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Thin ply composites: Experimental characterization and modeling ICCM19 Montréal 2013 In partnership with North-TPT, FHNW, RUAG Technology, RUAG space, and Connova Robin Amacher, Joël Cugnoni, John Botsis Ecole polytechnique fédérale de Lausanne, Switzerland

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o Advantages: Improved delamination resistance, higher onset of damage & ultimate failure Improved fatigue properties and in some case damage tolerance More design degrees of freedom = more optimal laminate Easier to design ply drops / small angle laminates / local reinforcements Can produce homogeneous laminates, no more dependency on stacking Can produce mixed thin & thick laminates for structures with large shell thickness variations. Thin ply materials are now commercially available o Challenges: o Characterization and understanding of ply-thickness effect for a wide range of constituents o Efficient and accurate models for prediction of thin ply composite performance o Flexible manufacturing (assembly of complex preform) & automation to reduce layup time o Develop & validate efficient design method to account for the performance benefit without blowing up the degree of complexity of the problem Relevant literature (small subset): [1] S. Sihn, R.Y. Kim, K. Kawabe, S. Tsai, Experimental studies of thin-ply laminated composites, Composites Science and Technology, 67, 2007 [2] M.R. Wisnom, B. Kahn, S.R. Hallet, Size effects in unnotched tensile strength of unidirectional and quasi-isotropic carbon/epoxy composites, Composite Structures, 84, 2008 [3] A. Arteiro, G. Catalanotti, J. Xavier, P.P. Camanho, Notched response of non-crimp fabric thin-ply laminates, Composites Science and Technology, 79, 2013 Why Thin Ply composites? Thin ply : below 125 g/m2, down to 15g/m2 today

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Objectives : Understand, characterize and model thin ply composites from the ply level to part level. Experimental study of thin ply size effects Constant specimen thickness UD prepregs of varying ply thickness & number of sub-laminate repetitions Material North TPT Thin ply composites UD prepreg with ATL production of complex laminates Thick = 300 (2x150) g/m 2 ~300 microns / ply Intermediate = 100 g/m 2 ~100 microns / ply Thin = 30 g/m 2 ~30 microns / ply M40JB fiber / NTPT TP80ep (80°C epoxy resin) from the same batches and production on the same machine the same week. Autoclave production, 55% fiber volume fraction Objectives & Method

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ThinPly composites: design performance advantages Lamina & Laminate Open Hole & Fatigue Bolted joint & Ageing Impact Damage Resistance Part Design & Analysis ? Click one of the icons below to learn more about Thin Ply composites performance in a specific area … or just continue reading normally to have a full overview

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Experimental characterization of Thin-ply size effects

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UD ply level properties o Thin Ply : more uniform microstructure and improved 0° compressive strength Compressive strength (ASTM D5467*) Thick 300 g/m2 Intermediate 100g/m2 Thin 30g/m2 o Overall no change in intrinsic lamina properties when reducing ply thickness o One exception: 0° compression

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Quasi isotropic laminate, tensile ASTM D3039, constant thickness, sub-laminate scaling: [+45°/90°/-45°/0°]ns with n=1 for thick, n=3 for interm. and n=10 for thin ply. Laminate level Damage (Acoustic emission) Applied stress

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QISO tensile properties Laminate level Ultimate strength: +42% [+45°/90°/-45°/0°] ns n=1 n=3 n=10 & n=3 Onset of damage: +227% Change of failure mode: Thick: extensive matrix cracking & delamination Thin: brittle rupture by fiber failure (max strain of fiber) Little effect of n

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Open Hole Compression Element level +18% Open Hole Compression [+45°/90°/-45°/0°]ns (ISO / ASTM D6484) OHC Strength [Mpa]

