TRUE STRESS VS ENGINEERING STRESS EXPLAINED !!!


Stress is defined as the resistance force acing per unit cross section area of the body. It is also defined as the ratio of applied load to the cross section area of the body.It is an internal property of the material.True stress and Engineering stresses are a bit different. 
Let us take the example of tensile test.Before doing the tension test we have to determine the length and diameter of the specimen and we assume these parameters remain constant throughout the whole experiment. So when we calculate the stress by the formula-

 Stress= Force/Area

we actually calculate the stresses (corresponding to different strains) for the given value of diameter which we have calculated in the beginning of experiment. These stresses are called Engineering Stresses.but as we do the experiment we observe a phenomenon called necking and till now we have been neglecting any such necking which takes place. So let's see what happens when there is necking.
So, As we can see as the bar is subjected to tensile forces just like what happens in a tension test experiment and after certain instant a neck like region forms. This shows the cross-section of the specimen has changed as the experiment proceeds further.
Hence whatever we  will get the stresses now will be different from the ones we got when we we assumed the cross-section remained constant. These stresses are called TRUE STRESSES. True stresses are thus force divided by the area of the section at that instant.
(It should be noted that only after the necking starts there is a considerable change in the cross-section and thus the variation between the two stresses becomes more prominent).

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