Page 398 - Marine Structural Design

374 Part III Fatigue and Fracture

20.1.2 Terminology

Some terms, applied in fatigue analysis, have specific meanings as defined below:
Mean Zero-Crossing Period: The mean zero-crossing period is the average time between
successive crossings with a positive slope (up crossings) of the zero axis in a time history.
Random Waves: represent the irregular-surface elevations and associated water particle
kinematics of the marine environment. Random waves can be represented analytically by a
summation of sinusoidal waves of different heights.
Regular Waves: are un-directional waves having cyclic water particle kinematics and surface
elevation.
Sea state: is an oceanographic wave condition, which can be characterized for a specified time
period as a stationary random process.
Significant Wave Height: is the average height of the highest 1/3 of all the individual waves
presented in a seastate.
Transfer Function: is defined to be the ratio of a structural response quantity to the wave-
height as a frequency function.
S-N curves: empirically represent relationships between stress range and number of cycles to
failure.
Nominal Stress: is the stress determined from member section properties and the resultant
forces and moments at the members end. The section properties must account for the existence
of thickened or flared stub ends.
Hot Spor Stress: is the stress located at the weld toe of a structural detail.

20.2 Spectral Fatigue Analysis

20.2.1 Fatigue Damage Acceptance Criteria
The fatigue damage assessment is based on Miner's rule:

(20.1)

where D,,, is the accumulated life time fatigue damage, q is the allowable damage ratio and
Ni is the number of cycles to failure at stress Si as defined by the S-N curve of the form:

N=K.S-" (20.2)
20.2.2 Fatigue Damage Calculated Using Frequency Domain Solution
Fatigue Damage for the i-th Sea-State
For narrow banded response, the accumulated damage of a sea-state may be expressed in the
continuous form:

(20.3)