The Composites Centre

Transcription

1 The Composites Centre for research, modelling, testing and training in advanced composites DELAMINATION RESEARCH: PROGRESS IN THE LAST TWO DECADES AND THE CHALLENGES AHEAD COMPTEST 06, 11 April 2006 Paul Robinson The Composites Centre Aeronautics Department Imperial College London

2 Introduction: An example The Composites Centre, Imperial College London

3 Data from Ei Engineering Village 2 delamination AND composites 7794 hits! The Composites Centre, Imperial College London number of publications year of publication How to cover this? See the review papers: Delamination-a damage mode in composite structures, Garg, A.C. (Dept. of Aeronaut. Eng., Indian Inst. of Technol., Bombay, India), Engineering Fracture Mechanics, v 29, n 5, 1988, p Characterization and analysis of delamination fracture in composites: an overview of developments from 1990 to 2001 Tay, T.E. (Dept. of Mech. Eng., Nat. Univ. of Singapore, Singapore), Applied Mechanics Review, v 56, n 1, Jan. 2003, p 1-32

4 The Composites Centre, Imperial College London Characterisation Materials development Courtesy of D Cartie, Cranfield University Modelling

5 The Composites Centre, Imperial College London Characterisation Crack Growth Modes Delamination growth in laminated composites can occur in Mode I, Mode II, Mode III and combinations of these modes. This is because the fibre reinforcement in the plies above and below the delamination can act to constrain the delamination to remain at the interface.

6 Characterisation current status Mode I DCB test: JIS K7086 (1993), ASTM D ( first published 1994) and ISO (2001) Mode II Many methods proposed : Clip gauge Stabilised ENF test JIS K7086 (1993), no international standard? The Composites Centre, Imperial College London Mode I/II Mode III? MMB test (1988) : ASTM D6671/D6671M-04e1 (first published 2001) ECT test has been investigated by ASTM; no standard has been established

7 Mode II Characterisation: problems? The Composites Centre, Imperial College London Friction Courtesy of B Blackman, Imperial College Geometric non-linearity Difficulty in measuring crack tip position From Sun & Davidson, Eng Frac Mech (2006) Courtesy of B Blackman, Imperial College + others! - fixture compliance, precracking, ply waviness

8 Mode II recent research - The Composites Centre, Imperial College London Davidson and colleagues* Friction Geometric non-linearity ENF & 4ENF Fixture compliance Additional moment due to horizontal force components Reduction in lever arm of vertical force components * Davidson & Sun, Effects of friction, geometry and fixture compliance., J Reinf Plast Compos 2005;24(15: Sun & Davidson, Numerical Evaluation of the effects of friction and geometric nonlinearities., Eng Frac Mech (2006) Davidson, Sun & Vinciquerra, Influences of friction, geometrical nonlinearities and fixture compliance., submitted to J Compos Mater

9 Davidson et al: Results Friction and geometric non-linearities with rigid fixture produce small changes in perceived toughness but larger for 4ENF Perceived toughness / True toughness The Composites Centre, Imperial College London ENF 4ENF coefficient of friction More recent work (yet to be published) indicates the above in combination with fixture compliance and load range for compliance calibration can give large errors again larger for 4ENF. (Expt l results for 4ENF typically 9-60% higher than for 3ENF.) ASTM likely to produce a test standard for 3ENF (i.e. initiation toughness only)

10 Mode II recent research - The Composites Centre, Imperial College London ESIS TC4 * ESIS TC4 have conducted round robin evaluations of various Mode II test specimens. Recent work has focused on two particular aspects of the ELS specimen. difficult in measuring crack length accurately variability induced by clamping arrangement * Blackman, Brunner & Williams, Mode II fracture testing of composites: a new look at an old problem, accepted for publication in Eng Fract Mech

11 CBT Data Reduction (from TC4 protocol) The Composites Centre, Imperial College London Corrected Beam Theory C = 3 3 ( a + Δ ) + ( L + Δ ) II 2bh 3 E 1 clamp 3 N a L G IIC 2 9P = 4b ( a + Δ ) 2 h 3 E II 1 2 F in which Δ II was obtained using 0.42Δ I obtained from a Mode I dcb test: (C/N) 1/3 Δ =X-axis intercept VIS Δ 0 crack length (a)