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Open Hole Tensile fatigue o Lower static ultimate strength. No damage around hole means no stress concentration relief but better predictability (Wisnom & al) -34% +31% Thick plies 300g/m 2, cycles, 316MPa Thin plies 30g/m 2, 316MPa [+45°/90°/-45°/0°] ns Ruin = -10% stiffness o Strong improvement in fatigue life ( 1M cycles) Open Hole Tensile: static and fatigue (ASTM D5766 & D7615, R=0.1) Element level

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Bolted joint bearing strength o Strength improvement for as 20°C +18% o Strength improvement for Hot 90°C +58% Single lap bearing test, standard and Hot Wet condition (ASTM 5961), fastener type EN-6115 Hot Wet cond. 95%RH/70°C, test 90°C br_ult = 156 MPa br_ult = 476 MPa br_ult = 573 MPa br_ult = 294 MPa br_ult = 584 MPa br_ult = 372 MPa Thick Ply 300g/m 2, n=2 Intermediate 100g/m 2, n=5Thin Ply 30g/m 2, n=18 Hot Wet 90°C As Produced, 20°C As produced, 20°C Element level [+45°/90°/-45°/0°] ns

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Low energy impact o Transition of failure mode from delamination to fiber failure o An optimal ply thickness can be found to achieve the smallest damage area o Thin-Ply technology allows tailoring the material properties wrt impact induced damage o Rectangular specimen clamped on the short sides; bending is dominant QI [0°/+45°/90°/-45°] ns, 300 x 140 x 2.4 mm Thick (300 g/m 2 ) n=1, Intermediate (100g/m 2 ) n=3, Thin (30g/m 2 ) n=10 o Energy: 11.5 J & 18J Backside THICK, n=1INTERMEDIATE, n=3THIN, n=10 delamination delamination & fiber failure mostly fiber failure Element level

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Modeling of Thin-ply size effects

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Ply thickness effects: 1.reduction of interlaminar shear stresses at the free edges & delayed free edge delamination 2.Constrained intra laminar transverse cracks => apparent 90° and in-plane shear strength increase => in-situ ply strength model (*) 3.Matrix cracking induced delamination also delayed by constraining plies 4.Other mechanisms (bridging?, plasticity of matrix?) Modeling thin-ply size effects Simulation 2D generalized plane strain or 3D continuum model In-situ shear / normal strength (*) + coarse 3D model Or high fidelity 3D continuum model + cohesive zone High fidelity 3D continuum model + cohesive zone (*) Camanho et al., Composites Part A, 37, 2006 Fracture mechanics : onset ~ 1/sqrt(t)

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Simulation of thin ply effects o Goal: capture the transition in dominant failure mode in order to understand and predict ply size effects with interacting damage modes o Hypotheses: no change in intrinsic properties of ply and interface wrt ply thickness First ply: 0° (symetry) User material with fiber failure (subroutine) UD mx. without fiber failure Cohesive elements > lateral cracking 2 nd ply: -45°3rd ply: 90°4th ply: +45° Between the layers: cohesive surfaces > delamination Simulation: force controlled (sigmoid ramp, quasi-static) o High fidelity 3D modeling of quasi isotropic unnotched tensile test in Abaqus Explicit, [45°/90°/-45°/0°]ns laminate. o All material properties coming from tests, no fitting parameters. Simulation

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Simulation of thin ply effects o Goal: capture the transition in dominant failure mode in order to understand and predict ply size effects with interacting damage modes o Hypotheses: no change in intrinsic properties of ply and interface wrt ply thickness First ply: 0° (symetry) User material with fiber failure (subroutine) UD mx. without fiber failure Cohesive elements > lateral cracking 2 nd ply: -45°3rd ply: 90° Between the layers: cohesive surfaces => delamination Simulation: force controlled (sigmoid ramp, quasi-static) o High fidelity 3D modeling of quasi isotropic unnotched tensile test in Abaqus Explicit, [45°/90°/-45°/0°]ns laminate. o All material properties coming from tests, no fitting parameters. Simulation Cohesive elements => Intralaminar matrix cracking 45° 90° -45° 0° User material with fiber failure (subroutine) Symmetry BC

