Lifetime assessment of cylinder heads for efficient heavy duty engines Part I: A discussion on thermomechanical and high-cycle fatigue as well as thermopyhsical properties of lamellar graphite cast iron GJL250 and vermicular graphite cast iron GJV450

Paper #:
  • 2017-01-0349

Published:
  • 2017-03-28
Abstract:
Increasing the efficiency of heavy duty internal combustion engines is directly related to increasing specific power and, thus, increasing combustion pressure and temperature. One key component of the engine is the cylinder head which must withstand these higher temperatures and higher pressures. The path of increasing loads intensifies design conflicts, as e.g. associated with the fire deck of cylinder heads: the deck should be as thin as possible to avoid critical thermal stresses during the low frequency thermal transients but sufficiently thick to avoid failures due to the high frequency combustion pressure. A superficial solution of the design conflict is the usage of superior cast iron materials. Vermicular graphite cast iron show higher strength and fatigue resistance than the classically used lamellar graphite cast iron. However, due to their lower thermal conductivity, higher thermal stresses will arise. In this paper (Part I), low-cycle isothermal fatigue and thermomechanical fatigue properties of the lamellar graphite cast iron GJL250 and the vermicular graphite cast iron GJV450 are investigated in uniaxial tests. Moreover, low-cycle fatigue tests with superimposed high-cycle fatigue loads are done representing the loading situation of combustion pressure loads during thermal transients. On the basis of the experimental results an advanced model for fatigue life prediction is developed which is based on elastic, plastic and creep fracture mechanics results of short cracks and which considers effects due to superimposed high-cycle fatigue loads. The results related to fatigue properties are discussed with respect to the variety of loading conditions of heavy duty cylinder heads taking also the thermophysical properties of both cast iron materials into account. The fatigue life model and the material properties are used to predict the fatigue life of cylinder heads by means of finite-element calculations (Part II of the paper).
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