PSI - Issue 33

Jesús Toribio et al. / Procedia Structural Integrity 33 (2021) 1131–1138 Jesús Toribio / Procedia Structural Integrity 00 (2021) 000–000

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weakest link posterior to the process zone, so that when a critical equivalent is achieved over this weakest link the unstable fracture takes place. According to the described formulation, the critical domain or fracture region is not constant, but depends on the stress triaxiality in the notched geometry under consideration (in contrast with the pure weakest-link approach in which the critical length was constant and equal to two cleavage facets). Although the new process-zone approach is more difficult due to microstructural considerations, it may also be implemented in a computer, since the size of the afore-said process zone can also be predicted by numerical analysis (a critical value of triaxiality below which the fracture is by MVC), as explained with regard to the MFMs. In a similar way, this fracture criterion on the basis of a process zone concept has been successfully applied to the modelling of the fracture process in hydrogen environment of notched samples of the same high-strength steel as used in this work (Toribio and Elices 1992). In this case of environment-sensitive fracture, the process zone in a hydrogen environment is the zone microstructurally affected by the hydrogen (the so called tearing topography surface or TTS, cf. Toribio and Elices (1992)) a special region that resembles micro-damage which can be evaluated and quantified by scanning electron microscopy, and whose asymptotic size is again predictable by numerical analysis. 6. Conclusions A process-zone fracture criterion is proposed for high-strength pearlitic steel notched bars in the following way: fracture will take place when the effective or equivalent stress in the von Mises sense reaches a critical value in the middle of the first cleavage facet after the process zone or MVC region. Whereas the critical value of the equivalent stress is a constant material's characteristic, the process zone size is a function of the stress triaxiality in the vicinity of the notch tip. References Beremin, F.M., 1980. Influence de la triaxialité des contraintes sur la rupture par déchirement ductile et la rupture fragile par clivage d'un acier doux. Journal de Mécanique Appliquée 4, 327-342. Beremin, F.M., 1981. Study of fracture criteria for ductile rupture of A508 steel. ICF5-Advances in Fracture Research , Francois, D. (Ed.). Pergamon, Oxford, pp. 809-816. Hancock, J.W., Brown, D.K., 1983, On the role of strain and stress state in ductile failure. Journal of the Mechanics and Physics of Solids 31, 1 24. Lewandowski, J.J. Thompson, A.W., 1986, Effects of the prior austenite grain on the ductility of fully pearlitic eutectoid steel. Metallurgical Transactions 17A, 461-472. Park, Y.J., Bernstein, IM., 1979. The process of crack initiation and effective grain size for cleavage fracture in pearlitic eutectoid steel. Metallurgical Transactions 10A, 1653–1664. Pineau, A., 1981, Review of fracture micromechanisms an a local approach to predicting crack resistance in low strength steels. ICF5-Advances in Fracture Research , Francois, D. (Ed.). Pergamon, Oxford, pp. 553-577. Toribio, J., 1996, Fracture of high-strength steel notched bars: a fractographic and numerical study . Localized Damage IV , Nisitani, H. et al ., (Eds.), Computational Mechancis Publications, Southampton, pp. 701-708. Toribio, J., Elices, M., 1992, The role of local strain rate in the hydrogen embrittlement of round-notched samples. Corrosion Science 33, 1387 1409. Toribio, J., Vasseur, E., 1996, Numerical analysis of micro-fracture maps in notched specimens. ECF11-Mechanisms and Mechanics of Damage and Failure , Petit, J. et al . (Eds.), EMAS, West Midlands, pp. 1951-1956.