Re: [MW:23883] Tensile testing

Definitely

construct a straight line parallel to the initial linear portion of the stress-strain curve, but offset from it by e = 0.002 (0.2%.) The yield strength is taken as the stress level at which this straight line intersects the stress–strain curve.

Generally the yield point is observed in all carbon steel material.
The reason behind yield point is Cottrell atmosphere. Its related to how dislocation pinned in mild steel metals by carbon and nitrogen interstitials. These atoms distort the lattice shortly and there will be associated residual stress field surrounding the interstitials. This stress field relaxed by interestitail atoms diffusing towords dislocation. Once the dislocation has become pinned a small extra force is required to unpinn and the dislocation prior the yielding and produce upper yield point in stress-strain strain diagram. After unpinning dislocation are free to move in Crystal which results means lower yield point and material will deform in more plastic manner.

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