Issue 9

C. M. Sonsino, Frattura ed Integrità Strutturale, 9 (2009) 3-12 ; DOI: 10.3221/IGF-ESIS.09.01 7 Figure 10 : Modification of the SN-curve and calculation of fatigue life (schematically). Figure 11 : Crack propagation law for steel in air and seawater. The crack propagation calculations were carried out using the single edge model, Fig. 12. The effect of possible tensile residual stresses was covered conservatively using the crack propagation data with R = 0 [9] even though the tests were carried out with R = -1. The FE-modelling of the critical area of the brace-chord connection was carried out to determine the hot-spot stresses and the modelling of the weld toe for the calculation of the local stresses for the reference radii r ref = 0.5 and 1.00 mm, Fig. 13. The hot-spot stress (von Mises) was calculated for the critical crack initiation region (  = 105°) by a FE-Model in the program system MARC using curved four-node “thick” shell elements. The stiffening effect of the weld seam on chord wall bending itself was not modelled. The notch stress was calculated by a plane cross sectional model (in finite elements) subject to prescribed end displacements (including end notations) taken from the three dimensional tubular joint model. The linear-elastic stress was determined for the plane strain condition for measured minimum weld toe radius of r = 0.5 2 105.1  3 104  cycles mm Stress intensity ΔK Artificial seawater (f = 1 Hz) Air (f = 1-10 Hz) K ｪ c,Air Crack propagation rate da/dN 10 -2 10 -3 10 -4 10 -5 10 -6 10 3 3 1.5 6 N/mm 3/2 28.3 m 105.7 C 86.2 m 103.5 C 1.0R )K(C dN da sea 14 sea Air 13 Air m            Material: Fe E355