The Multiaxial Stress State Analysis Research On High Stress Gradient Position Of Diesel Engine Cylinder Head

As one of the key parts of the internal combustion engine, cylinder head bears periodicfunction of the bolt force, the gas pressure and the heat thermal load, leading to the periodicchanges in stress and deformation. To cylinder head, the multiaxial stress state of high stressgradient position has a great effect on its fatigue failure. In order to solve this problem, thestructure statics and dynamic response analysis are conducted based on finite element theory,and multiaxial stress state characteristics of high stress gradient parts is detailedly researched ,which provides support for the non-proportional multiaxial fatigue life prediction.The three-dimensional geometry solid models of cylinder head, cylinder sleeve, cylinderblock, bolts and other parts are built with Pro/E software, then they are imported into theprofessional meshing software ANSA to set materials and properties for each part and selectrational unit types and unit size according to there respective structural features for meshing ,and ten contact pairs are defined between parts such as cylinder head and cylinder head gasket,cylinder head and bolts, cylinder block and cylinder sleeve, Valve seat ring and cylinder headand etc.Finally the finite element grid model of composite structure is completed under thepremise of ensuring the calculation accuracy. Boundary conditions including constraints, thebolt preloaded force and the breakout pressure are exerted on the grid model in the finiteelement analysis software ANSYS, and static calculation is carried out in three workingconditions which are preloaded force, preloaded-breakout pressure and preloaded-breakoutpressure overload 30%. The displacement and stress distribution provide theoretical basis forthe structureal design and improvement of cylinder head.On the basis of grid model for static analysis, preloaded force is applied on the bolt andthe periodic cylinder gas pressure is exerted on the fire surface and valve seat ring bearing bevel in the form of load steps through the self APDL loading procedure. And then initialconditions, damping coefficient, load type, nonlinear options, integral method and otheroptions are properly setted up. Lastly the dynamic response analysis is conducted in threeworking cycles based on the newmark implicit algorithm and the FULL method in ANSYSsoftware. The displacement field and stress field at the breakout pressure moment and near thetime, displacement time history curves of the large deformation positions and stress timehistory curves of the high-stress positions will help to understand and grasp the dynamicresponse characteristics of cylinder head, and provide a theoretical basis for the establishmentof the load spectrum in fatigue analysis.The APDL calculation program on first principal stress gradient is written based on theconcept of stress gradient. In the statics analysis post-processing stage, the first principalstress gradient is calculated and analysed with the method of combining the GUI way and theAPDL command stream,and the stress gradient path and the corresponding stress gradientvalues of the high stress positions are obtained, and the high stress gradient positions aredetermined. Based on the node stress results of dynamic response analysis, direction angles inX, Y, Z coordinate axis of three principal stress of the high stress gradient positions are gettedfrom stress unit vector, with which the principal stress direction is represented. The principalstress direction at the breakout pressure moment and near the time is calculated to research themultiaxial stress state characteristics caused by the principal stress direction changes withtime of the high stress gradient positions, which provides theoretical support for thenon-proportional multiaxial fatigue life prediction of cylinder head.