Properties of Metals
Soft steel, when tested in tension, frequently displays a peculiar
characteristic, known as a yield point. If the stress-strain curve is plotted,
a drop in the load (or sometimes a constant load) is observed although the
strain continues to increase. Eventually, the metal is strengthened by the
deformation, and the load increases with further straining. The high point
on the S-shaped portion of the curve, where yielding began, is known as
the upper yield point, and the minimum point is the lower yield point.
This phenomenon is very troublesome in certain deep drawing operations
of sheet steel. The steel continues to elongate and to become thinner at
local areas where the plastic strain initiates, leaving unsightly depressions
called stretcher strains or "worms."
The proportional limit is defined as the stress at which the stress-strain
curve first deviates from a straight line. Below this limiting value of
stress, the ratio of stress to strain is constant, and the material is said to
obey Hooke's Law (stress is proportional to strain). The proportional limit
usually is not used in specifications because the deviation begins so
gradually that controversies are sure to arise as to the exact stress at which
the line begins to curve.
The elastic limit has previously been defined as the stress at which plastic
deformation begins. This limit cannot be determined from the stress-strain
curve. The method of determining the limit would have to include a
succession of slightly increasing loads with intervening complete unloading
for the detection of the first plastic deformation or "permanent set." Like
the proportional limit, its determination would result in controversy.
Elastic limit is used, however, as a descriptive, qualitative term.
In many situations, the yield strength is used to identify the allowable stress to which a material
can be subjected. For components that have to withstand high pressures, such as those used in
pressurized water reactors (PWRs), this criterion is not adequate. To cover these situations, the
maximum shear stress theory of failure has been incorporated into the ASME (The American
Society of Mechanical Engineers) Boiler and Pressure Vessel Code, Section III, Rules for
Construction of Nuclear Pressure Vessels. The maximum shear stress theory of failure was
originally proposed for use in the U.S. Naval Reactor Program for PWRs. It will not be
discussed in this text.