ULTIMATE STRENGTH UNDER PLASTIC TENSILE INSTABILITY.

The fundamental concept of plastic tensile instability is the basic phenomenon for the determination of maximum load or ultimate strength of metals. Basically the phenomenon is due to the fact that as a tensile specimen is loaded into the plastic range, a point is eventually reached at which the relative decrease in cross-sectional area, owing to the Poisson contraction effect, exactly balances the relative increase in true stress owing to strain hardening. At this point, instability occurs and the load attains its maximum value; this point is called the tensile instability point. The ultimate tensile strength at the point of instability is the strength, in terms of engineering stress, that is theoretically achievable. It can be reached in practice, even in modern high strength, low ductility alloys. If this strength is not achieved in a uniaxially loaded member, some other mechanism of failure, such as brittle fracture or creep rupture, may be acting. Though the maximum ultimate tensile strength, which occurs at the point of instability, depends on various engineering parameters the present investigations conclude that the determination of this ultimate strength becomes easier if the material constant is evaluated from its stress-strain data. A brief discussion of the paper is appended.