This study deals with the numerical estimation of the fatigue life represented in the form of strength-life (S-N, or Wöhler) curves of metals with different crystallographic structures, namely body-centered cubic (BCC) and face-centered cubic (FCC). Their life curves are determined by analyzing the initiation of a short crack under the influence of microstructure and subsequent growth of the long crack, respectively. Micro-models containing microstructures of the materials are set up by using the finite element method (FEM) and are applied in combination with the Tanaka-Mura (TM) equation in order to estimate the number of cycles required for the crack initiation. The long crack growth analysis is conducted using the Paris law. The study shows that the crystallographic structure is not the predominant factor that determines the shape and position of the fatigue life curve in the S-N diagram, but it is rather the material parameter known as the critical resolved shear stress (CRSS). Even though it is an FCC material, the investigated austenitic stainless steel AISI 304 shows an untypically high fatigue limit (208 MPa), which is higher than the fatigue limit of the BCC vanadium-based micro-alloyed forging steel AISI 1141 (152 MPa).

A B S T R A C T The influence of welding residual stresses in stiffened panels on effective stress intensity factor (SIF) values and fatigue crack growth rate is studied in this paper. Interpretation of relevant effects on different length scales such as dislocation appearance and microstructural crack nucleation and propagation is taken into account using molecular dynamics simulations as well as a Tanaka-Mura approach for the analysis of the problem. Mode I SIFs, K I , were calculated by the finite element method using shell elements and the crack tip displacement extrapolation technique. The total SIF value, K tot , is derived by a part due to the applied load, K appl , and by a part due to welding residual stresses, K res . Fatigue crack propagation simulations based on power law models showed that high tensile residual stresses in the vicinity of a stiffener significantly increase the crack growth rate, which is in good agreement with experimental results.Keywords dislocation; fatigue crack growth rate; microstructurally small cracks; residual stress. N O M E N C L A T U R Ea = half crack length a 0 = initial crack length a fin = final crack length C = material constant of the Paris equation CRSS = critical resolved shear stress d = slip band length da/dN = crack growth rate E = Young's modulus F → r; t ð Þ = interatomic force F max = maximum applied force F min = minimum applied force G = shear modulus K = stress intensity factor (SIF) K appl = stress intensity factor due to the applied load K res = stress intensity factor due to weld residual stresses K th = stress intensity factor threshold K tot = total stress intensity factor m = atomic mass m = material constant of the Paris equation N = number of stress cycles for the fatigue crack propagation N f = number of stress cycles for fatigue failure N g = number of stress cycles required for crack nucleation in a single grain N ini = number of stress cycles needed for the initiation of a small crack R = stress ratio R eff = effective stress intensity factor ratio U → r ; t ð Þ = interatomic embedded atom method pair potential W c = specific fracture energy per unit area Correspondence: Ž. Božić.

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