Dear Reader,

I hope you are doing well. I am writing this newsletter from my holiday destination where I am spending time with my family. It has been quite refreshing spending the time together. I will just write a quick and short newsletter and will return to a more extensive reflection next week. The content of the newsletter will be a technical reflection only.

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Technical Reflection

How to get stress-strain plot in ABAQUS by probing element regions

The question of obtaining Stress-Strain plots in ABAQUS is something a lot of people often ask questions about on my YouTube channel. I have discussed extensively about it and it continues to come up regularly. There are also newsletter articles that I have written about it. The video I filmed about a year ago about this has gathered over 5,581 views which again is a testament of the interest in this problem.

One of the easiest way of obtaining stress-strain plot is simply by probing the value of stress or strain at specific regions in the model. This can be at element level where you choose an element and extract the stress-history there. This is what I refer to as the localization approach which requires the user to locate an element and extract the stress- or strain-history there. I published a YouTube video this week precisely on this. It was scheduled to be released whilst I was on holiday and as expected was premiered at 6:00 am on Wednesday, 26th July 2023. If you have not seen that video then, watch it below.

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Why is it difficult to get stress-strain plot in ABAQUS: This could be a question you might be thinking of since it should not really be that difficult. But, the reality is that it is not as straight forward as presssing a button and get the stress-strain plot of any simulation you are doing in ABAQUS. Here are my reasons for why it is difficult to get stress-strain plot in ABAQUS.

1. ABAQUS CAE does not have a button you can press to generate stress-strain plot. If this exists, at least I do not know of it.
2. Stress and strain are variables that are defined at element level so each element has got these values defined within it. It is possible to track these variables but that will have to be either for all elements or just a sub-section/sub-region of the model.
3. Stress and strain variables are many, and based on stress-tensor for a 3D model, you will have 9 stress-terms in the model. Which one to choose is the choice of the user and so ABAQUS leaves this decision to the user to make.
4. Simulations outputs of stress- and strain are often time-dependent values due to the variables generated as incremental values of time-steps. As a result, to get stress-strain plot, you have to combine the variables without the time-element.

What strategies can I use to extract stress-strain plot in ABAQUS: I have found three major ways for obtaining stress-strain plots and here are they:

1. Localization approach: I already described this method above. It involves identifying an element and probing its values of stress- and strain and then plotting the stress-strain plot based on the values. You can simply use the field output values of the stress and strain variables.
2. Volume-averaged approach: This is an improvement on the localization approach in that it requires the user to identify a region (finite, say gauge section), and then write to the memory of the computer during the simulation via the history variables of the computer. You then would have to average the values of the stress- and stress- across all elements that make up the region/gauge section.
3. Experimental-equivalence approach: This is comparable to a physical experiment. The user specifies a reference point on the gripped end of the test specimen. The using kinematical constraint equation within ABAQUS, you will link th reference point to the whole gripped region (where load cell is attached). During the simulation, you will record as history variables the reaction force (RF) and displacement (U) which you then use to generate a force-displacement profile. Then subsequently using cross-sectional area, A and gauge length, L0, then you can obtain the corresponding stress- and strain values.

I published a video in the past about this and you can watch it here:

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That is it for this week - a truly short reflection - highlighting two videos about getting stress-strain plot from an ABAQUS simulation. I wish you a wonderful weekend and I will catch up with you next week.

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