Diffusion Calculations

NairnMPM can do diffusion calculations coupled with stresses and strains through concentration expansion.

Activating Diffusion

Activates diffusion calculations. Without this element in the MPM Header, no diffusion calculations will be done. When diffusion is activated, you can set material diffusion and solvent expansion constants, initial particle concentrations, and impose concentration or flux boundary conditions. All calculations are done in terms of a concentration potential from 0 to 1 where 1 is the saturation concentration of a material type. When concentration and concentration gradients are archived, however, they are converted to actual concentration in weight fraction using the material's saturation concentration setting. The reference attrbute is used to set a reference concentration potential (between 0 and 1) that corresponds to zero strain. (Default values: diffusion off and reference = 0)

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. 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.