Type of metals effect to mechanical properties because each metals are different in structure. Cross-section area effect to stress (= FA) ,but length after load effect to strain (=LL0) of materials. Each materials have specific properties. From relationship of stress strain curve of each other ,yield point of materials are different.
The stress-strain results that were read from the machine were inaccurate. One inaccuracy is that the experiment used the ‘nominal stress’ of the sample rather than the ‘true stress’. However, the difference between the two are very small, particularly in the elastic region of the test, and could not cause such a large difference between the experimental and theoretical value of yield stress.