Life Prediction Approach for Stainless Steel Exhaust Manifold

Stainless steel grades are now widely used for automotive exhaust systems, driven by the need to increase their durability and to reduce their weight. Exhaust Manifolds are subjected to more severe conditions and peak gas temperatures of 1000°C could be reached in new downsized gasoline engines. Also, longer guaranties are now required. This evolution is a direct consequence of the effort to decrease automotive pollutant emissions with new environmental regulations throughout the world. The paper will deal with the thermal-mechanical fatigue (TMF) damage prediction of fabricated automotive exhaust manifold fixed to the engine. A dedicated lifespan prediction approach was created based on elasto-viscoplastic behavior and damage models identification from different thermal-mechanical tests. The experimental procedure will be presented, especially the high temperature cyclic plasticity tests developed on thin sheet specimens and the V-shape thermal fatigue test that simulates the TMF process observed on manifolds subjected to engine bench tests. The Chaboche model was chosen for the elasto-viscoplastic model including a kinematic hardening plastic law coupled with the Norton viscous law. Our damage model is based on the Taira non-isothermal low-cycle-fatigue LCF criteria that has recently been completed with a Larson-Miller creep damage law in order to take into account of mean stress effects due to static loading on the manifold. The paper will also present the implementation of the models in the post-processing software named XhaustLife and for Abaqus finite element simulations.