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In this article they mention that Cauchy stress is defined by studying the forces acting on the deformed body, and these forces are constant in space. But if a body is subjected to pure rotation, the actual values of stress component will change, meaning now there is a full tensor with both normal and shear stress components. Why is this something we dont want - do they mean cauchy stress is inaccurate since forces are fixed in space, when in reality we have all sorts of normal and shear stress due to rotation?

enter image description here

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Imagine we're reviewing the strength of a glass table. I say, "This table can withstand a pushing force of 1000 N." You say, "I understand—a maximum force of 1000 N." I say, "Oh no, not for you. You're not standing directly over the table as I am, so your pushing force will be slightly angled, so you'll need to take the vector components, and then you'll need to incorporate the possibility of friction and your hand slipping, and..."

You'd say, "Stop, stop, stop. It's not useful for you to incorporate where I'm standing. When we talk about pushing on the table, it's most convenient if we always assume that the person is standing above the table, pushing down vertically on the center. You might think that your approach of considering where we're individually standing right now might seem more objective, but it's ultimately introducing more complexity. It's neither what I'd expect or what I'd want to use."

This is what the authors mean when they say, for example, "It is much more plausible that you want to see the stress in the fiber direction, even if the component is rotated." In other words, for convenience, we often want our stress tensor to rotate with our object, especially if our material is anisotropic and we're interested on the shear stress applied to laminate interfaces, for instance. The Second Piola-Kirchhoff stress tensor (with appropriate initial alignment) would always contain a component that describes this exact shear stress; with the least little bit of rotation, the Cauchy stress tensor would not. Does this make sense?

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  • $\begingroup$ The table = infinitesimal area element in the deformed body? And we want whats on the right in the picture, not the left? imgur.com/a/Tv85LQV $\endgroup$ Jun 30 at 0:51
  • $\begingroup$ What does the arrow mean in that image? The forces don't change, but with a material tensor field, the orientation of the infinitesimal element we use to define the stress tensor can change. $\endgroup$ Jun 30 at 1:26

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