PSI - Issue 41

Jesús Toribio et al. / Procedia Structural Integrity 41 (2022) 712–717 Jesús Toribio / Procedia Structural Integrity 00 (2022) 000–000

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Keywords: crack/fracture profile; crack tip blunting; crack re-sharpening; macro-crack path; micro-crack path; microstructurally-induced crack path; crack path micro-roughnmess; crack path micro-deflections; locally multi-axial crack path; zig-zag crack path; deflected crack path; bifurcated crack path; kinked crack path; crack shape; crack aspect ratio; preferential crack path; path near a crack; path-indpependent integral; J -integral.

1. Introduction In fracture mechanics approaches, damage tolerance analyses and structural integrity assessment, the basic idea of crack path plays a substantially relevant role. This paper advances towards a new concept of crack path covering different length scales ( multi-scale approach ) and distinct points of view ( multi-perspective approach ). Both approaches are formulated by re-visiting examples of previous research results by the author in the field of fatigue and fracture of progressively cold drawn pearlitic steels. 2. Multi-scale approach to the concept of crack path With regard to the crack/fracture profile in a vertical plane of analysis (i.e., perpendicular to the crack front, on assuming a crack contained in a horizontal plane), it can be evaluated by means of a fracto-materialographic (or fracto-metallographic) analysis to obtain the physical crack path that can be evaluated at different length scales (macro- & micro-), as explained in the following sections of the paper. The fracto-materialographic analysis is two-fold: (i) on one side, it studies the appearance of the fracture path (fractographic approach); (ii) on the other side, it analyzes the interaction between specific crack paths and material microstructure that is revealed after cutting and polishing (along a plane perpendicular to the crack front) to show the microstructural material features affecting the crack path profile . 2.1. Macro-crack path profile Fig. 1 shows the evolution of crack paths in pearlitic steels under hydrogen assisted cracking (HAC) conditions, where a progressive change appears in the macro-crack path profile as the drawing degree increases (Toribio, 2017). Fig. 1a offers a 3D-view of these fracture surfaces, showing that crack path deflection appears from a certain cold drawing level (mixed mode crack growth) starting just at the fatigue crack tip, i.e., with no subcritical HAC propagation distance in mode I. Fig. 1b shows the geometric parameters describing the crack path, whereas the evolution of the fracture profile as the degree of cold drawing increases is given in Fig. 1c. In the first drawing steps (slightly drawn steels 0 and 1) the crack growth develops in mode I during HAC. In steels 2 and 3 there is a sudden deflection in the hydrogen-assisted crack (more pronounced in steel 3) generating a wavy crack shape. In the most heavily drawn steels (4 to 6) the crack deflection takes place suddenly after the fatigue precrack and the deviation angle is even higher. Crack branching is seen just after the fatigue precrack tip, i.e., there are two pre-damage directions (crack embryos ), only one of which becomes the final fracture path. Fig. 2 shows the evolution of crack paths in pearlitic steels under localized anodic dissolution (LAD) conditions, where a progressive change appears in the macro-crack path profile as the drawing degree increases (Toribio, 2017). Fig. 2a offers a 3D-view of these fracture surfaces, showing that crack path deflection appears from a certain cold drawing level (mixed mode crack growth) starting from a certain mode I LAD propagation distance from the fatigue crack tip. Fig. 2b shows the geometric parameters describing the crack path, whereas the evolution of the fracture profile as the degree of cold drawing increases is given in Fig. 2c In the first drawing steps (slightly drawn steels 0, 1 and 2) the crack growth develops in mode I during LAD. In steels 3 there is a crack path deflection in the stress-corrosion crack, thereby evolving from mode I during fatigue to a mixed mode propagation. In the most heavily drawn steels (4 to 6) the crack deflection takes place after a certain mode I LAD propagation distance x I ( short but not null ) after the fatigue precrack, the crack deflection angle is even higher. and a mixed mode propagation takes place over a distance x II (horizontal projection).