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Technical Paper

Cylinder Head Design Process to Improve High Cycle Fatigue Performance

Cylinder head design is a highly challenging task for modern engines, especially for the proliferation of boosted, gasoline direct injection engines (branded EcoBoost® engines by Ford Motor Company). The high power density of these engines results in higher cylinder firing pressures and higher operating temperatures throughout the engine. In addition to the high operating stresses, cylinder heads are normally heat treated to optimize their mechanical properties; residual stresses are generated during heat treatment, which can be detrimental for high-cycle fatigue performance. In this paper, a complete cylinder head high cycle fatigue CAE analysis procedure is demonstrated. First, the heat treatment process is simulated. The transient temperature histories during the quenching process are used to calculate the distribution of the residual stresses, followed by machining simulation, which results in a redistribution of stress.
Technical Paper

Cylinder Head Thermo-Mechanical Fatigue Risk Assessment under Customer Usage

For aluminum automotive cylinder head designs, one of the concerning failure mechanisms is thermo-mechanical fatigue from changes in engine operating conditions. After an engine is assembled, it goes through many different operating conditions such as cold start, through warm up, peak power, and intermediate cycles. Strain alternation from the variation in engine operation conditions change may cause thermo-mechanical fatigue (TMF) failure in combustion chamber and exhaust port. Cylinder heads having an integrated exhaust manifold are especially exposed to this failure mode due to the length and complexity of the exhaust gas passage. First a thermo-mechanical fatigue model is developed to simulate a known dynamometer/bench thermal cycle and the corresponding thermo-mechanical fatigue damage is quantified. Additionally, strain state of the cylinder head and its relation to thermo-mechanical fatigue are discussed. The bench test was used to verify the TMF analysis approach.
Technical Paper

Field Risk Assessment Based on Cylinder Head Design Process to Improve High Cycle Fatigue Performance

In a separate SAE paper (Cylinder Head Design Process to Improve High Cycle Fatigue Performance), cylinder head high cycle fatigue (HCF) analysis approach and damage calculation method were developed and presented. In this paper, the HCF damage calculation method is used for risk assessment related to customer drive cycles. Cylinder head HCF damage is generated by repeated stress alternation under different engine operation conditions. The cylinder head high cycle fatigue CAE process can be used as a transfer function to translate engine operating conditions to cylinder head damage/life. There are many inputs, noises, and design parameters that contribute to the cylinder head HCF damage CAE transfer function such as cylinder pressure, component temperature, valve seat press fit, and cylinder head manufacturing method. Material properties and the variation in material properties are also important considerations in the CAE transfer function.
Journal Article

Effects of Non-Associated Flow on Residual Stress Distributions in Crankshaft Sections Modeled as Pressure-Sensitive Materials under Fillet Rolling

In this paper, the evolution equation for the active yield surface during the unloading/reloading process based on the pressure-sensitive Drucker-Prager yield function and a recently developed anisotropic hardening rule with a non-associated flow rule is first presented. A user material subroutine based on the anisotropic hardening rule and the constitutive relation was written and implemented into the commercial finite element program ABAQUS. A two-dimensional plane strain finite element analysis of a crankshaft section under fillet rolling was conducted. After the release of the roller, the magnitude of the compressive residual hoop stress for the material with consideration of pressure sensitivity typically for cast irons is smaller than that without consideration of pressure sensitivity. In addition, the magnitude of the compressive residual hoop stress for the pressure-sensitive material with the non-associated flow rule is smaller than that with the associated flow rule.