UDMGINI

User subroutine to define the damage initiation criterion.

User subroutine UDMGINI:

can be used to specify a user-defined damage initiation criterion;
allows the specification of more than one failure mechanism in an element, with the most severe one governing the actual failure;
can be used in combination with several Abaqus built-in damage evolution models, with each model corresponding to a particular failure mechanism;
will be called at all integration points of elements for which the material definition contains user-defined damage initiation criterion;
can call utility routine GETVRM to access material point data; and
is currently available only for enriched elements.

The following topics are discussed:

User subroutine interface
Variables to be defined
Variables that can be updated
Variables passed in for information
Example: User-defined damage initiation criterion with two different failure mechanisms

User subroutine interface

      SUBROUTINE UDMGINI(FINDEX,NFINDEX,FNORMAL,NDI,NSHR,NTENS,PROPS,
     1 NPROPS,STATEV,NSTATEV,STRESS,STRAIN,STRAINEE,LXFEM,TIME,
     2 DTIME,TEMP,DTEMP,PREDEF,DPRED,NFIELD,COORDS,NOEL,NPT,LAYER,
     3 KSPT,KSTEP,KINC,KDIRCYC,KCYCLELCF,TIMECYC,SSE,SPD,SCD,SVD,
     4 SMD,JMAC,JMATYP,MATLAYO,LACCFLA,CELENT,DROT,ORI)
C
      INCLUDE 'ABA_PARAM.INC'

C
      DIMENSION FINDEX(NFINDEX),FNORMAL(NDI,NFINDEX),COORDS(*),
     1 STRESS(NTENS),STRAIN(NTENS),STRAINEE(NTENS),PROPS(NPROPS),
     2 STATEV(NSTATV),PREDEF(NFIELD),DPRED(NFIELD),TIME(2),JMAC(*),
     3 JMATYP(*),DROT(3,3),ORI(3,3)

      user coding to define FINDEX,  and  FNORMAL

      RETURN
      END

Variables to be defined

FINDEX(NFINDEX): A Vector defining the indices for all the failure mechanisms.
FNORMAL(NDI, NFINDEX): An Array defining the normal direction to the fracture plane (three dimensions) or line (two dimensions) for each failure mechanism.

Variables that can be updated

STATEV: An array containing the user-defined solution-dependent state variables at this point. This array will be passed in containing the values of these variables at the start of the increment unless the values are updated in user subroutine USDFLD. They can be updated in this subroutine to their values at the end of the increment. You define the size of this array by allocating space for it (see Allocating space for more information).
SSE,SPD,SCD,SVD,SMD: Specific elastic strain energy, plastic dissipation, “creep” dissipation, viscous, and damage energy, respectively, passed in as the values at the start of the increment and should be updated to the corresponding specific energy values at the end of the increment. They have no effect on the solution, except that they are used for energy output.

