The material's elastic limit
Precisionmetal parts Within the elastic range, the limiting value of the stress up to which the stress and strain are proportional, is called the limit of proportionality. In this region, the metal obeys Hooke’s law, which states
that the stress is proportional to strain in the elastic range of loading (the material completely regains its original dimensions after the load is removed). In the actual plotting of the curve, the proportionality limit is obtained at a slightly lower value of the load than the elastic limit. This may be attributed to the time-lag in the regaining of the original dimensions of the material.
Precisionmetal parts This effect is very frequently noticed in some no-ferrous metals. While iron and nickel exhibit clear ranges of elasticity, copper, zinc, tin, etc, are found to be imperfectly elastic even at relatively low values of stresses. Actually the elastic limit is distinguishable from the proportionality limit more clearly depending upon the sensitivity of the measuring instrument.
When the load is increased beyond the elastic limit, plastic deformation starts.
Simultaneously the specimen gets work-hardened. A point is reached when the deformation starts to occur more rapidly than the increasing load. This point is called the yield point Q. the metal which was resisting the load till then, starts to deform somewhat rapidly, i. e. , yield. The yield stress is called yield limit.
Precisionmetal parts The elongation of the specimen continues form Q to S and then to T. the stess-strain relation in this plastic flow period is indicated by the portion QRST of the curve. At T the specimen breaks, and
this load is called the breaking load. The value of the maximum load S divided by the original cross-sectional area of the specimen is referred to as the ultimate tensile strength of the metal or simply the tensile strength. Logically speaking, once the elastic limit is exceeded, the metal should starts to yield, and finally break, without any increase in the value of stress. But the curve records an increased stress even after the elastic limit is exceeded. Two reasons can be given for this behavior:
1. the strain hardening of the material;
2. the diminishing cross-sectional area of the specimen, suffered on account of the plastic deformation