UXFEMCRACK

User subroutine to define the interactions between the cracked element surfaces for enriched elements using a small sliding formulation.

User subroutine UXFEMCRACK:

can be used to specify the crack opening offset for contact of cracked element surfaces;
can also be used to define the mechanical (normal and shear) interactions between the cracked element surfaces;
is called at all the points of intersection between the crack surface and the element edges;
can include the effects of slurry concentration and the proppant particle size; and
is available only for contact of cracked enriched element surfaces using a small-sliding formulation.

This page discusses:

User Subroutine Interface
Variables to Be Defined
Variables That Can Be Updated
Variables Passed in for Information
Example: User-Defined Offset for Enriched Cracked Element Surfaces

User Subroutine Interface

 SUBROUTINE UXFEMCRACK( OFFSETOLD, OFFSETNEW, DELTA,
     1 OPENMAX, NOEL, NPT, COORDS, KSTEP, KINC, TIME1, DTIME1, TEMP, DTEMP,
     2 PREDEF, DPRED, NFIELD, STATEV, NSTATV, PROPS, NPROPS, JMAC, JMATYP,
     3 MATLAYO, LACCFLA, SDEG, GAPINI, DV, STRESSOLD, STRESSNEW,
     4 STIFFNESS, DIAMETERP, CV, JFLGOPT, NTENS, NDIR, NSHR, MCRD,
     5 NIARRAY, I_ARRAY, NRARRAY, R_ARRAY, NCARRAY, C_ARRAY)
C
      INCLUDE 'ABA_PARAM.INC'
C
      DIMENSION COORDS(*), TIME1(*), TEMP(*), JMAC(*), JMATYP(*), STRESSOLD(MCRD),
     1 STRESSNEW(MCRD), STIFFNESS(MCRD,MCRD), DELTA(*), PROPS(NPROPS),
     2 STATEV(NSTATV), PREDEF(NFIELD), DPRED(NFIELD), I_ARRAY(NIARRAY),
     3 R_ARRAY(NRARRAY), C_ARRAY(NCARRAY)

C
      user coding to define JFLGOPT, OFFSETNEW, STIFFNESS, STRESSNEW, 
      and, if necessary, STATEV

      RETURN
      END

Variables to Be Defined

OFFSETNEW

The offset at the end of the increment, $δ_{o f f}$ . If this value is not defined, it is assumed to be zero.

JFLGOPT

An integer flag that must be set to either 1 or 2.

Set JFLGOPT=1 to update OFFSETNEW and the penalty stiffness matrix, STIFFNESS.

Set JFLGOPT=2 to update OFFSETNEW; the penalty stiffness matrix, STIFFNESS; and the stress at the end of the increment, STRESSNEW.

Variables That Can Be Updated

STIFFNESS

Penalty stiffness of the cracked surfaces.

STIFFNESS is initialized to a penalty stiffness, based on the bulk stiffness of the underlying elements and the element characteristic length, before UXFEMCRACK is called. Abaqus uses these values for all subsequent calculations if STIFFNESS is not updated in UXFEMCRACK.

STRESSNEW

Traction vector on the cracked surfaces.

This variable is initialized to the nominal traction vector at the beginning of the increment and can be updated to the corresponding values at the end of the increment.

If you set JFLGOPT=1, Abaqus computes STRESSNEW based on the offset and the stiffness matrix, STIFFNESS. Any updates you make to STRESSNEW in this case are ignored.

If you set JFLGOPT=2, you must update STRESSNEW.

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. The size of the array is defined as described in Allocating Space for Element Solution-Dependent Variables.

