Page 420 - Mechanical Behavior of Materials

Section 9.2 Definitions and Concepts 421

y P
t P
σ S =
y wt
σ y
(a) x
x
0 w
w

M
y
t 6M σ
σ S = y S
y w 2 t (yielding)
σ
y
(b) x 0 w x 0 x

w

P
σ k S
t
σ y
t y k S (yielding)
t
y
P
S =
(c) x (w – d)t S
d
w σ y
0 x 0 x
σ x w – d
2

Figure 9.3 Actual and nominal stresses for (a) simple tension, (b) bending, and (c) a notched
member. Actual stress distributions σ y vs. x are shown as solid lines, and hypothetical
distributions associated with nominal stresses S as dashed lines. In (c), the stress distribution
that would occur if there were no yielding is shown as a dotted line.

9.2.3 Stress Versus Life (S-N) Curves
If a test specimen of a material or an engineering component is subjected to a sufficiently severe
cyclic stress, a fatigue crack or other damage will develop, leading to complete failure of the
member. If the test is repeated at a higher stress level, the number of cycles to failure will be smaller.