Dear all, 

I have another my stupid question, but I just can't find the error. I started to implement a UMAT for a rheological model. My first step is to implement the linear elasticity, which is orthotropic. So I created my model, tested it with the Abaqus standard materials which works. So now I want to use my UMAT (see below), but the analysis aborts with the error messages "Too many attempts made for this increment" and in the message file with "FORCE EQUILIBRIUM NOT ACHIEVED WITHIN TOLERANCE". Could someone please help?

SUBROUTINE UMAT(STRESS,STATEV,DDSDDE,SSE,SPD,SCD,
1 RPL,DDSDDT,DRPLDE,DRPLDT,
2 STRAN,DSTRAN,TIME,DTIME,TEMP,DTEMP,PREDEF,DPRED,CMNAME,
3 NDI,NSHR,NTENS,NSTATV,PROPS,NPROPS,COORDS,DROT,PNEWDT,
4 CELENT,DFGRD0,DFGRD1,NOEL,NPT,LAYER,KSPT,JSTEP,KINC)
C
INCLUDE 'ABA_PARAM.INC'
C
CHARACTER*80 CMNAME
DIMENSION STRESS(NTENS),STATEV(NSTATV),
1 DDSDDE(NTENS,NTENS),DDSDDT(NTENS),DRPLDE(NTENS),
2 STRAN(NTENS),DSTRAN(NTENS),TIME(2),PREDEF(1),DPRED(1),
3 PROPS(NPROPS),COORDS(3),DROT(3,3),DFGRD0(3,3),DFGRD1(3,3),
4 JSTEP(4)
 
PARAMETER (ZERO = 0.D0,ONE = 1.D0,TWO = 2.D0)
 
E1 = PROPS(1)
E2 = PROPS(2)
E3 = PROPS(3)
v12 = PROPS(4)
v13 = PROPS(5)
v32 = PROPS(6)
G12 = PROPS(7)
G13 = PROPS(8)
G23 = PROPS(9)
 
C --------------------------------------------------------------------  
C Stiffness Matrix  
v21 = v12*E2/E1
v31 = v13*E3/E1
v23 = v32*E2/E3
 
Y = ONE/(ONE-v12*v21-v23*v32-v31*v13-TWO*v21*v32*v13)
DO I = 1,NTENS
DO J = 1,NTENS
DDSDDE(I,J) = 0.0D0
END DO
END DO
 
DDSDDE(1,1) = E1*(ONE-v23*v32)*Y
DDSDDE(2,2) = E2*(ONE-v13*v31)*Y
DDSDDE(3,3) = E3*(ONE-v12*v21)*Y
DDSDDE(1,2) = E1*(v21+v31*v23)*Y
DDSDDE(1,3) = E1*(v31+v21*v32)*Y
DDSDDE(2,3) = E2*(v32+v12*v31)*Y 
DDSDDE(2,1) = E2*(v12+v32*v13)*Y
DDSDDE(3,1) = E3*(v13+v12*v23)*Y
DDSDDE(3,2) = E3*(v23+v21*v13)*Y 
DDSDDE(4,4) = G12
DDSDDE(5,5) = G13
DDSDDE(6,6) = G23
 
DO I = 1,NTENS
DO J = 1, NTENS
STRESS(I) = STRESS(I) * DDSDDE(I,J)*DSTRAN(J)
END DO
END DO
 
RETURN
END