Fatigue Life Assessment of Welded Joints Considering Crack Propagation Based on Hot Spot Stress

In this study, fatigue test and crack propagation analysis of welded joints in ships and offshore structures were performed. The test models have two types of welded joints, T-joint and double plate. Test data is presented in S-N curves obtained by the Hot Spot Stress (HSS) approach. The S-N data for each length of crack were appraised to define the fatigue life of crack initiation and propagation. In the analysis of crack propagation of welded joints the effect of residual stresses is to be considered when evaluating effective Stress Intensity Factor (SIF) range. To analyze the residual stress field, thermal elastic-plastic analysis was performed considering temperature dependent material properties. The test results of propagation life and crack shape were compared with the data predicted from numerical analysis. Two sets of results were proven to be completely comparable. INTRODUCTION Fatigue design guidance for ship structures in Korea is based on BS5400, where S-N curves for various joint classes of bridges are suggested. Use of this standard may cause problems with inaccurate fatigue life evaluations of ship structures as their welded joints and loading types are much more complex than bridges. These S-N curves provide no standard for determining the amount of damage to structural members or what crack size is allowed for the safe operation of ship 1 structures. The quantification of fatigue damages requires definition of the stresses on S-N curves. In this study fatigue tests of welded joint were performed for T-joint and double plate of ship structures. S-N curves were obtained for various crack lengths by the HSS approach including geometric stress concentration. Life of fatigue crack growth could be predicted by numerical analysis. 1 MATERIAL AND TEST MODELS For fatigue assessment, T-joint and double plate models were prepared as presented in Fig.1. A 15 mm thick base plate was made from KR (Korea Register of Shipping) steel, grade AH32. This is high tensile steel widely used in shipbuilding. Chemical compositions and mechanical properties of AH32 are given in Table 1 and Table 2. A Flux Cored Arc Welding (FCAW) method was used for these models and the welding conditions are listed in Table 3. Table 1 Chemical composition of base material (10 %) C Si Mn P S Al Ti Ni Cu Cr Mo 14 32 131 21 12 43 1 1 1 1 1 Table 2 Mechanical properties of base material (AH32) Yield stress Tensile stress Elongation 330 MPa 505 MPa 29 % Copyright © 2001 by ASME Table 3 Welding conditions Model Current (A) Voltage (V) Welding speed (mm/min.) Method T-joints 240 27 357 FCAW Double plate 280 31 372 FCAW