I need to implement a constitutive model in Abaqus subroutine. It is used for high strain rate and large plastic deformation in which elastic strain could be omitted.

Now Im perplexed by some questions:

1: Is it ok to just define the material plasticity model in VUHARD and input E and v in material Elastic definition box? Or, do I need to write VUMAT to define the yield criterion and update the stresses explicitly?

///////////////////////////////////////////////////////////////////

2: If only VUHARD is enough, could the yield strength updated explicitly by the analytical model? Suppose we use a simplified model: FlowStress=K*(PlasticStrain^n)*(StrainRate^m), could we write the code as:

yield(k)=K*(eqps(k)**n))*(eqpsRate(k)**m)

or do we have to use Newton Iteration like:

IF(EQPLAS .GT. STATEV(1)) THEN

DO 10 KEWTON=1,NEWTON

c DF is the derivative of F with respect to yield strength

F=YieldOld-K*(eqps(k)**n))*(eqpsRate(k)**m)

DF=1-K*(eqps(k)**n))*(eqpsRate(k)**m)

DeltaYield=YieldOld-F/DF

IF(ABS(F).LT.TOLER) GOTO 20

10 CONTINUE

3: It seems that in VUHARD, the derivatives of the yield stress (or yield surface size in combined hardening models) must be defined with respect to the appropriate independent variables, such as strain, strain rate, and temperature.i.e. dyieldDeqps(nblock,1)/dyieldDeqps(nblock,2), etc. Othwise, the material looks to be very soft under shock pressure.

If use a simplied model: FlowStress=K*(PlasticStrain^n)*(StrainRate^m), could we define dyieldDeqps(nblock,1)=K*n*(PlasticStrain^(n-1))*(StrainRate^m) and dyieldDeqps(nblock,2)=K*m*(PlasticStrain^n)*(StrainRate^(m-1))?

Thank you very much for your help!

Stanley