PSI - Issue 62

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

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recombination poisons such as sulphides, thiocyanates or arsenic or selenium compounds is critical. The duration of this stage can be more or less long depending on the characteristics of the materials and the environment. Once the hydrogen penetrates the steel it tends to concentrate in the areas of maximum tensile stress, e.g. in the areas close to defects present on the surface of the material (e.g. the pit istelf) or notches. Indeed, the FIP recommendations prescribe that there are no longitudinal defects on the surface of the material up to a depth of 1% of the diameter of the wire or bar, recommending not exceeding 50 µm. That despite, the presence of widespread forms of corrosion (up to a certain extent) cannot be considered a reason for rejecting the material as indicated in the ASTM A416/A416M (Standard specifications for low relaxation, seven wire steel strand for prestressed concrete) that at point “8. Workmanship, finish and appearance” prescribes that slight rusting, provided it is not sufficient to cause pits visible to a person with normal or corrected vision, shall not be cause of rejection”. Similarly in the Manual for Quality Control for Plants and Production of Precast and Prestressed Concrete Products published by PCI, under Section 2.2.2, it is reported that the presence of light rust on strand does not necessarily shall be considered a problem because not detrimental to the bond (Sason, 1992). When hydrogen enters the metal matrix it inhibits dislocation slide and promotes metal matrix embrittlement. Near the crack tip triaxial stress condition dilates the crystal lattice and attracts atomic hydrogen from less stressed zones. When the quantity of hydrogen and the tensile stresses in front of the defects reach sufficiently high values, a microfracture is triggered which gives rise to actual cracks. The cracks then propagate with the same mechanism (Toribio & Kharin, 1997).

Fig. 2. HAC promoted by SCN - anions on steel strand (optical image, upper, and SEM image, lower) (Messina et al., 2021)

Actually, crack tip is blunted by plastic flow and a rather moderate stress level is always there maintained which can never reach the lattice strength and thus cause local rupture. The maximum tensile stress is achieved beyond the tip where nucleation of a separate micro-crack is facilitated (W. W. Gerberich et al., 1991) by dislocation instabilities, micro localizations and incompatibilities of plastic strains associated with dislocation arrays, slip bands promoted by hydrogen enhanced plasticity (Hirth, 1994). Hydrogen concentration achieves a maximum at the points