PSI - Issue 62

Edoardo Proverbio et al. / Procedia Structural Integrity 62 (2024) 285–298 Author name / Structural Integrity Procedia 00 (2019) 000–000

290

6

of extreme hydrostatic and tensile stress. Following widely accepted views of the HAC phenomenon, the critical distance from the crack tip where hydrogen concentration and stress are maxima correspond to the crack advancement in a jump-like propagation mechanism. The parameter controlling the sub-critical crack propagation is the stress intensity factor K ( = √ ), where a is the defect size, β a geometrical constant between 0.5 and 2 and the stress in the undisturbed area. Following the jump-like propagation approach crack propagation consists of a series of loops (cycles) hydrogen accumulation – local rupture – crack advance. Crack initiation is possible when K reaches a threshold value K th . K th declines systematically with increasing amount of H absorbed on the crack tip (Gangloff, 2008) and depends on metallurgical as well as environmental factors. When the intensity factor is close to K th the crack propagation rate is very low. The final fracture (third step) occurs when due to crack growth the intensity factor K reaches the material toughness (Kc), in such a condition the critical defect size is

2 2 2

=

(2)

In heavily cold drawn steel elements (i.e. wires) surface fracture is characterized by a typical mixed mode crack propagation pattern (mode I and mode II) with a multi-terrace appearance with shear cracking (SC) of the cementite lamellae and the mechanism of hydrogen enhanced decohesion (HEDE). The micro mechanism of fracture evolves from predominantly SC in slightly drawn steel to predominantly HEDE in heavily drawn steel (Proverbio & Longo, 2007). Secondary cracks due to internal stresses can be also present (Fig. 3).

Fig. 3. Surface fracture of cold drawn prestressing steel wire following HAC testing (Proverbio & Longo, 2007)

In the presence of non-alkaline electrolytes containing nitrates (pH value < 9), unalloyed and low-alloy steels can undergo anodic stress corrosion cracking. The formation and subsequent propagation of cracks are due to a selective dissolution of the metal (e.g. along the grain boundaries of the steel structure) with a simultaneous effect of high