A “Toolbox” for Forensic Engineers

Materials in Distress 49
Stress Amplitude/MPa
500
400
300
200
100
Cycles to Fatigue Fracture
Alloy steel A
Low carbon steel B
Aluminium alloy C
10 4 10 5 10 6 10 7 10 8
Figure 2.14 S-N for two steels and heat-treated aluminum alloy. Notice the
leveling off at 40% of tensile strength for the steels.
task is usually to identify the mode and cause of a particular failure and this,
more often than not, is because a product has been exposed to unusual
conditions or was abused in some way that the designer never intended or
foresaw. However, it is useful to refer to the form of S-N curve, as depicted
in Figure 2.14. These diagrams summarize data from a series of tests in which
standard specimens are subjected to cyclic loads of a particular stress amplitude,
S, until fracture occurs. The number of cycles to failure, N, is represented
on a logarithmic scale. Tests are carried out over a range of stress
levels, all below the elastic limit of the material. S-N curves supply useful
data for design, provided that the frequency and maximum amplitude of
cyclic stresses likely to be encountered in service can be estimated. There are
various ways of taking into account the variations in stress amplitude and
frequency likely to be experienced by a specific component in a particular
application.
For steels the S-N curves level off at approximately 40% of their tensile
strength, and this is defined as the “fatigue limit.” In principle steels should
survive indefinitely if a component is used in situations where the maximum
level of cyclic stress is unlikely to exceed this fatigue limit, but there are
complications in relying on this, not the least of which is because the level
of stress at all positions within a shaped component under different applied
loads is difficult to predict, even using finite element analysis methods. However,
for nonferrous metals and most other solids used in engineering there
A
B
C