Recent emissions standards have become more restrictive in terms of CO2 and NOx reduction. This has been translated into higher EGR rates at higher exhaust gas temperatures with lower coolant flow rates for much longer lifetimes. In consequence, thermal load for EGR components, specially EGR coolers, has been increased and thermal fatigue durability is now a critical issue during the development.
Consequently a new Thermo-Mechanical Analysis (TMA) procedure has been developed in order to calculate durability. The TMA calculation is based on a Computational Fluid Dynamics simulation (CFD) in which a boiling model is implemented for obtaining realistic temperature predictions of the metal parts exposed to possible local boiling. The FEM model has also been adjusted to capture the correct stress values by submodeling the critical areas.
Life calculation is based on a Multiaxial Fatigue Model that has also been implemented in FEM software for node by node life calculation. This model takes into account the effect of temperature using the stresses obtained in linear elastic FEM analyses and the temperature dependent properties of the metal. Material Fatigue Properties have been obtained from an extensive test campaign with fatigue specimens for different materials and temperatures. To verify the proposed methodology several EGR cooler prototypes were tested under different thermal-stress cycles.
The results have shown good accuracy between the tests and the predicted life expectancy when using the methodology developed.
