fem - elements, materials
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FEM
- elements, materials Tomáš Janda
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Outline• Finite elements
• Structural elements
• Contacts
• Constitutive models (materials)
– Elastic model, Modified elastic
– Mohr-Coulomb, Drucker-Prager
– Advanced models
• Driven tunnels
• Solution strategies, solver parameters
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Finite elements
• Plane elements – two nodal degrees of freedom
• Beam elements – three nodal degrees of freedom (Mindlin beam theory)
• Interface elements – two nodal degrees of freedom (zero thickness element)
• Rod elements – anchor, geo-reinforcement or propAUTHORISED RESELLER
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Structural element – beams• Beams in 2D = Walls in 3D
• Bending and normal stiffness per running meter
• Predefined types of cross section and materials
• Output: bending moment, normal and shear force, deflection
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Structural element – beams• Beams in 2D = Walls in 3D
• Bending and normal stiffness per running meter
• Predefined types of cross section and materials
• Output: bending moment, normal and shear force, deflection
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Structural element – beams• Beams in 2D = Walls in 3D
• Bending and normal stiffness per running meter
• Predefined types of cross section and materials
• Output: bending moment, normal and shear force, deflection
AUTHORISED RESELLER
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Structural element – beams• Beams in 2D = Walls in 3D
• Bending and normal stiffness per running meter
• Predefined types of cross section and materials
• Output: bending moment, normal and shear force, deflection
AUTHORISED RESELLER
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Structural elements – anchors• Pre-stressed rod elements
• Free elements, not connected to FE mesh,
– independent on topology, add at any stage
• Defined by
– Pre-stress force, tensile strength, tensile stiffness
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Structural elements – anchors
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Structural elements - reinforcements• Geotexiles, geogrids
• Free rod elements
• Independent on finite element mesh
• Only axial stiffness, active in tension only
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Contacts
Mohr-Coulomb failure criterion Contact element
Initial stiffness
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Constitutive models• Define stress-strain relationship
• Elastic models
• Elasto-plastic models of Mohr-Coulomb family
• Models of critical stateAUTHORISED RESELLER
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Elastic model• Elastic model – Hooke’s law
Constant stiffness, 𝐸, 𝜈
Infinite strength
Numerically stable
• Modified elastic model
Initial loading modulus 𝐸
Unloading/reloading modulus 𝐸𝑢𝑟Rule of thumb:
𝐸𝑢𝑟 = 3𝐸
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Mohr-Coulomb family of models• Elasto-plastic models
• Elastic region given by yield surface in stress space
• Constant stiffness in elastic region
• Limit deviatoric stress 𝐽 increases with compressive mean stress −𝜎𝑚
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Angle of dilatancy• Controls the volumetric plastic strain
• Dense soil tend to dilate
• Default value 𝜓 = 0°
• Typical range 𝜓 = 0°, 5°
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M-C hardening/softening• Advanced material model feature
• Strength parameters depend on plastic strain
• 𝜑 = 𝜑(𝐸𝑑,𝑝𝑙), 𝑐 = 𝑐(𝐸𝑑,𝑝𝑙)
• Defined as piecewise linear function
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Tension cutoff• Limits the tensile stress that is allowed due to
cohesion
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Critical state model• They define “state” of the soil
– Degree of „compactness“
– Void ratio 𝑒 =𝑉𝑝
𝑉𝑠
• Shearing of dense sand → dilatation
• Shearing of loose sand → compaction
• Stiffness depends on
– Void ratio
– Stress
• Non-standard material parameters, lab. testsAUTHORISED RESELLER
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Critical state models• Cam clay
• Hypoplastic model for clay
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Critical state models• Cam clay
• Hypoplastic model for clay
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Constitutive models - application• Elastic model
– Linear stress/strain relation, infinite strength
• Modified elastic model
– Unloading/reloading modulus, infinite strength
• Mohr-Coulomb
– Shear failure, most standard model
• Modified M-C, Drucker-Prager
– Smooth yield surface → better numerical stability
• Critical state models
– Very loose/dense soil, significant volumetric strain AUTHORISED RESELLER
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Effect of mode size
Simple elastic analysis of a shallow excavation
Elastic analysis of tunnel excavation
Slope stability analysis
Plot of horizontaldisplacements
Plot of vertical displacements
Plot of an equivalent plastic strain
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Driven tunnels• How to model clearly 3D structure in 2D?
• Two limit states in 2D
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Driven tunnels• Convergence confinement method (𝜆-method)
• This function is called “Excavations” in Geo5 FEM
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Driven tunnels• Sequential application of “excavation forces”
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Nonlinear solver
Newton-Raphson iteration scheme
• Most often used• May fail when close to the limit load• May fail when the load increment is too large
Arc-length method
• Useful when searching for the maximum or limit load
• In general more time consuming• May fail when the load increment is too large
Both methods can be combined with the Line search method to stabilize the iteration process
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