Transversely Isotropic Viscoelastic Material

Constitutive Law

(This material is available only in OSParticulas because it is still in development)

This anisotropic MPM material is a small strain, linear viscoelastic material that extends the Viscoelastic Material to model anisotropic viscoelasticity. The stress (σ) and strain (ε) are related by:

      [math]\displaystyle{ \sigma(t) = \mathbf{C}(t) * \varepsilon(t) }[/math]

Here [math]\displaystyle{ * }[/math] indicates convolution (or Boltzman's superposition) between time-dependent stiffness tensor ([math]\displaystyle{ \mathbf{C}(t) }[/math]) and strain tensor. In Voight-notation with unique axis in the z direction, the time-dependent stiffness tensor is

      [math]\displaystyle{ \mathbf{C}(t) = \left[\begin{array}{cccccc} K_T(t)+G_T(t) & K_T(t)-G_T(t) & \ell(t) & 0 & 0 & 0 \\ K_T(t)-G_T(t) & K_T+G_T & \ell(t) & 0 & 0 & 0 \\ \ell(t) & \ell(t) & n(t) & 0 & 0 & 0 \\ 0 & 0 & 0 & G_A(t) & 0 & 0 \\ 0 & 0 & 0 & 0 & G_A(t) & 0 \\ 0 & 0 & 0 & 0 & 0 & G_T(t) \end{array}\right] }[/math]

Here [math]\displaystyle{ K_T(t) }[/math] is the plane strain, bulk modulus, [math]\displaystyle{ G_T(t) }[/math] is the transverse shear modulus, [math]\displaystyle{ G_A(t) }[/math] is the axial shear modulus, and [math]\displaystyle{ n(t) }[/math] and [math]\displaystyle{ \ell(t) }[/math] give time-dependence of the C₃₃ and C₁₃=C₂₃ elements of the stiffness tensor (as defined here). The time dependence of each property is modeled with a sum of exponentials:

      [math]\displaystyle{ K_T(t) = K_{T0} + \sum_{k=1}^{N_{KT}} K_{Tk} e^{-t/\tau_{KT,k}} \qquad G_T(t) = G_{T0} + \sum_{k=1}^{N_{GT}} G_{Tk} e^{-t/\tau_{GT,k}} \qquad G_A(t) = G_{A0} + \sum_{k=1}^{N_{GA}} G_{Ak} e^{-t/\tau_{GA,k}} }[/math]

      [math]\displaystyle{ n(t) = n_0 + \sum_{k=1}^{N_n} n_k e^{-t/\tau_{n,k}} \qquad\qquad \ell(t) = \ell_0 + \sum_{k=1}^{N_\ell} \ell_k e^{-t/\tau_{\ell,k}} }[/math]

TIViscoelastic 1 and 2

TIViscoelastic 1 and TIViscoelastic 2 give identical materials but with different initial orientations. TIViscoelastic 1 has the unrotated axial direction along the z (or θ if axisymmetric) axis (see above tensors) while TIViscoelastic 2 has unrotated axial direction along the y (or Z if axisymmetric) axis (exchange y and z directions in above tensors). You can change the unrotated direction to any other orientation when defining material points by selecting rotation angles. For 2D analyses, the two options are needed to allo for axial direction in the x-y (or R-Z if axisymmetric) analysis plane (TIViscoelastic 2) or normal to that plane (TIViscoelastic 1). For 3D analyses, only TIViscoelastic 1 is allowed (and it in the only one needed).

Elastic Fiber Direction

Some materials, such as unidirectional composite materials or wood, can be modeled with the fiber direction as the axial direction. Because this direction is typically much stiffer than the isotropic plane direction, it might be expected to have little or no viscoelasticity compared to other directions. If one assumes that axial modulus ([math]\displaystyle{ E_A }[/math]) and Poisson's ratio ([math]\displaystyle{ \nu_A }[/math]) are independent of time, then [math]\displaystyle{ n(t) }[/math] and [math]\displaystyle{ \ell(t) }[/math] are determined by [math]\displaystyle{ K_T(t) }[/math] by setting:

      [math]\displaystyle{ N_n=N_\ell=N_{KT}, \quad \tau_{n,k}=\tau_{\ell,k}=\tau_{KT,k}, \quad n_0 = E_A + 4K_{T0}\nu_A^2, \quad n_k = 4K_{Tk}\nu_A^2, \quad \ell_0 = 2K_{T0}\nu_A, \quad \ell_k = 2K_{Tk}\nu_A }[/math]

Despite the connect of these three properties, you still need to enter them separately.

