Are our Stress & Strain calculations reliable?

It has been taught that Load and deflection across specimens can vary. But not the stress and strain. True! And that is why while characterizing /testing a material, we express load and deflection in terms of stress and strain for the convenience of comparison and standardization.

BUT what one should realize is that the stress and strain are user dependent parameters in real time tests. You are inputting area A and gauge length GL for stress and strain respectively. Have you thought of the variation in stress or strain, per unit error in the area or length that you are calculating?

What Gauge Length is to be taken for a typical tensile test?

Gauge Length (GL) is basically the length of interest for which one measures the deflection during a standard tensile test. As the name suggests, you 'gauge' the ength of your interest for the study being conducted.

Several standards give specifications for the GL to be considered for these tests. What has been observed in American codes is a standard length of 5D where D is the diameter of the steel rod tested. IS code defines it as 5.65*sqrt(A) where A is the Area of cross section of steel. But if you substitute A in terms od D as pi*D*D/4, it turns out to be same as 5xD.

So why can't we take a longer or shorter GL?
Too long a GL?
Your strain is going to be averaged if you increase your GL. So you need to be sure that the length you are considering is enough to e the influential length of plastic deformation (In elastic region, nothing to worry about as we assume all the whole bar to deform at the same strain rate. But note that that the strain% is very low at the elastic limit which is typically 0.2% compared to the ultimate strains of 16-20% or more)

Too less a GL?
Well, you need to make sure that you are not focusing in too much in the complex stress area. You need to have a distance wherein the the stress flow becomes uniform, for which distance it is standardized to be considered for tensile test. It was 4xD initially which was revised to 5xD(Said to be based on test results by the Standardization Agency)