Explanation

When materials starts plastic deformation (dislocation; multiplication and movement), the applied stress suddenly drops or no longer increases, which is called yield point phenomenon. The stress drop point is called the yield point, the maximum value when the stress starts to drop is called the upper yield point (upper yield limit), and the lowest value when the stress I drops is called the lower yield point. For material I showing yield point, the stress corresponding to the yield point on the stress-strain curve is the yield strength. For materials with obvious upper and lower yield point, the stress corresponding to the lower yield point is the yield strength. The yield point phenomenon can be explained by the i theory of air mass and the theory of dislocation multiplication. For bcc material I containing interstitial impurities (such as low-cardon steel), dislocation is pinned by impurity air mass, so the external stress required for dislocation to move off the I pin is large, corresponding to the yield point. Once the nail is removed, the external stress required for the continued movement of the dislocation becomes smaller, which corresponds to the lower yield point. The theory of dislocation proliferation has more general significance.

Classification