13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- The local concept to assess weldment with help of nano-indentation and FEM simulation Jie Fang, Huang Yuan* Department of Mechanical Engineering, University of Wuppertal, Germany * Corresponding author: h.yuan@uni-wuppertal.de Abstract In this work the local mechanical behavior and fatigue resistance of the welding material were investigated with help of nano-indentation. By assuming a power-law strain hardening, the elastoplastic properties of the welding joint material were identified from inverse nano-indentation analysis with help of finite element simulation. With the known mechanical property of the weldment the fatigue should be described by the local stresses and strains. A critical plane based Cruse-Meyer model was introduced to predict the fatigue life of weldment. The predictions agree with experiments. Combined with critical distance concept the local fatigue life model was extended to predict fatigue life of holed specimens. The present work attempted to establish more reliable and more accurate a local fatigue and damage description of the weldment for engineering structures. Keywords Nano-indentation, Weldment, Notched Fatigue, Extended Cruse-Meyer Model, Critical Distance Method. 1. Introduction With the development of aero engines, welding has been widely used to manufacture aircraft engine components for better performance. Many welding methods such as tungsten inert gas welding, plasma welding, electron beam welding (EBW), diffusion welding and friction welding have been developed and used for the welding of Inconel alloys. Compared with traditional fusion welding technology, the electron beam welding has a narrow heat-affected zone with little distortion and low residual stresses [1]. Very high quality welds can be made during the manufacturing of complex components. Being a fusion welding process, the weld joint of EBW has significantly different properties to the base material. Nano-indentation is a method to extract the mechanical properties of materials from load-displacement measurements [2]. Hardness and Young’s modulus can be obtained from the indentation curve based on the Oliver and Pharr method [3] to characterize micromechanical behavior of material. More depth knowledge about mechanical properties can be gained with help of finite element simulation through an inverse analysis procedure [4]. The local mechanical property of the weldment makes it possible to establish fatigue life prediction based on local stress and strain variations. In design of a gas turbine engine, notches and holes are not avoidable, for instance, in disks and blades, which cause local stress concentrations and lead to significant reduction in fatigue life. In industrial design the stress concentration factor, Kt, is often used to characterize the stress concentration. Under cyclic loading, the fatigue notch factor, Kf, is introduced to describe the change of fatigue strength [5]. Notched fatigue strength depends not only on Kt, but also on the stress gradient which relates the material volume with the high stress. Peterson [6] and Neuber [7] try to describe the difference between Kf with Kt through empirical relations. Siebel and Stieler [8] proposed using the stress gradient at a notch root for evaluation of the notch effect. Taylor developed the former works and proposed the theory of critical distance (TCD) [9]. Susmel and Talyor [10,11,12] successfully extended the TCD to the LCF regime. By employing an elasto-plastic critical plane approach, the TCD is extended to consider multi-axial fatigue. In this work the stress-strain relation and tensile fatigue resistance of the EB weld joints are investigated. This work aims to develop a method to determine the local elastoplastic properties of
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