ssc-367 - Ship Structure Committee

‘b =
fatigue strength of a joint applicable to T curve
for 32 & wall thickness (N/mm2)
t = wall thickness of a joint under consideration (nnn)
Although the tubular joint test data available may be insufficient
to document the size effect throughout the range of plate
thicknesses in use, the data available has been grouped, analyzed
and relative fatiguestrengthdata documented. Tolloczko and Lalani
(Reference7.8) report that sizeeffect is adequatelyrepresentedin
the Guidance Notes (Reference 1.6) and that none of the more than
300 datapoints fall below the applicableS-N curves.
Test results show that plate thickness or scale increases can
adversely affect fatigue strength, perhaps due to increase in weld
toe stresses with an increase in plate thickness. S-N curves
modified to account for thickness-effect of thick plates often
substantially affect the fatigue lives computed. Some experts
consider the applicable plate thickness correction to be mild for
typical nodes. However, additionalwork by Maddox (Reference 7.9)
indicates that thickness correction may be too severe if only the
primary plate thickness is increased. His work on cruciform-type
joints (Figure7-2) indicatesthat the joint proportions ratio (L/B)
has greater effect on fatigue strength than does the primary plate
thickness.
.-
While Maddox’s encouraging results are applicable to joints
subjected to axial tension, increased primary plate thickness
subjected to bending stresses still adversely affects the fatigue
life. A typical joint in most marine structures is likely to be
subjected to substantial bending stresses. Thus, before any
relaxation of plate thickness effect on the S-N curves is attempted
further data are necessary for a range of geometries and combined
loading conditions.
7-7