Plastic-Hardening model: Internal parameter calculation fails (small-strain active)

FLAC2D
,
1

Hi all,

I’m currently using FLAC2D v9 to implement the Plastic-Hardening constitutive model with small-strain stiffness activated. However, I consistently receive the following error : *** Plastic-Hardening model: Internal parameter calculation fails.. In zone …
Here are the key parameter values I use:
[global _slot = ‘Soil’]
[global _name = ‘L8_4’]
[global _condition = ‘Undrained’]
[global _Gred = 1.0]
[global _coh = 163000]
[global _phi = 0.1]
[global _m = 0.8]
[global _pref = 148.4E03]
[global _E50 = 66000000]
[global _Eoed = 66000000]
[global _Eur = 199000000]
[global _poisson = 0.2]
[global _strain70 = 0.000192]
[global _OCR = 1.0]
[global _K0ref = 0.9]
[global _void = 0.5]
[_Fn_Assign_PHSmall]

fish define _Fn_Assign_PHSmall
global _slot, _name, _condition
global _coh, _phi, _S_min, _m, _pref, _E50, _Eoed, _Eur, _poisson, _strain70, _OCR, _void, _K0ref
local _pz, _Vs, _G, _K, _rho, _s1, _s3, _s, _pp, _es1, _es3, _esmean, _strength, _pgp, _ngp, _sat, _Z1, _Z
loop foreach _pz zone.list
if zone.group(_pz,_slot) == _name
; Material properties
_rho = zone.density(_pz) ; Density
_Vs = zone.extra(_pz,2)
_G = _rho*_Vs^2.0
_K = 2.0*_G*(1+_poisson)/(3.0*(1.0-2.0*_poisson))

        ; Compute stress measures
        _s1   = zone.stress.min(_pz)
        _s3   = zone.stress.max(_pz)
        _es1 = zone.stress.min(_pz) + zone.pp(_pz)
        _es3 = zone.stress.max(_pz) + zone.pp(_pz)
        _esmean = zone.stress.int(_pz) + zone.pp(_pz)
        
        ; Stress-Dependent Modulus parameter, Z
       
        _Z1 = ((_coh*(1/math.tan(_phi*math.degrad)))-_s3)/((_coh (1/math.tan(_phi*math.degrad)))+_pref)
        _fcut = 0.1 ;The default cut-off factor
        _Z = math.max(_Z1,_fcut)

        zone.model(_pz) = 'plastic-hardening'
        zone.prop(_pz,'Shear')    = _G
        zone.prop(_pz,'Bulk')     = _K
        zone.prop(_pz,'cohesion') = _coh
        zone.prop(_pz,'friction') = _phi
        zone.prop(_pz,'dilation') = 0.0
        zone.prop(_pz,'exponent') = _m
        zone.prop(_pz,'stress-1-effective') = _es1
        zone.prop(_pz,'stress-2-effective') = _esmean
        zone.prop(_pz,'stress-3-effective') = _es3
        zone.prop(_pz,'pressure-reference') = _pref
        zone.prop(_pz,'stiffness-50-reference') = _E50
        zone.prop(_pz,'stiffness-oedometer-reference') = _Eoed
        zone.prop(_pz,'stiffness-ur-reference') = _Eur
        zone.prop(_pz,'poisson') = _poisson
        zone.prop(_pz,'over-consolidation-ratio') = _OCR
        zone.prop(_pz,'coefficient-normally-consolidation') = _K0ref
        zone.prop(_pz,'void-initial') = _void
        
        ;Small Strain Parameters
        zone.prop(_pz,'flag-smallstrain') = 'on'
        zone.prop(_pz,'stiffness-0-reference') = (_G*2*(1+_poisson))/(_Z^_m)
        zone.prop(_pz,'strain-70') = _strain70

    endif
endloop

end

What I’ve tried:

  • Verified zone group membership is correct
  • Used model large-strain off
  • Tried using pref = 100 kpa → still fails
  • Tried different parameter combinations (like _coh = 100kpa , _phi=30 etc.)→ still fails
  • Removed small-strain parameters → still fails
  • Tried math.cos/sin based Z formula → still fails

At this point I suspect some issue with the discrepancy of parameters or zone stress state, but it’s unclear why FLAC throws the “internal parameter calculation” error.
Any insight would be appreciated! I can provide the full script if needed. Thanks in advance.

– Emirhan

2

Bulk and Shear are read-only properties, you should not specify those two values.

3

With the input parameters, FLAC performed an internal oedometer simulation (a housekeeping check) that failed to converge to the specified K0 value. This suggests that the parameters are internally inconsistent. A quick review suggests that adjustments such as increasing the stiffness-oedometer-reference or decreasing K0 may help resolve the issue.