ANSYS and arc length method for plastic collapse history analysis

I am computing the plastic collapse load of pressure vessel by using ANSYS, and it is no difficulty in calculating the plastic deformation from zero load step to the load steps before the numerical divergency occurs by using Newton-Raphson method. Now I have difficulty in using the arc length method of ANSYS to compute the plastic deformation of the pressure vessel from zero load step until after the numerical divergency occurs, namely the load versus deformation history including the post collapse failure stage. The difficulty is that the load step with small load values will not converge toward the load value specified and tend to decrease to negative values by using arc length method, contrast to the Newton-Raphson method which converges toward the specified load values.

the FEA model of the presuure vessel and the pre collapse load-deformation curve are in the attachement.

The main APDL code of My FEA analysis model is as follows.

/title,plastic collapse analysis of pressure vessel
!*material property parameters
pressure=4.585 !internal pressure of the vessel
sts1=124.1 !yield strength（MPa）
C2=661 !plastic modulus
ex1=6.9e4 !elastic modulus（MPa）
!*structural geometrical parameters
!*****************************************
!*modelling of the pressure vessel
!*meshing
ET,1,SHELL43
ET,2,MASS21
R,3,0,0,0,0,0,0,
R,1,TH, , , , , ,
R,2,TN, , , , , ,
!*****************************************
!*billinear material
TB,Biso,1,1,,0,
TBTEMP,reft1,1
TBDATA,1,sts1,c2,,,,
!*****************************************
!*the initial load step analysis by using arc length method
/solu
antype,static
nlgeom,on
arclen,on,4
rescontrol,,all,1,2
outres,all,all
solcontrol,1,,,
autots,on
pressure=fstp*dtp
*SET,pressure1,DN*pressure/4/TN
!*applying surface pressure
allsel,all,area
sfa,all,1,pres,pressure,
!*
lsel,s,line,,15,18,3,
lsel,a,line,,35,37,2,
sfl,all,pres,-pressure1,,,
ALLSEL ，，
sbctran
solve
save
fini
!*****************************************
!*the multi frame restart analysis by using arc length method
*do,istp,2,fstp,1
astp=astp+1
*if,istp,lt,rstp,then
nsbstp=5
*else
nsbstp=100
*endif
!*****************************************
!*
/FILNAME,arcp,
resume,
/solu
antype,,rest
pressure=dtp*astp
*SET,pressure1,DN*pressure/4/TN
nsubst,nsbstp,,
!*applying surface pressure
allsel,all,area
sfa,all,1,pres,pressure,
!*
lsel,s,line,,15,18,3,
lsel,a,line,,35,37,2,
sfl,all,pres,-pressure1,,,
!*
ALLSEL ，，
sbctran
solve
save
fini
*enddo

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