PSI - Issue 37

Jesús Toribio et al. / Procedia Structural Integrity 37 (2022) 1013–1020 Jesús Toribio / Procedia Structural Integrity 00 (2021) 000 – 000

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3. Experimental programme As reported by Toribio and Toledano (1998, 2000), hot rolled and slightly drawn steels behave isotropically, i.e., cracking develops in mode I following the initial plane of fatigue crack propagation (Fig. 2a), while heavily drawn steels exhibit a clearly anisotropic fracture behaviour in the form of crack deflection after the fatigue precrack (and some mode I propagation in certain cases) with a deviation angle of almost 90º from the initial crack plane and further propagation in a direction close to the initial one (Fig. 2b).

Fig. 2. Crack paths (propagation profiles) produced by axial fracture in air environment in steels with 0 (a) and 6 (b) cold-drawing steps; f: fatigue crack growth; I: mode I propagation; II: mixed mode propagation (90º propagation step in heavily drawn steels); F: final fracture.

In both steels, the load-displacement ( F - u ) plot, Fig. 3, presents an initial linear phase that becomes progressively curved with cold drawing. In general terms, the curves were characterised by two parameters: the load defining the end of linear behaviour ( F e ) and the final fracture load ( F max ); in the case of heavily drawn steels a third parameter were required, the pop-in load ( F Y ) associated with a sudden and small decrease of load, probably caused by a small burst, tearing or fracture at the finest microstructural level.

F max

F max

F Y

F e

F e

F

F

u

u

Fig. 3. F - u plot: slightly drawn steel bar (left) and heavily drawn steel wire (right).

4. Discussion 4.1. Methods for evaluation of the critical stress intensity factor Four characteristic stress intensity values (candidates to fracture toughness) may be evaluated from test results: (i) K IY * at fracture initiation ( pop-in ) using the load F Y and the crack depth a . (ii) K IY ** at fracture initiation ( pop-in ) using the load F Y , the crack depth a and the aspect ratio a/b . (iii) K I max * at the final fracture instant using the load F max and the crack depth a. (iv) K I max ** at the final fracture instant using the load F max , the crack depth a and the aspect ratio a/b . These four stress intensity factor values (candidates to fracture toughness of the considered steels) are represented in Fig. 4 as a function of the degree of cold drawing (number of cold drawing steps undergone by each steel).