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V. Tvergaard., Frattura ed Integrità Strutturale, 1 (2007) 25-28 28 energy assumed in the traction separation law. The dif- ferent curves correspond to different values of the peak stress ˆ σ for the traction separation law, normalised by the initial yield stress. The curves show two typical fea- tures of such results, that the fracture toughness level is very sensitive to small increases of the peak stress, and that the curves have minima for near mode I conditions at the crack-tip. Figure 3: Steady-state interface toughness as a function of the local mixity measure 0 ψ , for 1 1 / 0.003 Y E σ = and 2 Y σ ∞ , considering different values of 1 / , Y σ σ 2 1 / 2 E E = . (From [23]). 4 REFERENCES [ 1] A.L. Gurson, Engng. Mater. Technol., 99 (1977) 2. [2] V. Tvergaard, Advances in Applied Mechanics , Academic Press, Inc., 27 (1990) 83. [3] V. Tvergaard, Acta Metallurgica, 32 (1984) 1977. [4] J. Lemaitre, A Course on Damage Mechanics. Springer-Verlag, (1992). [5] V. Tvergaard, J.W. Hutchinson, J. Mech. Phys. Sol- ids, 41 (1993) 1119. [6] V. Tvergaard, J. Mech. Phys. Solids, 30 (1982) 399. [7] V. Tvergaard, Int. J. Fracture, 17 (1981) 389. [8] V.Tvergaard, A.Needleman, Acta Metall., 32 (1984) 157. [9] V. Tvergaard, A. Needleman, J. Mech. Phys. Solids, 40 (1992) 447. [10] K.K. Mathur, A. Needleman, V. Tvergaard, J. Mech. Phys. Solids, 44 (1996) 439. [11] V. Tvergaard, Modelling Simul. Mater. Sci. Eng., 9 (2001) 143. [12] V. Tvergaard, A.Needleman, 3D Charpy Specimen Analyses for Welds, to appear in Proc. Charpy Centenery Conf. (2001). [13] J.B. Leblond, G. Perrin, J. Devaux, J. Appl. Mech. 61 (1994) 236. [14] V. Tvergaard, A. Needleman, Int. J. Solids Struc- tures 32 (1995) 1063. [15] A. Needleman, V.Tvergaard, Eur. J. Mech., A/Solids, 17 (1998) 421. [16] D.R. Hayhurst, P.R.Brown, C.J. Morrison, Philoso- phical Transactions, Royal Society London A311 (1984) 131. [17] M.F. Ashby, B.F. Dyson, National Physical Labora- tory, Report DMA(A), (1984) 77 . [18] V. Tvergaard, Int. J. Fracture, 42 (1990) 145. [19] E. van der Giessen, V. Tvergaard, Acta Metall. Mater., 42 (1994) 959. [20] P.R. Onck, E. van der Giessen, Mech. Mater, 26 (1997) 109. [21] N. Ohno, J.-D. Wang, Int. J. of Plasticity, 9 (1993) 375. [22] V. Tvergaard, T.Ø. Pedersen, Arch. Mech., 52 (2000) 799. [23] V. Tvergaard, J. Mech. Phys. Solids, 49 (2001) 2689.