Mechanical properties measurement and modelling
Transcript of Mechanical properties measurement and modelling
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