A Thermomechanical Fatigue Analysis on a Ductile Cast Iron Exhaust Manifold

An engine exhaust manifold undergoes repeated thermal expansion and contraction due to temperature variation. Thermomechanical fatigue (TMF) arises due to the boundary constraints on thermal expansion so that mechanical strain is introduced. Therefore, TMF evaluation is very important in engine design. In this work, the mechanical properties important for TMF assessment and modeling of a silicon (Si)- and molybdenum (Mo)-containing ductile cast iron used for exhaust manifold have been evaluated. Tensile, creep, isothermal low cycle fatigue (LCF), and TMF tests have been conducted. Parameters for material modeling, such as the viscoplastic constitutive model and the Neu-Sehitoglu TMF damage model, have been calibrated, validated, and used to evaluate the TMF life of the exhaust manifold. A transient temperature profile created from computational fluid dynamics (CFD) simulation and correlated to thermal survey under exhaust manifold durability (EMD) dynamometer test is used in stress/strain analysis during a thermal cycle. TMF damage is evaluated using the Neu-Sehitoglu TMF model. Although the ferrite-based cast iron does not fully satisfy the requirements of the Neu-Sehitoglu TMF model due to the intergranular embrittlement at 400°C, low ductility at low temperatures, and phase transformation at 820°C-850°C, it is found that the TMF predictions are correlated well to the EMD dynamometer tests. An additional analysis is suggested to address the 400°C intergranular embrittlement.