Stress.
Stress is the intensity at a point in a
body of the internal forces or components of force that act on a
given plane through the point. Stress is expressed in force per unit
of area (pounds per square inch, kilograms per square millimeter,
etc.).
There are three kinds of stress:
tensile, compressive, and shearing.
Flexure involves a combination of
tensile and compressive stress. Torsion involves shearing stress. It
is customary to compute stress on the basis of the original
dimensions of the cross section of the body, though “true stress”
in tension or compression is sometimes calculated from the area of
the time a given stress exists rather than from the original area.
Strain.
Strain is a measure of the change, due
to a force, in the size or shape of a body referred to its original
size or shape. Strain is a nondimensional quantity but is frequently
expressed in inches per inch, etc.
Under tensile or compressive stress,
strain is measured along the dimension under consideration. Shear
strain is defined as the tangent of the angular change between two
lines originally perpendicular to each other.
Stress-Strain Diagram.
A stress-strain diagram is a diagram
plotted with values of stress as ordinates and values of strain as
abscissas. Diagrams plotted with values of applied load, moment, or
torque as ordinates and with values of deformation, deflection, or
angle of twist as abscissas are sometimes referred to as
stress-strain diagrams but are more correctly called load-deformation
diagrams.
The stress-strain diagram for some
materials is affected by the rate of application of the load, by
cycles of previous loading, and again by the time during which the
load is held constant at specified values; for precise testing, these
conditions should be stated definitely in order that the complete
significance of any particular diagram may be clearly understood.
Modulus of Elasticity.
The modulus of elasticity is the ratio
of stress to corresponding strain below the proportional limit. For
many materials, the stress-strain diagram is approximately a straight
line below a more or less well-defined stress known as the
proportional limit.
Since there are three kinds of stress,
there are three moduli of elasticity for a material, that is, the
modulus in tension, the modulus in compression, and the modulus in
shear.
The value in tension is practically the
same, for most ductile metals, as the modulus in compression; the
modulus in shear is only about 0.36 to 0.42 of the modulus in
tension.
The modulus is expressed in pounds per
square inch (or kilograms per square millimeter) and measures the
elastic stiffness (the ability to resist elastic deformation under
stress) of the material.
