Problem is using static/riks option

sushil — Tue, 29 Apr 2008 21:52:22 -0400

I have some problem in defining the material properties using UMAT. I want to simulate the cubic stress-strain traction-sepration law. Solution doesn't converge after max load is applied (i.e zero slope of stress-strain plot). I am using riks solver. I have written my stress -strain relationship as below in UMAT file. Please tell me how to converge the solution even after zero slope of stress-strain relationship. Can i make my load factor(lambda) negative in riks option.

SMAX=5
EMAX=0.001
STRAIN=STRAN(1)+DSTRAN(1)
ENOM=STRAIN/EMAX
IF(STRAIN.LT.0.00) THEN
STRESS(1)=(27/4)*(SMAX/EMAX)

*ENOM
ELSE IF(STRAIN.LE.EMAX) THEN
STRESS(1)=(27/4)*(SMAX/EMAX)*(1-2*ENOM+ENOM**2)*STRAIN
ELSE
STRESS(1)=1.0D-20
ENDIF
STRESS(2)=STRESS(2)+200E3*DSTRAN(2)
STRESS(4)=STRESS(4)+200E3*DSTRAN(4)/2/1.3

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Problem is using static/riks option