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

*ask,dtp,'please input increment of iterated load step',

*ask,fstp,'please input last load step',

!*applying loads start

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,,,

!*applying loads end

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 loads start

!*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