iMechanica - experimentally, stresses cannot be measured directly - Comments

Is microscopic stress even an observable?

ravi maranganti — Thu, 15 Mar 2007 14:30:37 -0400

I agree with Dr Tan's statement that stresses cannot be measures directly. Forces and displacements are the quantities one usually measures.

On a related note, if one defines the spatially varying microscopic stress density to be a tensor field whose divergence is the vectorial force density then this stress field density is rendered ambiguos because then one can add a divergence free quantity e.g. the curl of an arbitrary tensor field to the stress density without affecting the "physical" force. This issue of uniqueness of stress has been discussed at length in the literature on the subject of defining stress in a quantum-mechanical framework and a common consesus does not exist.

Not even in the simple cases

Temesgen Markos — Sat, 10 Mar 2007 15:52:13 -0500

Dear Bin Liu, even in a uniaxial tension test, is it not the force that is measured and then divided by the crossectional area to find the stress? Whether the area is the original area or the current area can result in different names for the measured stress.

In some simple cases, such

Bin Liu — Fri, 09 Mar 2007 17:47:48 -0500

In some simple cases, such as in the uniaxial tension test, the stresses can be measured directly. Then with these incomplete experimental results, the relation between stresses and strains is established to predict more complex stresses when the strains are known. I think this is a typical strategy people use.

disc fluid pressure

MichelleLOyen — Thu, 08 Mar 2007 07:49:16 -0500

Since hydrated soft tissues are poroelastic, this could be only a direct measure of the pore pressure in the fluid, which is only one component of the total Terzaghi effective stress. The stress in the solid phase (in disc, a collagen-proteoglycan composite) is also potentially important in this tissue.

soft tissue experiments

Zaoyang Guo — Thu, 08 Mar 2007 06:29:51 -0500

Dr. Adams in Bristol uses probe to measure fluid pressure in intervertebral disc and treats it as the stress component in the soft tissue. Maybe this is not considered as a direct measurement.

Is it stress that photoelasticity measures?

Ajit R. Jadhav — Thu, 08 Mar 2007 05:26:35 -0500

No, not quite. One can take photoelasticity to measure *strains*. In fact the argument that it is strains that photoelasticity measures, would be more sound.

So, essentially, Henry Tan's beginning position is correct. Stress can never be measured directly in any experiment. It's always an inferred (i.e. indirectly defined) quantity.

I have posted a separate entry in my blog on this issue. Please check it there. If necessary, I will move that entry to the main forum as well.

oh, thanks

Henry Tan — Wed, 07 Mar 2007 15:24:49 -0500

oh, thanks

photoelasticity gives you

niteshjain — Wed, 07 Mar 2007 15:14:34 -0500

photoelasticity gives you maximum shear stress contours

experimentally, stresses can never be measured directly

Henry Tan — Wed, 07 Mar 2007 13:16:14 -0500

Correct me if I am wrong.

experimentally, stresses cannot be measured directly

Henry Tan — Wed, 07 Mar 2007 13:02:05 -0500

Experimentally, loading to a mechanical system can be applied either through the displacement control or the force control.

However, the responses of the system can only be measured in displacements, and hence strains.

The stresses can never be measured directly.