PSI - Issue 39

R. Yarullin et al. / Procedia Structural Integrity 39 (2022) 364–378 Author name / Structural Integrity Procedia 00 (2021) 000–000

365

2

tension/compression, bending and/or torsion, and their failure can lead to catastrophic consequences. Most often part through flaws appear on the free surface of the cylinder, often with a geometry that can be approximately considered as semi-elliptical. Prediction of surface fatigue crack growth from critical zones of cylindrical components, subjected to multiaxial cyclic loading conditions, is one of the most important parts of the so-called damage tolerance approach. Damage tolerance design requires accurate prediction of fatigue crack growth under service conditions based on wide experimental background. Even in recent years, research under mixed mode loading conditions usually focused on the targeted coupling of two crack modes and often on the superposition of cyclic and static loads. In many cases specimens with through thickness cracks are considered as a subject of studies. Giannella et al. (2017) applied a FEM-DBEM hybrid methodology to crack propagation simulations in order to optimize computational effort and accuracy. FEM was used for stress evaluation of the uncracked domain, whereas, the fracture analysis on a submodel embedding the cracked zone was demanded to DBEM (Dual Boundary Element Method). The proposed case study was based on a shaft/hub coupling undergoing three different loading conditions: combined ‘‘bending” and ‘‘press-fit”, ‘‘shear” and ‘‘torque”. The material was a common steel with isotropic mechanical properties. An overview about theories, experiments and simulations of cracks and crack growth under Mixed Mode loading was given by Richard et al. (2014). Furthermore, several Mixed Mode fracture specimens, especially the CTS-, AFM- and the CTSR specimen and loading devices were presented delivering new experimental findings for unstable and stable crack growth. It was proposed that for 2D as well as 3D Mixed Mode crack problems different fracture criteria exist. Several criteria were considered for description of crack kinking and twisting during propagation. For the fracture surface curve especially under prevailing Mode III loading a non-negligible discrepancy between the theories and the experiments was stated. Martins et al. (2016) carried out mode III and mode I fatigue loading tests on standard compact tension (CT) specimens. It was noticed that slant/twisting crack growth began from outer lateral surfaces and propagated to the interior region of CT specimens, resulting in a smoothly V-shape fracture surface. Equivalent stress intensity factor values at the crack tip were calculated using Pook (2015), Richard or Schöllmann et al. (2004) criterion. Later Köster et al. (2020) and Ayhan and Demir (2021) investigated the superposition of mode I, II and III at once. For this purpose, appropriate fracture mechanical investigations were performed in order to obtain further basic knowledge and to get a better understanding of the relevant phenomena. The resulting crack surfaces indicated that friction effects are of great importance. To evaluate the SIF solutions along the crack front for all tested mixed mode conditions successful combination of ABAQUS and FRANC3D tools was demonstrated. It was shown that especially for highly mixed mode conditions, the developed criterion predicts the crack growth surface better than the MTS criterion. Yang et al. (2006) studied variation of crack growth behavior for straight-fronted edge crack in a round bar under cyclic axial loading with steady torsion. Results showed that Mode III loading superimposed on the cyclic Mode I lead to a significant reduction in the crack growth rates. It was also found that the retardation effect of the torsion is reduced when the tension loading increases. The fatigue crack growth rates in shafts were presented by Fonte et al. (2006) for short as well as long cracks for two types of testing: rotary or alternating bending combined with steady torsion. These multiaxial fatigue loading conditions (K I +K III ) often occur in power rotor shafts, for example, in electric power plants or propeller shafts of screw ships. It was shown that a superimposed steady Mode III loading to a crack growing in cyclic Mode I lead to a significant fatigue crack growth retardation. At the same time, different fatigue crack growth rates were observed at the two sides of the symmetry axis during the testing. Some studies considered crack growth under different mixed mode conditions – axial tension/compression, pure torsional and multiaxial in phase mode (e.g., Citarella et al. 2018, Citarella et al. 2014, Poczklán et al. 2019, Geng et al. 2020). It was found that cycling in axial mode leads to the fastest crack growth rate and the shortest fatigue lives. On the contrary, cycling in torsional mode leads to the slowest crack growth rate and the longest fatigue lives. Different criteria for the crack path assessment (Minimum Strain Energy Density, Maximum Principal Stress and Approximate Energy Release Rate) and for the SIF evaluation (COD and J-integral) were adopted. Satisfactory agreements between computational results and experimental findings were found.