We are trying to code a UMAT subroutine in ABAQUS. It is clear from the
manuals that we need to supply the Cauchy stress, but I am not sure what to
give for the stiffness matrix, DDSDDE. Does anyone out there know an exact
expression that we can use to derive this? For example, in several books I
have seen an expression that gives the elasticity tensor equal to the
derivative of the Second Piola-Kirchoff stress tensor with respect to the
left Cauchy-Green strain tensor (see Holzapfel's "Nonlinear Solid
Mechanics" (2000), sec. 6.6), but I tried this and it doesn't seem to work.
Any advice would be most appreciated.
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