Thermal Fatigue Accelerated Life-Testing and Field Damage Analysis in EGR Coolers 2016-01-1088

The increasingly restrictive emission standards in the automotive industry require higher thermal requirements in the EGR loop in terms of gas mass flow, gas temperature and lower coolant flow rate. Also, their performance has to be sustained over a longer period of time. Therefore, thermal load for EGR components, especially EGR coolers, has been increased and thermal fatigue durability is now a critical issue during their development.

One of the most challenging issues during product validation is to define a thermal fatigue test with the same field cumulative fatigue damage in order to guarantee durability during vehicle life. A new analytical procedure has been developed in order to define the equivalent thermal fatigue test which has the same cumulative damage as the real application in the field or to estimate durability in the field on the basis of a previous thermal fatigue test result. This methodology is based on a load parameter that is calculated with the EGR cooler working parameters and is linearly related with thermal stress in the part allowing movement along the S-N curve using working parameters. The aim of this work is not to calculate the absolute value of stress for a given design in a given working condition, but estimate how stress values change when working conditions vary based on a given stress value.

This approach allows the realization of accelerated life-tests simply by modifying the working parameters in order to reduce the number of cycles and is the basis of a cumulative damage analysis procedure in which field cumulative damage is translated into a single thermal fatigue test by means of the Rainflow-counting algorithm and Miner’s rule. The calculated lives show a good correlation with several failures that occurred in field.