Page 344 - Marine Structural Design

320 Part 111 Fatigue and Fracture

Number of cycles to failure N

Figure 16.2 Illustration of Fatigue Life( Initiation and Propagation
Stages)
The first two factors in the above list are explicitly accounted for in fatigue design codes.

16.4 Cumulative Damage for Variable Amplitude Loading
Much of the fatigue data discussed so far was generated from constant amplitude and constant
frequency tests. However, these results are not realistic in actual field service conditions.
Many structures are subjected to a range of load fluctuations and frequencies. In order to
predict the fatigue life of a structural detail subjected to a variable load history based on
constant amplitude test data, a number of cumulative damage theories have been proposed. For
instance, the Palmgren-Miner cumulative damage law (Miner, 1945) states that.
i"=1 (1 6.2)
i=t Ni
where,
k = Number of stress range levels in the block of load spectrum

Si = ith stress range level
ni = Number of stress cycles applied at Si
Ni =Fatigue life at Si
The hypothesis of Miner was originally based on several assumptions (Fricke et al, 1997):
sinusoidal load cycles
purely alternating load
crack initiation as the failure mode
No contribution to damage by load cycles below the endurance limit
Sequence of load cycles not considered