13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- persistent slip band. [7, 8, 9, 10, 11, 12, 13]. Non-metallic inclusions, which are present in commercial materials as a result of the production process, can also act as potential sites for fatigue crack nucleation. In the high cycle regime fatigue cracks initiate from inclusions and defects on the surface of a specimen or component. For very high cycle fatigue, fatigue cracks initiate from defects located under the surface of the specimen [14,15]. Micro-crack nucleation can be analyzed by using the Tanaka-Mura model or some of its modifications [16, 17, 18]. Fatigue crack growth prediction models based on fracture mechanics have been developed to support the damage tolerance concepts in metallic structures, [19]. A well known method for predicting fatigue crack propagation under constant stress range is a power law described by Paris and Erdogan [20]. Dexter et al. [21, 22] analysed the growth of long fatigue cracks in stiffened panels and simulated the crack propagation in box girders with welded stiffeners. He conducted cyclic tension fatigue tests on approximately half-scale welded stiffened panels to study propagation of large cracks as they interact with the stiffeners. Measured welding residual stresses were introduced in the finite element model and crack propagation life was simulated. Sumi at al. [23] studied the fatigue growth of long cracks in stiffened panels of a ship deck structure under cyclic tension loading. For that purpose fatigue tests were carried out on welded stiffened panel specimens damaged with a single crack or an array of collinear cracks. This paper presents a study of the influence of welding residual stresses in stiffened panels on effective stress intensity factor values and fatigue crack growth rate. Mode I SIF values, KI, were calculated by the FE software package ANSYS using shell elements and the crack tip displacement extrapolation method in an automatic post processing procedure. A total SIF value, Ktot, was obtained by a linear superposition of the SIF values due to the applied load, Kapp, and due to weld residual stresses, Kres. The effective SIF value, Keff, as defined by Elber [24], was considered as a crack growth driving force in a power law model. Simulated fatigue crack propagation life was compared with the experimental results as obtained by Sumi at al. [23]. The molecular dynamics (MD) simulation was implemented to analyze dislocation development in an iron cuboid model with a triangular notch tip. Numerical simulations of the fatigue crack initiation and growth for martensitic steel, based on modified Tanaka-Mura, were carried out. 2. Molecular dynamics (MD) simulation of dislocation development in iron 2.1. Methods and model Taking a close look on dislocation development leads to the necessity of atomistic scale simulation methods. Therefore, we used for the present work the molecular dynamics (MD) simulation code IMD [1]. It was developed at the Institute of Theoretical and Applied Physics (ITAP) belonging to the University of Stuttgart. In MD the atoms are seen as mass m points at the position ݎ Ԧ for which Newton´s equations of motion: ܨ ሺ ݎ Ԧ, ݐ ሻൌ݉ ∗డ డ ² ² ௧ Ԧ (1) are solved in every time step. The force ܨ ሺ ݎ Ԧ, ݐ ሻ is given by the derivative of the interatomic embedded atom method (EAM) [2] pair potential ܷ ሺ ݎ Ԧ , ݐ ሻ (Eq. 2): ܨ ሺ ݎ Ԧ, ݐ ሻ ൌെܷ ሺ ݎ Ԧ, ݐ ሻ (2) The system we investigated contains about half a million iron atoms. They form a cuboid of the size
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