drawing direction and an initial mode I growth evolves towards a mixed mode propagation (strength anisotropy with regard to HAC). Figure 4: Evolution with cold drawing of fracture surfaces in HAC, from slightly to heavily drawn steels (left to right). Increasingly anisotropic behaviour with cold drawing (f: fatigue; I: mode I propagation; II: mixed mode propagation; F: final fracture by cleavage). FRACTOGRAPHIC ANALYSIS A fractographic analysis by scanning electron microscopy (SEM) was carried out on the fracture surfaces to elucidate the microscopic modes of fracture. Figures 5 and 6 offer respectively the microscopic topographies for hot rolled bar (steel 0 which is not cold drawn at all) and for a heavily drawn prestressing steel (steel 6 which has suffered six steps of cold drawing). In the hot rolled bar (steel 0; cf. Figure 5) there is a subcritical crack growth in mode I by tearing topography surface (TTS, cf. [9-12]) before the unstable cleavage-like propagation associated with mechanical final fracture. This special microscopic fracture mode is undoubtedly associated with hydrogen-assisted micro-damage in steels with pearlitic microstructure [11-12]. In the heavily drawn prestressing steel wire (steel 6; cf. Figure 6) the microscopic fracture mode is a kind of very deformed TTS, the deformation axis coincident with the cold drawing direction. This topography is produced as a consequence of the macroscopic crack deflection with mode II propagation, as if a previous TTS mode had been strained by shear in its own fracture plane (quasi-parallel to the drawing axis). Figure 5: Microscopic fracture mode in steel 0. Figure 6: Microscopic fracture mode in steel 6. The microscopic fracture modes of Figures 5 and 6 may be classified as hydrogen damage topographies (HDT). The appearance of this generic topography is a function of the degree of drawing so that it can be noted as HDT(i), where i is the number of cold drawing steps undergone by the steel. Thus the topography HDT(i) evolves form pure TTS (which corresponds exactly with the mode described in [9-12]) in steel 0 to HDT(6) (a kind of TTS very
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