13th International Conference on Fracture June 16–21, 2013, Beijing, China -9- 4. Discussions One of the reasons that the crack was closed and healed as shown in Figs. 6 and 7 is thought to be thermal compressive stress due to Joule heating caused by the high-density electric current field formed at the clack tip. When the electric current is applied as crossing a crack, it flows along the crack because of the electric resistance on the crack surface. Therefore, the high-density electric current field is formed at the crack tip. The area at the tip and the vicinity of the crack is heated rapidly and expands due to Joule heating. On the other hand, the outside area of the crack tip where the high-density electric field is not caused remains intact. Therefore, the direction of expansion is restricted and the thermal compressive stress toward crack closure is caused due to the thermal expansion and it is thought that the crack is closed. Moreover, it is thought that the crack surfaces are easily bonded because the oxide layer preventing the bonding between each crack surfaces is eliminated by the surface-activated pre-coating technique, and the Ni film works as the well-adhesion inner layer. In addition, for every application of the electric current, the crack is closed, and the crack tip transfers to the direction to the notch. Therefore, the area of the current concentration continuously transfers to the direction of the notch and the whole crack was closed by the application of electric current. On the other hand, the crack growth rate decreased temporarily as shown Fig. 11. The reason the crack growth was decreased is thought that the crack closure and bonding between the crack surfaces influenced on fatigue crack growth by applying electric current. The crack closure reduces the driving force for the crack propagation because of the decrease of the crack opening displacement. The bonding between the crack surfaces also makes the crack opening suppressive. 5. Conclusion The technique to heal fatigue crack treated with surface-activated pre-coating method by controlling high-density electric current field was researched. The closure of the crack and the bonding between crack surfaces were realized by applying high density electric current. The crack was closed 75-97 % from original crack by improving adhesion between crack surfaces and applying the multiple electric current. Moreover, it was observed that the crack growth rate was decrease temporarily after the application of the electric current. As a result, it was indicated that the technique with the electrical stimulation has the potential to heal a fatigue crack. Acknowledgements This work was supported by the Japan Society for the Promotion of Science under Grant-in-Aid for Young Scientists (A) 23686021. References [1] X. Chen, M.A. Dam, K. Ono, A. Mal, H. Shen, S.R. Nutt, K. Sheran and F. Wudl, A thermally re-mendable cross-linked polymeric material, Science, 295 (2002) pp. 1698-1702. [2] S.R. White, N.R. Sottos, P.H. Geubelle, J.S. Moore, M.R. Kessler, S.R. Sriram, E.N. Brown and S. Viswanathan, Autonomic healing of polymer composites, Nature, 409 (2001) pp. 794-797. [3] K. Ando, Y. Shirai, M. Nakatani, Y. Kobayashi and S. Sato, (Crack-healing + proof test): a new methodology to guarantee the structural integrity of a ceramics component, J Eur Ceram Soc, 22 (2002) pp. 121-128. [4] G.V. Karpenko, O.A. Kuzin, V.I. Tkachev and V.P. Rudenko, Influence of an electric current upon the low-cycle fatigue of steel, Sov Phys Dokl, 21 (1976) pp. 159-160. [5] Y.I. Golovin, V.M. Finkel and A.A. Sletkov, Effects of current pulses on crack propagation
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