PSI - Issue 41

Jesús Toribio et al. / Procedia Structural Integrity 41 (2022) 728–735 Jesús Toribio / Procedia Structural Integrity 00 (2022) 000–000

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4. Application to slow strain rate tests (SSRT) The concept of CTSR was applied to the kinematic modelling of SSRT on precracked samples under HAC environmental conditions. The tests were performed on a high-strength eutectoid pearlitic steel, supplied in bar form of 12 mm diameter. The mechanical properties were Young’s modulus E = 195 GPa, yield strength  Y = 725 MPa, ultimate tensile strength (UTS)  R = 1300 MPa and elongation at UTS  UTS = 8.0 %. The specimens were transverse precracked rods of the same diameter (12 mm) as the commercial bar. The maximum stress intensity factor (SIF) during fatigue pre-cracking was 0.28 K IC , where K IC is the fracture toughness (53 MPa m 1/2 for this steel). Control of the SIF during the precracking process was accomplished by a method based on the compliance of the sample. The relationship between crack depth a and cylinder diameter D was a / D = 0.3. The samples were subjected to SSRT, covering a broad range of crosshead speeds or applied displacement rates from 10 –9 to 10 –5 ms –1 . The aggressive environment was an aqueous solution of 1 g/l Ca(OH) 2 plus 0.1 g/l NaCl (pH=12.5). All tests were carried out at constant potential and room temperature by using a potentiostat and three electrodes with E= –1200 mV SCE (saturated calomel electrode), i.e., cathodic regime and pure HAC conditions. Fig. 2 offers the macroscopic results of the SSRT under HAC conditions, expressed as a plot representing failure load in the solution F c (divided by failure load in air F o ) vs. the applied displacement rate, showing the classical trend of HAC tests: the higher the displacement rate, the lower the embrittlement, since the hydrogen has less time to enter and diffuse towards the inner parts of the sample. For very low values of the applied displacement rate ( quasi-static tests), the stationary situation for the hydrogen diffusion is reached, and thus the embrittlement effect does not increase further as the displacement rate decreases, but remains constant at a value F c /F o =0.41, as shown by the horizontal line in the plot of Fig. 2.

Fig. 2. Macroscopic results of the SSRT under HAC conditions. The fractographic analysis of all samples by scanning electron microscopy showed evidence of hydrogen-assisted micro-damage (HAMD) in the close vicinity of the crack tip, in the form of tearing topography surface (TTS), a term used to describe a variety of hydrogen-affected zone consisting of micro-tearing at the finest microscopic level, as shown in Fig. 3. It has been proved (Toribio, 2012) that the TTS zone is associated with HAC in pearlitic steel. In addition, Toribio and Vasseur (1997) and Toribio (2012) analyzed the evolution of HAMD in the form of different sub-modes of TTS region depending on the level of hydrogenation. Furthermore, Toribio (1997c) performed a fracture mechanics approach to HAMD in pearlitic steel, showing that the extension (progress) of this micro-damage (TTS) in cracked specimens can be modeled as a macroscopic crack prolonging the original fatigue precrack and involving LEFM principles.