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True and engineering stress

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I am a student  in mechanical engineering and we graph a stress strain curve. I know what these values are and I even know how to calculate it given the engineering stress and strain, but I just don't understand why it is necessary in the first place to calculate the true stress and strain. Can anyone help?

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Hi,
In theoretically, we consider the section of the tensile sample does not change against increased strain values. However, in reality, it is obvious that the sample's section is going to be reduced because it is elongating. 

 

Posted by: @nevofam

Hi,
In theoretically, we consider the section of the tensile sample does not change against increased strain values. However, in reality, it is obvious that the sample's section is going to be reduced because it is elongating. 

 

I agree plus I would say during the tensile test, the width and thickness shrink as the length of the test sample increases. Although these dimensional changes are not considered in determining engineering stress, they are of primary importance when determining true stress. At any load, the true stress is the load divided by the cross-sectional area at that instant.

You can maybe check the Holloman calculation.

 

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