Variables Passed in for Information

OFFSETOLD: Offset at the end of the previous increment.
OPENMAX: $δ_{o f f}^{\max}$ , the maximum value of $δ_{o f f}$ at an interaction point between the crack surface and the element edge over the deformation history.
NDIR: Number of direct components of the underlying enriched element at this contact point.
NSHR: Number of shear components of the underlying enriched element at this contact point.
NTENS: Number of total components (NDI + NSHR) of the underlying enriched element at this contact point.
MCRD: Number of coordinate directions at the contact point.
DELTA: Current crack opening
NOEL: Element number.
NPT: Integration point number.
COORDS: An array containing the current coordinates of this point.
KSTEP: Step number.
KINC: Increment number.
TIME1(1): Value of step time at the beginning of the current increment.
TIME1(2): Value of total time at the beginning of the current increment.
DTIME1: 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.
NSTATV: Number of solution-dependent state variables that are associated with this material type (defined as described in Allocating Space for Element Solution-Dependent Variables).
PROPS(NPROPS): User-specified array of material constants associated with this material.
NPROPS: User-defined number of material constants associated with this material.
JMAC: Variable that must be passed into the GETVRM utility routine to access an output variable.
JMATYP: Variable that must be passed into the GETVRM utility routine to access an output variable.
MATLAYO: Variable that must be passed into the GETVRM utility routine to access an output variable.
LACCFLA: Variable that must be passed into the GETVRM utility routine to access an output variable.
SDEG: Damage variable at the end of the last increment.
STRESSOLD: Stresses at the end of previous increment.
DV: Integration point volume.
DIAMETERP: Diameter of slurry particles
CV: Slurry concentration at the end of previous increment.
GAPINI: Initial crack opening.
NIARRAY: Size of array I_ARRAY.
I_ARRAY: Not used.
NRARRAY: Size of array R_ARRAY.
R_ARRAY: Not used.
NCARRAY: Size of array C_ARRAY.
C_ARRAY: Not used.

Example: User-Defined Offset for Enriched Cracked Element Surfaces

The following is a simple example of how to specify the offset for cracked element surfaces by using UXFEMCRACK:

SUBROUTINE UXFEMCRACK( OFFSETOLD, OFFSETNEW, DELTA,
     1     OPENMAX, NOEL, NPT, COORDS, KSTEP, KINC, TIME1, DTIME1, TEMP, DTEMP,
     2     PREDEF, DPRED, NFIELD, STATEV, NSTATV, PROPS, NPROPS, JMAC, JMATYP,
     3     MATLAYO, LACCFLA, SDEG, GAPINI, DV, STRESSOLD, STRESSNEW,
     4     STIFFNESS, DIAMETERP, CV, JFLGOPT, NTENS, NDIR, NSHR, MCRD,
     5     NIARRAY, I_ARRAY, NRARRAY, R_ARRAY, NCARRAY, C_ARRAY )

      INCLUDE 'ABA_PARAM.INC'

C
      DIMENSION COORDS(*), TIME1(*), TEMP(*), JMAC(*), JMATYP(*), STRESSOLD(*),
     1 STRESSNEW(MCRD), STIFFNESS(MCRD,MCRD), DELTA(*), PROPS(NPROPS),STATEV(NSTATV),
     2 PREDEF(NFIELD),DPRED(NFIELD), I_ARRAY(NIARRAY), R_ARRAY(NRARRAY), 
     3 C_ARRAY(NCARRAY)

CC   THIS EXAMPLE DEMONSTRATES HOW TO SPECIFY AN OFFSET IN STEP 3 TO KEEP THE
CC   FRACTURE “PROPPED OPEN” WHEN EXTERNAL LOADING CONDITIONS WOULD
CC   OTHERWISE HAVE LED TO CLOSURE OF THE FRACTURE 


     JFLGOPT = 1
     IF( KSTEP .LE. 2 ) THEN
         OFFSETNEW = 0.0
     ELSE
        OFFSETNEW = 0.15
	IF( OPENMAX .gt. OFFSETNEW .and. DELTA(1) .gt. OFFSETNEW ) THEN
	   DO II = 1,MCRD
             STIFFNESS(II,II) = 0.0
           END DO     
        END IF
     END IF
C
     RETURN
     END