.04 0.05 � 0.01 40.8 � 0.9 CD MD = B ? CD = A ? 18 downloaded from: http://www.mtm.kuleuven.ac.be/Research/C2/poly/index.htm

N ew results. Vf = 56% Fabric areal density, g/sq.m 540 Fabric layers in the laminate 4 Plies in the laminate 8 Stacking sequence MD [+45/-45,+45/-45,-45/+45;-45/+45] Vf, % 56 Matrix Toughtened Tests in BD+ and BD- directions BD + = A BD+ BD- BD- = B Stacking sequence the same as with Mattsson et al 19 downloaded from: http://www.mtm.kuleuven.ac.be/Research/C2/poly/index.htm

F irst series –some effect is present Tensile Failure E modulus Damage initiation Test direction strength (MPa) strain (%) (GPa) strain (%) +45 � (BD+) 840 � 77 1.5 � 0.3 68 � 6 0.40 � 0.05 -45 � (BD-) 1115 � 55 1.57 � 0.05 72 � 2 0.36 � 0.03 Six tests each direction BD- BD + 20 downloaded from: http://www.mtm.kuleuven.ac.be/Research/C2/poly/index.htm

S plitting of the outer plies in BD + samples BD + BD- Images taken just before final failure of the samples 21 downloaded from: http://www.mtm.kuleuven.ac.be/Research/C2/poly/index.htm

D amage development 0.9% 1.3% 0.3% 0.5% + BD + - BD - BD- No difference in damage development; BD+ No major delaminations / fibre breakage 22 downloaded from: http://www.mtm.kuleuven.ac.be/Research/C2/poly/index.htm

A coustic emission: cumulative event energy 300000 AE sensors 250000 removed - BD cumulative AE event en - 200000 150000 100000 + BD + 50000 0 0 0.2 0.4 0.6 0.8 1 1.2 strain, % 23 downloaded from: http://www.mtm.kuleuven.ac.be/Research/C2/poly/index.htm

S ummary 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… 24 downloaded from: http://www.mtm.kuleuven.ac.be/Research/C2/poly/index.htm

S econd series: almost no effect… Stiffness Strength Strain to ST R EN G T H BD+ (GPa) (MPa) failure 1 70 1037 0,013 + BD 1200 2 64 969 / + Strength (MPa 3 65 994 0,015 900 4 66 1013 0,016 5 68 985 0,015 600 - BD 6 65 998 0,016 7 73 958 0,013 300 - 8 69 979 0,015 0 average 68 992 0,015 BD+ BD- stdev 3 25 0,001 ST R AIN T O FAILU R E Stiffness Strength Strain to BD- (GPa) (MPa) failure 0.02 1 / 1010 / Strength (MPa 2 65 970 0,015 0.015 3 67 1032 0,015 4 67 1029 0,015 0.01 5 67 1058 0,016 6 65 1091 0,017 0.005 average 66 1036 0,016 stdev 1 41 0,001 0 BD+ BD- 25 downloaded from: http://www.mtm.kuleuven.ac.be/Research/C2/poly/index.htm