Viscoelastic Material
Constitutive Laws
This MPM material has separate constitutive laws for deviatoric stress and pressure.
Deviatoric Constitutive Law
The deviatoric constitutive law is always a small-strain, linear viscoelastic material with time-dependent shear modulus, G(t), which is given by a sum of n exponentials:
[math]\displaystyle{ G(t) = G_0 + \sum_{i=1}^n G_i e^{-t/\tau_i} }[/math]
Here G0 is the long-time shear modulus and the short-time shear modulus is the sum:
[math]\displaystyle{ G(0) = \sum_{i=0}^n G_i }[/math]
The updates from components of the deviatoriic stress become
[math]\displaystyle{ ds_{ij} = 2\left( G(0) de_{ij} - \sum_{k=1}^n G_k d\alpha_{ij,k} \right) }[/math]
where αij,k are a series of internal variables that are tracked in history variables on each particle.
Pressure Constitutive Law
The pressure constitutive law has two options. The first in to use a small strain linear elastic law with pressure increment of
[math]\displaystyle{ dP = -K(d\varepsilon_{xx} + d\varepsilon_{yy} + d\varepsilon_{zz} - 3d\varepsilon_{res}) }[/math]
where K is the time-independent bulk modulus and other terms are applied or residual strain increments.
The second option is to use then [Isotropic, Hyperelastic-Plastic Mie-Grüneisen Material#Mie-Grüneisen Equation of State|Mie-Grüneisen equation of state (MGEOS)[.
Material Properties
The unusual task for this material is to use multiple Gk and tauk properties (all with the same property name) to enter a material with multiple relaxation times.
| Property | Description | Units | Default |
|---|---|---|---|
| ntaus | The number of relaxation times. This property is only needed in XML files and must come before any Gk or tauk properties. In scripted files, the number is automatically determined from the number of relaxation times you provide. | none | none |
| G0 | The long term (or fully-relaxed) shear modulus | pressure units | 0 |
| Gk | The shear modulus for the next relaxation time. Enter multiple Gk properties to have multiple relaxation times. | pressure units | none |
| tauk | The the next relaxation time. Enter multiple tauk properties to have multiple relaxation times. | time units | none |
| alpha | Thermal expansion coefficient | ppm/K | 40 |
| pressureLaw | Picks the constitutive law use for time independent pressure. The options are 0 to linear elastic law and 1 to use MGEOS equation of state. | none | 0 |
| K | Time-independent bulk modulus (when using linear elastic law) | pressure units | none |
| C0 | The bulk wave speed | alt velocity units | 4004 |
| gamma0 | The γ0 parameter | none | 1.64 |
| S0 | The S0 parameter | none | 1.35 |
| S1 | The S1 parameter | none | 0 |
| S2 | The S2 parameter | none | 0 |
| S3 | The S3 parameter | none | 0 |
| (other) | Properties common to all materials | varies | varies |
The total number of Gk and tauk properies must be equal. In XML files, that total number must match the supplied ntaus property.
This material can emulate a liquid (i.e., similar to a Tait Liquid Material) by setting G0 to zero, using a single relaxation time with tauk short (on time scale of the simulation), and setting the one Gk modulus (in pressure units) to:
[math]\displaystyle{ G_1 = 10^{-9}\left({\eta\over \tau_1}\right) }[/math]
where [math]\displaystyle{ \eta }[/math] is desired viscosity of the liquid in cP and [math]\displaystyle{ \tau_1 }[/math] is the chosen relaxation time in seconds.
History Variables
This material tracks internal history variables (one for each relaxation time and each component of stress) for implementation of linear viscoelastic properties, but currently none of these internal variables are available for archiving.