Deformation and fatigue behaviors of carburized automotive gear steel and predictions

The fatigue behavior of carburized components such as automotive transmission gears is very complex due to hardness and microstructure difference, residual stresses and multi-axial stress states developed between the case and the core. In addition, automotive gears in service, commonly used in helical type, are actually subjected to complex stress conditions such as bending, torsion, and contact stress states. This study presents experimental and analytical results on deformation behavior of carburized steels, widely used in automotive gears, under cyclic stress conditions including axial and torsion loadings. Axial fatigue tests and rotating bending fatigue tests are also included. Predictions of cyclic deformation and fatigue behaviors of the carburized steel with two-layer model are compared with experimental results. The carburized steel investigated in this study exhibited cyclic softening under both axial loading and torsional loading. Predicted results with simple two-layer model for the cyclic deformation and fatigue behaviors were comparatively similar to the experimental data

Interaction Between Normal and Shear Stresses and Its Effect on Multiaxial Fatigue Behavior

Interaction between normal and shear stresses plays an important role in multiaxial fatigue damage. The aim of this study was to investigate this interaction effect on fatigue behavior of shear failure mode materials under multiaxial loading conditions. In order to model the influence of normal stress on fatigue damage, the present study introduces a method based on the idea that the normal stress acting on the critical plane orientation causes two types of influence, first by affecting roughness induced closure, and second, by a fluctuating normal stress affecting the growth of small cracks in mode II. The summation of these terms could then be used in shear-based critical plane damage models, for example FS damage model, which use normal stress as a secondary input. In order to investigate the effect of the method, constant amplitude load paths with different levels of interaction between the normal and shear stresses were designed for an experimental program. The proposed method was observed to result in improved fatigue life estimations where significant interactions between normal and shear stresses exist

Fatigue analysis of ductile and brittle behaving steels under variable amplitude multiaxial loading

The analysis of fatigue behavior under multiaxial variable amplitude stress states, despite its wide applicability, has not been fully studied. Issues such as varying degrees of nonproportionality of the load history, cycle counting, damage accumulation, failure behavior of the material, and mean stress fluctuations which can significantly affect the results of these analyses have not been well understood. In this study, a methodology for the analysis of fatigue behavior under multiaxial variable amplitude loading conditions is employed which accounts for the aforementioned issues. At its core, the applied methodology uses critical plane analysis based on the failure behavior of each material to assess the fatigue damage. In order to evaluate the performance of the analysis method, axial, torsional, and combined axial-torsional variable amplitude tests were performed on one ductile and one brittle behaving steel. The applied methodology resulted in close estimation of the experimental fatigue life for both ductile and brittle behaving steels

On the interaction of normal and shear stresses in multiaxial fatigue damage

One of the important issues in assessing multiaxial fatigue damage is interactions between different components of stress such as normal and shear stresses. The present study investigated this interaction effect on the fatigue behavior of materials with shear failure mode when subjected to multiaxial loading conditions. A method is introduced to model this interaction based on the idea that two types of influence are caused by the normal stress acting on the critical plane orientation. These two types of influence are affecting roughness induced closure, as well as fluctuating normal stress which affects the growth of small cracks in mode II. Shear-based critical plane damage models which use normal stress as a secondary input, such as FS damage model, could then use the summation of these terms. In order to investigate the effect of the method, constant amplitude load paths with different levels of interaction between the normal and shear stresses, as well as variable amplitude tests with histories both taken from service loading conditions and generated using random numbers were designed for an experimental program. The proposed method was observed to result in improved fatigue life estimations where significant interactions between normal and shear stresses exist

Evaluation of Estimation Methods for Shear Fatigue Properties and Correlations with Uniaxial Fatigue Properties for Steels and Titanium Alloys

The goal of this study was to evaluate the accuracy of different methods in correlating uniaxial fatigue properties to shear fatigue properties, as well as finding a reliable estimation method which is able to predict the shear fatigue behavior of steels and titanium alloys from their monotonic properties. In order to do so, axial monotonic as well as axial and torsion fatigue tests were performed on two types of steel and a Ti-6Al-4V alloy. The results of these tests along with test results of 23 types of carbon steel, Inconel 718, and three types of titanium alloys commonly used in the industry were analyzed. It was found that von Mises and maximum principal strain criteria were able to effectively correlate uniaxial fatigue properties to shear fatigue properties for ductile and brittle behaving materials, respectively. Also, it was observed that for steels and Inconel 718 obtaining shear fatigue properties from uniaxial fatigue properties which are in turn calculated from Roessle-Fatemi estimation method resulted in reasonable estimations when compared to experimentally obtained uniaxial fatigue properties. Furthermore, a modification was made to the Roessle-Fatemi hardness method in order to adjust it to fatigue behavior of titanium alloys. The modified method, which was derived from uniaxial fatigue properties of titanium alloys with Brinell hardness between 240 and 353 proved to be accurate in predicting the shear fatigue behaviors

