numero25

J. Toribio et alii, Frattura ed Integrità Strutturale, 25 (2013) 124-129 ; DOI: 10.3221/IGF-ESIS.25.18 127 D ISCUSSION o analyze the results of the EAC tests on the basis of crack tip mechanics, the critical stress intensity factor for EAC to proceed K QEAC is evaluated from the experimental failure loads (Fig. 1) as K QEAC = (F EAC /F C ) K IC (air). For LAD-controlled fracture , fatigue pre-cracking may cause a strong protective effect characterized by the fracture loads ratio F EAC /F C (Fig. 1). Considering mechanical factors of EAC, the normal stresses at the crack tip surface (at x = 0) and the crack tip plastic strains may influence LAD processes [2]. Evolutions of the crack tip mean normal stress  (x = 0) during EAC after different pre-cracking regimes (Fig. 5a) are practically insensitive to the cyclic load level K max . Stresses in the interior at x > 0 must be irrelevant for LAD since it is a surface dissolution reaction. Therefore, no difference should be expected for LAD from the residual stresses produced at different K. Toughening effect of the pre-cracking on LAD-driven EAC may be associated with accelerated dissolution of the cyclic plastic zone due to the inherently higher chemical activity of its damaged crystalline structure [2]. Due to cyclic damage accumulation not only ahead of the tip but also aside of the main crack path (Fig. 3), lateral strain-enhanced dissolution may allow chemical crack blunting to compete with dissolution-induced crack extension. Then, fracture load must increase together with the LAD-driven crack blunting. The LAD process may be supposed to involve a domain with a cumulative plastic strain above a certain level. This region must be proportional to the zone of accumulated cyclic plastic strain  x Y , or probably to the active plastic size x APZ . Fig. 5b displays this correspondence according to the EAC tests and numerical data about plastic zones. Figure 5 : Mechanical factors of the crack growth by LAD: (a) evolutions of the crack tip hydrostatic stress  during EAC test after fatigue pre-cracking at K max /K IC = 0.45 (dashed line) and 0.80 (solid line); (b) sizes of the plastic zones associated with EAC tests: the terminal active plastic zone during the LAD test (x APZ at K QEAC ) which surpasses the cyclic and the monotonic plastic zones created during fatigue pre-cracking of the specimens,  x Y (K max ) and x Y (K max ) respectively. For HAC-controlled fracture , hydrogen transport to prospective rupture sites ahead of the crack tip may be supplied by two different mechanisms [8,9]: (i) sweeping by moving dislocations within the active plastic zone during load rise; (ii) diffusion in metal driven by hydrostatic stress gradient   towards the maximum tensile stress locations. With regard to the first, hydrogenation and hydrogen damage area must be about as extensive as the active plastic zone. However, experimental data on the TTS width x TTS as an indicator of hydrogen damage (Fig. 2) do not agree with calculated plasticity extent x APZ after different pre-cracking regimes (Fig. 6a). Although TTS overpassing the plastic zone size at lower K max levels in Fig. 6a may be attributed to the subcritical crack growth and plastic (damage) zone displacement next to the crack tip, it cannot be at K max = 0.8K IC when x TTS << x APZ even for stationary crack. This fact indicates that dislocational transport of hydrogen to rupture sites must not be the responsible for HAC in this case. Considering stress-assisted diffusion, the near tip distributions of the hydrostatic stress  (x) displayed in Fig. 4 indicate that, during the rising load EAC tests, initially compressive stresses  (x) < 0 and accompanying negative gradients d  /dx < 0 induced by fatigue pre-cracking delay hydrogen penetration towards rupture sites, and this effect is more pronounced for the heaviest fatigue regimes (i.e., with the highest K max ). This is supported by comparison of the evolutions of the average value of the stress gradient <   > over the range 0 < x < x  + (K QEAC ) (Fig. 6b) which delays hydrogen transportation into the metal as far as the respective component of the diffusion flux is J  proportional to   [8,9]. This scale of averaging x  + (K QEAC ) corresponds to the maximum tensile stress position and possible location of the stress- T