Learning Forum

[helen.durand]

Hello!

In the two sources listed at the end of this post, the authors describe using instantaneous annealing and creep to represent resetting the stress and strain to zero (i.e., removing the stress and strain history) when a portion of the simulated geometry surpasses a specified relaxation temperature. This is done to represent the effects of the material melting and re-solidifying involved in an additive manufacturing process.

I have a similar simulation, where my simulation involves a one-way coupling between transient thermal and transient structural. In transient thermal, I am using APDL command objects to represent a moving heat source, and in transient structural I want to represent the material melting and re-solidifying as the temperature changes due to the heat source (using APDL command objects). I am using nonlinear material proerties and a multilinear, temperature-dependent stress-strain relation. Currently I am having issues that, when I use EALIVE to bring back killed elements, the deformation is very high and the simulation diverges. I suspect that I need to do something similar to what is described in the sources below to address this.

How do I accomplish something similar in my simulations as was done in the sources listed below? Does it seem like this will address my issue?

 

 

One of my previous posts asked a related question (here: /forum/forums/topic/setting-plastic-strain-and-stress-to-zero-above-a-stress-relaxation-temperature/), but I tried implementing the given suggestion and the results of the simulation were unchanged (high deformations and divergence still occurs). 

I also found a similar post on the forums (here: /forum/forums/topic/applying-instantaneous-stress-and-strain-relief/), but I do not fully understand the answer that was given. 

 

 

SOURCES:

1). Denlinger, Erik R., Jarred C. Heigel, and Panagiotis Michaleris. "Residual stress and distortion modeling of electron beam direct manufacturing Ti-6Al-4V." Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture 229.10 (2015): 1803-1813.

2). Song, X., et al. "Residual stresses and microstructure in powder bed direct laser deposition (PB DLD) samples." International Journal of Material Forming 8 (2015): 245-254.

 

Thank you!

 

[John Doyle]

You mentioned element birth and death to simulating an "instantaneous annealing".  As you know, EKILL will erase the element solution history, but the elements are still in the model. They are just reduced to negligible stiffness (see ESTIF command) and no results are saved to the rst file.   When they are re-introduced (via EALIVE), the ESTIF knockdown factor is removed and results begin to be saved to the rst file.  

It is hard to say what is causing the convergence trouble without closer examination of the application, but any abrupt status change or disruption in the stiffness is going to present a potential convergence challenge.  Perhaps you need to be more strategic in how many elements are reintroduced (via EALIVE) in one load step.  Perhaps you need to use smaller time increments.  You should also take care that the elements are sufficiently supported both while killed and after they are reactivated, especially if they are in a primary load path when reactivated.

[helen.durand]

Thank you for the reply! Could you please clarify what you mean by the following statements? 

1). "EKILL will erase the element solution history"

Does this mean that, when an element is set to be alive again, that it will have zero plastic strain?

2). "You should also take care that the elements are sufficiently supported both while killed and after they are reactivated"

Does 'sufficiently supported' mean that each element has enough nearby elements?

 

[John Doyle]

 

1. From the MAPDL Commands Manual for EKILL we document the following: "Any solution-dependent state variables (such as stress, plastic strain, creep strain, etc.) are set to zero. Deactivated elements contribute nothing to the overall mass (or capacitance, etc.) matrix, and do not generate a load vector (pressures, convections, gravity, etc.)."
2. It is just a general tip that killed elements will not offer any resistence, so if they are part of the primary load path, this might be a problem for convergence.