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Steel Designers' Manual - 6th Edition (2003)
262 Fracture and fatigue
(1) The operational environment of a structure and the relationship between the
environment and the actual forces on it;
(2) The internal stresses at a critical point in the structure induced by external
forces acting on the structure;
(3) The time to failure due to the accumulated stress history at the critical point.
There are three approaches to the assessment of fatigue life of structural components.
The traditional method, called the S–N approach, was first used in the mid
19th century.This relies on empirically derived relationships between applied elastic
stress ranges and fatigue life. A development of the S–N approach is the strain–life
method in which the plastic strains are considered important. Empirical relationships
are derived between strain range and fatigue life. The third method, based on
fracture mechanics, considers the growth rate of an existing defect. The concept of
defect tolerance follows directly from fracture mechanics assessments.
7.6.2 Loadings for fatigue
Fluctuating loads arise from a wide range of sources. Some are intentional, such
as road and rail traffic over bridges. Others are unavoidable, such as wave loading
on offshore oil rigs, and some are accidental – a car striking a kerb, for example.
Occasionally the loads are entirely unforeseen: resonance of a slender tower under
gusting wind loads can induce large numbers of small amplitude loads. References
12–14 are useful sources of information on fatigue loading.
The designer’s objective is to anticipate the sequence of service loading throughout
the life of the structure. The magnitude of the peak load, which is vital for static
design purposes, is generally of little concern as it represents only one cycle in
millions. For example, highway bridge girders may experience 100 million significant
cycles in their lifetime. The sequence is important because it affects the stress
range, particularly if the structure is loaded by more than one independent load
system.
For convenience, loadings are usually simplified into a load spectrum, which
defines a series of bands of constant load levels and the number of times that each
band is experienced, as shown in Fig. 7.8.
7.6.3 The nature of fatigue
Materials subject to a cyclically variable stress of a sufficient magnitude change
their mechanical properties. In practice a very high percentage of all engineering
failures are due to fatigue. Most of these failures can be attributed to poor design
or manufacture.
Fatigue failure is a process of crack propagation due to the highly localized cyclic
plasticity that occurs at the tip of a crack or metallurgical flaw. It is not a single