Difference between revisions of "Diffusion Calculations"
| Line 24: | Line 24: | ||
To be able to model diffusion in composite materials where different phases may absorb different amounts of solvent, all diffusion calculations are done in terms of a chemical potential for the solvent in the material, where chemical potential, μ, is approximate by | To be able to model diffusion in composite materials where different phases may absorb different amounts of solvent, all diffusion calculations are done in terms of a chemical potential for the solvent in the material, where chemical potential, μ, is approximate by | ||
| |||
<math>\mu= {c\over c_{sat}}</math> | |||
When concentration and concentration gradients are archived, however, they are converted to actual concentration in weight fraction using the material's saturation concentration setting. | where <tt>c</tt> is the weight fraction of solvent absorbed in the material and <tt>c<sub>sat</sub></tt> is the saturation solvent weight fraction for that material (which is specified in the [[Material Models|material definition]]. This concentration potential varies from 0 to 1. When concentration and concentration gradients are archived, however, they are converted to actual concentration in weight fraction using the material's saturation concentration setting. | ||
== Diffusion Boundary Conditions == | |||
Note that setting initial particle concentrations different than the reference concentration will cause strains to immediately evolve toward the changed state. The net effect will be an instantaneous "impact" of concentration change that might cause undesirable dynamic effects. | |||
When diffusion is activated, you can set material diffusion and solvent expansion constants, initial particle concentrations, and impose concentration or flux boundary conditions. | When diffusion is activated, you can set material diffusion and solvent expansion constants, initial particle concentrations, and impose concentration or flux boundary conditions. | ||
Revision as of 10:06, 4 November 2013
NairnMPM can do diffusion calculations coupled with stresses and strains through concentration expansion.
Activating Diffusion
In scripted files, diffusion is activated with the command
Diffusion (YesOrNo),<(refConc)>
In XML input files, diffusion is activated with the Diffusion command, which must be within the <MPMHeader>:
<Diffusion reference = '(refConc)'/>
where
- (YesOrNo) must be "Yes" or "No" to activate or not activate diffusion calculations. In XML input files, the presence of a <Diffusion> command activates diffusion. The default is "No".
- (refConc) is used to set a reference concentration potential (between 0 and 1) that corresponds to zero strain. All diffusion calculations are done in terms of a concentration potential from 0 to 1 where 1 is the saturation concentration of a material type. The default (refConc) is 0.
Diffusion Material Properties
Concencentration changes are coupled to stress and strains through concentration expansion coefficients defined for the materials. By default, all moisture expansion coefficients are zero which decouples diffusion and strains. By entering non-zero values, the coupling will occur. Isotropic materials have a single solvent expansion coefficient while anisotropic will have two or three solvent expansion coefficients.
The rate of diffusion is controlled by the solvent diffusion constant defined for each material. Isotropic materials have a single solvent diffusion constant while anisotropic will have two or three solvent diffusion constants.
To be able to model diffusion in composite materials where different phases may absorb different amounts of solvent, all diffusion calculations are done in terms of a chemical potential for the solvent in the material, where chemical potential, μ, is approximate by
[math]\displaystyle{ \mu= {c\over c_{sat}} }[/math]
where c is the weight fraction of solvent absorbed in the material and csat is the saturation solvent weight fraction for that material (which is specified in the material definition. This concentration potential varies from 0 to 1. When concentration and concentration gradients are archived, however, they are converted to actual concentration in weight fraction using the material's saturation concentration setting.
Diffusion Boundary Conditions
Note that setting initial particle concentrations different than the reference concentration will cause strains to immediately evolve toward the changed state. The net effect will be an instantaneous "impact" of concentration change that might cause undesirable dynamic effects.
When diffusion is activated, you can set material diffusion and solvent expansion constants, initial particle concentrations, and impose concentration or flux boundary conditions.