Bistable Isotropic Material

Constitutive Law

This MPM material (or FEA material) is two small-strains materials (each with same constitutive law as an isotropic material). The two material states are linked by transition rules and the transition between the two states can be reversible or irreversible.

The transition between the two states is determined either by a "dilation" rule, a "distortion" rule, or a "Von Mises" stress rule (as determined by the transition property). With a dilation rule, the transition occurs when the volumetric strain

      [math]\displaystyle{ {\Delta V\over V} = \varepsilon_{xx}+\varepsilon_{yy}+\varepsilon_{zz} }[/math]

reaches the entered critical value. WIth a distortion rule, the transition occurs when the second strain invariant:

      [math]\displaystyle{ I_2 = \sqrt{{1\over 2}\sum_{i,j}\varepsilon_{ij}'\varepsilon_{ij}'} }[/math]

where [math]\displaystyle{ \varepsilon_{ij}' }[/math] is deviatoric strain tensor, reaches a critical value. By a Von Mises stress rule, the transition occurs when the Von Mises stress

      [math]\displaystyle{ \sigma_{VM} = \sqrt{\sum_{i,j}\sigma_{ij}'\sigma_{ij}'} }[/math]

where [math]\displaystyle{ \sigma_{ij}' }[/math] is is deviatoric stress, reaches a critical value. When using a dilation rule, the new stress-strain relation can include a changed offset in volumetric strain corresponding to stress-free conditions at a non-zero dilation relative to the initial state (see DeltaVOffset property). This change normally leads to an instantaneous change in stress upon transition. When using a distortion or Von Mises stress rule, the offset is ignored and the change is only a change in slope of mechanical properties.

Material Properties

Property	Description	Units	Default
K0	Bulk modulus for initial state	MPa	none
G0	Shear modulus for initial state	MPa	none
a0	Shear modulus for initial state	MPa	none

Property	Description	Units	Default
beta	Solvent expansion coefficient	1/(wt fraction)	0
D	Solvent diffusion constant	mm2/sec	0
kCond	Thermal conductivity	W/(m-K)	0
(other)	Properties common to all materials	varies	varies

History Variables

None