PSI - Issue 47

Ilia Nikitin et al. / Procedia Structural Integrity 47 (2023) 617–622 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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specimen used for VHCF torsion tests, Nikitin (2015). The loading conditions are like experimental one, Nikitin (2016-b). The excitation of the specimen was realized at the first mode of torsion vibration by applying a circumferential displacement. The value of displacements is corresponding to rotation angle about 20 m rad. The elastic problem solution was built up by using Ansys software. The results of numerical simulation were used to calculate the value of damage function and associated degradation of material moduli at each spatial node. This procedure was realized by in-house developed code integrated into the FEM calculations. This numerical procedure is looped with the following step in number of cycles coming from convergencies criterion: (4) The results of numerical simulation on smooth VHCF torsion specimens are presented on Fig.3 and Fig.4. The Fig.3 shows the result of simulation of the specimen without any artificial defects. The gray color indicate zone with critical value of damage function (‘quasi - crack’). min0.5 t  = t k k N N  , 1 1 2(1 )  − / (1 )    − − − / 2/ (1 ) / t k    −  t k N  = B   

(a) (c) Fig. 3. (a) Fracture surface with (a) surface and (b) subsurface crack initiation (c). (b)

The result of numerical simulation shows a single crack with clear stage of propagation on the maximum shear stress plane (90-degree). After reaching a certain length the crack spontaneously bifurcates to further propagation on the plane of maximum normal stress. The precise analysis on damage function accumulation via two mechanisms proved that at the initial stage the dominant mechanism was shear, Fig.3-b. The crack growth on the plane of maximum normal stress is driving by Mode I crack opening. The second numerical experiment was performed on the same CAD model with the same loading conditions, but a ‘hart element’ (artificial defect) was introduced. The ‘hard element’ is the finite element with elastic moduli twice higher compared to the rest of material. The spatial shape of VHCF torsion crack was obtained for different locations of this ‘hard element’ in the bulk of the specimen, Fig.4.

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Fig. 4. (a) Fracture surface with (a) surface and (b) subsurface crack initiation (c).