stress- strain

An outstanding problem in plastic deformation of solids has been: Is it
external stress or external strain that we could prescribe at local
material points?
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Amit Pandey
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There are two questions.... 1) Which of these (stress, strain) can you more accurately predit? 2) Which of these actually drives whatever process you are studying?
In general, in a simple tensile bar loaded without moments, you tend to accurately know the stress from P/A, but the strain depends on whatever mathematic formula you have concocketed to represent the stress strain behavior.
In a thermal stress problem, by trial and error of a lot of Finite Element Methods, you find that you know the strain fairly well, no matter what the stress-strain law you input. However, the predicted stresses vary widely with the assumed stress-strain law.
This opens up the discussion of what you can more accurately predict... stress of strain. Without and understanding of this you are babbling.
There is a long history of "stress analysis" which is often based on isotropic materials and "stress functions". This colors the debate of what is fundamental. Isotropic materials aren't fundamental.
If you made a pile of the parts of your body which are isotropic, it would be fairly small, and liquids would form the majority.
Perhaps, only by working with anisotropic materials for all theoretical materials mechanical behavior frameworks, we can avoid self deceptions.
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