12 Comparison of data reduction methods The Composites Centre, Imperial College London Compliance calibration (using measured crack lengths) New method (using calculated crack lengths) A recent numerical study has confirmed the effectiveness of the proposed appoach*. * De Moura & de Morais, Equivalent crack-based analyses of ENF and ELS tests, EUROMECH Colloquium 473 Fracture of Composite Materials, 2005, Porto

13 Nesting in unidirectional laminates The Composites Centre, Imperial College London Nesting: In two adjacent plies, fibres from one ply nest in valleys in the other ply cure fibre direction Nominal pre-cure state Post-cure state Effect on Mode II delamination growth: Profiles match in the unloaded state Upper and lower half profiles do not match causing opening if crack is to continue propagating

14 The Composites Centre, Imperial College London Mode II load-displacement plots (4ENF) Film starter method Natural starter crack

15 G IIc results GIIc (J/m 2 ) Film Foil Spray Natural Starter Method The Composites Centre, Imperial College London Initiation values GIIc (J/m 2 ) Propagation values 0. Film Foil Spray Natural Starter Method

16 Materials development The Composites Centre, Imperial College London Resin system improvement : rubber toughening (G Ic ) Fracture Energy, J/m Single-Component (1K) Hybrid Anhydride/Epoxy + CTBN Rubber Epoxy 9% rubber The effect of silica nano particles and rubber particles on the toughness of multiphase thermosetting epoxy polymers Journal of materials science [ ] Kinloch yr:2005 vol:40 iss:18 pg:

17 How tough? The Composites Centre, Imperial College London G Ic for Sellotape = 110J/m 2

18 Materials development The Composites Centre, Imperial College London Rubber Toughening Mechanisms - Fracture Surface The rubbery-phase particle does not debond under the triaxial stress field in the vicinity of the crack tip but instead internally cavitates. This internal void allows plastic void expansion in the epoxy polymer to occur. Courtesy of AJ Kinloch, Imperial College

19 The Composites Centre, Imperial College London Materials development : nano reinforcement of resin Nano-Silica Composites (SiO 2 particles are formed In-situ during a sol-gel manufacturing process) Added Nano-Particles << Kinloch & Taylor, J. Materials Sci., 37, 433, 2002 >> TEM of a cured nano-silica/epoxy.

20 The Composites Centre, Imperial College London Materials development : nano reinforcement of resin (G Ic ) Fracture Energy, J/m Single-Component ( 1K ) Epoxy/Anhydride with Nano-SiO2 (wt.%) But why do the nano-silica particles increase the toughness? Epoxy 4% nano 8% nano 11% nano 15% nano 20% nano The effect of silica nano particles and rubber particles on the toughness of multiphase thermosetting epoxy polymers Journal of materials science [ ] Kinloch yr:2005 vol:40 iss:18 pg:

21 The Composites Centre, Imperial College London Materials development : nano reinforcement of resin 100 nm From FEG SEM Studies Courtesy Dr. I.A. Kinloch (Univ. Cambridge)

22 The Composites Centre, Imperial College London Materials development : composite reinforcement through-thickness Many forms : stitching, weaving, tufting, z-pins, cm True G True Ic G (J/m 2 IC ) Developed bridged zone Crack length (m) crack length a (m) Bridged zone Z-pinned composites: Pin testing and interlaminar toughness data reduction strategies,paul Robinson, Shumit Das & Marcin Fert, presented at 4 th Int Conf on Fract of Polymers, Composites and Adhesives, Les Diablerets, Sept 2005.

23 Modelling development The Composites Centre, Imperial College London Simple formulae Axisymmetric isotropic circular plate containing mid-plane circular delamination G II 2 9P 1 ν = 2 64π Eh 2 3 P = 8π c 3 G Eh IIc 2 1 ν 3 Davies, G.A.O. and Robinson, P. "Predicting Failure By Debonding/ Delamination", Debonding/Delamination Of Composites, AGARD : 74th Structures & Materials Meeting, (AGARD_CP_530), Patras, Greece, May Davies, G.A.O., Robinson, P., Robson J. and Eady D. Shear driven delamination propagation in two dimensions", Composites Part A, 28A, 1997,

24 The Composites Centre, Imperial College London Computational modelling development: Direct application of fracture mechanics Step 1. Calculate the energy available to drive the delamination growth Detail of VCC* for 2-D mesh Determine F z A and Fz B the vertical stress resultants acting at node 3 due to elements A and B Determine w 3 and w 4, the vertical displacements of nodes 3 and 4 So, for Mode I: Similarly for Mode II: *Rybicki and Kanninen A Finite Element Calculation of stress intensity factor by modified crack closure integral, Engng Fract Mech, 9, , 1977.