Cyclic deformation and fatigue behavior of carburized automotive gear steel and predictions including multiaxial stress states

The fatigue behavior of carburized components such as automotive transmission gears is very complex due to hardness and microstructure difference, residual stresses and multi-axial stress states developed between the case and the core and/or at stress concentrations. In addition, automotive gears in service, commonly used in helical type, are actually subjected to complex stress conditions such as bending, torsion, and contact stress states. This study presents experimental and analytical results on deformation behavior of carburized steels, widely used in automotive gears, under cyclic stress conditions including axial, torsional and combined axial-torsion loadings. Axial fatigue and rotating bending fatigue, as well as torsional fatigue and in-phase axial-torsional fatigue tests are also included. Predictions of cyclic deformation and fatigue behaviors of the carburized steel with two-layer model are compared with experimental results. Predicted results with simple two-layer model for the cyclic deformation and fatigue behaviors were comparatively similar to the experimental data

Torsional fatigue behavior of wrought and additive manufactured Ti-6Al-4V by powder bed fusion including surface finish effect

Additive manufacturing (AM) is a state of the art technology enabling fabrication of complex geometries, in addition to providing other advantages as compared to the traditional subtractive manufacturing methods. Powder bed fusion (PBF) is one of the most commonly used metal AM processes that uses laser to melt particles on the bed of metallic powder. Ti-6Al-4V is a common alloy made by this process and has applications in many industries, in particular aerospace and medical industries. Understanding mechanical performance of the additively manufactured materials and components for critical load bearing applications is still in early stages. As such components are typically subjected to cyclic loading, fatigue failure is a major consideration in their design. In addition, due to the multiaxial nature of the loading and/or complex geometries manufactured by AM, the state of stress often includes both normal and shear stresses. However, all the studies so far on fatigue behavior of additive manufactured metals have only considered axial loading, resulting in normal stresses. This study is on torsional fatigue behavior producing shear stresses and, therefore, addresses a major gap in understanding the mechanical behavior of additive manufactured metals in general and for PBF Ti-6Al-4V alloy in particular. In this study, thin-walled tubular specimens made of both wrought and AM Ti-6Al-4V were subjected to monotonic as well as cyclic torsional loads to study and compare their shear deformation and fatigue behaviors. Failure mechanisms in different life regimes and the effects of heat treatment and surface finish were also evaluated

Multiaxial fatigue behavior of wrought and additive manufactured Ti-6Al-4V including surface finish effect

Additive manufacturing (AM) technology has enabled efficient fabrication of parts with complex geometries and laser-based powder bed fusion (L-PBF) is one of the most commonly used AM fabrication process. In many applications, the loading condition of AM parts is multiaxial. Even under uniaxial loading conditions, due to the geometry complexity or interaction of residual stresses, the stress state may still be multiaxial. Therefore, an understanding of cyclic deformation and fatigue behaviors of AM materials under multiaxial stress states is critical to the expected performance of such parts. These behaviors were investigated in this study using thin-walled tubular specimens of Ti-6Al-4V alloy made of a common PBF process. To compare with the conventional material performance, wrought Ti-6Al-4V alloy was also investigated. The loadings considered included axial, torsion, in-phase axial-torsion, and 90° out-of-phase axial-torsion loads. The surface roughness effect was also studied by considering both the as-built and the machined and polished surface conditions of the AM specimens. The ductility of the AM material was found to be significantly lower than the wrought material due to the martensitic microstructure as well as the presence of defects. AM specimens had significantly shorter lives compared to the wrought specimens under all loading conditions and regardless of the surface finish. However, machining improved the fatigue performance of AM specimens. For all loading conditions brittle fracture of AM specimens was observed with cracking on maximum tensile plane, and ductile fracture of wrought specimens with shear cracking. Consequently, fatigue test results of the wrought material were correlated using a shear-based critical plane model, while the AM specimen test data were correlated based on the maximum principal stress criterion

Fatigue behavior of AHSS lap shear and butt arc welds including the effect of periodic overloads and underloads

The effect of periodic overloads and underloads, which is a common occurrence in service load histories, on the fatigue behavior of AHSS (Advanced High Strength Steel) single lap shear and butt arc welds is investigated. A series of constant amplitude tests under tensile loading condition (R = Pmin/Pmax = 0.1) were performed to obtain load amplitude-life curves for each type of arc welded joint. Variable amplitude load history tests, one with periodic overloads and another with periodic underloads were then conducted for each type of welded joint. Additional constant amplitude fatigue tests at a high load ratio (R ≈ 0.6) representing the small cycles in the periodic underload loading were also performed. The validity of linear cumulative damage rule in the prediction of the fatigue life of these structures under variable amplitude loading is investigated. The results of the tests show that while periodic overloads did not affect the fatigue life of the welded joints, periodic underloads decreased the fatigue life. Finite element analyses were conducted to simulate the stress and strain states at the weld locations under overloads and underloads, the results of which were used to explain the observed experimental observations