DAMAGE INITIATION AND DEVELOPMENT IN CARBON-EPOXY TRIAXIAL BRAIDED - PowerPoint PPT Presentation

DAMAGE INITIATION AND DEVELOPMENT IN CARBON-EPOXY TRIAXIAL BRAIDED COMPOSITE AND FINE STRUCTURE OF DAMAGE D. Ivanov, S.V. Lomov, I. Verpoest, F.Baudry, H.Xie Department MTM, Katholieke Universiteit Leuven, Belgium ECCM-2006, 01.09.2006 D.S. Ivanov 1

Cont ent � Geometry of the composite � Tensile test program � Strain mapping measurements � Damage stages � Crack geometry � Conclusions ECCM-2006, 01.09.2006 D.S. Ivanov 2

Geom et ry of braided t ex t ile c om posit e: in-plane BD 14.4 mm MD CD BD Carbon-epoxy composite, 4 layers Experiments on the composite: Tensile test in 3 directions (MD, BD, CD): accompanied by: examined posterior by: Strain mapping measurements X-ray Acoustic emission measurements Cross-sectioning ECCM-2006, 01.09.2006 D.S. Ivanov 3

Int ernal geom et ry of t he c om posit e Inlay yarns Layer borders 3 . 2 5 m m 20 mm Local fibre distribution at the yarn edges; ECCM-2006, 01.09.2006 D.S. Ivanov 4

Int ernal geom et ry of t he c om posit e Fibre volume fraction in the inlay yarns along their path 71 % Location of maximum fibre volume fraction in inlay 63 55 Four inlay yarns in different layers 47 I II III IV V VI VII 1.05 1.00 0.95 0.90 Position across the yarn, % 0.85 Position of the -50 -40 -30 -20 -10 0 10 20 30 40 50 measurements of Normalised and symmetrised fibre volume fraction fibre volume fraction across inlay yarn (obtained with V.Koissin) along the path; ECCM-2006, 01.09.2006 D.S. Ivanov 5

Int ernal geom et ry of t he c om posit e 80 Cross section of the 4-layer triaxial braided composite in the machine direction. Maximum thickness of the layers up to 44% of the composite thickness. Minimum thickness of the layers - 18% of the composite thickness). 1. Surface fields are influenced by inner layers 2. Strong nesting and “interpenetration” of the layers may blur the structural features of the surface field ECCM-2006, 01.09.2006 D.S. Ivanov 6

Cont ent � Geometry of the composite � Tensile test program � Strain mapping measurements � Damage stages � Crack geometry � Conclusions ECCM-2006, 01.09.2006 D.S. Ivanov 7

Ex perim ent Specimens: Width: 40 mm (2 unit cell) Thickness: 3 mm Total length: 250 mm Strain mapping (Aramis 4.7): Facet size: 1 mm Facet step: 1 mm Inspection area: 40 mm (width direction), 55 mm (length direction) Load step size: 0.033% of applied strain Acoustic emission: Distance between the sensors: 110 mm Loading: Speed: 1 mm/min ECCM-2006, 01.09.2006 D.S. Ivanov 8

Loading in different m at erial direc t ion Tensile diagrams BD MD CD BD � critical Acoustic MD BD CD emission E, Gpa 32.6 � 1.1 36.8 � 1.8 15.9 � 0.7 diagrams 0.73 � 0.06 0.07 � 0.02 0.39 � 0.03 � � 1 ,% 0.29 � 0.09 0.36 � 0.05 0.33 � 0.07 � 1 ,% 0.55 0.45 0.43 � 2 ,% � critical ,% 1.45 � 0.15 1.25 � 0.18 1.11 � 0.03 ECCM-2006, 01.09.2006 D.S. Ivanov 9 � 1 - damage initiation; � 2 – AE intense grow;

Cont ent � Geometry of the composite � Tensile test program � Strain mapping measurements � Damage stages � Crack geometry � Conclusions ECCM-2006, 01.09.2006 D.S. Ivanov 10

St rain m apping m easurem ent s Strain field � MD : Strain history of a point: � MD, % � < � MD > , % < � MD > = 0.063% Natural requirements to the results: � Proportionality of strain fields on each load step to the applied strain . � Periodicity: expected at least in the direction of loading. � Smoothness: A thin layer of matrix covers all the surface of the composite => no inner interface boundary on the interface. ECCM-2006, 01.09.2006 D.S. Ivanov 11

St rain fields at different load st eps loc ,% MD � 3 ave ,% � MD 0.189 2 0.122 1 0.063 Local strain fields along a line in the direction of loading (MD). Load steps: 1 - at 0.063%; 2- at 0.122%; 3- at 0.189%; ECCM-2006, 01.09.2006 D.S. Ivanov 12

Filt ered fields along a line � MD, % Initial deformation F iltered fields along a line field (resolution 3mm); Filtered (least square Period of the x, mm regression, resolution 3mm); � MD, % structure x, mm � MD, % Filtered (least square regression, resolution 5 mm); x, mm Probable position of the line ECCM-2006, 01.09.2006 D.S. Ivanov 13