25 The Composites Centre, Imperial College London Direct application of fracture mechanics (cont.) Step 2. The energy release rate is tested against a growth criterion, which involves the experimentally determined critical energy release rates (G Ic, G IIc etc). Step 3. The delamination front is advanced where the growth criterion is satisfied. - Move mesh - Disconnect nodes

26 The Composites Centre, Imperial College London Direct application of fracture mechanics (cont.) Advancing crack front by moving mesh example Hitchings D, Robinson, P. and Javidrad, F., A Finite Element Model for Delamination Propogation in Composites, Computers and Structures, Vol. 60, No 6, pp , Nillson F.K. and Giannakopoulos A.E., Finite Element Simulation of Delamination Growth, 1 st Int Conf on Computer-Aided Assessment and Control of Localized Damage, 1990, Rinderknecht S. and Kroplin B. A Finite Element Model for the Delamination in Composites Plates, Mechanics of Composites and Structures, V1, No2, 1994.

27 The Composites Centre, Imperial College London Direct application of fracture mechanics (cont.) Advancing crack front by moving mesh -problems Meshing problems as delamination fronts approach each other. How do delaminations merge?

28 The Composites Centre, Imperial College London Direct application of fracture mechanics (cont.) Advancing crack front by disconnecting nodes Problems : mesh only approximates the mesh front in a stepped fashion. Calculation of accurate energy release rates for stepped front very difficult Ko A.W.L. An investigation in the use of a stationary mesh approach to simulate delamination growth in composite laminates, PhD Thesis, Imperial College, London, 2002 Kutlu Z. and Chang F.K. Composite Panels containing multiple through-the-width delaminations and subjected to compression. Part I, Composite Structures, 31, pp , Zie D and Biggers S.B. jr. Strain energy release rate calculation for a moving delamination front of arbitrary shape based on the virtual crack closure technique Part I: Formulation and validation, Eng Fract Mech, Volume 73, Issue 6, April 2006, p

29 Direct application of fracture mechanics (cont.) ABAQUS implementation of VCCT The Composites Centre, Imperial College London

30 The Composites Centre, Imperial College London Computational modelling development: Indirect application of fracture mechanics (Often called the interface element approach or Cohesive Zone Model approach*.) Step 1. A finite element model is constructed in which interface elements are embedded between the layers of elements which are likely to delaminate. interface elements G c *Numerical simulation of free edge delamination in graphite-epoxy laminates under uniaxial tension Schellekens, J.C.J. (Delft Univ of Technology); De Borst, R., International Conference on Composite Structures, 1991, p 647 Interlaminar interface modelling for the prediction of delamination, Allix, O. (Lab. de Mecanique et Technol., GRECO/GIS Calcul des Structures, Cachan, France); Ladeveze, P., Composite Structures, v 22, n 4, 1992, p

31 The Composites Centre, Imperial College London Indirect application of fracture mechanics (cont.) Step 2. A non-linear finite element analysis is performed. Example zone in a softened state* ( ) 2 EG c /σ *An Engineering Solution for using Coarse Meshes in the Simulation of Delamination with Cohesive Zone Model, Turon A, Davila CG, Camanho PP & Costa J, NASA/TM , March 2005 Progressive delamination using interface elements Mi, Y. (Imperial Coll); Crisfield, M.A.; Davies, G.A.O.; Hellweg, H.-B., Journal of Composite Materials, v 32, n 14, 1998, p

32 The Composites Centre, Imperial College London Indirect application of fracture mechanics (cont.) Influence of mesh refinement on load displacement plot Progressive delamination using interface elements Mi, Y. (Imperial Coll); Crisfield, M.A.; Davies, G.A.O.; Hellweg, H.-B., Journal of Composite Materials, v 32, n 14, 1998, p The softened zone can be made artificially larger by reducing the strength σ of the interface element but this will alter the initiation of the delamination growth process.

