definition of strain

[gabor1]

hi Guys!

Vikram and I were reviewing the code in get_delta_plus_epsilon(), because we wanted to understand exactly what was being calculated. Here are some suggestions:

1. you compute the deformation matrix, which is indeed delta+epsilon, as suggested by the name of the function, but then the name of the variable into which it is accumulated is just called epsilon, which is confusing.

2. The deformation matrix that is thus computed is not symmetric, so can include rotations. we suggest to take those out, using M.T*M

3. To actually get a strain, one has to subtract the identity, so we suggest that is done, so the final thing returned should be 0.5 (M.TM - I) where M is the thing accumulated in get_delta_plus_epsilon()

[pastewka]

I agree that the name Delta+epsilon is confusing because it suggests small strain. It should probably be called deformation gradient. However, I think conversion to Green-Lagrangian strain should be up to the user - but adding a function that returns that would probably be useful.

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--- Prof. Lars Pastewka University of Freiburg Department of Microsystems Engineering Office: +49 761 203-67480 http://www.imtek.de/laboratories/simulation

________________________________ Von: gabor1 <notifications@github.com> Gesendet: Freitag, 20. Juli 2018 16:48 An: libAtoms/matscipy Cc: Subscribed Betreff: [libAtoms/matscipy] definition of strain (#12) hi Guys! Vikram and I were reviewing the code in get_delta_plus_epsilon(), because we wanted to understand exactly what was being calculated. Here are some suggestions: 1. you compute the deformation matrix, which is indeed delta+epsilon, as suggested by the name of the function, but then the name of the variable into which it is accumulated is just called epsilon, which is confusing. 2. The deformation matrix that is thus computed is not symmetric, so can include rotations. we suggest to take those out, using M.T*M 3. To actually get a strain, one has to subtract the identity, so we suggest that is done, so the final thing returned should be 0.5 (M.TM - I) where M is the thing accumulated in get_delta_plus_epsilon() - You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub<#12>, or mute the thread<https://github.com/notifications/unsubscribe-auth/ACacVwifqsZBvAs4wcCVFU7HdyuEc9hEks5uIe2YgaJpZM4VYKI6>.

[gabor1]

Sure. Perhaps also rename the function to atomic_deformation_gradient?

…

-- Gábor

On 21 Jul 2018, at 09:29, Lars Pastewka ***@***.***> wrote: I agree that the name Delta+epsilon is confusing because it suggests small strain. It should probably be called deformation gradient. However, I think conversion to Green-Lagrangian strain should be up to the user - but adding a function that returns that would probably be useful. --- Prof. Lars Pastewka University of Freiburg Department of Microsystems Engineering Office: +49 761 203-67480 http://www.imtek.de/laboratories/simulation ________________________________ Von: gabor1 ***@***.***> Gesendet: Freitag, 20. Juli 2018 16:48 An: libAtoms/matscipy Cc: Subscribed Betreff: [libAtoms/matscipy] definition of strain (#12) hi Guys! Vikram and I were reviewing the code in get_delta_plus_epsilon(), because we wanted to understand exactly what was being calculated. Here are some suggestions: 1. you compute the deformation matrix, which is indeed delta+epsilon, as suggested by the name of the function, but then the name of the variable into which it is accumulated is just called epsilon, which is confusing. 2. The deformation matrix that is thus computed is not symmetric, so can include rotations. we suggest to take those out, using M.T*M 3. To actually get a strain, one has to subtract the identity, so we suggest that is done, so the final thing returned should be 0.5 (M.TM - I) where M is the thing accumulated in get_delta_plus_epsilon() - You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub<#12>, or mute the thread<https://github.com/notifications/unsubscribe-auth/ACacVwifqsZBvAs4wcCVFU7HdyuEc9hEks5uIe2YgaJpZM4VYKI6>. — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub, or mute the thread.