What is the difference between engineering stress and true stress?

Answer 1

The difference between true stress & engineering stress is summarised as follows:

Engineering stress assumes that the area a force is acting upon remains constant, true stress takes into account the variation in the cross sectional area as a result of the stress induced deformation (strain) of a material.

For example a steel bar in tension once its yield point or stress is reached will start to "neck". Necking is the localized concentration of strain in a small region of the material, causing a reduction in cross sectional area at this point.

To calculate the engineering stress in the above case, the applied load is divided by the original cross sectional area, however the true stress would be equal to the load divided by the new deformed cross sectional area. Therefore true stress is likely to be significantly higher than engineering stress. Note that while the material is deforming elastically before the yield point is reached there will be some difference between true and engineering stress (as the material is changing shape) but it will be much smaller than the difference after the yield point is reached.

A rock core in a uni-axial compression test will typically expand radially under loading. Therefore in this case, the engineering stress (based on the original diameter) will be larger than the true stress within the material.

Answer 2

True stress accounts for the change in area as the material is stressed:

Assuming homogenous plastic deformation at a constant volume, sigma_true=sigma_engineering*(1+strain)

However, as far as I know, that assumption is no longer correct once necking occurs, as the stresses within that region become unknown equations based on the deformation. I suppose with the right measuring equipment one could possibly find it, but I'm assuming that you'd be at a far more advanced level than my expertise could provide if that were true.

Answer 3

When you apply tensile pull on a specimen ex: rod, its length increases at the same time its cross sectional area decreases.when find stress by taking original area of cross section than it is known as engg stress. If you take reduced area than it becomes true stress.

Answer 4

As the material begins to fail, it stretches and yields and the cross sectional area is reduced. Thus, the true stress (load over area) is higher since area is smaller than the original area, on which engineering stress is based.

To illustrate this you can take a stick of soft Chewing Gum and stretch it; you will see the area reduce significantly before failure

Answer 5

because as failure approaches the cross sectional area necks down (reduces) then true stress is load divided by the reduced area. Engineering stress uses the initial area, and is the usual way to report stress

Answer 6

see following question:

What_the_difference_between_true_strain_and_engineering_strain

Answer 7

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