PSI - Issue 39

Jesús Toribio et al. / Procedia Structural Integrity 39 (2022) 470–474 Author name / Procedia Structural Integrity 00 (2021) 000–000

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The role of crack deflection in fatigue crack propagation has been studied by Suresh (1983) with implications in the matter of fatigue crack growth rates. With regard to plastic crack advance , the Laird-Smith mechanism of propagation by cyclic blunting and re-sharpening, transferring material from the crack tip towards its flanks, has been visualized by Toribio and Kharin (2013). Other studies about crack tip blunting have been performed by Handerhan and Garrison Jr. (1992) and Tvergaard (2004). The final aim of this research work is to study the retardation in plasticity-induced fatigue crack growth due to the micro-deflection in the near tip area, i.e., the deflection-induced fatigue crack growth retardation . 2. Numerical procedure For the study of fatigue propagation by plastic crack advance, a numerical simulation by the finite element method (FEM) under small scale yielding (SSY) was performed using the MSC.Marc software (nonlinear finite element code). Material was characterized as elastic–perfectly-plastic and the von Mises yield criterion was employed to define the plastic zone in the vicinity of the crack tip. Large strains and large geometry changes were used with an updated lagrangian formulation. Material properties (Young’s modulus E = 200 GPa, yield strength σ Y = 600 MPa and Poisson coefficient ν = 0.3) were those associated with high-strength steels. The geometry is a symmetric double-edge-cracked panel under remote tension fatigue (Fig. 1 shows the near-tip finite element mesh). The undeformed crack was a parallel-flanks slot, where the kink length l 0 (0.0012 times the total crack length) is deflected an angle α 0 (Fig. 2) and exhibits a semicircular shape, i.e., smooth blunting according to Handerhan and Garrison Jr. (1992) with b 0 = 5 µm, i.e., 0.055 l 0 . Four-node isoparametric quadrilateral elements (for plane strain applications) were used. Finally, a convergence study was performed to determine the optimal finite element mesh size and the most adequate number of steps required in the computations.

Fig. 1. Finite element mesh.

b 0

l

0

α

0

Fig. 2. Deflected crack kink.

The key variable analyzed in this research work is the deflection angle of the kink in relation to the main crack. The four values α 0 = 0, 15, 30 and 45º were used. The stress intensity factor (SIF) range used in the numerical procedure was Δ K = 25 MPam 1/2 (associated with the Paris regime of fatigue crack propagation).