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Monday, September 22, 2008
In recent years, Nickel Titanium based shape memory alloys (NiTi), have received much attention from scientific and engineering communities, owing to their unique characteristics, namely shape memory effect (SME) and superelastic effect (SE) [1]. These properties are due to a reversible solid state phase transformation between austenite and martensite; which can be activated by a temperature change (TIM, Thermally Induced Martensite) or by the application of external forces (SIM, Stress Induced Martensite). Up to now, a lot of researchers have studied the thermo-mechanical behavior of SMAs, but a very limited number of works has been devoted to study the fracture behavior of SMAs [2-4].
The evolution of stress induced martensitic transformation in front of the crack tip in a NiTi alloy is numerically analyzed in this investigation, by 2-D finite element simulations of single edge-crack specimens. In particular, the transformation start and finish contours, i.e. the boundaries of the transformation zone, were obtained by using plasticity concepts, and the effects of the temperature were taken into account by using the Clausius-Clapeyron relation. Furthermore, comparisons between numerical and analytical results, obtained by modified linear elastic fracture mechanics relations, were carried out. These comparisons show that the analytical approach is able to describe the stress field in the crack tip region outside the phase transformation zone, i.e. in the austenitic region.
References
[1] K. Otsuka and C.M. Wayman, in: Shape memory materials, edited by Cambridge University Press, Cambridge, UK (1998).
[2] S. Yi and S. Gao: Int. J. Solids Struct. Vol. 37 (2000), pag. 819
[3] K. Gall, N. Yang, H. Sehitoglu and Y.I. Chumlyakov: Int. J. Fract. Vol. 109 (2001), p. 271
[4] J.H. Chen, W. Sun and G.Z. Wang: Metall. Mater. Trans. Vol. 36A (2005), p. 941
The evolution of stress induced martensitic transformation in front of the crack tip in a NiTi alloy is numerically analyzed in this investigation, by 2-D finite element simulations of single edge-crack specimens. In particular, the transformation start and finish contours, i.e. the boundaries of the transformation zone, were obtained by using plasticity concepts, and the effects of the temperature were taken into account by using the Clausius-Clapeyron relation. Furthermore, comparisons between numerical and analytical results, obtained by modified linear elastic fracture mechanics relations, were carried out. These comparisons show that the analytical approach is able to describe the stress field in the crack tip region outside the phase transformation zone, i.e. in the austenitic region.
References
[1] K. Otsuka and C.M. Wayman, in: Shape memory materials, edited by Cambridge University Press, Cambridge, UK (1998).
[2] S. Yi and S. Gao: Int. J. Solids Struct. Vol. 37 (2000), pag. 819
[3] K. Gall, N. Yang, H. Sehitoglu and Y.I. Chumlyakov: Int. J. Fract. Vol. 109 (2001), p. 271
[4] J.H. Chen, W. Sun and G.Z. Wang: Metall. Mater. Trans. Vol. 36A (2005), p. 941