有关单元生死和循环的问题
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
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