[samuel_]

Hello,
I have the datas of 7 relaxation tests at 7 different temperatures (the names of the files are T0.txt, T1.txt .... T6.txt) where T0=250.65K is the reference temperature for the WLF shift function.

I want to calculate the values of the constants (C1 and C2) in the WLF shift function. To do that I followed this tutorial : https://www.sharcnet.ca/Software/Ans...tml#vmcftable1 (section 6.2. Viscoelastic Material Curve Fitting). However, I obtain bad results (negatives constants and bad fitting of the shift function) and when I change just one line on my code the results change a lot.

Here is my code :



/prep7
tbft,eadd,1,sdec,T0,txt
tbft,eadd,1,sdec,T1,txt
tbft,eadd,1,sdec,T2,txt
tbft,eadd,1,sdec,T3,txt
tbft,eadd,1,sdec,T4,txt
tbft,eadd,1,sdec,T5,txt
tbft,eadd,1,sdec,T6,txt

tbft,fcase,1,NEW,PVHE,fit_WLF
tbft,fadd,1,VISCO,pshear,3
tbft,fadd,1,VISCO,pbulk,3
tbft,fadd,1,VISCO,shift,wlf
tbft,fcase,1,FINI

tbft,set,1,CASE,fit_WLF,,15,250.65
tbft,set,1,CASE,fit_WLF,,3,1000
tbft,set,1,CASE,fit_WLF,,5,50000
tbft,set,1,CASE,fit_WLF,,7,250000
tbft,set,1,CASE,fit_WLF,,10,1000
tbft,set,1,CASE,fit_WLF,,12,50000
tbft,set,1,CASE,fit_WLF,,14,250000
tbft,fix,1,CASE,fit_WLF,,15,1

!tbft,set,1,visco,temp,,tdep,1
tbft,set,1,case,fit_WLF,,comp,pshear
tbft,set,1,case,fit_WLF,,tref,250.65
tbft,solve,1,CASE,fit_WLF,,1,10000,0.0001,0.0001
tbft,set,1,case,fit_WLF,,comp,pbulk
tbft,set,1,case,fit_WLF,,tref,250.65
tbft,solve,1,CASE,fit_WLF,,1,10000,0.0001,0.0001
tbft,set,1,case,fit_WLF,,comp,pvhe
tbft,solve,1,CASE,fit_WLF,,1,10000,0.0001,0.0001

!tbft,set,1,visco,temp,,tdep,1
tbft,set,1,case,fit_WLF,,comp,pshear
tbft,set,1,case,fit_WLF,,tref,253.15
tbft,solve,1,CASE,fit_WLF,,1,10000,0.0001,0.0001
tbft,set,1,case,fit_WLF,,comp,pbulk
tbft,set,1,case,fit_WLF,,tref,253.15
tbft,solve,1,CASE,fit_WLF,,1,10000,0.0001,0.0001
tbft,set,1,case,fit_WLF,,comp,pvhe
tbft,solve,1,CASE,fit_WLF,,1,10000,0.0001,0.0001

!tbft,set,1,visco,temp,,tdep,1
tbft,set,1,case,fit_WLF,,comp,pshear
tbft,set,1,case,fit_WLF,,tref,255.65
tbft,solve,1,CASE,fit_WLF,,1,10000,0.0001,0.0001
tbft,set,1,case,fit_WLF,,comp,pbulk
tbft,set,1,case,fit_WLF,,tref,255.65
tbft,solve,1,CASE,fit_WLF,,1,10000,0.0001,0.0001
tbft,set,1,case,fit_WLF,,comp,pvhe
tbft,solve,1,CASE,fit_WLF,,1,10000,0.0001,0.0001

!tbft,set,1,visco,temp,,tdep,1
tbft,set,1,case,fit_WLF,,comp,pshear
tbft,set,1,case,fit_WLF,,tref,263.15
tbft,solve,1,CASE,fit_WLF,,1,10000,0.0001,0.0001
tbft,set,1,case,fit_WLF,,comp,pbulk
tbft,set,1,case,fit_WLF,,tref,263.15
tbft,solve,1,CASE,fit_WLF,,1,10000,0.0001,0.0001
tbft,set,1,case,fit_WLF,,comp,pvhe
tbft,solve,1,CASE,fit_WLF,,1,10000,0.0001,0.0001

!tbft,set,1,visco,temp,,tdep,1
tbft,set,1,case,fit_WLF,,comp,pshear
tbft,set,1,case,fit_WLF,,tref,273.15
tbft,solve,1,CASE,fit_WLF,,1,10000,0.0001,0.0001
tbft,set,1,case,fit_WLF,,comp,pbulk
tbft,set,1,case,fit_WLF,,tref,273.15
tbft,solve,1,CASE,fit_WLF,,1,10000,0.0001,0.0001
tbft,set,1,case,fit_WLF,,comp,pvhe
tbft,solve,1,CASE,fit_WLF,,1,10000,0.0001,0.0001

!tbft,set,1,visco,temp,,tdep,1
tbft,set,1,case,fit_WLF,,comp,pshear
tbft,set,1,case,fit_WLF,,tref,283.15
tbft,solve,1,CASE,fit_WLF,,1,10000,0.0001,0.0001
tbft,set,1,case,fit_WLF,,comp,pbulk
tbft,set,1,case,fit_WLF,,tref,283.15
tbft,solve,1,CASE,fit_WLF,,1,10000,0.0001,0.0001
tbft,set,1,case,fit_WLF,,comp,pvhe
tbft,solve,1,CASE,fit_WLF,,1,10000,0.0001,0.0001

!tbft,set,1,visco,temp,,tdep,1
tbft,set,1,case,fit_WLF,,comp,pshear
tbft,set,1,case,fit_WLF,,tref,298.15
tbft,solve,1,CASE,fit_WLF,,1,10000,0.0001,0.0001
tbft,set,1,case,fit_WLF,,comp,pbulk
tbft,set,1,case,fit_WLF,,tref,298.15
tbft,solve,1,CASE,fit_WLF,,1,10000,0.0001,0.0001
tbft,set,1,case,fit_WLF,,comp,pvhe
tbft,solve,1,CASE,fit_WLF,,1,10000,0.0001,0.0001

!tbft,set,1,case,fit_WLF,,tref,250.65
!tbft,set,1,CASE,fit_WLF,,comp,
!tbft,solve,1,CASE,fit_WLF,,1,10000,0.0001,0.0001
tbft,fset,1,CASE,fit_WLF


If someone can help me, it would be very nice