Init ial and proc essed fields Initial field at applied Filtered field Filtered field strain 0.1%; (resolution 5 mm); (resolution 3 mm); Normalised strain fields � MD in the direction of loading (MD) ECCM-2006, 01.09.2006 D.S. Ivanov 14

Max im um and m inim um values of st rain field � min , % � max , % Initial experimental field - 0.877 1.836 Filtering by load steps 0.052 0.509 Filtering and area smoothing 0.142 0.491 ECCM-2006, 01.09.2006 D.S. Ivanov 15 Finite element analysis 0.162 0.524

Fourier analysis of filt ered and sm oot hed st rain Period of the structure Frequency of the signal 20 mm 20 mm Amplitude, mm Misalignment of textile orientation with loading direction causes an increase of apparent unit cell size ECCM-2006, 01.09.2006 D.S. Ivanov 16

Cont ent � Geometry of the composite � Tensile test program � Strain mapping measurements � Damage stages � Crack geometry � Conclusions ECCM-2006, 01.09.2006 D.S. Ivanov 17

St at ist ic al feat ures of t he noise Relative error for 16 points in square Strain history � MD of a point: � 1 � grid: � � � Relation of error < � > , % � � standard deviation to � average strain Noise � local strain deviation from its linear regression The distribution of noise is normal and has the same standard deviation for all the load steps Few locations in structure are characterised with “abnormal behaviour” ECCM-2006, 01.09.2006 D.S. Ivanov 18

Dam age init iat ion and developm ent in t ex t ile Relative error for 16 c om posit es Loading points in square grid � 2 direction � 1 Relation of error � � � 0.3 % 1 standard deviation to � � � average strain MD � 1 Stress BD Cumulative energy of acoustic emission CD Cumulative event curve ECCM-2006, 01.09.2006 D.S. Ivanov 19 of acoustic emission

Dam age m ec hanism in t riax ial braided c om posit e obtained with A.Salehi � 1 - damage initiation: transverse cracks (inter-fibre failure) Microscopy � 2 - damage propagation: inter-yarn delamination X-Ray � critical - damage propagation: fibre failure ECCM-2006, 01.09.2006 D.S. Ivanov 20

Cont ent • Geometry of the composite • Tensile test program • Strain mapping measurements • Damage stages • Crack geometry • Conclusions ECCM-2006, 01.09.2006 D.S. Ivanov 21

Crac k dist ribut ion: BD � 2 Line of the cross-section Braiding yarns Cross-section of the composite Inlay yarns X-ray image Cracks: in the yarns of upper and middle layers Sample loaded in the bias (BD) direction ECCM-2006, 01.09.2006 D.S. Ivanov 22

Crac k dist ribut ion: MD � 2 Cross-section of the composite Transverse cracks X-ray image Sample loaded in the machine (MD) direction ECCM-2006, 01.09.2006 D.S. Ivanov 23

Crac k dist ribut ion: CD � 2 Cross-section of the composite Inlay yarns Braiding yarns X-ray images Transverse cracks Sample loaded in the cross (CD) direction ECCM-2006, 01.09.2006 D.S. Ivanov 24

Crac k densit y at t he different load st ages � 2 Crack density, mm/dm 2 1600 1200 MD CD BD 800 � 1 400 % 0.29 0.33 0.37 0.41 0.45 0.49 0.53 0.57 ECCM-2006, 01.09.2006 D.S. Ivanov 25

Conc lusions AE initiation corresponds to occurrence of first cracks High energy of AE signal = multiple cracks or/and delamination Transverse cracks cross all the thickness of yarn with no exceptions A difference of damage accumulation braided composite with the process in laminates or non-crimp composite - finite length of transverse cracks At a first stage of damage accumulation: periodic increase of a crack pattern without crack length grow Maximum crack density is observed in a specimen subjected to macroscopic shear (braiding direction) ECCM-2006, 01.09.2006 D.S. Ivanov 26

Conc lusions No extensive micro cracking is observed Transverse cracks are always co-oriented to fibre direction An angle of the crack is 90 � 15 � ECCM-2006, 01.09.2006 D.S. Ivanov 27

Requirem ent s for dam age m odelling Geometry of the composite: yarn architecture yarn curvature fibre volume fraction in yarns Engineering application Criterion for damage initiation Criterion for damage stoppage Main criterion Criterion for inter-yarn delamination for the composite failure Criterion for delamination stoppage Criterion of fibre failure ECCM-2006, 01.09.2006 D.S. Ivanov 28

FE c alc ulat ions 1 Unit cell Rotation around the x 3 axis 2 Half of the UC: by � 3 Quarter of the UC Rotation around the x � 1 axis by � Loading in MD direction up to the critical strain �� MD � =0.3%, < � CD >= -0.02%. Stress index: (a) longitudinal L; (b) transverse T; (c) shear LT. Hoffman criterion predicts crack occurrence at �� MD � =0.2% ECCM-2006, 01.09.2006 D.S. Ivanov 29