IMECE 2008 -68209 DESIGN OF THERMAL MECHANICAL FATIGUE ACCELERATED LIFE TEST CRITERIA

ABSTRACT Product validation tests are essential all over the design stages of product development. In the automotive industry, laboratory fatigue testing is an accelerated test that is specifically designed to replicate fatigue damage and failure modes potentially found during field driving conditions. The work presented in this paper is mainly focused on hot components of automotive turbocharger, centered on failure due to thermal mechanical fatigue (TMF). A design of experiment (DOE) was run to define the relation between load parameters and the TMF damage. This fatigue damage model is based on the stress approach (Chaboche fatigue model), widely spread in use, to allow computing the damage under a complex duty cycle. A TMF damage transfer function was identified and it is used to select smart lab test parameters and to link that lab test damage, with customer field damage. This relation allows defining accelerated lab test criteria in order to demonstrate customer field reliability requirements. The lab test is composed of an elementary cycle repeated several times and the field duty cycle is defined via the so called "median customer". The median customer duty cycle is defined as a combination of several elementary driving patterns (country road, city, highway,…). For each driving sequence, an instrumented turbocharger allows measuring the relevant parameters such as gas temperature, metal temperature, rotating assembly speed… Finally, a smart lab test can be designed and a criterion defined to demonstrate, with a specific confidence level, that the customer reliability requirement is met

TMF DESIGN OPTIMISATION FOR AUTOMOTIVE TURBOCHARGERS TURBINE HOUSINGS

I : ABSTRACT Turbine housings for automotive turbochargers are complex castings that are subjected to transient thermal stress from exhaust gas temperature and flow variation under complex duty cycles. A fatigue model based on a cumulative damage approach ( Chaboche model To determine the transient stress and temperature distributions due to the high rates of convection from the gas, and the complexity of the design, conjugate heat transfer CFD simulation is performed. The tongue of the turbine housing is a critical region in which cracks initiate within a short time. Heat transfer coefficients ( HTC ) and bulk temperature predictions from CFD, in general, can be validated by thermal measurement. But because of the geometry and the location of the tongue, it is impossible to measure the metal temperature. A sensitivity study on predicted life shows that this difference results in no more than a 2 hr change for a total predicted life of 50 hrs. In the industrial approach this difference is quite acceptable. The design of a turbine housing is optimized based on this TMF methodology and shows very good results in testing, as presented here. II. INTRODUCTION Turbine housings are critical parts due to the complex geometry, and complicated stress and temperature. Cracks are mainly due to thermal cyclic load, where temperatures and stresses are varying consequently. Transient thermal stress analysis can indicate the crack region. The cracks in the turbine housing could be internal crack (Tongue), and / or external crack (Volute, V Band area, ..). It is easy to optimise the external region in order to increase the lifetime. Temperature and cracks could be very well investigated at the external regions, but at the internal regions such as tongue, it is complicated to measure the temperature and then assess the life. In this paper we will focus our TMF study only for the tongue where the damage is mainly due to the fatigue only, the creep and oxidation damage is not considered for this particular case Stress based fatigue Chaboche model will be presented for the case where the stress is temperature dependant. The lifetime for the crack initiation will be computed for the tongue region from two cases of Heat transfer Coefficients