Activities in Composites and Material Mechanics

Kompositsektionens Årsmøde, Assens 07 06 2013 Activities in Composites and Material Mechanics Povl Brøndsted Forskningsspecialist pobr@dtu.dk Institut for Vindenergi Sektion for Kompositter og materialemekanik Danmarks Tekniske Universitet Risø Campus Frederiksborgvej 399 4000 Roskilde Slide Contributions from colleagues: Vladimir Guzman, Justine Beauson, Tom L. Andersen, Christen Malte Markussen, Bo Madsen, Bent F. Sørensen, Leon Mishnaevsky, Erik M Lauridsen Risø Campus (DTU) formerly Risø National Laboratory 1

DTU Wind Energy Technical University of Denmark 230 240 persons (started 1/1 2012) Materials research Composites and Material Mechanics section Disciplines: Basic materials properties Fibers, Sizing, Resins, Laminate and sandwich architectures Processing Microstructure characterisation (microscopy) Modelling (micromechanical and materials modelling) Mechanical characterisation Damage detection (Non destructive evaluation and structural health monitoring) 2

Scientific vision Top down multiscale approach What controls this? Study at smaller scale Use answer at coarser scale This controls it Composite Parameters Fibre orientation/architecture Fibre properties Fibre/matrix interface properties Mechanical performance of composites Porosity Matrix properties Fibre content Fibre length/diameter Fibre packing ability 3

Fibers Single Fiber and Interface Properties Single fibre properties Fibre diameter Fibre E modul Fibre Strength Sizing Sizing Chemistry Thickness of Sizing Layer Interface shear strength OC Simulation of glass production through nozzles Interfaces and Sizing F. R. Jones, in Interface Design of Composites 28 th Risø International Symposium on Material Science, Interface Design of Composites, 2007 Sizing is organic based material which is coated onto the individual glass fibre. Function processability protection bonding with resin Challenge: solubility of protective layers in the resin. 4

Interface properties Shear strength, static and cyclic Fracture toughness Weak interface: debonding at the fibre/matrix interface Strong interface: crack propagates into the matrix M l d1 l d3 F l d2 DF l d4 Interface properties Interface adhesionimprovement atmospheric pressure plasma processing Surfaces of fibres Surfaces of composites Ultrasound enhanced treatment Continuous treatment Treatment in air 5

Quantification of Interface properties Mechanical testing: Single Fibre Fragmentation Tests Single Fibre Pull out Interlaminar Shear Strength Off Axis Tensile Test V Notch Shear Test Transverse Tensile Test Compression Test Fatigue tests, BIAX laminates Crack Opening Fibre bridging... Matrix resin properties Requirements Ductility, Toughness Viscosity (<500 cps) Processability (gel time>20 min) Compatibility to fibres, sizing and adhesives Low moisture absorption Low cost 6

Influence of resin on fatigue performance 1.2 1 Strain Max [%] 0.8 0.6 0.4 0.2 Epoxy Vinylester Polyester 0 10 4 10 5 10 6 10 7 Cycles to failure Fabric Architectures Stitched Fabrics (non crimp fabrics) Stitched fabrics use a lightweight fibre as a loop. The reinforcement tow remains aligned in the plane without crimp (so these reinforcements are commonly referred to as non crimp fabrics (NCF)). The fabrics may be single layer multiple layers as either biaxial (0 /90 or ±45 cross plied) multi axial (e.g. 0 /45 / 45 90 ) 7

Fabric Architectures Stitched Fabrics (non crimp fabrics) Schnabel, A. and T. Gries. Production of noncrimp fabrics. In non crimp fabrics composites. Ed. S.V. Lomov. Wooghead Publishing in Materials. 2011 Laminate architecture Example: Hybrid Glass/Carbon, Filament Wound CXG01 0:1 Glass only Increasing carbon fibre content CXG06 1:6 Hybrid CXG03 1:4 Hybrid CXG05 1:2 Hybrid CXG02 1:4 Hybrid (carbon twisted) CXG07 1:4 Hybrid (thick plate) CXG08 1:4 Hybrid (ply-ply) CXG04 1:0 Carbon only 8

Microstructure characterization Optical microscopy material quality evaluation Reinforcement (Glass) Stitching yarn Matrix rich area Porosity Unwetted fibres Fiber orientation - defects (cracks) - damage evaluation UD Composites, Transverse 9

UD Composites, Longitudinal UD Composites, Tow bundles(rowings) Variety in Fibre content FWF 66.4% FWF 78.4% 0 (2400 Tex) side 90 (198 Tex), Mat (24 Tex) side 10

Laminate architecture Example: Biax fabrics 1200 g/m 2 Microstructure characterization Scanning electron microscopy mechanical test inside microscopy Fracture surface: microscale failure mode fibre/matrix debonding Testing in ESEM: tension, compression & bending Mode I & Mixed Mode crack growth 11

Microstructure characterization Crack growth experiments in ESEM understanding micromechanisms of fibre bridging Microstructure characterization Modelling understanding toughening by crack bridging i [Bent Sørensen] 4 EI 1/ W 1.3732 N i 3/ 4 Cohesive Stress, Opening, 12

Modelling Microscopy modelling of composites Finite element modelling: Micromechanical modelling tensile properties Fibre/matrix debonding Matrix properties (strain gradient plasticity) Compressive strength [Leon Mishnaevsky] [Lars P. Mikkelsen] Processing of composites Production facilities for polymer composites Process development, manufacturing of test materials, material quality determination and tooling specimens Filament winding Vacuum infusion process Press consolidation Autoclave technology Microscopy Density and porosity measurements Gluing/assembly methods Tooling 13

