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In this work, deviatoric tensile and shear-based damage models previously proposed by the authors [1, 2] are implemented within a maximum-damage critical plane approach [3]. These models have been shown to improve predictions of mean or peak normal stress effects on the initiation lives of both tensile and shear-driven cracks, assuming the peak deviatoric stress normal to the crack plane controls them. The Deviatoric Smith-Watson-Topper (DSWT) is the adopted tensile-based multiaxial fatigue damage model [1], based on Kujawski’s deviatoric damage parameter. For the initiation of shear-driven cracks, the Deviatoric Fatemi-Socie (DFS) multiaxial fatigue damage model is used instead [2], to describe eventual detrimental effects of static axial compression loads…
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In this work, deviatoric tensile and shear-based damage models previously proposed by the authors [1, 2] are implemented within a maximum-damage critical plane approach [3]. These models have been shown to improve predictions of mean or peak normal stress effects on the initiation lives of both tensile and shear-driven cracks, assuming the peak deviatoric stress normal to the crack plane controls them. The Deviatoric Smith-Watson-Topper (DSWT) is the adopted tensile-based multiaxial fatigue damage model [1], based on Kujawski’s deviatoric damage parameter. For the initiation of shear-driven cracks, the Deviatoric Fatemi-Socie (DFS) multiaxial fatigue damage model is used instead [2], to describe eventual detrimental effects of static axial compression loads under cyclic torsion, a challenging modeling problem. Moreover, to describe increasing mean normal stress effects in high-cycle fatigue without needing an ill-defined variable normal stress sensitivity parameter, the DFS model has been modified through the application of the peak deviatoric stress correction term only to the elastic component of the shear strain ranges. This Modified Deviatoric Fatemi-Socie (MDFS) model is then used in a maximum-damage critical plane search to predict the initiation of both in-plane and out-of-plane shear-driven cracks, while the DSWT model is adopted for tensile-driven cracks initiating perpendicular to the surface, using an efficient search algorithm detailed in this work. The proposed maximum-damage critical plane approach is evaluated comparing its predictions with 7075-T651 [4] and 2024-T3 [5] aluminum data from the literature, including more than two hundred uniaxial and torsional specimens that developed either tensile or shear-driven cracks, see Fig. 1. It is found that the use of separate tensile and shear-based models, along with the efficient maximum-damage critical plane search algorithm proposed here, enables the prediction of cracking initiation life, mode, and even direction, see Fig. 2, as well as the observed transition from shear to tensile-dominated behavior for longer fatigue crack initiation lives. This methodology also provides a more accurate description of multiaxial fatigue lives for materials exhibiting both ductile and brittle behaviors in the low and high-cycle regimes, respectively. The DSWT model data-fitting constants are easily calibrated from zero-mean uniaxial strain-life data. The capability of this tensile-based model to handle challenging scenarios involving compression-compression loadings is verified, as well as tension-compression histories with highly compressive mean stresses. When subjected to zero or positive mean normal stresses, it yields predictions comparable to those of the classic Smith-Watson-Topper (SWT) model. The shear-based MDFS model is able to explain a detrimental effect of static axial compression in cyclic torsion, observed in two aluminum alloys for axial shear cracks that did not branch into the tensile modes within the considered definition of their initiation lives. Furthermore, it is noteworthy that the initiation of both in-plane and out-of-plane shear-driven cracks can be predicted using the same torsional calibration of the MDFS model. This shear-based model also enhances the representation of the observed increase in peak stress effects in the high-cycle fatigue regime, eliminating the necessity for variable normal stress sensitivity parameters. Finally, the DSWT model, combined with the MDFS with a constant normal stress sensitivity value of one, successfully correlated nearly all measured fatigue lives within a three times scatter band, validating the proposed deviatoric critical-plane approach.