# [aspect-devel] reference strain rate for diffusion dislocation material

Bob Myhill myhill.bob at gmail.com
Tue Oct 11 02:52:04 PDT 2016

Hi John,

If I understand you correctly, you're just wanting to change the prefactors
for diffusion creep, is that correct?
At any given state, if you halve the prefactor you halve the strain rate.

Have I misunderstood your question? Changing the prefactors, or indeed any
of the other values relevant to the diffusion creep flow law in the .prm
file (listed in source/material_model/diffusion_dislocation.cc and copied
below) is just a matter of refitting the parameters in the governing
equation (found in the manual and l.154 of diffusion_dislocation.cc, also
copied below) to whatever data you have.

Best wishes,
Bob

// Power law creep equation
// edot_ii_i = A_i * stress_ii_i^{n_i} * d^{-m}
\exp\left(-\frac{E_i^* + PV_i^*}{n_iRT}\right)
// where ii indicates the square root of the second invariant and
// i corresponds to diffusion or dislocation creep

// Rheological parameters
// Diffusion creep parameters (Stress exponents often but not
always 1)
prefactors_diffusion = get_vector_double("Prefactors for
diffusion creep", n_fields, prm);
stress_exponents_diffusion = get_vector_double("Stress exponents
for diffusion creep", n_fields, prm);
grain_size_exponents_diffusion = get_vector_double("Grain size
exponents for diffusion creep", n_fields, prm);
activation_energies_diffusion = get_vector_double("Activation
energies for diffusion creep", n_fields, prm);
activation_volumes_diffusion = get_vector_double("Activation
volumes for diffusion creep", n_fields, prm);
// Dislocation creep parameters (Note the lack of grain size
exponents)
prefactors_dislocation = get_vector_double("Prefactors for
dislocation creep", n_fields, prm);
stress_exponents_dislocation = get_vector_double("Stress
exponents for dislocation creep", n_fields, prm);
activation_energies_dislocation = get_vector_double("Activation
energies for dislocation creep", n_fields, prm);
activation_volumes_dislocation = get_vector_double("Activation
volumes for dislocation creep", n_fields, prm);

On 11 October 2016 at 09:59, John Armitage <armitage at ipgp.fr> wrote:

> Hi,
>
> This is perhaps a quick question with a one line answer:
>
> I am want to change the coefficients for the diffusion dislocation
> material option. From previous experience with implementing a dislocation
> creep rheology I have calculated the "Prefactors for diffusion creep" based
> on a 'reference strain rate'. Is this the case here? And if so, what is the
> reference strain rate?
>
> Merci,
> John
>
>
> --
> John Armitage
> --
> --
> Institut de Physique du Globe de Paris
> 1 rue Jussieu
> 75005 Paris
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> +33 (0)1 83 95 78 12
>
>
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