Thermomechanical Fatigue Analysis on a Ductile Cast Iron Exhaust Manifold

Paper #:
  • 2018-01-1215

Published:
  • 2018-04-03
Abstract:
An engine exhaust manifold undergoes repeated thermal expansion and contraction due to temperature variation. Thermomechanical fatigue (TMF) arises due to the boundary limitation 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 CAE simulation of a Silicon and Molybdenum containing ductile cast iron used for exhaust manifold have been evaluated. Tensile, creep, isothermal low cycle fatigue (LCF), and thermomechanical fatigue (TMF) tests have been conducted. Parameters for material modeling, such as viscoplastic constitutive model and Sehitoglu TMF damage model have been calibrated, validated, and used to evaluate the TMF life of the exhaust manifold. A transient temperature profile created using CFD simulation and correlated to thermal survey under exhaust manifold durability test is used in stress/strain analysis during a thermal cycle. TMF damage is evaluated using Sehitoglu TMF model. Although the cast iron does not fully satisfy the requirements of Sehitoglu model due to the brittle-to-ductile transition at around 400C and a phase transformation at around 730C, it is found that the TMF predictions are correlated well to the exhaust manifold durability tests. Additional analysis is suggested to address the brittle-to-ductile transition.
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