Analytical Correction of Nonlinear Thermal Stresses Under Thermomechanical Cyclic Loadings

Abstract

Automotive turbocharger components frequently experience complex thermomechanical fatigue (TMF) loadings wiiich require estimation of nonlinear plastic stresses for fatigue life calculations. These field duty cycles often contain rapid flucttiations in temperatures and consequently transient effects become important. Although current flnite element (FE) software are capable of performing these nonlinear flnite 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 veiy 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 thermomechanical loading. In this paper a detailed study is conducted with two different existing analytical approaches (Neuber's rule and Hojfman-Seeger) to estimate the multiaxial nonlinear stresses from linear elastic stresses. For the Neuber's approach, the multiaxial 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 multiaxiai stress state from plastic equivalent stress estimated using Neuber's method for uniaxial stress. The novelty in the present work is the estimation of noniinear 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 nonproportional 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.