为什么卸载后，残余应力不为零？

/PREP7
ET,1,PLANE42,,,2
MP,EX,1,1.25E5
MP,NUXY,1,0.3
TB,MISO,1,1,7
TBPT,DEFI,2E-3,250
TBPT,DEFI,4E-3,268
TBPT,DEFI,6E-3,278
TBPT,DEFI,8E-3,284
TBPT,DEFI,10E-3,288
TBPT,DEFI,12e-3,290
TBPT,DEFI,200E-3,650
TBPLOT,MISO,1
cyl4,,,500,,518,90
lsel,s,,,2,4,2
lesize,all,,,4
lsel,s,,,1,3,2
lesize,all,,,200
alls
amesh,all
/solu
nlgeom,on
outres,all,all
autots,on
nsel,s,loc,x,0
d,all,ux
nsel,s,loc,y,0
d,all,uy
allsel
sfl,3,pres,2
nsub,20,50,5
solve
/solu
nlgeom,on
outres,all,all
autots,on
sfl,3,pres,4
nsub,20,50,5
solve
/solu
nlgeom,on
outres,all,all
autots,on
sfl,3,pres,6
nsub,20,50,5
solve
/solu
nlgeom,on
outres,all,all
autots,on
sfl,3,pres,8
nsub,20,50,5
solve
/solu
nlgeom,on
outres,all,all
autots,on
sfl,3,pres,9.68
nsub,20,50,5
solve
/solu
nlgeom,on
outres,all,all
autots,on
sfl,3,pres,10.5
nsub,100,200,5
solve
/solu
nlgeom,on
outres,all,all
autots,on
sfl,3,pres,11.5
nsub,100,200,5
solve
/solu
nlgeom,on
outres,all,all
autots,on
sfl,3,pres,12.5
nsub,100,200,5
solve
/solu
nlgeom,on
outres,all,all
autots,on
sfl,3,pres,13.5
nsub,100,200,5
solve
/solu
nlgeom,on
outres,all,all
autots,on
sfl,3,pres,14
nsub,100,200,5
solve
/solu
nlgeom,on
outres,all,all
autots,on
sfl,3,pres,14.5
nsub,100,200,5
solve
/solu
nlgeom,on
outres,all,all
autots,on
sfl,3,pres,12
nsub,50,100,5
solve
/solu
nlgeom,on
outres,all,all
autots,on
sfl,3,pres,10
nsub,100,200,5
solve
/solu
nlgeom,on
outres,all,all
autots,on
sfl,3,pres,0
nsub,50,100,5
solve
/post1
rsys,1
set,last
ples,nl,epeq
/post26
eSOL,2,1,1,S,eqv
ESOL,3,1,1,EPEL,y,
ESOL,4,1,1,EPPL,y,
ADD,5,3,4, , , , ,1,1,0,
xvar,5
/axlab,x,total-strain
/axlab,y,s-eqv
plvar,2
卸载后，剩余的等效应力有80多MPa，估计是卸载的过程不对，请教！