PSI - Issue 41

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

737

2

1. Introduction Environmentally assisted fracture (EAF) is a time-dependent process where kinematic events play a relevant role An important particular case of EAF is the material degradation phenomenon known as hydrogen embrittlement (HE), hydrogen degradation (HD) or hydrogen assisted fracture (HAF) in general or hydrogen assisted cracking (HAC) in the presence of cracks, i.e., when sharp defects appear in the material. The key role of kinematic variables in EAF processes has been analyzed in the past by Kim and Wilde (1979), Scully (1980), Scully and Moran (1988); Ford and Silverman (1980), Hinton and Procter (1983), Herbsleb and Schwenk (1985), Burnell et al. (1987), Mayville et al. (1987, 1989), Rieck et al. (1989) and Magnin et al. (1990). This paper analyzes hydrogen embrittlement of high-strength pearlitic steels in the presence of notches, by formulating a kinematic fracture criterion based on the notch tip strain rate . 2. On the notch tip strain rate (NTSR) With regard to the calculation of local strain rate at a crack tip ( crack tip strain rate CTSR) previous research was performed by Lidbury (1983), Congleton et al. (1985), Maiya and Shack (1985), Maiya (1987), Parkins (1987, 1989, 1990) and Andresen and Ford (1988). An inherent limitation of all these approaches for the CTSR is that they do not take into account the constitutive equation of the material. Another important inherent fault of previous models is the lack of proper definition of the reference length for evaluating the CTSR. In the matter of the calculation of local strain rate at a notch tip ( notch tip strain rate NTSR), innovative formulations were proposed by Toribio and Elices (1992), Toribio (1997a), Toribio (1997b) and Toribio (1998) comprising a rigorous definition of the global and local reference lengths to calculate the global and local strains, as well as accounting for material factors governing the process (through the plastic zone spreading), thereby inducing a clear yielding-induced increase of NTSR. 3. Calculation of the notch tip strain rate (NTSR) To analyze very different levels of notch tip strain (an thus quite distinct values of NTSR), four axisymmetric round-notched specimens of very different notch geometries were considered (Fig. 1). The dimensions of the samples were chosen as follows: -------------------------------------------------- Sample R/D A/D -------------------------------------------------- A 0.03 0.10 B 0.05 0.39 C 0.36 0.10 D 0.40 0.39 -------------------------------------------------- where R is the notch radius, A the notch depth and D the sample diameter. Cylindrical coordinates r,  and z were used. The radius of the net section was called a (therefore a = D/2-A), and the distances from the notch tip were measured through the auxiliary coordinate x (with x = a-r). Calculations were performed using the finite element method and an elastic-plastic code on the basis of the incremental plasticity theory. The material selected for the computations was a high-strength eutectoid pearlitic steel whose stress-strain curve fits the following Ramberg-Osgood equation: (1) where E = 199 GPa is the elastic modulus, and P and n the Ramberg-Osgood parameters (P = 2100 MPa, n = 4.9).  =  E +      P n