FCC vs BCC

[DANISH KHAN]

Hi Reza,

I am currently trying to use PRISMS to try few elementary things to get hold of CPFEM modelling technique. I am using Rate dependent formulations. I did two trial simulations, one for each FCC and BCC single crystal using a single element, single grain model having the same orientation, for the same amount of compressive strain along X-axis, and same set of material properties. Only difference was that respective  slipDirections.txt, slipNormals.txt and LatentHardeningRatio.txt files were used for each crystal type. On carrying out the simulation, I get identical results for both the crystals; overlapping macroscopic stress-strain curve (pic attached) and same final deformed shape (pic attached for both). Is it something correct and we would expect or am I doing something wrong here?

I can understand the same macroscopic response considering that for the model there is no difference between FCC and BCC (both have 12 slip-systems) as long as the hardening parameters are same.  But I would expect different final deformed shape, considering their slip systems are different.

Kindly let me know if I am doing something wrong here.

Many thanks again!

Regards,

Danish

[Krzysztof Stopka]

Hey Danish,

I am glad you are using PRISMS to better understand elementary aspects, so what you are doing is great. I have a few comments:

1) I would not expect your final deformed shape to look like that (but I could be wrong...). Reza may be able to comment further but it looks like one of the boundary conditions may be incorrect.

2) I would recommend you examine other state variables throughout the simulation. For example, you can inspect the current value of slip on each slip system, i.e., "gamma". If you set an orientation of (0,0,0) and apply compression or tension in the X axis, then you should expect slip on only eight of the 12 available FCC slip systems (and you can make sure you are in fact getting slip on only the correct eight by examining your slipDirections.txt and slipNormals.txt files). The BCC structure contains at least 12 slip systems (6 planes each with two slip directions), which at an orientation of (0,0,0) should also result in slip on 8 of the available 12 slip systems. Once again, you should examine the state variable of each slip system to make sure they match up with the slipDirections.txt and slipNormals.txt files.

3) Now, I haven't looked into this exactly, but I'm not sure if I would expect the two responses here to be IDENTICAL, although they should certainly be SIMILAR. I think Reza can elaborate further. :) 

Best regards,

Kris

[DANISH KHAN]

Dear Kris,

Thanks for your reply. These are great comments and suggestions. Specially, looking at individual activity of individual slip systems. While we wait to hear back from Reza as well, I have some follow up questions:

1) Why do you think the deformed shape is wrong? To clarify, the boundary condition involves completely encastering the negative X plane and applying loading on the positive X plane. 

2) Pardon my ignorance :), but what makes you deduce that loading along 1,0,0 direction would only activate 8 slip systems, both for FCC and BCC?

Regards,

Danish 

[Krzysztof Stopka]

Hey Danish,

1) Mainly because the two opposite corners of one side of the element are stretched so significantly. Can you upload your files in a .zip folder and share here please?

2) This is easier to explain for the FCC slip system: if you apply a load along the X, Y, or Z direction with the (0,0,0) orientations, then the slip direction on four slip systems is perpendicular to the direction of load and hence the resolved shear stress on these slip systems will be 0. Thus, there should be no slip. I will email you slides I put together a while back (they are targeted at simulations in ABAQUS, but the analysis on the second half holds true in any CPFEM software). I hope this is helpful!

Best regards,

Kris

[Mohammadreza Yaghoobi]

Hi Danish,

Can you share your input files here as well. I want to see the  type of BCs you applied for.

I think part of the deformation problem is with visualization as well. I'm not sure why your sample deformed like that.

Thanks

Reza

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[DANISH KHAN]

Dear Kris and Reza,

Please find attached the .zip file containing the folders for each FCC and BCC files, i used for creating this results. I think Reza is right. In the snapshots, the deformed geometry is with scale factor 10. I think that makes the two opposite corner stretched out significantly. In any case, kindly let me know: (1) if there is any other issue with my files (2) Are both FCC and BCC crystal structures supposed to give almost similar response of deformed geometry and macroscopic stress-strain response(as long as their material parameters remain the same)?.

 In these files, I have applied a compressive displacement along X axis, keeping the other X face as completely encastre. 

[DANISH KHAN]

Hi Reza and Kris,

I realised that you also need my updateAfterIncrement.cc for running these files. Please find the updateAfterIncrement.cc file with this email.

Regards,

Danish

[Itsuki Fujita]

Hi Danish,

I am experiencing the same issue.
Were you able to resolve it?

In my case, I tried to compare the results between fcc and bcc by modifying only the SlipDirections.txt and SlipNormals.txt files in the fcc sample case.
However, I ended up getting results that were essentially the same as the original fcc case.

Any advice would be greatly appreciated.

Best regards,
Itsuki

2021年12月28日火曜日 1:23:15 UTC+9 DANISH KHAN:

[DANISH KHAN]

Hi Itsuki,

Honestly, I do not remember since completely its more than 3 years now ;). I think in my case it was some issue of the boundary conditions that I was ending up getting overly deformed, similar structures. The similar homoegnsied stress-strain response in the two cases can be understandable since its just an averaged quantity and can be the same for the two cases. In case of slip-system level response, if the material parameters remain the same, it all boils down to the Schmid factors (SF) of the slip-systems. If the SF remain the same, you will get the same response (I think). 

Regards,

Danish

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