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Mechanical viscosity:

*Material, ...
*Mechanical viscosity = <mechanical viscosity model>
<parameter 1>, ... , <parameter n>

This parameter is used to overlay the mechanical model with an additional viscous constitutive model. This can improve the numerical stability for very fast deformations at vanishing mean effective stress (e.g. \(p = |\text{tr}(\boldsymbol{\sigma})/3| < 2\) F/A). The subsequent lines <parameter 1>, ... , <parameter n> depend on the model chosen:

  • *Mechanical viscosity=linear: for the linear viscosity model, the viscous stress is calculated from \(\boldsymbol{\sigma}^{{\scriptsize vis}} = \lambda \boldsymbol{1} \text{tr}(\dot{\boldsymbol{\varepsilon}}) + 2\mu \dot{\boldsymbol{\varepsilon}}\). To ensure a smooth transition, \(\lambda\) and \(\mu\) are assumed to increase linearly with vanishing mean effective stress \(p\). The command takes the following arguments: 

    *Mechanical viscosity=linear
p_{min}, p_{max}, lambda_{min}, lambda_{max}, mu_{min}, mu_{max}

    where \(\lambda_{min}\) [F/A\(\cdot\)s], \(\lambda_{max}\) [F/A\(\cdot\)s], \(\mu_{min}\) [F/A\(\cdot\)s] and \(\mu_{max}\) [F/A\(\cdot\)s] are material parameters. Note that \(\lambda_{min} \leq \lambda_{max}\) and \(\mu_{min} < \mu_{max}\) are required. Viscosity can be defined in conjunction with the following mechanical models:

    Model Section
    Hypoplasticity + Intergranular Strain 5.5.2.3
    Hypoplasticity + Intergranular Strain Anisotropy 5.5.2.6
    Sanisand 5.5.2.7
    Modified Cam Clay 5.5.2.10
    Anisotropic Viscous ISA 5.5.2.12
    High Cycle Accumulation model for sand 5.5.2.9
    ... coupled with Hypoplasticity + Intergranular Strain
    ... coupled with Sanisand
    High Cycle Accumulation model for clay 5.5.2.16

    The (viscous) stresses from this phantom viscosity can be requested for output using the following keyword PHANTOM-VISCOSITY. In Paraview these stresses are labeled Stress-Vis 11, ...

  • *Mechanical viscosity=bulk_explicit: introduces damping in explicit dynamics in terms of volumetric straining. It is switched on by default in explicit analyses, but the two parameters \(a,b\) associated with this type of viscosity can be modified. The default values are \(a= 0.06\) and \(b=1.2\).