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Chapter 27 Fatigue Reliability 489
27.5 Fatigue Reliability Application - Fatigue Safety Check
27.5.1 Target Safety Index for Fatigue
The basic design requirement is that the safety index describing the reliability of a component
exceeds the minimum allowable, or target, safety index.
P 2 Po (27.30)
The value of 00 and the statistics on the design variables are used to derive the expression for
the target damage level.
For a safety check expression, it is necessary to specify a minimum allowable safety index (or
target safety index), PO. The target safety index for each of the categories was chosen to be
compatible with the values selected for other similar applications:
Table 27.2 Target Safety Index (Mansour, 1997)
I Target Safety Index,
Description 00
Category 1
Category 2 2.5
and will not result in pollution; repairs will be
relatively expensive
A significant fatigue crack is considered to
compromise the integrity of the ship and put the crew
Category 3 at risk and/or will result in pollution. Severe 3.0
economic and political consequences will result from
significant growth of the crack
27.5.2 Partial Safety Factors
An alternative approach to developing probability-based design criteria for the fatigue limit
state is to use partial safety factors. Eq. (27.22) may be expressed as
N=- KA (27.31)
BmSem
Letting the cycles to failure N equal the service life, Ns; and assuming 2 = 1 .O, Eq. (27.3 1)
may be re-written as
(27.32)
