Effects of Holding Time on Thermomechanical Fatigue Properties of Compacted Graphite Iron Through Tests with Notched Spe
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CAST iron components in engines, such as cylinder blocks and heads, are exposed for long periods of time to elevated temperatures and are subjected to large numbers of heating and cooling cycles. In complex components, these cycles can lead to localized cracking of the cast iron due to stresses that develop as a result of thermal gradients and thermal mismatch. This phenomenon is known as thermomechanical fatigue (TMF). ‘‘In a combustion engine, the TMF phenomenon is related to the start-operate-stop cycles and not to the combustion cycles.’’[1] Especially the valve bridges in cylinder heads, the areas between intakes and outtakes are severely subjected to so called out-of-phase (OP) SEPIDEH GHODRAT, Ph.D. Research Student, is with the Materials Innovation Institute (M2i), 2628 CD Delft, The Netherlands, and also with the Department of Materials Science and Engineering, Delft University of Technology, 2628 CD Delft, The Netherlands. Contact e-mails: [email protected]; [email protected] TON A.C. RIEMSLAG, Consultant of Mechanical Testing & Behaviour, MICHAEL JANSSEN, Associate Professor, and JILT SIETSMA, Professor, are with the Department of Materials Science and Engineering, Delft University of Technology. LEO A.I. KESTENS, Professor, and ROUMEN H. PETROV, Associate Professor, are with the Department of Materials Science and Engineering, Delft University of Technology, and also with the Department of Materials Science and Engineering, Ghent University, 9052 Gent, Belgium. Manuscript submitted March 26, 2012. Article published online July 20, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
loading, where a maximum tensile stress occurs at the minimum temperature level of the cycles, and vice versa.[1] According to Lo¨he et al.,[2] multiple mechanisms take place during a thermomechanical out-of-phase cycle: plastic deformation, creep, oxidation, coarsening of the microstructure, and crack initiation and propagation. However, these damage mechanisms significantly depend on the material, temperature, frequency, mean stress level, and environment.[3] The main damage mechanisms are mechanical damage by fatigue, viscous phenomena by stress relaxation/creep, and environmental damage by oxidation.[4] Gocmez et al.[4] point out that in damage models the oxidation process is defined as a function of strain range, strain rate, straintemperature phasing, and oxidation kinetics. They claim that creep damage is based on stress, temperature, strain-temperature phasing, and time. Both mechanical and oxidation damage mechanisms are temperature and time dependent, which is relevant for TMF since this takes place during prolonged periods of time at various temperatures. In addition to these phenomena, in the case of cast iron, the graphite particles affect the deformation. As discussed by Seifert and Riedel[5] and Seifert et al.,[5,6] graphite particles in cast iron weaken the material in tension by decreasing the stiffness since the graphite particles partly delaminate from the matrix. In compression, interfacial microcracks are c
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