Isotropic with Time-Dependent Bulk Modulus

The available isotropic viscoelastic material is limited to materials with time-independent bulk modulus because that is a good approximation for most isotropic, viscoelastic materials. This transversely, isotropic materials, however, does not place any restrictions on which properties are time dependent. As result, it can model an isotropic material with a time-dependent bulk modulus as a special case. Imagine an isotropic material with K(t) and G(t) as time-dependent bulk and shear moduli, respectively. To model using a transversely isotropic material, choose G_A(t) = G_T(t) = G(t) along with:

      [math]\displaystyle{ K_T(t) = K(t) + \frac{G_T(t)}{3},\quad n(t)= K(t) + \frac{4G_T(t)}{3},\quad {\rm and}\quad \ell(t) = K(t) - \frac{2G_T(t)}{3} }[/math]

Finally, any other material properties (such as thermal expansion coefficients) should be set to the special cases for an isotropic material.

Material Properties

The unusual task for this material is to use multiple terms to define the exponential series used for up to five material properties.

Property	Description	Units	Default
GT0	The long term (or fully-relaxed) transverse shear modulus	pressure units	none
GA0	The long term (or fully-relaxed) axial shear modulus	pressure units	none
KT0	The long term (or fully-relaxed) plane-strain bulk modulus	pressure units	none
en0	The long term (or fully-relaxed) C33 element of the stiffness tensor	pressure units	none
ell0	The long term (or fully-relaxed) C13=C23 elements of the stiffness tensor	pressure units	none
ntaus	The number of relaxation times of the previous long-term property that was entered. This property is only needed in XML files and must come before any subsequent Pk or tauk properties. In scripted files, the number is automatically determined from the number of relaxation times you provide.	none	none
Pk	The next property in the series for the previous long-term property that was entered. Use multiple Pk values for each term on the series.	pressure units	none
tauk	The next relaxation time in the series for the previous long-term property that was entered. Enter multiple tauk values for each term on the series.	time units	none
TI Properties	Enter thermal expansion, solvent expansion, and poroelasticity properties as usual for transversely isotropic materials. When fibrous is 1, also enter EA (required) and nuA (default 0.33).	varies	varies
(other)	Properties common to all materials	varies	varies

The material properties need to define the time dependence of 3 properties (when fibrous is 1) or 5 properties (when fibrous is 0). The process for each one is to enter the long-term value first (GT0, GA0, KT0, en0, ell0) and then to follow each one by ntaus (only needed in XML files) and by one Pk and tauk value for each term in the series.

When fibrous is 1, you instead enter time-independent values for EA and nuA. When fibrous is 0, their initial values calculated from:

      [math]\displaystyle{ \nu_A(0) = {\ell(0)\over 2K_T(0)} \qquad {\rm and} \qquad E_A(0) = n(0) - 4K_T(0)\nu_A^2 = n(0) - \frac{\ell(0)^2}{K_T(0)} }[/math]

The remaining, initial transverse properties are:

      [math]\displaystyle{ \frac{1}{E_T(0)} = \frac{1}{4K_T(0)} +\frac{1}{4G_T(0)} + {\nu_A(0)^2\over E_A(0)} \qquad {\rm and} \qquad \nu_T(0) = \frac{E_T(0)}{2G_T(0)}-1 }[/math]

The above initial values, or the properties at time zero, are sums of all Pk terms for that property. For example

      [math]\displaystyle{ G_T(0) = G_{T0} + \sum_{k=1}^{N_{GT}} G_{Tk} }[/math]

For valid modeling, the initial Poisson's ratios must satisfy

      [math]\displaystyle{ -1\lt \nu_T(0)\lt 1 \qquad -\sqrt{E_A(0)\over E_T(0)} \lt \nu_A(0) \lt \sqrt{E_A(0)\over E_T(0)} \qquad {E_T(0)\nu_A(0)^2\over E_A(0)} \lt {1-\nu_T(0)\over 2} }[/math]

These relations must apply for all time, but only the initial values are validated before starting a simulation.

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.