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Fatigue Damage

Theory

The damage due to stress range k in seastate i, direction j, as defined by the Palmgren-Miner Rule, is found by dividing the actual number of occurrences of stress range Sk, that is nijk, by the number of cycles of this stress range required to cause failure. This latter quantity is denoted N(Sk) or Nk, and is found from the appropriate S-N curve. Denoting the damage due to stress range k in seastate i, direction j, as dijk we write:

               (1)

and the accumulated damage in the response to seastate i due to all stress ranges and directions, denoted di, is given by:

               (2)

The accumulated damage in one year due to all seastates, which is denoted d1, is given by:

               (3)

According to the Palmgren-Miner Rule the fatigue life at a particular hot spot is 1/d1 years. This is the procedure used to predict fatigue life in LifeFrequency.

Relevant Keywords

•*S-N CURVE is used to define fatigue analysis S-N curves.

•*FATIGUE DATA is used to assign fatigue data to hot spot sets.