Stress Density Material

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This command is used to construct a multi-dimensional stress density material object for modeling sand behaviour following the work of Cubrinovski and Ishihara (1998a,b). Notee that as of January 2020 this material is still undergoing verification testing.

nDMaterial stressDensity $matTag $mDen $eNot $A $n $nu $a1 $b1 $a2 $b2 $a3 $b3 $fd $muNot $muCyc $sc $M $patm <$ssl1 $ssl2 $ssl3 $ssl4 $ssl5 $ssl6 $ssl7 $hsl $pmin>
$matTag integer tag identifying material
$mDen mass density
$eNot initial void ratio
$A constant for elastic shear modulus
$n pressure dependency exponent for elastic shear modulus
$nu Poisson's ratio
$a1 peak stress ratio coefficient (etaMax = a1 + b1*Is)
$b1 peak stress ratio coefficient (etaMax = a1 + b1*Is)
$a2 max shear modulus coefficient (Gn_max = a2 + b2*Is)
$b2 max shear modulus coefficient (Gn_max = a2 + b2*Is)
$a3 min shear modulus coefficient (Gn_min = a3 + b3*Is)
$b3 min shear modulus coefficient (Gn_min = a3 + b3*Is)
$fd degradation constant
$muNot dilatancy coefficient (monotonic loading)
$muCyc dilatancy coefficient (cyclic loading)
$sc dilatancy strain
$M critical state stress ratio
$patm atmospheric pressure (in appropriate units)

Optional steady state line parameters (default values shown for each, be careful with units)

<$ssl1> void ratio of quasi steady state (QSS-line) at pressure $pmin (default = 0.877)
<$ssl2> void ratio of quasi steady state (QSS-line) at 10 kPa (default = 0.877)
<$ssl3> void ratio of quasi steady state (QSS-line) at 30 kPa (default = 0.873)
<$ssl4> void ratio of quasi steady state (QSS-line) at 50 kPa (default = 0.870)
<$ssl5> void ratio of quasi steady state (QSS-line) at 100 kPa (default = 0.860)
<$ssl6> void ratio of quasi steady state (QSS-line) at 200 kPa (default = 0.850)
<$ssl7> void ratio of quasi steady state (QSS-line) at 400 kPa (default = 0.833)
<$hsl> void ratio of upper reference state (UR-line) for all pressures (default = 0.895)
<$pmin> pressure corresponding to $ssl1 (default = 1.0 kPa)

The material formulations for the stressDensity object are "PlaneStrain"

Code Developed by Saumyashuchi Das, University of Canterbury. Maintained by Chris McGann

General Information

This nDMaterial object provides the stress density model for sands under monotonic and cyclic loading as set forth by Cubrinovski and Ishihara (1998a,b). The original formulation for this model was applicable to plane strain conditions and this is the only currently available formulation.

Notes

Usage Examples

The following usage example provides the input parameters for dry pluviated Toyura sand (with initial void ratio e = 0.73) after Cubrinovski and Ishihara (1998b). The units of this analysis are Mg, kN, s, and m. 

# mass density
set mDen  1.8
# atmospheric pressure
set patm  98.1
# stress density model parameters
set eNot  0.730
set A     250.0
set n     0.60
set a1    0.58
set b1    0.023
set a2    230.0
set b2    65.0
set a3    79.0
set b3    16.0
set fd    4.0
set muNot 0.22
set muCyc 0.0
set sc    0.0055
set M     0.607
nDMaterial stressDensity 1  $mDen $eNot $A $n $nu $a1 $b1 $a2 $b2 $a3 $b3 $fd $muNot $ muCyc $sc $M $patm

References

Cubrinovski, M. and Ishihara K. (1998a) 'Modelling of sand behaviour based on state concept,' Soils and Foundations, 38(3), 115-127.

Cubrinovski, M. and Ishihara K. (1998b) 'State concept and modified elastoplasticity for sand modelling,' Soils and Foundations, 38(4), 213-225.

Example Analysis

Element test with pure shear loading starting from isotropic initial state of stress. 

