Fall 2012 ME 395 - GSI Josh Lacey

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 Correlation between displacement and strain; which curve to get material properties from

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GSI Overlord
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PostSubject: Correlation between displacement and strain; which curve to get material properties from   Sat Sep 29, 2012 12:16 am

I received the following two questions:

Do we need to put error bar on the curve? Because there are many points on the graph, the error bars are impossible to be seen. Also when drawing calibration curve, should we use all the four data sets, or should we only use the data set obtained in our experiment?

Should we get yield stress and tensile stress from nominal stress strain curve or true stress strain curve?
Also for the true stress strain curve, do we need to draw the necking part? Because the equation relating nominal stress, strain with the true stress, strain is invalid when necking happens, so we can't get true stress, strain from the experimental data.

To answer the first question, I would say first off you probably won't need to use so many data points to make the strain vs. displacement curve. I think the only error you need to quantify here is how well your correlation fits the data (how close the data points are to the best-fit curve). It should be fine to just use the data collected for your section to develop that relationship.

For the second question, if you refer back to your lecture slides, you'll see from page 3 that you will get all your material properties (elastic modulus, yield strength and ultimate tensile strength) from your nominal stress-nominal strain curve.
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wchengh



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PostSubject: Re: Correlation between displacement and strain; which curve to get material properties from   Sat Sep 29, 2012 12:43 am

I am still a little confused. So, I don't need to include error bar for each point on the graph? I only use the 15%-30% data to calibrate the displacement and strain. Is it appropriate? When calibrating, is it ok to include unloading and loading data, or should we get rid of these data points?
Thank you for your help and patience~
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GSI Overlord
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PostSubject: Re: Correlation between displacement and strain; which curve to get material properties from   Sat Sep 29, 2012 1:37 am

Well, potentially you'll have a bit of uncertainty on your strain versus displacement curve, as there is some resolution error from the measurement instrument.

The reason I mentioned using the last piece of the loading/unloading curve is that the 30% strain point is where you need to tack on the last failure part, hence it makes sense that you probably want to develop the strain displacement relationship using strain data as close to that point as you can get (up to the 30% strain point).

When I plotted the data from the labs to make the stress vs. strain plot, I used the entire 30% strain file to get the strain vs. displacement curve, but realistically that was probably more data than I needed; I could have left off the loading and unloading data. Ultimately, it's up to you to decide what you think will be best.
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wchengh



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PostSubject: Re: Correlation between displacement and strain; which curve to get material properties from   Sat Sep 29, 2012 1:43 am

Thank you very much. I get it. The last quick question:
For the true stress-strain curve, do we need to draw the necking part? Because the equation relating nominal stress, strain with the true stress, strain is invalid when necking happens, so we can't get true stress, strain from the experimental data. Right?
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PostSubject: Re: Correlation between displacement and strain; which curve to get material properties from   Sat Sep 29, 2012 2:34 am

You can reference your lecture slides for a nice example of a true stress-true strain curve, and you'll notice that there is an 'x' where the failure occurs...you're right that anything you try to calculate past this point is basically non-physical as your normal strain equation is invalid in this region.
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