Page 161 - Finite Element Analysis with ANSYS Workbench

152 Chapter 8 Failure Analysis

(a) Soderberg criterion states that a structural part or
machine component is safe if,
 a   m  1
S e  y n
where S is the endurance limit stress,  is the yield stress and n
e
y
is the design safety factor.
(b) Goodman criterion states that a structural part or
machine component is safe if,
 a   m  1
S e  u n

where  is the ultimate stress.
u
(c) Gerber criterion states that a structural part or
machine component is safe if,
 a    m  2  1
S e    u   n
The three criteria when n  1 are plotted as shown in

the figure. The Soderberg criterion is the most conservative
measure while the Goodman and Gerber criteria are the lesser ones,
respectively.
 a

Gerber
S e

Goodman

Soderberg
 y  u  m

If the computed stress does not meet one of the criteria
above, the structural part or machine component may fail at a
limited time. Its limited life is normally estimated in form of the