Issue 40

L. Zou et alii, Frattura ed Integrità Strutturale, 40 (2017) 137-148; DOI: 10.3221/IGF-ESIS.40.12 138 I NTRODUCTION s a traditional processing technique, welding has been widely used in many fields, such as mechanical manufacturing, aerospace, transportation, etc. The fatigue analysis and life prediction of a welding joint are directly related to the stability and safety of the whole structure. Currently, the nominal stress method and the nodal based structural stress method are two most commonly used welding fatigue analysis and prediction methods. The nominal stress method is the first routine way to get theoretical and experimental research in fields of engineering fatigue design, strength assessment and life prediction of welded structures. In various industry fields, its method and data has been widely standardized, and it has been maturely applied to the actual project. But because of the existence of various preconditions and regulations, choice of the S-N curves is uncertain in this method. How to accurately select the S-N curve and to calculate the stress are the most important problems which cannot be solved in this method. The nodal force based structural stress method is a new type of fatigue life prediction technology for welded structure proposed by Dong [1]. In this method, the finite element technique is used to compute the structural stress through nodal force. Currently, the nodal force based structural stress method is one of the most striking engineering technologies for fatigue analysis of welded structures due to its mesh-insensitive hot spot stress calculation, higher fatigue life prediction accuracy and the broad applicability [2]. Dong et al. reprocessed thousands of fatigue test data of the steel welded joints in the last 50 years [3]. According to the linear regression analysis, the main S-N curve of fatigue design based on equivalent structural stress (Eq. SS) range is determined. In this study, first of all, fatigue data of aluminum alloy welded joints is collected and the fatigue database is obtained from related literatures. Then, S-N curves are fitted based on the nodal force based structural stress and the scatter degree of fatigue data is computed. Subsequently, neighborhood rough set theory is used for knowledge reduction to find the core among the many factors which influence the fatigue life of aluminum alloy welded joints. Finally, the fatigue characteristics domain is established according to the reduction result of neighborhood rough set theory and S-N curves are fitted subsequently in each domain. R ELATED W ORKS -N curve is the main tool to analyze and predict fatigue lifetime of a metallic material, component or structure. A large number of domestic and foreign scholars have devoted themselves to the study of the S-N curve modeling method. Monotonic test based empirical fatigue formulae and a Wholer field mathematical model is combined and a new formula for developing full range stress life curves for medium strength steels is proposed [4]. The importance of employing material specific S-N curves with appropriate stress concentration factors for special connection details and correct damage accumulation methods is highlighted. The fatigue crack growth of a double fillet weld with the existence of a semi-elliptical crack is studied [5]. The constant amplitude loading is applied where the influence of the load ratio over the fatigue life is presented. A new probabilistic model is proposed [6], where the model parameters are estimated with an EM algorithm for which the Maximisation step combines Newton-Raphson optimization method and Monte Carlo integrations. A new method that assumes linear change of scatter according to stress levels is developed in [7]. The algorithm derives from maximum likelihood estimation and general Newton's method. A study has been carried out to establish which confidence level in the estimation of the characteristic S-N curve from limited data [8].The results of the study provide a new way to optimize fatigue design whenever it is costly or time-consuming to achieve many reliable test data. A unified statistical model which can take into account any number of failure mechanisms and the possible presence of the fatigue limit is presented [9]. The adaptability of the statistical model to the S-N curves proposed in the open literature is demonstrated by qualitative numerical examples. Generally speaking, fatigue behavior of welded components is influenced by many factors such as temperature, material type, load type, ratio and etc. Up to now, many researchers have devoted themselves to this research and initial achievements have been obtained. For example, plate thickness factor is considered and a new analytical formula of fracture toughness is proposed based on the energy theory and linear elastic mechanics [10], which would significantly reduce the calculation cycle of remaining life of structures in structural integrity assessment of welded structures ． Crack initiation potential in materials containing defects is investigated numerically by focusing on defect types, size, shape, location, and residual stress influences [11]. Results show that the crack initiation potency is higher in case of serious property mismatching between matrix and defects, and higher strength materials are more sensitive to soft inclusions. Near-threshold fatigue crack growth tests are conducted at various stress ratios and different pre-cracking locations of a 25Cr2Ni2MoV welded joint by using load-shedding procedure at room temperature to investigate the transition behavior of fatigue crack growth curve [12]. A S