In the development of the basic stress equations for tension, compression, bending, and
torsion, it was assumed that no geometric irregularities occurred in the member under
consideration. But it is quite difficult to design a machine without permitting some
changes in the cross sections of the members. Rotating shafts must have shoulders
designed on them so that the bearings can be properly seated and so that they will take
thrust loads; and the shafts must have key slots machined into them for securing pulleys and gears. A bolt has a head on one end and screw threads on the other end, both
of which account for abrupt changes in the cross section. Other parts require holes, oil
grooves, and notches of various kinds. Any discontinuity in a machine part alters the
stress distribution in the neighborhood of the discontinuity so that the elementary stress
equations no longer describe the state of stress in the part at these locations. Such discontinuities are called stress raisers, and the regions in which they occur are called
areas of stress concentration. Stress concentrations can also arise from some irregularity not inherent in the member, such as tool marks, holes, notches, grooves, or threads.
1 in
T
18
in
Figure 3–28
The cross-section of a thin strip
of steel subjected to a torsional
moment T.
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Load and Stress Analysis 111
A theoretical, or geometric, stress-concentration factor Kt or Kts is used to relate
the actual maximum stress at the discontinuity to the nominal stress. The factors are
defined by the equations
K
t =
σ
max
σ0
K
ts =
τ
max
τ0
(3–48)
where Kt is used for normal stresses and Kts for shear stresses. The nominal stress σ0 or
τ0 is the stress calculated by using the elementary stress equations and the net area, or
net cross section. Sometimes the gross cross section is used instead, and so it is always
wise to double check the source of Kt or
1