PSI - Issue 41

Jesús Toribio et al. / Procedia Structural Integrity 41 (2022) 736–743 Jesús Toribio / Procedia Structural Integrity 00 (2022) 000–000

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Fig. 5. Results of the SSRT under HAC conditions performed on notched samples,: fracture stress as a function of the NTSR. 6. Conclusions 1. In slow strain rate testing (SSRT) with notched samples promoting strain localization in the vicinity of the notch tip, the local strain rate at the notch tip ( notch tip strain rate NTSR) –and not externally applied displacement rate– is the variable governing the environmentally assisted fracture (EAF) process. 2. This paper provides definitions of the local and global strain rates in notched geometries, by considering both the mathematical convergence of the method and the physical congruence between the continuum mechanics model and the physical reality of the material on the basis of its microstructure. 3. Yielding development is relevant for the evolution of the local strain rate at the notch tip. The NTSR is thus markedly increased by the spreading of the plastic zone. The importance of the sample geometry and the constitutive equation of the material is thus emphasized. 4. Local strain rate at the notch tip depends not only on the material properties and sample geometry, but also on the loading process, that is, on the global strain externally applied on the sample ends. Therefore, the relationship between local and global strain rates is not constant, but increases with time, even for constant global strain rate. 5. NTSR results are directly applicable to the modelling of EAF. Usual fracture criteria under aggressive environments can be improved by replacing global strain rates by local ones. For hydrogen assisted fracture (HAF) environmental conditions, space-time averages of the local strain rate are representative. 6. Results of SSRT on notched samples of high strength pearlitic steel in a hydrogen environment can be plotted as a single curve (geometry-independent) by expressing the critical fracture parameter of the material as a function of the local strain rate in the vicinity of the notch tip or NTSR. 7. A fracture criterion for notched samples of high strength steel under HE is formulated: fracture takes place when the Von Mises effective or equivalent stress reaches a critical value over a critical region characteristic of material microstructure ( tearing topography surface TTS). Both the TTS zone size and the critical equivalent stress of the material depend on the amount of hydrogen that penetrates the sample, and therefore on the strain rate. 8. The NTSR plays a relevant role in HE of round notched samples. The critical equivalent stress of the material is a direct function of the local strain rate in the vicinity of the notch tip or NTSR. Such a function is (within certain limits) geometry-independent , which makes the kinematic formulation of the fracture criterion consistent. 9. The asymptotic values of the critical equivalent stress of the material for quasi-static tests are obtained. Such values are a decreasingly monotonic function of the stress triaxiality level achieved in each geometry, and they limit the applicability range of the kinematic formulation of the fracture criterion in aggressive environment.