Variables passed in for information

NFINDEX: Number of indices for all failure mechanisms.
NDI: Number of direct stress components at this point.
NSHR: Number of engineering shear stress components at this point.
NTENS: Size of the stress or strain component array (NRI + NSHR).
PROPS(NPROPS): User-specified array of material constants associated with this user-defined failure criterion.
NPROPS: User-defined number of material constants associated with this user-defined failure criterion.
NSTATV: Number of solution-dependent state variables associated with this material (specified when space is allocated for the array; see Allocating space).
STRESS(NTENS): An Array passed in as the current stress tensor. If a local orientation is used at the same point as user subroutine UDMGINI, the stress components will be in the local orientation; in the case of finite-strain analysis, the basis system in which stress components are stored rotates with the material.
STRAIN(NTENS): An Array containing the current total strains. If a local orientation is used at the same point as user subroutine UDMGINI, the strain components will be in the local orientation; in the case of finite-strain analysis, the basis system in which strain components are stored rotates with the material.
STRAINEE(NTENS): An Array containing the current elastic strains. If a local orientation is used at the same point as user subroutine UDMGINI, the elastic strain components will be in the local orientation; in the case of finite-strain analysis, the basis system in which elastic strain components are stored rotates with the material.
LXFEM: An integer flag to indicate an enriched element.
TIME(1): Value of step time at the beginning of the current increment.
TIME(2): Value of total time at the beginning of the current increment.
DTIME: Time increment.
TEMP: Temperature at the start of the increment.
DTEMP: Increment of temperature during the time increment.
PREDEF: An array containing the values of all of the user-specified predefined variables at this point at the start of the increment.
DPRED: An array containing the increments of all of the predefined variables during the time increment.
NFIELD: Number of user-specified predefined variables.
COORDS: An array containing the current coordinates of this point.
NOEL: Element number.
NPT: Integration point number.
LAYER: Layer number (for composite shells and layered solids).
KSPT: Section point number within the current layer.
KSTEP: Step number.
KINC: Increment number.
KDIRCYC: Iteration number in a direct cyclic analysis.
KCYCLELCF: Cycle number in a direct cyclic low-cycle fatigue analysis.
TIMECYC: Time period in one loading cycle in a direct cyclic analysis.
JMAC: Variable that must be passed into the GETVRM utility routine to access a material point variable.
JMATYP: Variable that must be passed into the GETVRM utility routine to access a material point variable.
MATLAYO: Variable that must be passed into the GETVRM utility routine to access a material point variable.
LACCFLA: Variable that must be passed into the GETVRM utility routine to access a material point variable.
CELENT: Characteristic element length, which is a typical length of a line across an element for a first-order element; it is half of the same typical length for a second-order element. For beams and trusses it is a characteristic length along the element axis. For membranes and shells it is a characteristic length in the reference surface. For axisymmetric elements it is a characteristic length in the (r, z) plane only. For cohesive elements it is equal to the constitutive thickness.
DROT(3,3): Rotation increment matrix. This matrix represents the increment of rigid body rotation of the basis system in which the components of stress (STRESS) and strain (STRAIN) are stored. It is provided so that vector- or tensor-valued state variables can be rotated appropriately in this subroutine: stress and strain components are already rotated by this amount before UDMGINI is called. This matrix is passed in as a unit matrix for small-displacement analysis and for large-displacement analysis if the basis system for the material point rotates with the material (as in a shell element or when a local orientation is used).
ORI(3,3): Material orientation with respect to global basis.

Example: User-defined damage initiation criterion with two different failure mechanisms

As a simple example of the coding of user subroutine UDMGINI, consider a damage initiation criterion based on two different failure mechanisms: the maximum principal stress and the quadratic traction-interaction.

      SUBROUTINE UDMGINI(FINDEX,NFINDEX,FNORMAL,NDI,NSHR,NTENS,PROPS,
     1 NPROPS,STATEV,NSTATEV,STRESS,STRAIN,STRAINEE,LXFEM,TIME,
     2 DTIME,TEMP,DTEMP,PREDEF,DPRED,NFIELD,COORDS,NOEL,NPT,
     3 KLAYER,KSPT,KSTEP,INC,KDIRCYC,KCYCLELCF,TIMECYC,SSE,SPD,
     4 SCD,SVD,SMD,JMAC,JMATYP,MATLAYO,LACCFLA,CELENT,DROT,ORI)
C
      INCLUDE 'ABA_PARAM.INC'
CC
      DIMENSION FINDEX(NFINDEX),FNORMAL(NDI,NFINDEX),COORDS(*),
     1 STRESS(NTENS),STRAIN(NTENS),STRAINEE(NTENS),PROPS(NPROPS), 
     2 STATEV(NSTATEV),PREDEF(NFIELD),DPRED(NFIELD),TIME(2),
     3 JMAC(*),JMATYP(*),DROR(3,3),ORI(3,3)
      
      DIMENSION PS(3), AN(3,3), WT(6)
      PS(1)=0.0
      PS(2)=0.0
      PS(3)=0.0
C
C ROTATE THE STRESS TO GLOBAL SYSTEM IF THERE IS ORIENTATION
C
      CALL ROTSIG(STRESS,ORI,WT,1,NDI,NSHR)
C
C MAXIMUM PRINCIPAL STRESS CRITERION
C
      CALL SPRIND(WT,PS,AN,1,NDI,NSHR)
      SIG1 = PS(1)
      KMAX=1
      DO K1 = 2, NDI
         IF(PS(K1).GT.SIG1) THEN
            SIG1 = PS(K1)
            KMAX = K1
         END IF
      END DO
      FINDEX(1) = SIG1/PROPS(1)
      DO K1=1, NDI
      	FNORMAL(K1,1) = AN(KMAX,K1)
      END DO
C
C QUADRATIC TRACTION-INTERACTION CRITERION
c
      FINDEX(2)=(STRESS(1)/PROPS(2))**2.0+(STRESS(NDI+1)/
     $     PROPS(3))**2.0+(STRESS(NDI+2)/PROPS(4))**2.0
C
      FINDEX(2)=sqrt(FINDEX(2))
C	
      DO K1=1, NDI
      	FNORMAL(K1,2)=ORI(K1,1)
      END DO
      RETURN
      END