c
c User subroutine VFRIC_COEF to define isotropic friction coefficient
c
      subroutine vfric_coef (
c Write only - 
     *   fCoef, fCoefDeriv, 
c Read only - 
     *   nBlock, nProps, nTemp, nFields, 
     *   jFlags, rData, 
     *   surfInt, surfSlv, surfMst, 
     *   props, slipRate, pressure, 
     *   tempAvg, fieldAvg )
c     
c Write-only variable fCoefDeriv is not used in Explicit
c
c Surface names surfSlv and surfMst are not available in 
c general contact (set to blank).
c
      include 'vaba_param.inc'
      dimension fCoef(nBlock), 
     *   fCoefDeriv(nBlock,3),
     *   props(nProps),
     *   slipRate(nBlock),
     *   pressure(nBlock),
     *   tempAvg(nBlock), 
     *   fieldAvg(nBlock,nFields)
c
      parameter( iKStep   = 1,
     *           iKInc    = 2,
     *           nFlags   = 2 )
      parameter( iTimStep = 1,
     *           iTimGlb  = 2,
     *           iDTimCur = 3,
     *           nData    = 3 )
      dimension jFlags(nFlags), rData(nData)
      character*80 surfInt, surfSlv, surfMst 
c
      us = props(1)
      uk = props(2)
      dc = props(3)
      du = us - uk
      
      do k=1, nBlock
         fCoef(k) = uk + du*exp( -dc*slipRate(k) )
      enddo
c
      return
      end