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

Vision of PhD projectThe 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.

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Outline

1. STRAIN GAUGE MEASUREMENTS OF SOFT MATERIALS

2. Plastic zone and shear band formation around notches

3. Single interface region study between fibre and matrix

4. Interaction between multiple fibre/matrix interfaces

5. Correlation between microscopic and macroscopic behaviour

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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.

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Experimental testingInterface 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

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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.

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1. STRAIN GAUGE MEASUREMENTS OF SOFT MATERIALS

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:

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o

LLRRGF//

Resistivity changeStrain

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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?

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Recognition of problem• Experimentally observed discrepancy between different strain measurement

methods:

Why SG, clip on and laser extensometer measurements show different strain values?

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Strain distortions by ABAQUS

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DIC measurements

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Parameter studyVariables:

Elastic modulus of specimen

Specimen dimension Strain gauge dimension

Elastic and plastic deformation

Length1.5 - 10

mmThickness

3.8 - 5.0 µm

Pattern modification

(elongation of end-loops)Length

25 – 150 mm Width10 – 25 mm

Thickness1 – 30 mm

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2 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

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Gauge factor correction

Gauge factor (GF)

Correction coefficient (C) – ratio between actual and SG determined strain:

specsg

sg

spec

sg

spec

EEC 1

Gauge factor correction:

actualalcalibrated

osg

speco

GFGF

RRRR

C 1/

/

C

GFGF calibratedactual

o

o

LLRR//

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.

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Parameter study results

Specimen thickness Strain gauge length & stiffness

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Correlation between specimen thickness and strain gauge length

THINTHICK thickness

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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.

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