Issue 21

C. Maletta et alii, Frattura ed Integrità Strutturale, 21 (2012) 5-12; DOI: 10.3221/IGF-ESIS.21.01 12 and an associated reduction in the amount of stabilized martensite present resulting from the increase of transformation stresses with temperature. C ONCLUSIONS he indentation response of a pseudoelastic NiTi shape memory alloy (SMA) has been analyzed in this study, by experimental measurements and numerical simulations. Single quasi-static indentation tests have been carried out and load-displacement data have been analyzed to obtain valuable information on the pseudoelastic response of the alloy. In addition, numerical simulations have been carried out to understand better the microstructural evolution occurring during the indentation process, as well as to analyze the effects of test temperature on the indentation response of the SMA. The main results of this study are summarized as follows:  Stress induced transformation mechanisms occur in the indentation region, as demonstrated by the preliminary FE simulations, which significantly affect the indentation response of SMAs with respect to conventional elastic plastic materials;  A spherical indenter should be used in order to promote a large stress-induced transformation zone in the indentation region and, consequently, to avoid local effects due to microstructural variations. This allows the overall macroscopic response of the alloy to be measured;  The volume fraction of stabilized martensite immediately beneath the indented surface increases with increasing indentation load in single quasi-static tests, which results in an overall reduction of the shape recovery during unloading;  The functional behavior of NiTi superelastic alloys is clearly governed by an energy balance between martensite formation and the plastic deformation involved in the indentation process.  Systematic finite element (FE) studies revealed a significant effect of the test temperature and the corresponding transformation stress on the indentation response of the alloy in terms of both maximum and residual depth. R EFERENCES [1] K. Otsuka, X. Ren, Progr. Mater. Sci., 50 (2005) 511. [2] W.C. Oliver, G.M. Pharr, J. Material Res., 7 (1992) 1564. [3] G. Satoh, A. Birnbaum, Y.L. Yao, In: Proc. Of Int. Congress on Applications of Lasers and Electro-Optics, Temecula CA (2008). [4] P.D. Tall, S. Ndiaye, A.C. Beye, Z. Zong, W.O. Soboyejo, H.J. Lee, A.G. Ramirez, K. Rajan, Mater. Manuf. Processes, 22 (2007) 175. [5] A.K. Nanda Kumar, C.K. Sasidharan Nair, M.D. Kannan, S. Jayakumar, Mater. Chem. Phys., 97 (2006) 308. [6] G.A. Shaw, W. C. Crone, Mater Res Soc Symp Proc., 791 (2003) 215. [7] W.C. Crone, G.A. Shaw, D.S. Stone, A.D. Johnson, A.B. Ellis, In: Society for Experimental Mechanics, SEM Annual Conference Proceedings, Carlotte, NC (2003). [8] G.A. Shaw, D.S. Stone, A.D. Johnson, A.B. Ellis, W.C. Crone, Appl. Phys. Lett., 83 (2003) 257. [9] C. Liu, Y.P. Zhao, T. Yu, Mater. Design, 26 (2005) 465. [10] C. Liu, Y. Zhao, Q. Sun, T. Yu, Z. Cao, J. Mater. Sci., 40 (2005) 1501. [11] A.J. M. Wood, T.W. Clyne, Acta Materialia, 54 (2006) 4607. [12] A.J.M. Wood, J.H You, T.W. Clyne, Proc. SPIE , 5648(39) (2205) 216. [13] W. Yan, Q. Sun, X.Q. Feng, L. Qina, Int. J. Solids Struct., 44 (2007) 1. [14] M. Arciniegas, Y. Gaillard, J. Pena, J.M. Manero, F.J. Gil, Intermetallics, 17 (2009) 784. [15] R. Liu, D.Y. Li, Y.S. Xie, R. Llewellyn, H.M. Hawthorne, Scripta Materialia, 41(7) (1999) 691. [16] M. Saeedvafa, A Constitutive Model for Shape Memory Alloys, Internal MSC Report (2002). [17] M. Saeedvafa, R.J Asaro, LA-UR-95-482, Los Alamos Report, (1995). [18] A.C. Fisher-Cripps, Nanoindentation, Second Edition, Springer (2002). T