有关单元生死和循环的问题

zhchp

我做的是一个简单的结构分析，将结构中应力小的单元杀死后，重新计算结构的应力，再将结构中应力小的单元杀死，如此反复，直到满足约束条件和收敛条件。但为什么在第三次循环时，结构中所有单元的等效应力都近似为零？（该结构是一薄壁箱体中密布筋板，要求KILL筋板中应力小的单元）请高手帮忙解答！
程序如下：
*CREAT,FENXI1,mac
/SOL
ANTYPE,0
NLGEOM,ON
NROPT,FULL,,ON

ALLSEL,ALL
SOLVE   
FINISH
*END

*CREAT,FENXI2,mac  
/POST1
ETABLE,SEQV,S,EQV
ESORT,ETAB,SEQV,1,1 　　　　　　　
*GET,SMAX,SORT,0,MAX
ESEL,S,REAL,,1
ETABLE,SEQV1,S,EQV
ESEL,S,ETAB,SEQV1,,0.01*SMAX
FINISH
/SOL
EKILL,ALL
FINISH
*END

FINISH
/CLEAR

/PREP7  
  
ET,1,SHELL63

R,1,0.01, , , , , ,

R,2,0.01, , , , , ,
  
MP,EX,1,2E11   
MP,PRXY,1,0.3

BLOCK,0,1.8,0,0.3,0,0.2,
   
VDELE,       1  
  
AGEN,36,5, , ,0.05, , , ,0   

APTN,ALL   
APLOT   
ASEL,S,LOC,X,-0.01,0.01
ASEL,A,LOC,X,1.79,1.81  
ASEL,A,LOC,Y,-0.01,0.01
ASEL,A,LOC,Y,0.29,0.31  
ASEL,A,LOC,Z,-0.01,0.01
ASEL,A,LOC,Z,0.19,0.21  
APLOT   
TYPE,   1   
MAT,       1
REAL,       2   
ESYS,       0   
SECNUM,

AESIZE,ALL,0.05,   
MSHAPE,0,2D
MSHKEY,0

AMESH,ALL   

ASEL,INVE   

TYPE,   1   
MAT,       1
REAL,       1   
ESYS,       0   
SECNUM,

AESIZE,ALL,0.05,   

AMESH,ALL   

DL,9, ,ALL,
DL,10, ,ALL,
   
ALLSEL,ALL  

SFA,75,1,PRES,500000  

SFA,143,1,PRES,700000
FINISH

*DIM,SM,,50000
*DO,I,2,50000,1
  FENXI1
  FENXI2
  /POST1
  ESEL,ALL  
  SM(I)=SMAX
  SM(1)=0
  *IF,SM(I),GE,5000000,THEN
    A=SM(I)
    *EXIT
  *ELSEIF,SM(I)-SM(I-1),ABLT,5
    B=SM(I)
    *EXIT
   *ELSE
    *CYCLE
  *ENDIF
*ENDDO
ESEL,S,REAL,,1
ESEL,R,LIVE
EPLOT
/VIEW,1,1,1,1   
/ANG,1  
/REP,FAST