110 Steels: Metallurgy and Applicationsformation and failure could occur at a lower stress than would cause failure by
the single application of the stress. The design of any structure must, therefore,
take the possibility of fatigue damage into account. This is done by estimating
fatigue life at critical positions in any structure and by modifying the design, if
necessary, to ensure that the life is adequate for the intended application. The
critical positions are usually at discontinuities such as holes, joints or comers
where the local stress is high. The resulting estimate will depend on an interac-
tion between the local stresses developed in the structure and the properties of
the material used in its formed or welded condition.
Total fatigue failure usually occurs in one of two stages. The first is crack
initiation and the second is crack propagation. The total fatigue life or number of
strain cycles to failure Nt may, therefore, be estimated by combining the crack
initiation life Ni with the crack propagation life Np. These two contributions may
be estimated using data obtained from different types of specimen. The motor
industry is, however, more concerned with crack initiation. 161
Fatigue crack initiation
The crack initiation fatigue life may be estimated using data obtained from smooth
specimens, often in the form of a small tensile specimen with a gauge length of
only a few millimetres. For tests involving both tension and compression, the
ratio of length to thickness must be small enough to enable sufficient strain
to be given in compression without leading to buckling. Each specimen may
be subjected to a cyclic strain of constant amplitude, alternating between fixed
amounts of deformation in tension and compression and often using a triangular
waveform. It is often found that the cyclic stress developed during each test varies
gradually during the test, particularly for the higher strains, and then reaches a
constant value. It finally falls rapidly as the first cracks are initiated at which point
the test is terminated. For lower strains which are almost completely elastic, the
stress developed remains substantially constant up to the stage of crack initiation.
The number of cycles to crack initiation always increases as the applied strain
decreases and these parameters may be plotted against each other to give what is
called a strain-life curve. Examples of this relationship are given in Figure 1.110.
Plastic deformation occurs at the higher values of strain and the number of cycles
to give crack initiation becomes relatively small, depending on the strength and
other characteristics of the steel. At very low values of applied strain, suffi-
cient cycles to give crack initiation can never be applied. For practical purposes,
therefore, Ni becomes infinite.
The stresses and strains developed half way through the various tests and in
the stable region may be plotted against each other to form what is known as
the cyclic stress-strain curve for the steel. It may be used to calculate the local
strain that would be developed as a result of any localized applied cyclic stress.
In service, any component may be subjected to a fairly random sequence of
strains and it is clearly necessary to know the expected stress history of any
critical part of a structure before any estimate of fatigue life can be made. The
local strain history may then be calculated using the cyclic stress-strain curve. A
method must also be available to be able to calculate the contribution to fatigue
damage that would arise from each of the strains in the expected strain history.