Page 364 - Marine Structural Design

340 Part III Fatigue and Fracture

Model with 20-node solid elements
Structural Detail

Model with 8-node shell elements (size: t x t)
Figure 17.5 Examples of Modelling (NTS, 1998)

17.3.2 Determination of SCF by Experimental Measurement
Determination of the SCF by using strain measurements in fatigue tests is the most reliable
method. However, it is important to decide exactly where to locate strain gauges to ensure that
the value obtained is compatible with the chosen design S-N curve. If this is not achieved,
gross error may occur.
The existing method of defining SCF for use in the S-N curves is established based on the
extrapolation to the weld toe from an area of linear stress data, which would include varying
proportions of the notch SCF depending on the weld detail and the geometric stress
concentration. This is basically due to the fundamental assumption in hotspot stress concept
since the structural geometry effects may not be completely separated from the local weld
geometry effects. Size effects, weld profiles, residual stresses, and stress distributions are
usually the sources of this variation. The weld profile effect in tubular joints, is not primarily
due to the weld shape itself, it is due to the position of the weld toe on the chord, which
significantly affects the hot spot stress at the weld toe. Therefore, a consistent stress recovery
procedure should be developed in SCF measurement.

17.3.3 Parametric Equations for Stress Concentration Factors
Given that a variety of SCFs need to be estimated on any given tubular joint, SCF
determinations have to rely more on sets of parametric equations, which account for the joint
geometry configurations and applied loading.
A stress concentration factor may be defined as the ratio of the hot spot stress range over the
nominal stress range. All stress risers have to be considered when evaluating the stress
concentration factors (SCF). The resulting SCF is derived as:
SCF = SCF, . SCF, * SCF,, . SCE;I, . SCF, (17.1 1)