PSI - Issue 59

Jesús Toribio et al. / Procedia Structural Integrity 59 (2024) 137–144 Jesús Toribio / Procedia Structural Integrity 00 ( 2024) 000 – 000

138

2

1. Introduction In the framework of fracture mechanics, damage tolerance and structural integrity, experimental evaluation of stress corrosion cracking (SCC) or, more generally speaking, environmentally assisted cracking (EAC) of materials is commonly performed in a laboratory by testing pre-cracked specimens. In this procedure, a pre-crack in the specimen is required for posterior SCC/EAC testing, and it is usually generated by fatigue (cyclic) loading in air environment. The procedure of fatigue preloading inevitably produces ambiguous mechanical effects in the near-tip area and, depending on the intensity of loading, a sort of overload retardation effect on SCC behaviour, as described elsewhere (Toribio and Lancha, 1996; Toribio, 1998), since the cyclic loading regime affects the further plastic zone development and thus controls the evolution of stress-strain fields in the close vicinity of the crack tip after the process of loading/unloading the specimens, thereby generating compressive residual stresses in the near-tip area after the unloading phase of cyclic preloading. This paper analyzes experimental results of slow strain rate tests (SSRT) on high-strength steel in aqueous environments under cathodic electrochemical conditions promoting hydrogen assisted cracking (HAC), a deleterious phenomenon that has received many names form the classical hydrogen embrittlement (HE) to others such as hydrogen degradation (HD) or hydrogen assisted fracture (HAF). Emphasis is placed on the effect of the fatigue pre cracking procedure, which influences dramatically the behaviour of the steel in the SSRT 2. Experimental programme The wide experimental programme is based on previous research by the author and co-workers, as described elsewhere (Toribio et al., 1991a; 1991b; 1992; 1993; Toribio and Lancha, 1992; 1993; 1996; 1998). A high-strength pearlitic steel was studied whose chemical composition and mechanical properties are given in Tables 1 and 2. The EAC experiments were SSRT with pre-cracked specimens in aqueous solution. The tests analyzed in this paper were performed at cathodic potentials to evaluate the HAC phenomenon as a key mechanism of EAC. Fig. 1 shows the experimental results of the failure load in solution F HAC (divided by the reference value at rupture in air F C ) as a function of the ratio K max / K IC . The mechanical effect of fatigue pre-cracking is beneficial for the HAC resistance of the steel, since the fracture load in aggressive (hydrogen) environment is an increasing function of the maximum stress intensity factor (SIF) K max during fatigue, i.e., the higher the fatigue pre-cracking intensity (level) K max , the higher the critical (fracture) load F HAC in a hydrogen environment, in a certain overload retardation effect on SCC behaviour (similar to that happening in fatigue), as described by Toribio and Lancha, (1996) and Toribio (1998).

Table 1. Chemical composition (wt %) of the steel. C Mn Si P S

Cr

Ni

Mo

0.74

0.70

0.20

0.016

0.023

0.01

0.01

0.001

Table 2. Mechanical properties of the steel. Young's Modulus E (GPa) Yield Strength  Y (MPa) UTS  R (MPa)

Fracture Toughness K IC (MPam 1/2 )

Ramberg-Osgood parameters*  p < 1.07  p > 1.07 P I (MPa) n I P II (MPa)

n II 17

195

725

1300

53

2120

5.8

2160

* P,n: Ramberg-Osgood Parameters  =  e +  p =  / E +(  / P ) n

In the matter of microscopic fracture modes, a special topography associated with hydrogen effects was found in the fractographic analysis: the tearing topography surface (TTS), a term coined by Thompson and Chesnutt (1979) and Costa and Thompson (1982). The relationship between the TTS zone/region and the experimental evidence of hydrogen-assisted micro-damage (HAMD) in pearlitic microstructures has been demonstrated thoroughly in the past