Page 358 - Marine Structural Design

334 Part III Fatigue and Fracture

structures. The latest developments in the research on S-N curves may be found from Maddox
(2001).
In the API Rp 24 the X' curve is used for welded connections without profile control,
thickness correction applies if wall-thickness is greater than 0.625 inches (16 m). In API
W2A, thickness correction exponent k is taken as 0.25. The X curve is used for welded
connections with profile control, and wall-thickness correction factor applies when the wall-
thickness is greater than 1 inch (25 mm). However, after thickness correction, the X-curve
shall not be reduced to be lower than the X' curve.
MI S-N curves are single (not bi-linear) and have an endurance limit. The endurance limit for
the X curve and X' curve is 35 MPa and 23 MPa respectively. K and m for the X curve are
1.15E15 and 4.38 respectively. For the X' curve, K and m are 2.50E13 and 3.74 respectively.
The classification societies define fatigue criteria in their Rules and guidancelguidelines.
IACS requirements for fatigue assessment have been developed by unifying the requirements
of individual classification societies for ship structural assessment. The fatigue S- curves for
ship structures are mainly based on UK Den basic S-N curves and IIW S-N curves.
The IIW S-N curves assume that the slope of a1 S-N curves is m=3 and the change in slope (m-
5) occurs for N=5x10E6 cycles (Hobbacher, A. (1996). These S-N curves are based on
nominal stress range and correspond to non-corrosive conditions, are given for mean minus
two standard deviations. Their fatigue class is characterized by the fatigue strength at 2xIOE6
cycles, e.g. the stress range corresponding to 2xlOE6 cycles (FAT) are 160,140,125,112,100,
90,80,71,63,56,50,45,40,36, see Table 17.1.
BV (1998) proposed corrections of the design S-N curves to account for the various factors
such as:
Influence of static and residual stresses: Tensile residual stress of the magnitude of yield
stress will reduce fatigue life, and in such cases the maximum stress should be assumed to
be yield stress irrespective of the amount of actual maximum stress. Post-weld treatment
may improve the weld geometry and fatigue capacity.
Influence of compressive stresses: To account for the less damaging effect of
compressive stresses while the stress range is greater than the yield stress, the calculated
local stress range S,,,may be corrected using the British Standard 5400: defining stress
range as:

S = O, +0.6(Sl,,, -0,) for o,, S SlOeal 5 20, (1 7.6)
S = 0.8Sl,,, for > 20, (1 7.7)

. Influence of plate thickness
Influence of the material: the fatigue strength of welded joints is nearly independent on
the material properties such as material grades. However, for machined plates the effect of
yield strength is large.
Influence of the environment
Workmaship: S-N curves have been derived for standard workmanship and welding
procedures. In some instances, the effect of imperfection and misalignment should be
taking into account when determining the hot spot stresses.