Processing of composites Filament winding unidirectional laminates 0 -laminat Processing of composites Vacuum infusion process dry lay up of fibres in concealed vacuum bag Outlet Inlet Investigation of new fiber and matrix materials 14

Processing of composites Autoclave high quality composites reference material Capabilities: 500 C 20 bar vacuum Processing of composites Controllable trailing edge flaps casting of rubber parts Controllable trailing edge 15

Mechanical properties Measurements of mechanical properties Accredited test laboratory Test machines: 10 servohydraulic test machines 4 spindle driven test machines 1 electromagnetic resonnans test machine 6 fixtures for ESEM testing Test methods: Tension Compression Bending Shear Fracture mechanics Evaluation: Ultrasonic scanning X ray Acoustic emission Digital Image Correlation (DIC) Thermovision Conditions: Static and cyclic loading Controlled temperature Mechanical properties Test specimens manufacturing and tooling 16

Statistical Representative Volume Element Optimising Test Specimen Geometries Optimised test geometry for UD laminates (DTU Wind) Mechanical testing Overall objective is experimentally to measure the mechanical properties of the material. Testing on different scales: Micro Scale fibre inteface testing Coupon Scale Fabric and Laminate testing Subcomponent Scale Sandwich testing, adhesive joints etc Component Scale substructures Full Scale Blades, Full Wind Turbine Challenges on all scales in order to perform representative tests Selection of test specimen geometry Selection of test method Analysis of test data 17

Statistical Representative Volume Element Optimization for Virtual and Experimental Testing Scope What is the smallest volume of material that statistically represents the full volume properties? Scaling effects To what extent can a full size component be represented by downscaling > subcomponents > plain panels > test coupons > microscopy >... Requirements Microstruture such as fibre packing, fibre diameters, bundle packing and bundle features must be statistically representative Defects must be must be statistically representative Deformation and Failure modes must be statistically representative In the cases where this cannot be fulfilled each volumen must be consideres as as subcomponent with special substructure and defects, and must be evaluated using fracture mechanics or Damage Mechanics. crodefects Compression test 18

Test specimen with back to back mounted extensometers Compression: Stress/strain curves Carbon : Glass = 1 : 4 Effect of ply/ply versus fibre/fibre structure 3 mm laminate 6 mm laminate fibre/fibre ply/ply 3 mm laminate 6 mm laminate 3 8 19

Mechanical properties Fracture toughness damage tolerance DCB test Fatigue testing R=0.1 20

Geometry Effects R = 0.1 Only trends there might be dependence of material quality Precaution: Only trends there might be dependence of material quality Large vs. Small R = 0.1 Optimat Blades project (WMC) 21

Combining different techniques Mechanical fatigue test Stiffness degradation Damping evolution Fiber content Porosity FWF / FVF Acoustic Emission Crack initiation Crack growth characteristics Crack growth localization (1D) Improve understanding and detection of fatigue damage. Infrared video Damage localization Damage evolution Microscopy Material microstructure quality Crack type observation (fiber fracture, matrix fracture, debonding) Mechanical properties Tension Tension fatigue thermal imaging ISO 527 4 geometry (..\..\JZAN_GPV 10.avi) Risø Butterfly (..\..\GPV 05.avi) 22

Mechanical properties Tension Compression fatigue thermal imaging Infra red camara reveals heating ind grips 720 638A 02 50 Sample ID: 720-638A-02 150 Stiffness [GPa] 40 30 20 100 50 Stiffness [MPa/mm] Strain 1 Strain 2 Strain Avg. Position 10 0 0 0 0.5 1 1.5 2 2.5 3 3.5 Cycle x 10 4 23

413-575A-02 413-575A-03 4 7 17 J ne NDE and structural health monitoring Defect detection ultrasound scanning Air-coupled through-transmission scan of a sandwich panel with a debond between skin layer and foam core sandwich specimen [Kaj K. Borum] 24

NDE and structural health monitoring Damage detection acoustic emission (AE) AE in a blade on a turbine Static load blade test Teaching DTU Wind Energy master 45201 Experimental Characterization of Materials 45211 Manufacturing of Advanced Fiber Composites Processing fibre, matrix, coating proces parameters Properties stress-strain traction-separation Microstructure fibre volume & distrubution porosity Modelling micromechanical modelling property prediction 45208 Advanced Finite Element Simulations using Abaqus 45206 Composite Materials and Fibres 25

Teaching example: parts for eco car [C. Malte Markussen] Vacuum infusion of carbon fiber parts Bio based composites Small rotor blades use of dififferent competances in KOM Wind turbine car Rotor blade of natural fibre composites Materials processing Materials selection Materials testing Materials properties Materials modelling 26

Running projects Forskningsprojekter Genvind (Innovationskonsortioum) RenVind (Reichhold) Blade King (Højteknilogifonds projekt) Resist (Forsvarets materialetjenester) EUPD Flexible Trailing Edge EUDP SHMT Kompositcenter Ph.d. projekter Erhvervs ph.d. LM, Rockwool (Jens, Lucie) SHMT (Reidar) Interfaces DTU Nano (Helga) Modelling (Sanita) Kommercielle kontrakter 27