# intended number of cycles in the test
set nCycles  120
# shear strain increment for the test
set dg 0.0001
set wg [expr 2.0*$dg]
# max number of steps
set maxStep 20000

# initial confinement pressure (kPa)
set pNot -95.0
# max/min shear stress in the test (kPa)
set CSR 0.2
set maxShear [expr -$CSR*$pNot]

wipe

model BasicBuilder -ndm 2 -ndf 2

# Create nodes
node 1	0.0	0.0
node 2	1.0	0.0
node 3 	1.0	1.0	
node 4	0.0	1.0

# Create fixities
fix 1 	1 1
fix 2 	1 1
fix 3	1 1
fix 4 	1 1

# atmospheric pressure
set patm  98.1
# mass density 
set mDen  1.8
# steady state line void ratio
set ssl1  0.832
set ssl2  0.832
set ssl3  0.810
set ssl4  0.796
set ssl5  0.776
set ssl6  0.756
set ssl7  0.735
# hydrostatic state line void ratio
set hsl   0.852
# reference pressures for state lines
set p1    1.0
# stress density model parameters
set A     250.0
set m     0.60
set nu    0.20
set a1    0.592
set b1    0.021
set a2    291.0
set b2    55.0
set a3    98.0
set b3    13.0
set fd    4.0
set muNot 0.15
set sc    0.0055
set M     0.607
# initial void ratio
set emax  0.885
set emin  0.541
set Dr    0.54
set eNot  [expr $emax - $Dr*($emax-$emin)]
set muCyc 0.0

# Create material
nDMaterial stressDensity 2 $mDen $eNot $A $m $nu $a1 $b1 $a2 $b2 $a3 $b3 $fd $muNot $muCyc \
                           $sc $M $patm $ssl1 $ssl2 $ssl3 $ssl4 $ssl5 $ssl6 $ssl7 $hsl $p1

nDMaterial InitStress 1 2 $pNot 2

# Create element
element SSPquad 1  1 2 3 4  1 PlaneStrain 1.0 0.0 0.0

# Create recorders
recorder Element -file stress.out -time stress
recorder Element -file strain.out -time strain
recorder Node -file disp.out -time -dof 1 2 disp

set dt 0.1
# Create analysis
constraints Penalty 1.0e18 1.0e18
algorithm   Linear
numberer    RCM
system      ProfileSPD
integrator  LoadControl $dt
analysis    Static

set dMax [expr 0.6/$wg]
eval "timeSeries Path 400 -time {0 0.1 0.2 300.2} -values {0 0 0 $dMax} -factor 1.0"
pattern Plain 400 400 { 
    sp  3 1 $wg
    sp  4 1 $wg
}
analyze 1

setParameter -value 1 -ele 1 materialState

analyze 1

# counter for max number of steps
set count  0
set cCount 0
set cyc    1
puts "Beginning of Cycle 1"
# loop through the total number of cycles
for {set i 1} {$i <= [expr 2*$nCycles]} {incr i} {
    if {$cCount == 2} {
        set cyc [expr $cyc+1]
        puts "Beginning of Cycle $cyc"
        set cCount 0
    }
    # loop within each cycle
    for {set j 1} {$j < 5000} {incr j} {

        # abort if count is greater than max number of steps
        if {$count >= $maxStep} {break}

        # analyze single step and get the current stress
        analyze 1
        set count [expr $count + 1]
        # get stress from element
        set stress [eleResponse 1 stress]
        # shear stress is component 2
        set tau [lindex $stress 2]


        # signal change in loading direction if needed
        if {[expr abs($tau)] >= $maxShear} {
            # get strain from element
            set strain [eleResponse 1 strain]
            set gamma [lindex $strain 2]
            puts "direction change required: tau = $tau; gamma = $gamma"

            # get current displacements of shearing nodes
            set f [expr 2.0*[nodeDisp 3 1]]
            set b [expr 2.0*[nodeDisp 4 1]]
            #puts "current displacement of front row is $f"
            #puts "current displacement of back row is $b"

            # get number of steps required to reach current disp from zero
            set nStep [expr round(abs($b/$wg))]
            #puts "there are $nStep steps needed to get back to neutral loading"

            # get current time
            set cTime [getTime]
            #puts "current time is $cTime"
            # set an end time for the load patterns
            set zTime [expr $cTime + $nStep*$dt]
            set eTime [expr $zTime + 100.0*$nStep]
            #puts "end time for the new load pattern is $eTime"

            remove loadPattern [expr 400+$i-1]

            eval "timeSeries Path [expr 400+$i] -time {$cTime $eTime 1e10} -values {1 -1000 -1000}"
            pattern Plain [expr 400+$i] [expr 400+$i] {
                sp 3 1 $b
                sp 4 1 $b
            }
            set cCount [expr $cCount + 1]
            break
        }
    }
}

wipe
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