Experimental Research On Thermo-mechanical Fatigue Life Of Cylinder Head

To meet the requirements of high specific power and low emission for diesel engines, the pressures and the gas temperatures in the cylinders have the inevitable tendency to increase, which aggravates the thermal loading conditions as well. Cylinder head, as one of the components with most complex structures and loadings in the engine, is facing prominent fatigue problems, which in some cases directly affects the reliability of the whole engine.In this paper, the stress-strain conditions of cast iron cylinder heads are analyzed to determine the failure of valve bridge areas belongs to thermo-mechanical fatigue; experimental schemes are provided to simulate the thermo-mechanical conditions of cylinder heads’ valve bridge areas; then, fake block is designed accordingly; based on the coupled simulation of electromagnetic induction heating process, test guidelines for thermo-mechanical fatigue test is established, and the tests are completed.The main content can be summarized as follows:(1)Based on finite element simulation method, the stress-strain of the cylinder head under different loads are analyzed to verify the conclusions that the valve bridge areas(including E-E, E-I, I-I) on fire-deck are in uniaxial stress state, and accordingly SWT formula is selected to predict corresponding fatigue lifetimes; By analyzing and comparing the impact of cylinder heads’ bolt preloads on constraint conditions and fatigue lifetimes of valve bridge areas, the need for design thermo-mechanical fatigue tests is putting forward.(2) The analysis of electromagnetic induction heating process. Corresponding to the experimental induction heating conditions of cylinder heads, the simplified model of cylinder heads is established, and the relative parameters on heating process are analyzed regularly. Finally, the influence of heating time, maximum temperature, temperature amplitude on the maximum stress amplitude and mean stress are researched by orthogonal tests.(3)The simulation of experimental conditions and the implement of thermo-mechanical fatigue life tests. A test cycle is divided into three processes: heating, maintaining and cooling,(wherein the heating is considered as thermal electromagnetic process, the thermal maintaining is considered as two alternating phases including electromagnetic phase and free convection heat transfer phase, the cooling is mandatory convection cooling process) a correspondence between the tests and simulation is determined by comparing the Mises stresses, and the experimental scheme which mirrors the practical conditions is established. Based on the test experimental scheme, fatigue tests are carried out to obtain the initiation and growth of cracks on valve bridge areas.This paper studies the influence of relevant test parameters on the stresses of critical areas, test guidelines for thermo-mechanical fatigue test is established. By designing, implementing and evaluating the thermo-mechanical fatigue test, a reference to establish future thermo-mechanical fatigue life model is provided.