13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Effect of thermally activated energy on dislocation emission from a blunted crack tip X. Zeng1, Q.H. Fang*1, 2, Y.W. Liu1 1 College of Mechanical and Vehicle Engineering, Hunan University, Changsha, 410082 Hunan Province, PR China 2 School of Mechanical and Manufacturing Engineering, The University of New South Wales, NSW 2052, Australia * Corresponding author: Fangqh1327@tom.com Abstract Thermally activated processes are of fundamental importance for the understanding and modeling of strength of structural materials. The effect of thermally activated on dislocation emission from an elliptically blunted crack tip is investigated. Critical stress intensity factors were simulated for edge dislocation emission from a blunt crack under mode I and mode II loading conditions at high temperature. The results show the impact of thermally activated processes is remarkable, which can affect the value of the critical stress intensity factors for dislocation emission, but doesn’t alter the emission angle. The applied electric load will increase with decreasing temperature. Keywords thermally activated, blunted crack, dislocation emission 1. Introduction Dislocation emission from a crack tip is one of the most fundamental process for understanding the ductile-brittle behavior in crystalline materials [1]. Once dislocations are emitted, they move out of the crack tip area, leaving behind a dislocation-free zone. An internal back stress due to the dislocations emitted from crack tip accommodates the stress intensity due to applied load, causing an increase in fracture toughness of materials[2]. The previous formulations [3-5] assume the crack tip to be sharp, a situation that is not observed experimentally [6]. However, a real crack in a material is always of finite length and the radius of curvature of the crack tip is never small enough to be zero [7,8]. Physically, an atomically sharp crack will be blunted by one atomic plane. On the other hand, the emission of a dislocation from crack tip results in crack blunting [9,10]. Lung and Wang [11], and Lee [7] calculated the image force on the dislocation near a blunt crack tip. Critical stress intensity factors were developed for edge dislocation emission from a blunt crack [12, 13]. In this paper, we investigated the effect of thermally activated on the emission of edge dislocation from a blunted crack tip. It is also well known that plastic deformation of nanocrystalline and ultrafine-grained metals is thermally activated. Furthermore, thermal activation parameters, such as activation volume and strain-rate sensitivity exponent, strongly depend on grain size and temperature [14-17].When thermally activated deformation processes are to be discussed, the applied stress is generally considered to have two components, i.e., ( ) , a T τ τ τ γ ∗ = + & (1) where τ is the applied shear stress, aτ is the long-range internal stress and is athermal in nature. The thermal component τ∗ is often called the effective stress and depends on test temperature T and shear strain rate γ&. The shear strain rate is formally written as an Arrhenius-type rate equation
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