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.