Maxwell Model: Oedometer Test
Note
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This example compares numerical and analytical solutions of an oedometer test carried out on a Maxwell substance. In this test, the base of the sample is fixed, lateral deformations are prevented, and a constant vertical load, P, is applied at the top of the specimen.
The analytical solution for vertical strain and stresses is
where a, b, and c are the three constants:
K and G are bulk and shear modulus of the substance, and η is the viscosity.
The numerical simulation is carried out simultaneously on three samples, represented by one zone, using the Maxwell model, the Burgers model and the Burgers-Mohr model.
Because no value is assigned to the property viscosity-kelvin for the Burgers model and the Maxwell-Mohr model, the Kelvin cell logic is not taken into account by the models. Also, the cohesion property is set to a high value to prevent triggering of the plasticity logic for the Burgers-Mohr model. The initial state is obtained by cycling the model to elastic equilibrium. For the viscous response, velocities are reset to zero, and the initial timestep is set to a small value (Δt = 10-4) compared to the ratio of viscosity over shear modulus (ηM/GM = 2.0). With the choice of default automatic creep timestep parameter settings used in the example, the timestep increases by a factor of 1.01 when the out-of-balance force ratio is less than 10-3, until Δt = 10-2. A state of hydrostatic stress is reached at the end of the test. Figure 1 and Figure 2 show the agreement between analytical solutions and numerical predictions for stresses and strains in the three samples.

Figure 1: Oedometer test on a Maxwell substance: analytical and numerical stress values versus time.

Figure 2: Oedometer test on a Maxwell substance: analytical and numerical strain values versus time.
Data File
;------------------------------------------------------------
; Oedometer test -- Maxwell substance
;-------------------------------------------------------------------
model new
model large-strain off
fish automatic-create off
model title "Oedometer test --- 'Maxwell' substance"
model configure creep
program call 'parameter'
[ini_cons]
; --- model ---
zone create brick size 5 1 1
zone cmodel assign maxwell range position-x 0 1
zone cmodel assign burgers range position-x 2 3
zone cmodel assign burgers-mohr range position-x 4 5
zone null range union position-x 1 2 position-x 3 4
;
zone property density 1 bulk [c_bu]
zone property shear [c_sh] viscosity [c_vi ] ...
range position-x 0 1
zone property shear-maxwell [c_sh] viscosity-maxwell [c_vi]...
range position-x 2 3
zone property shear-maxwell [c_sh] viscosity-maxwell [c_vi] ...
cohesion 1e20 tension 1e20 ...
range position-x 4 5
;
zone gridpoint fix velocity-x
zone gridpoint fix velocity-y
zone gridpoint fix velocity-z range position-z 0
zone face apply stress-normal -1.0 range position-z 1
; --- histories ---
history interval 1
zone history stress-yy position (0.5,0.5,0.5)
zone history stress-yy position (2.5,0.5,0.5)
zone history stress-yy position (4.5,0.5,0.5)
fish history [ana_syy]
zone history stress-zz position (0.5,0.5,0.5)
zone history stress-zz position (2.5,0.5,0.5)
zone history stress-zz position (4.5,0.5,0.5)
fish history [ana_szz]
zone history displacement-z position 0 0 1
zone history displacement-z position 2 0 1
zone history displacement-z position 4 0 1
fish history [ana_ezz]
model history creep time-total
; --- elastic equilibrium ---
model solve ratio 1e-4
; --- reset velocities to zero ---
zone gridpoint initialize velocity (0,0,0)
; --- viscous behaviour ---
model creep timestep starting 1.e-4
model creep timestep minimum 1.e-4
model creep timestep maximum 1.e-2
model solve time-total=25.
model save 'maxwell'
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