The stress-strain diagram for a ductile material like mild
steel is shown in Fig. 1.13. The curve starts from the origin , showing thereby that there is no initial stress of strain in the specimen. Upto point A, Hooke s law is obeyed and stress isproportional to strain.Therefore, OA is a straight line. Point A is called the limit of proportionality. Upto point B, the material remains elastic, i.e. on removal of the
load, no permanent set is formed. AB is not a straight line. Point B iscalled the elastic limit point. Beyond point B, the material goes to the plastic stage until the upper yield point C is reached. At this point the cross-sectional area of the material starts decreasing and the stress decreases to a lower value to point D, called the lower yield point. Between DE, the specimen elongates by a considerable amount without any increase in stress. From point E onwards, the strain hardening phenomena becomes predominant and the strength of the material increases thereby requiring more stress for deformation, unitl point F is reached. Point F is called the ultimete point and the corresponding stress is called the ultimate strength. At point F, necking of the material begins and the cross-sectional area decreases at a rapid rate. The apparent stress deceases but the actual or true stress goes on increasing until the specimen breaks at point C, called the point of fracture. The fracture of ductile material is of the cup and cone type.The phenomena of yielding and necking is not exhibited by brittle materials. The ulimate strenght is calculated at 0.2 per cent of maximum strain.