• Τhe value of δ1 - δ2 extrapolated for Δatear → 0 is the unloaded CTOD at cracking initiation, δ* c. A linear regression was found adequate in the various cases addressed until now (see Ref. [7,10]). Equation (4) can be used in order to evaluate the partial crack closing during unloading and determine the true CTOD at cracking initiation under load. The applied correction is significant when SSY or close to SSY situations prevail during original loading until cracking initiation. LSY during loading and large strainhardening exponents decrease the importance of the correction. Bauschinger effects, low strain hardening, or LSY during unloading increase the relative importance of the correction. ACKNOWLEDGEMENTS The work of T. Pardoen was supported by a fellowship from F.N.R.S., Belgium, and by a grant from the B.A.E.F. Inc. Suggestions by Professor J. W. Hutchinson (Harvard University) about the reverse plasticity problem, and the contribution of M. Scibetta and R. Chaouadi (SCKCEN, Mol, Belgium) for the precracking of the specimens are gratefully acknowledged. REFERENCES 1. ASTM Standard (1996). E1737-96, Standard Test Method J-Integral Characterization of Fracture Toughness. In Annual Book of ASTM Standards, Section 3, Volume 03.01, American Society for Testing and Materials, Philadelphia. 2. ESIS (1992). ESIS Procedure for Determining the Fracture Behaviour of Materials. European Structural Integrity Society. 3. Tvergaard, V. and Hutchinson, J.W. (1992). J. Mech. Phys. Solids 40, 1377. 4. Li, D.M. and Bakker, A. (1997). Engng. Fract. Mech. 57, 1. 5. Pluvinage, G. and Lanvin, A. (1996). Fatigue Fract. Engng. Mater. Struct. 16, 955. 6. Ebrahimi, F. and Seo, H.K. (1996). Acta Mater. 44, 831. 7. Pardoen, T. and Delannay, F. (2000). Engng. Fract. Mech. in press. 8. Rice, J.R. (1967). In Fatigue Crack Propagation, ASTM STP 415, American Society for Testing and Materials, 247-311. 9. Budiansky, B. and Hutchinson, J.W. (1978). J. Appl. Mech. 45, 267. 10. Pardoen, T., Scibetta, M., Chaouadi, R. and Delannay, F. (1999). Int. J. Fract., in press. 11. Shih, C.F. (1983). Tables of Hutchinson-Rice-Rosengren Singular Field Quantities, MRL E-147, Division of Engineering, Brown University, Providence, R.I. 12. Hutchinson, J.W. (1979). Nonlinear Fracture Mechanics, Department of Solids Mechanics, The Technical University of Denmark. 13. Anderson, T.L., Fracture Mechanics – Fundamentals and Applications, CRC Press, Boca Raton, 1995. 14. Irwin, G.R. (1960). In Proceedings of the Seventh Sagamore Ordinance Materials Conference, Syracuse University, IV.63-IV.78. 15. Scibetta, M. and Chaouadi, R. (1997). In Small Specimen Test Techniques, ASTM STP 1329, W. R. Corwin, S. T. Rosinski and E. van Walle (Eds.). American Society for Testing and Materials, Philadelphia. 16. Pardoen, T. and Delannay, F. (1998). Metall. Mater. Trans. A 29A, 1895. 17. Joyce, J.A. and Link, R.E (1995). In Fracture Mechanics: 26th Volume, ASTM STP 1256, American Society for Testing and Materials, 142-177.
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