33 Modelling Validation: Centre-loaded plate The Composites Centre, Imperial College London Circular plate with central point load Interface elements at the mid-plane 5 P (kn) Analytical 0 All numerical curves displ. (mm) Courtesy of S Pinho, Imperial College

34 Modelling Validation: double crack dcb test The Composites Centre, Imperial College London Implanted delaminations ABAQUS Benchmark case ( 10 3 ) Delamination growth prediction using a finite element approach Robinson, P., Besant, T. Hitchings, D., 2nd ESIS TC4 Conference on Polymers and Composites, Les Diablerets, 1999, p

35 Modelling Validation: double crack dcb test The Composites Centre, Imperial College London Reaction force [N] Experiment t o1 =110 MPa t o1 = 66MPa t0 I = 3.3MPa S.Z. S.Z. S.Z t o1 =3.3 MPa t o1 =33MPa t0 I = 33.0 MPa S.Z Displacement [mm] S.Z. S.Z. Influence of interface strength t o1 t 0 I σ I Blue interface: 0 < G < ½G Ic Red interface: G Ic δ I ½G Ic < G < G Ic Grey interface: G = G Ic fully broken δ I 0 δ I, c ELRIPS WP7 Composite Bonded Repairs: Static and Fatigue Performance, B. G. Falzon Imperial College, 19 October 2005 & R. T. Tenchev, Progress presentation,

36 Modelling Validation: pressure-loaded plate annular mid-plane The Composites Centre, Imperial College London Axisymmetric isotropic circular plate subjected to uniform pressure, p delamination of width l ( υ ) ( 2R l)l p = Shear driven delamination propagation in two dimensions,davies, G.A.O.; Robinson, P.; Robson, J.; Eady, D., Composites - Part A: Applied Science and Manufacturing, v 28, n 8, 1997, p τ 4 2G IIc Et 3 1 τ 0 G IIc Predicting delamination and debonding in modern aerospace composite structures Davies, G.A.O.; Hitchings, D.; Ankersen, J., Composites Science and Technology, v 66, n 6, May, 2006, Advances in Statics and Dynamics of Delamination, p

37 Challenges ahead Characterisation Complete Mode II development (initiation test standard soon?, propagation.?) Mode III, test standards for non 0 /0 interfaces, tests for new materials (NCFs, 3-D woven preforms ), fatigue Modelling Robust methods for selection of interface element parameters, extension of the technique Alternative modelling approaches? The Composites Centre, Imperial College London Address the reality of practical delamination growth multiple delamination, migration.

38 Thank you The Composites Centre, Imperial College London

39 DAMAGE ZONE

40

41

42 CRACK MIGRATION

43

44 Schematic of crack path in a Mode I DCB test at a 0 /90 interface Micrograph of the crack path in a mode II ELS test at a 0 /90 interface

45

46 TRANSVERSE σ

47

48 NESTING

49 Microscope stage testing in mode II Film starter crack propagation The Composites Centre, Imperial College London 2000μm Natural starter crack propagation 2000μm

50 Crack Opening Profile The Composites Centre, Imperial College London Crack Opening, μm Film Foil Natural Spray Hor. distance from crack tip, (μm)

51 BAE TESTING

52

53

54

55

56 OTHER MODELS

57 Mesh-independent discrete numerical representations of cohesive-zone models Rene de Borst, Remmers, J.J.C.; Needleman, A. Source: Engineering Fracture Mechanics, v 73, n 2, Jan. 2006, p

58 FATIGUE

59 ( Dg= ε ~,) Ce αd ε ~ ( Dg= ε ~,) Ce αd ε ~ Fatigue damage model for the interface element In continuum damage mechanics the damage rate can be expressed as: Peerling s law exponential law: (Note: even if the initial damage is zero, the damage starts to accumulate) dd = dt Ce ~ ε is an equivalent positive strain measure. ε β ε λ D ~ ~ Modified Peerling s law: dd dt = Ce λd δ δ c β δ δ c

60 DCDCB

61

62 Z PINS

63 The Composites Centre, Imperial College London Materials development : composite reinforcement through-thickness Many forms : stitching, weaving, tufting, z-pins,.. 1cm Longitudinal cracks in z-pin Z-pinned composites: Pin testing and interlaminar toughness data reduction strategies,paul Robinson, Shumit Das & Marcin Fert, presented at 4 th Int Conf on Fract of Polymers, Composites and Adhesives, Les Diablerets, Sept 2005.

64