1. Micro-Scale Experiments and Models for Composite Materials PhD project duration: 1. January 2012 - 31. December 2014 Project type & funding: PhD-A project, DCCSM Core (DSF) PhD-student: Sanita Zike Supervisors: Lars P. Mikkelsen, DTU Wind Energy, Section of Composites and Materials Mechanics Bent F. Sørensen, DTU Wind Energy, Section of Composites and Materials Mechanics Viggo Tvergaard, DTU Mechanical Engineering, Section of Solid Mechanics

2. Add Presentation Title in Footer via ”Insert”; ”Header & Footer” Vision of PhD project The target of the PhD project is to establish coupled modelling- experimental approaches for bridging the understanding of composite material properties from micro to macro scale length. 2

3. Add Presentation Title in Footer via ”Insert”; ”Header & Footer” Outline 3

4. Add Presentation Title in Footer via ”Insert”; ”Header & Footer” 2. - 4. Plastic zone and shear band formation around notches & fibre/matrix interface Modelling and experimental determination of plasticity zone by formation of shear bands in polymer material around notches, single and multiple fibre/matrix interfaces. References: 1. Wang, G.F. & Van der Giessen, E., 2004. Fields and fracture of the interface between a glassy polymer and a rigid substrate. European Journal of Mechanics - A/Solids, 23(3), pp.395-409. 2. Jeong, H.Y. et al., 1994. Slip lines in front of a round notch tip in a pressure-sensitive material. Mechanics of materials, 19(1), pp.29–38. 4

5. Add Presentation Title in Footer via ”Insert”; ”Header & Footer” Experimental testing Interface study between glass and polymer introducing DCB testing method in optical microscope and ESEM Glass – Polymer - Glass Cohesive laws References: 1. Sørensen, B.F. et al., 1998. Fracture resistance measurement method for in situ observation of crack mechanisms. Journal of the American Ceramic Society, 81(3), pp.661–669. 2.Sørensen, B.F. et al., 2010. Cohesive laws for assessment of materials failure: Theory, ezperimental methods and application. Doctor of Technices thesis, DTU. 3.. Goutianos, S., Frandsen, H.L. & Sørensen, B.F., 2010. Fracture properties of nickel-based anodes for solid oxide fuel cells. Journal of the European Ceramic Society, 30(15), pp.3173- 3179. Plasticity zone 5

6. Add Presentation Title in Footer via ”Insert”; ”Header & Footer” 5. Correlation between microscopic and macroscopic behaviour The ending of research project involves understanding the correlation between the observed micro- and macro-scale properties of composite materials. In micro-scale materials can sustain higher loads, therefore show better strength properties than the same materials in macro-scale. The project intention is to develop approaches, which can be used to predict macroscopic behaviour knowing the micro-scale properties. 6

7. 1. STRAIN GAUGE MEASUREMENTS OF SOFT MATERIALS

8. Add Presentation Title in Footer via ”Insert”; ”Header & Footer” Strain gauge as strain measuring device Strain gauge electrical resistance is changed with small deformations of inner grids. Calibration of strain gauges has to be done to obtain gauge factor: Resistivity change Strain 8

9. Add Presentation Title in Footer via ”Insert”; ”Header & Footer” Aim and tasks Purpose is to determine the measurements accuracy of strain gauges used in soft materials testing. Study involves: 1)development of numerical model in FEM program ABAQUS; 2)in situ micromechanical measurements under optical microscope incorporating digital image correlation (DIC) system Aim: Obtain correction methods for strain gauge measurements Tasks: –How measurement error varies with strain gauge type? –How much strain gauge measurements are influenced by specimen geometry and stiffness? –What is the impact of plastic deformation on strain gauge measurements? 9

10. Add Presentation Title in Footer via ”Insert”; ”Header & Footer” Recognition of problem Experimentally observed discrepancy between different strain measurement methods: Why SG, clip on and laser extensometer measurements show different strain values? 10

11. Add Presentation Title in Footer via ”Insert”; ”Header & Footer” Strain distortions by ABAQUS 11

12. Add Presentation Title in Footer via ”Insert”; ”Header & Footer” DIC measurements 12

13. Add Presentation Title in Footer via ”Insert”; ”Header & Footer” Parameter study Variables: Elastic modulus of specimen Specimen dimension Strain gauge dimension Elastic and plastic deformation Length 1.5 - 10 mm Thickness 3.8 - 5.0 µm Pattern modification (elongation of end-loops) Length 25 – 150 mm Width 10 – 25 mm Thickness 1 – 30 mm 13

14. Add Presentation Title in Footer via ”Insert”; ”Header & Footer” 2 D model2 D model MODELLING FEATURES: -SG: uniform foil with ½ thickness (2D), elastic- plastic, back-to-back SGs -Specimen: ¼ symmetry (3D), elastic, elastic- plastic -Parts: solid, homogeneous, deformable -Elements: plane stress & 3D stress -Load: displacement boundary 3 D model 14

15. Add Presentation Title in Footer via ”Insert”; ”Header & Footer” Gauge factor correction Gauge factor (GF) Correction coefficient (C) – ratio between actual and SG determined strain: Gauge factor correction: Manufacturers provided strain gauges are calibrated on stiff material - steel. Usage of strain gauges on softer material than constantan, requires new calibration or gauge factor correction. 15

16. Add Presentation Title in Footer via ”Insert”; ”Header & Footer” Parameter study results Specimen thicknessStrain gauge length & stiffness 16

17. Add Presentation Title in Footer via ”Insert”; ”Header & Footer” Correlation between specimen thickness and strain gauge length THINTHICK thickness 17

18. Add Presentation Title in Footer via ”Insert”; ”Header & Footer” Conclusions Sufficiently large errors are observed even for relatively stiff specimens Parametric study indicates major impact by gauge length and specimen thickness: –Shorter strain gauges are subjected to larger errors as strain distortions more affect measuring grid –Thinner specimens more affected by stiffening Correction coefficient can be used to modify manufacturers provided gauge factor. Two correction coefficient values can be distinguished depending on specimen thickness. At large strains, up to 5%: –Strain gauge reinforcement decreases due to plastic deformation of constantan. –Total reinforcement can either increase or decrease depending on specimen stiffness reduction during plastic deformation. 18