Many critical components in turbocharger are subjected to rapid temperature changes during operations. Thermal gradient within these components produces internal stresses and the repetition of these thermal cycles may cause a component to fail due to Thermal Mechanical Fatigue (TMF). Turbine Wheel, Turbine Housing and Manifold are subjected to TMF; these are the most expensive components of the turbocharger and have very complex geometric shapes. The maximum exhaust gas temperature could reach 1050°C. To assess TMF failure, it is very critical to accurately estimate metal temperature of these components subjected to complex duty cycles where exhaust gas temperatures vary significantly with time. Evaluating metal temperature and stress components from finite element analysis for complex duty cycles is a very time consuming process, particularly for complex geometries and approximately requires more than 3 weeks of time to complete analysis for different field complex duty cycles (driving conditions: city, highway and road). Several of these analysis cases are required to consider the impact of the real driving condition. In the present work we have developed an analytical methodology that is accurate and faster to predict the metal temperature and stresses in turbocharger components for complex duty cycles. This method was applied to evaluate the fatigue damage of turbine housing under actual condition.

Automotive turbocharger components frequently experience complex Thermo-Mechanical Fatigue (TMF) loadings which require estimation of nonlinear plastic stresses for fatigue life calculations. These field duty cycles often contain rapid fluctuations in temperatures and consequently transient effects become important. Although current FE software are capable of performing these nonlinear finite element analyses, the turnaround time to compute nonlinear stresses for complex field duty cycles is still quite significant and detailed design optimizations for different duty cycles become very cumbersome. In recent years, a large number of studies have been made to develop analytical methods for estimating nonlinear stress from linear stresses. However, a majority of these consider isothermal cases which cannot be directly applied for thermo-mechanical loading. In this paper a detailed study is conducted with two different existing analytical approaches (Neuber’s rule and Hoffman-Seeger) to estimate the multi-axial nonlinear stresses from linear elastic stresses. For the Neuber’s approach, the multi-axial version proposed by Chu was used to correct elastic stresses from linear FE analyses. In the second approach, Hoffman and Seeger’s method is used to estimate the multiaxial stress state from plastic equivalent stress estimated using Neuber’s method for uniaxial stress. The novelty in the present work is the estimation of nonlinear stress for bilinear kinematic hardening material model under varying temperature conditions. The material properties including the modulus of elasticity, tangent modulus and the yield stress are assumed to vary with temperature. The application of two analytical approaches were examined for proportional and non-proportional TMF loadings and suggestions have been proposed to incorporate temperature dependent material behavior while correcting the plasticity effect into linear stress. This approach can be effectively used for complex geometries to calculate nonlinear stresses without carrying out a detailed nonlinear finite element analysis.

Aircraft and pressure vessel components experience stresses that are negative biaxial or multiaxial in nature. Biaxiality is defined as the ratio of stress applied parallel and normal to the crack front. In recent years, many experimental studies have been conducted on fatigue crack growth (FCG) under various biaxial loading conditions. Biaxial loadings affect crack front stresses and strains, fatigue crack growth rate and direction, and crack tip plastic zone size and shape. Many of these studies have focused on positive biaxial loading cases. No conclusive study has been reported out yet that accurately quantifies the influence of negative biaxiality on fatigue crack growth behavior. Lacking validation, implementation on real life problems remains questionable. To ensure safe and optimum designs, it is necessary to better understand and quantify the effect of negative biaxial loading on fatigue crack behavior. This paper presents the results of a study to quantify the effect of biaxial load cases ranging from B = −0.5 to 1.0 on fatigue crack growth behavior. Also, a simplified approach is presented to incorporate the effect of biaxiality into da/dN curves generated from uniaxial loading using an analytical approach without conducting expensive biaxial crack growth testing. Sensitivity studies were performed with existing test data available for AA2014-T6 aluminum alloy. Detailed elastic-plastic finite element analyses were performed using the different stress ranges and stress ratios with various crack sizes and shapes on notched and unnotched geometries. Constant amplitude loads were applied for the current work and comparison studies were made between uniaxial and different biaxial loading cases. It was observed from the study that negative biaxiality has a very pronounced effect on the crack growth rate and direction for AA2014-T6 if the externally applied load equal to 30% of the yield strength as compared with 40% of externally applied load for steel alloy quoted in the literature.

Aircraft and pressure vessel components experience stresses that are negative biaxial or multiaxial in nature. Biaxiality is defined as the ratio of stress applied parallel and normal to the crack front. In recent years many experimental studies have been conducted on fatigue crack growth under various biaxial loading conditions. Biaxial loadings affect crack front stresses and strains, fatigue crack growth rate and direction, and crack tip plastic zone size and shape. Many of these studies have focused on positive biaxial loading cases. No conclusive study has been reported out yet that accurately quantifies the influence of negative biaxiality on fatigue crack growth behavior. Lacking validation, implementation on real life problems remains questionable. To ensure safe and optimum designs, it is necessary to better understand and quantify the effect of negative biaxial loading on fatigue crack behavior. In this paper, attempts were made to quantify the effect of biaxial load cases ranging from B = −0.5 to 1.0 on fatigue crack growth behavior. Also an attempt has been made to establish a simplified approach to incorporate the effect of biaxiality into da/dN curves generated from uniaxial loading using an analytical approach without conducting expensive biaxial crack growth testing. Sensitivity studies were performed with existing test data available for AA2014-T6 aluminum alloy. Detailed elastic–plastic finite element analyses were performed with different stress ranges and stress ratios with various crack sizes and shapes on notched and un-notched geometries. Constant amplitude loads were applied for the current work and comparison studies were made between uniaxial and different biaxial loading cases. It was observed from the study that negative biaxiality has a very pronounced effect on the crack growth rate and direction for AA2014-T6 if the externally applied load exceeds 20% of the yield strength as compared with 40% of externally applied load for alloy of steel quoted in the literature.

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