13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Healing of Fatigue Crack Treated with Surface-Activated Pre-Coating Method by Controlling High-Density Electric Current Atsushi Hosoi, Tomoya Kishi, Yang Ju* Department of Mechanical Science and Engineering, Nagoya University, Nagoya 464-8603, Japan * Corresponding author: ju@mech.nagoya-u.ac.jp Abstract A technique to heal a fatigue crack for a stainless steel by controlling a high-density electric current field was studied. The high-density electric current was applied at the crack tip using electrodes. A surface-activated pre-coating technique was used in order to improve adhesion of the crack surface. The crack on the specimen surface was observed by scanning electron microscope (SEM) before and after the application of the high-density electric current to examine the effect of the fatigue crack healing. The experimental results showed that the fatigue crack was closed and the crack growth rate of a healed specimen was decreased by the electrical stimulation. Keywords Crack Healing, Fatigue, Crack Closure, Electric Current, Stainless Steel 1. Introduction Fatigue is the main reason of failure accidents in metallic structures. To improve long term durability and reliability of structures, various methods are studied. The techniques to improve the fatigue strength, such as high-frequency quenching, carburizing, nitriding and shot peening, had been developed. They can prolong the fatigue life of materials by suppressing the crack initiation on the material surface. However, the advantages of these methods are limited for the materials where a crack had been existed. If damage of materials can be healed, it is possible to improve the reliability and durability of the industrial materials remarkably. Recently, some researchers have studied crack healing techniques in polymer materials and ceramic materials [1-3]. However, a technique to heal essentially a fatigue crack detected in metallic materials has not been established. The studies to improve the mechanical properties of metallic materials have been conducted by the application of the electric current for a few decades. Karpenko et al. [4] showed that the fatigue life of a steel was prolonged by the application of the electric current during fatigue loadings. Golovin et al. [5] studied the effect of the high-density pulse current on the crack propagation of a silicon iron during the dynamic tensile loading. They showed that the crack propagation halted when the high-density pulse current was applied just before the crack initiation. Conrad and coauthors [6-10] investigated the influence of high-density pulse current on materials in detail. They showed that the action of drift electrons influenced the dislocation mobility. As other effects, the following phenomena are known, such as the generation of Joule heating [11], the cause of the compressive stress due to Joule heating [12-14], the induction of Lorentz force [15], the cause of the electron wind force due to the flow of the electric current [16]. Although some studies to improve the mechanical and material properties by applying the electric current in metallic materials have been conducted, a method for the essential healing of a fatigue crack has not been established yet. Therefore, the authors developed the technique to heal a fatigue crack in a stainless steel by controlling the high-density electric current field [17, 18]. In these researches, it is revealed that the crack was closed and the bridging by partial melting was formed between the crack surfaces. However, the adhesion between the crack surfaces is prevented due to the oxide layer on the crack surfaces. In this study we proposed to improve the adhesion of the crack surfaces by treating surface-activated pre-coating technique, which eliminates oxide layer and coats Ni on the crack surfaces. In addition, we evaluated quantitatively the healing effect of the
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