Thermomechanical Fatigue Behavior of Stainless Steel Grades for Automotive Exhaust Manifold Applications

摘要

This paper describes a study performed on the thermomechanical fatigue behavior of stainless steel grades for exhaust systems, especially for exhaust manifolds. A testing rig has been developed to characterize the thermomechanical fatigue resistance of bent stainless steel sheets. In particular, the behaviors of ferritic and austenitic stainless steels are compared. After the identification of the elasto-viscoplastic constitutive law of the materials, the numerical simulation of the test is performed, enabling the computation of the viscoplastic stiain accumulated during each cycle. Examination of the failed specimens indicated that cracks could be mainly attributed to out-of-phase thermomechanical fatigue in the case of ferritic grades and to in-phase thermomechanical fatigue in the case of austenitic grades. A Taira type damage criterion is proposed, which correlates the amplitude of the viscoplastic strain and the number of cycles to failure. But due to a coupling with oxidation and creep during in-phase thermomechanical fatigue, the phasing between the thermal and the mechanical loads has to be taken into account in the criteria. The hydrostatic pressure at maximum temperature is proposed as such a phasing factor.