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Mechanical properties measurement andmodelling
Ruzena Chamrova
Laboratory of Construction MaterialsEcole Polytechnique Federale de Lausanne
Lausanne, Switzerland 2008
Chamrova, Scrivener, Guidoum Project 10 - Mechanical properties measurement and modelling
Scientific view differs from the industrial
I scientific view - fitting a curveI industrial view - same porosity, considerable difference in
strength
porosity
com
pres
sive
str
engt
h [M
Pa]
Chamrova, Scrivener, Guidoum Project 10 - Mechanical properties measurement and modelling
The aim is to study mechanical performance ofhydrating cement paste
Aim of the project
I measurement and prediction at early ageI development of in-house FEM code for the predictionI study relationship between microstructural development
and mechanical performance
Chamrova, Scrivener, Guidoum Project 10 - Mechanical properties measurement and modelling
Mechanical properties of microstructure are computedby FEM
Mechanicalproperties
Mesh + FEM
Experiment
Model
Chamrova, Scrivener, Guidoum Project 10 - Mechanical properties measurement and modelling
Factors influencing the FEM prediction
Mechanical propertiesprediction
Microstructural input
Porosity approach
Connectedness approach
Mesh
Boundaryconditions
Chamrova, Scrivener, Guidoum Project 10 - Mechanical properties measurement and modelling
Unstructured meshes capture the geometry better
UnstructuredSemi-structuredStructured
Smoothing
Octree
DelaunayVoxel
Regular tetrahedral
Chamrova, Scrivener, Guidoum Project 10 - Mechanical properties measurement and modelling
Semi-structured meshes offer a trade-off
UnstructuredSemi-structuredStructured
Smoothing
Octree
DelaunayVoxel
Regular tetrahedral
Chamrova, Scrivener, Guidoum Project 10 - Mechanical properties measurement and modelling
Delaunay mesh does not seem to be practical atpresent
I mesh improvement - spike reduction - is time consumingI memory limitations and complexity of overlaps allow to go
up to 50000 particlesI this approach might be necessary e.g. for transport
properties modelling
Chamrova, Scrivener, Guidoum Project 10 - Mechanical properties measurement and modelling
Structured tetrahedral mesh offers trade-off betweensimplicity of generation and capturing of geometry
Tetrahedral meshVoxel mesh
Subdivision into 6 tetrahedra
I mesh is stillunnecessarilyjagged
Chamrova, Scrivener, Guidoum Project 10 - Mechanical properties measurement and modelling
Problem of the subdivision into 6 lies in the orientationof elements
I orientation ofelements notsuitable forspheres
I rotating elementswould meanincompatibility forFEM
Chamrova, Scrivener, Guidoum Project 10 - Mechanical properties measurement and modelling
2D example shows that subdivision into 5 tetrahedra ismore suitable
Voxel mesh Subdivision - 5
Subdivision - 6
Chamrova, Scrivener, Guidoum Project 10 - Mechanical properties measurement and modelling
Tetrahedral meshes produce higher E (10 - 15 %)
degree of hydration [%]
Ela
stic
Mod
ulus
[GP
a]tetrahedra – 6tetrahedra – 5voxels
E for various types of meshes
Chamrova, Scrivener, Guidoum Project 10 - Mechanical properties measurement and modelling
Tetrahedral meshes produce higher E (10 - 15 %)
degree of hydration [%]
voxelstetrahedra – 5tetrahedra - 6
Poi
sson
's r
atio
[-]
Chamrova, Scrivener, Guidoum Project 10 - Mechanical properties measurement and modelling
On the contrary volume ratios differ by 1 %
Possible explanations
I solely volume ratios cannot precisely explain thisphenomenon
I the connections in the structure got denser with thetetrahedral mesh
I linear tetrahedral element (CST) might not be comparablewith the same size of a hexahedral element
Chamrova, Scrivener, Guidoum Project 10 - Mechanical properties measurement and modelling
And we are still not happy with the geometry...
Under implementation
Smoothing Octree
Chamrova, Scrivener, Guidoum Project 10 - Mechanical properties measurement and modelling
Factors influencing the FEM prediction
Mechanical propertiesprediction
Microstructural input
Porosity approach
Connectedness approach
Mesh
Boundaryconditions
Chamrova, Scrivener, Guidoum Project 10 - Mechanical properties measurement and modelling
More realistic way is not to mesh porosity
Porosity meshing
I sensitive to the elasticproperties assigned toporosity
Real porosity
I computational timeseveral times higher
Chamrova, Scrivener, Guidoum Project 10 - Mechanical properties measurement and modelling
Real porosity approach predicts lower E...
degree of hydration [%]
Ela
stic
Mod
ulus
[M
Pa]
meshed porosityreal porosity
Chamrova, Scrivener, Guidoum Project 10 - Mechanical properties measurement and modelling
...and lower Poisson’s ratio
degree of hydration [%]
K =2.18 GPareal porosity
Poi
sson
's r
atio
[-]
Chamrova, Scrivener, Guidoum Project 10 - Mechanical properties measurement and modelling
Factors influencing the FEM prediction
Mechanical propertiesprediction
Microstructural input
Porosity approach
Connectedness approach
Mesh
Boundaryconditions
Chamrova, Scrivener, Guidoum Project 10 - Mechanical properties measurement and modelling
Mechanical properties do not develop from the verybeginning of hydration
All phases intocomputation
Only connectedphases intocomputation
I unconnected phases are thrown away by the burningalgorithm which is provided by the microstructural model
Chamrova, Scrivener, Guidoum Project 10 - Mechanical properties measurement and modelling
Connectedness seems to be the most influential factor
degree of hydration [%]
All phasesOnly connected
Ela
stic
Mod
ulus
[M
Pa]
Chamrova, Scrivener, Guidoum Project 10 - Mechanical properties measurement and modelling
Factors influencing the FEM prediction
Mechanical propertiesprediction
Microstructural input
Porosity approach
Connectedness approach
Mesh
Boundaryconditions
Chamrova, Scrivener, Guidoum Project 10 - Mechanical properties measurement and modelling
There is little difference between BCs - RVEhypothesis correct
degree of hydration [%]
Ela
stic
Mod
ulus
[M
Pa]
displacement BCstatic BCperiodic BC
Chamrova, Scrivener, Guidoum Project 10 - Mechanical properties measurement and modelling
2D plain strain of slices is not a substitution for 3Dcomputation
Elastic Modulus of the microstructural slices – displacement BC
Slice of the microstructure
Ela
stic
Mod
ulus
[M
Pa]
direction Xdirection Ydirection Z3D structure
Chamrova, Scrivener, Guidoum Project 10 - Mechanical properties measurement and modelling
High E of some slices is caused by low porosity andhigh alite ratio
3D phase ratios 2D phase ratios
Comparison of the phase ratios
phas
e ra
tios
I for correct 2Dpredictionnecessary togenerate new 2Dmicrostructurebased on 3Ddistribution
Chamrova, Scrivener, Guidoum Project 10 - Mechanical properties measurement and modelling
Next plans
code development and prediction of
mechanical propertiesEPFL
porosity prediction, strengthtests - Aalborg
Prague – April 2008
code development EPFL
validation and thesis writingEPFL
Chamrova, Scrivener, Guidoum Project 10 - Mechanical properties measurement and modelling