Oedometric Compression Test

Finite Element (FE) representation

For the simulation of oedometric compression tests we perform a so-called ''single-element-simulation'' and by doing so enforce element test assumptions, i.e. a homogeneous distribution of stress/strain within the test sample. A schematic of the FE representation of the oedometric compression test is given below.

oedometer

An axisymmetric solid element with four nodes and linear shape functions for the displacement field is used
In a first initial step, the initial conditions are applied, i.e. initial stress (\(\sigma_2^0\) and, assuming \(K_0=1\), \(\sigma_1^0 = \sigma_2^0\)), initial void ratio \(e_0\) or any other initial state variable required by the constitutive model to be calibrated.
In the second loading step, the loading is applied by prescribing an additional vertikal stress \(\Delta \sigma_2\) on the top surface of the element. \(\Delta \sigma_2\) is increased linearly starting from \(\Delta \sigma_2=0\) until \(\Delta \sigma_2^{max}\) is reached.

Sample numgeo input file

An example input file for the simulation of an oedometric compression test using the hypoplastic constitutive model is shown below.

**=~~=~~=~~=~~=~~=~~=~~=~~=~~=~~=~~=~~=~~=~~=~~=~~=~~=~~=~~=~~=~~=~~=~~=~~=~~=
**                                 numgeo-ACT                                 
**                      Copyright (C) 2022 Jan Machacek                       
**=~~=~~=~~=~~=~~=~~=~~=~~=~~=~~=~~=~~=~~=~~=~~=~~=~~=~~=~~=~~=~~=~~=~~=~~=~~=

*Node
1 , 0.0 , 0.00
2 , 0.05, 0.00
3 , 0.05, 0.018
4 , 0.00, 0.018

*Nset, Nset=nall
1 , 2 , 3 , 4 
*Nset, Nset=nleft
1 , 4
*Nset, Nset=nright
2 , 3
*Nset, Nset=nbottom
1 , 2
*Nset, Nset=ntop
3 , 4

*Element, Type=U4-solid-ax
1 , 1 , 2 , 3 , 4

*Elset, Elset = eall
1

** ----------------------------------------

*Solid Section, Elset = eall, material=soil

** ----------------------------------------

*Material, name = soil, phases = 1

*Mechanical = Hypoplasticity
0.561,0.0,27771046.0,0.19,0.621,1.123,1.15,0.254
2.483,1.0,1.0,0.0001,0.2,2.0, 0.0

*Density
2.65

** ----------------------------------------

*Initial conditions, Type = stress, geostatic
eall, 0.0, -1.548, 0.1, -1.548, 0.46778501154445673, 0.46778501154445673

*Initial conditions, Type = state variables
eall, void_ratio, 0.98968
eall, int_strain22, -0.0001

** ----------------------------------------

*Amplitude, name = LoadingRamp, Type=ramp
0.0, 0.0, 1.0, 1.0

** ----------------------------------------

*Step, name = Geostatic, inc=1
*Geostatic

*Solver, simple

*Body force, instant
eall, grav, 0.0, 0., -1, 0.
*Dload, instant
eall, p3, -1.548

*Boundary
nleft, u1, 0.
nright, u1, 0.
nbottom, u2, 0.

*Output, print
*Element output, Elset = eall
S, E, void_ratio

*End Step

** ----------------------------------------

*Step, name = Loading, inc = 100000, maxiter = 16
*Static
0.00001, 1., 0.00001, 0.01

*Solver, simple

*Body force, instant
eall, grav, 0.0, 0., -1, 0.
*Dload, instant
eall, p3, -1.548
*Dload, amplitude = LoadingRamp
eall, p3, -405.541

*Boundary
nleft, u1, 0.
nright, u1, 0.
nbottom, u2, 0.

*Output, print
*Element output, Elset = eall
S, E, void_ratio

*End Step

** ----------------------------------------

*End Input