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from umatgen import *

GST=umat(Implicit=True, project='Tets_GST',
         outfile="GST_Incp", outroutine="GST_Incp")

C_Bar=GST.C_Bar

# calculate the isochoric GST invariant
E_i=I4_GST_kappa(C_Bar)

var('parK1, parK2, parK, parC')
# Isochoric strain energy function for dispersed fiber
W_isc=parK1/(2*parK2)*(exp(parK2*(E_i)^2)-1)

# Strain energy function for extra-cellular matrix
W_em=parC/2*(I1(C_Bar)-3)

# Volumetric strain energy function
W_vol=parK*1/2*(GST.Jacobian-1)^2

# Setting the strain energy function
GST.W=W_isc+W_vol+W_em
# Output the source code
GST.write()