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Simulation of thin ply effects o Damage models: cohesive interfaces between plies, cohesive elements for transverse cracking, continuum damage model for fiber failure o Mesh convergence study: 6 linear hex. element C3D8R per ply thickness, 0.5mm in plane elem. size, up to 650k elements, 2.1M dofs o Mass scaling & time step convergence study: dt = ~5e-6 s, ~ time steps Simulation

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Thick ply (300 g/m 2 ) QISO tensile Simulation Delamination damage Intra laminar cracking (matrix failure) Fiber failure

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Thick ply (300 g/m 2 ) QISO tensile Damage sequence: o cracking of 90° ply, then cracking of 45° plies & delamination from edges o Final failure after extensive delamination / damage of all off-axis plies o Ultimate strength: fiber failure in 0° plies with all other plies broken Simulation

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Intermediate ply (100 g/m 2 ), n=3 Damage sequence: o cracking of 90° plies then outer 45° ply then final failure (localization of damage starting from the free edges and fiber failure in 0° plies o Transverse normal and shear cracking delayed, Delamination nearly suppressed Simulation

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Simulation of thin ply effects Simulation

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Simulation of thin ply effects o Good predictions for thick & intermediate ply thickness for both onset of damage and ultimate strength. o High fidelity FE model can capture the change of damage mode sequence wrt ply thickness (with constant intrinsic properties) o Coarse 3D FE Models (1 elem/ply) with in-situ strength (scaled by 1/sqrt(t)) provide comparable results, could be extended towards shell modeling (work in progress) FE Analysis Intermediate 100 g/m2 Thick 300 g/m2

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Towards part level modeling and design Design ModelFirst ply failure0° ply failureExperimentDamageUlt strength CLT with damage 287 MPa609 MPa Thick ply 300g/m2 248 MPa595 MPa High fidelity 3D FE modeling Shell models or CLT with in-situ strength All damage modes with interactions o Thin Ply composites : closer to classical laminate theory as no delamination!! ModelFirst ply failure0° ply failureExperimentDamageUlt strength CLT with damage 287 MPa609 MPa Thick ply 300g/m2 248 MPa595 MPa CLT no damage 287 MPa819 MPa Thin ply 30g/m2 821 Mpa847 MPa Separate damage models for transverse cracking and delamination Coarse 3D FE modeling with in- situ strength Damage models for transverse cracking Delamination Delamination follows intra laminar cracking (no complex interaction) Ply thickness small enough to avoid delamination

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o Advantages: Improved delamination resistance, higher onset of damage & ultimate failure Improved fatigue properties and in some case damage tolerance More design degrees of freedom = more optimal laminate Easier to design ply drops / small angle laminates / local reinforcements Can produce homogeneous laminates, no more dependency on stacking Can produce mixed thin & thick laminates for structures with large shell thickness variations. Thin ply materials are now commercially available o Challenges: o Characterization and understanding of ply-thickness effect for a wide range of constituents o Efficient and accurate models for prediction of thin ply composite performance o Flexible manufacturing (assembly of complex preform) & automation to reduce layup time o Develop & validate efficient design method to account for the performance benefit without blowing up the degree of complexity of the problem Why Thin Ply composites? Thin ply : below 125 g/m2, down to 15g/m2 today

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Thin ply effects at different scales At the lamina level o More uniform microstructure, lower local variation of fiber Vf and finer voids o Improved longitudinal compressive strength At the laminate level o Delay or suppression of delamination as damage / failure mechanism o Delayed transverse cracking => apparent increase of transverse tensile strength o Increased QI laminate onset of damage and ultimate strength o Increased laminate fatigue strength At the structural component level o Notched tests: improved onset of damage and fatigue life increase; more brittle response but better predictability o Bolted joint strength: strong strength improvement in hot-wet conditions o Impact effects: from delamination to fiber failure, optimal ply thickness is « intermediate ». Summary Microstructure Change of failure mode (delayed delamination)

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