The Research Of Thermo-mechanical Fatigue Resistance Characterization Of Cylinder Head Material Based On Heat-flux Controlled Tests

With the continuous improvement of diesel engine,the thermo-mechanical fatigue problems of engine parts,such as cylinder head and exhaust manifold,have become more and more serious.Nowadays,an effective way to improve the fatigue resistance of cylinder head is the smart choice of material.For thermo-mechanical fatigue,standard tests usually allow comparing material on specimens with the same temperature and strain range.But this method ignores a key phenomenon.Under the same working conditions,temperature and strain amplitude are different for each specimens whereas heat flux is identical.Because their elastic-plastic and physical characteristics(thermal conductivity,thermal expansion coefficient,elastic modulus,etc.)are different.This is because the elastic-plastic and physical properties of different materials(thermal conductivity,thermal expansion coefficient,elastic modulus,etc.)are different.Therefore,it is not accurate to evaluate the thermal fatigue resistance of materials by standard thermo-mechanical fatigue test.In this paper,the material selection of cylinder head is studied and a thermo-mechanical fatigue test protocol based on heat flux is proposed,and the process of obtaining test load is obtained.Firstly,the actual heat load of the thermo-mechanical fatigue damage zone of the cylinder head is obtained,and the equivalent heat flux on the standard specimen is obtained by finite element analysis.Then ensure that the heat flux applied to different specimens are the same,so as to determine the loading value in the test.According to the thermo-mechanical fatigue test results,the ability of different materials to resist thermo-mechanical fatigue damage is evaluated.In this paper,three kinds of cylinder materials(vermicular cast iron,gray iron and nodular cast iron)are selected to verify this method.Firstly,the temperature field and stress field of the cylinder head are calculated by the finite element method,and the most critical area and fatigue type of the cylinder head are determined.Secondly,the effects of elastic-plastic and other physical characteristics on the finite element calculation are discussed to obtain more accurate simulation results.Finally,the heat flux is calculated according to the finite element method,and the temperature load of the three specimens is obtained.Then the thermal fatigue test is carried out under the loading condition,and thermal fatigue resistance of the three materials is sorted according to the experimental results.The results show that the method could evaluate the thermal fatigue resistance of different materials,and can be applied to other parts of the engine.It can provide reference for the design of engine components in the future.