Anisotropic, Elastic-Plastic Material
Constitutive Law
This MPM material is identical to an orthotropic material in the elastic regime, but can plastically deform according to a built-in, anistropic Hill yielding criterion. The Hill plastic yield criterion is:
[math]\displaystyle{ \sqrt{F(\sigma_{yy}-\sigma_{zz})^2 + G(\sigma_{xx}-\sigma_{zz})^2 + H(\sigma_{yy}-\sigma_{xx})^2 + 2L\tau_{yz}^2 + 2M\tau_{xz}^2 + 2N\tau_{xy}^2} = H(\alpha) }[/math]
where σ and τ are normal and shear stresses in the material axis system after rotation from the analysis coordinates, [math]\displaystyle{ H(\alpha) }[/math] is a hardening law, and [math]\displaystyle{ \alpha }[/math] is a plastic hardening variable. The remaining constants are determined by the yield stresses:
[math]\displaystyle{ F = {1\over 2}\left({1\over \sigma_{Y,yy}^2} + {1\over \sigma_{Y,zz}^2} - {1\over \sigma_{Y,xx}^2}\right) \qquad\qquad L = {1\over 2\tau_{Y,yz}^2} }[/math]
[math]\displaystyle{ G= {1\over 2}\left({1\over \sigma_{Y,xx}^2} + {1\over \sigma_{Y,zz}^2} - {1\over \sigma_{Y,yy}^2}\right) \qquad\qquad M = {1\over 2\tau_{Y,xz}^2} }[/math]
[math]\displaystyle{ H = {1\over 2}\left({1\over \sigma_{Y,xx}^2} + {1\over \sigma_{Y,yy}^2} - {1\over \sigma_{Y,zz}^2}\right) \qquad\qquad N = {1\over 2\tau_{Y,xy}^2} }[/math]
where σY and τY are yield stresses for loading in the indicated direction. The yield stresses have to be selected such that the plastic potential is positive semidefinite. Analysis shows that all tensile yield stresses must satisfy:
[math]\displaystyle{ \left|{1\over \sigma_{Y,ii}^2} - {1\over \sigma_{Y,jj}^2} \right| \le {1\over \sigma_{Y,kk}^2} \le {1\over \sigma_{Y,ii}^2} + {1\over \sigma_{Y,jj}^2} }[/math]
where i, j, and k are any combination or x, y, and z. In more practical terms, if two yield stresses are related by some ratio:
[math]\displaystyle{ R = {\sigma_{Y,ii}^2\over \sigma_{Y,jj}^2} \quad\rm{or} \quad {1\over \sigma_{Y,jj}^2} = {R\over \sigma_{Y,ii}^2} }[/math]
and [math]\displaystyle{ R\le1 }[/math] (i.e., [math]\displaystyle{ \sigma_{Y,ii}\le\sigma_{Y,jj} }[/math]) then the third yield stress is bracketed by:
[math]\displaystyle{ {\sigma_{Y,ii}\over 1+R} \le \sigma_{Y,kk} \le {\sigma_{Y,ii}\over 1-R} }[/math]
Some examples in various types of materials follow. If one direction is prevented from yielding by setting its yield strength to ∞, then
[math]\displaystyle{ {\rm if\ }R = 0, \ \sigma_{Y,kk} = \sigma_{Y,ii},\ \sigma_{Y,jj} = \infty }[/math]
In other words, the other two direction must have the same yield stress. If two directions have the same yield strength, then the third direction must greater than half that value:
[math]\displaystyle{ {\rm if\ }R = 1, \ \sigma_{Y,kk} \ge {\sigma_{Y,ii}\over 2} }[/math]
If two directions are similar, such as in in-plane, random fiber composites (e.g., medium density fiber board, particle board, fiberglass composites, or paperboard), the yield direction in the thickness direction must still be:
[math]\displaystyle{ {\rm if\ }R \sim 1, \ \sigma_{Y,zz} \gtrapprox \frac{1}{2}\min(\sigma_{Y,xx},\sigma_{Y,yy}) }[/math]
Because the thickness direction may be much weaker than the in-plane direction, this limitation limits modeling thickness-direction plasticity. Either Hill plasticity needs to be replaced with another anisotropic yielding model or it is realistic the thickness direction of such materials has very little plasticity. Note that it still may model shear plasticity.
Hardening Laws
There hardening laws are available:
[math]\displaystyle{ {\rm Nonlinear\ 1:} \qquad H(\alpha) = 1 + K \alpha^n }[/math]
[math]\displaystyle{ {\rm Nonlinear\ 2:} \qquad H(\alpha) = (1 + K \alpha)^n }[/math]
[math]\displaystyle{ {\rm Exponential:} \qquad H(\alpha) = 1 + \frac{K}{\beta}(1-\exp(-\beta\alpha)) }[/math]
Note, that because [math]\displaystyle{ H'(\alpha) }[/math] in the first law is infinite for [math]\displaystyle{ \alpha=0 }[/math] when n is less than one. This steep slope causes this law to become unstable for n less than about 0.7.
Material Properties
| Property | Description | Units | Default |
|---|---|---|---|
| yldxx | Yield stress for axial loading in the x direction | pressure units | ∞ |
| yldyy | Yield stress for axial loading in the y direction | pressure units | ∞ |
| yldzz | Yield stress for axial loading in the z direction | pressure units | ∞ |
| yldxy | Yield stress for shear loading in the x-y plane | pressure units | ∞ |
| yldxz | Yield stress for shear loading in the x-z plane | pressure units | ∞ |
| yldyz | Yield stress for shear loading in the y-z plane | pressure units | ∞ |
| Khard | Hardening law K parameter | dimensionless | 0 |
| nhard | Hardening law n parameter. If n>0, the Nonlinear 1 law is used; if n<0, the Nonlinear 2 law is used with abs(n). | dimensionless | 1 |
| exphard | Hardening law β parameter in the exponential hardening law | dimensionless | 0 |
| largeRotation | If used, this setting is ignored and material always uses 1 | dimensionless | 1 (fixed) |
| (other) | All other properties are identical to the properties for an othotropic material. | varies | varies |
Although default tensile yield stresses are all infinite, one of them must be finite to use this material. Similarly, the combination of properties must satisfy conditions for positive definiteness described above. If not, an error message will appear and simulation will not run.
If only nhard is provided, the modeling will use Nonlinear 1 or Nonlinear 2 hardening law. If only exphard is provided, it will use the exponential hardening law. If both are given, the last one provided determines the hardening law.
History Variables
The one history variable is the value of plastic hardening variable [math]\displaystyle{ \alpha }[/math]. This variable can be archived as history variable 1.