Research On Fundamental Problems Of Thermal Elasto-Plastic Contact And Its Application

Contact problem, which is one of the most challenging problems on applied mathematics and mechanics research, plays an important role in modern engineering design. The theoretical model construction of thermal elasto-plastic contact problem on rough surfaces, and quantitative calculation and analysis of its contact mechanics characteristics, can provide effective guidance in matching design of materials, structure, rigidity in joint parts of internal combustion engines and other thermal mechanical products. At present, the effect of temperature on the behavior of elastic-plastic contact mechanics is still a lack of research, which results that the effect of thermal stress caused by temperature changes on thermal elasto-plastic contact problem of rough surfaces is still unclear. Therefore it is necessary tofurther research on the basic problems of thermal elasto-plastic contact.A new fractal model of normal contact load is established. Then the effect of main parameters on the results is analyzed. After that the influence of bolt preload and thermal state of cylinder head on contact problem between cylinder head and cylinder head gasket is studied. Finally the simulation results and the theoretical values are compared and analyzed. Specifically, the main work and conclusion are as follows:1. Research of simulation of rough surface topography and its characterization. The rough surface topography is characterized respectively by statistics and fractal theory, and the inadequacies of previous work are detected and improved. According to surface height distribution type, rough surface profiles are simulated by poisson and gaussian respectively after process design and programming. This is statistical characterization. According to single argument w-m function, dimensional fractal curves with different fractal dimension are simulated. Anisotropic dimensional fractal rough surfaces with different fractal dimension are also simulated according to dual independent variable w-m function. That is fractal characterization.Results show that fractal simulation is an effective method to characterize fractal rough surfaces. Compared to the statistical characterization methods, fractal method has more advantages.2. Theoretical model research of the thermal elasto-plastic contact problem. Based on the normal contact mechanics model adopting anisotropic fractal geometrical theory, and combined it with rough surface thermodynamic properties built by thermal elasto-plastic contact theory, new fractal models of normal load and normal contact stiffness are established respectively. Besides, another expression of the model is proposed by introducing mechanical load scale factor and the model is validated through a special case.Results show that the thermal elasto-plastic contact normal fractal models are effective. The normal fractal models of thermal elasto-plastic contact which exclude mechanical load scale factor apply both to the thermodynamic factors considered case, also apply to purely mechanical situation. This model expands basic theory and applications of traditional models. The normal fractal models of thermal elasto-plastic contact which include mechanical load scale factor apply only to the thermodynamic factors considered case. As another form of model expression, they are more concise and easier to calculate.3. Mechanical characteristics research of thermal elasto-plastic contact. The effect of main parameters on the normal load and normal stiffness of thermal elasto-plastic contact of joint interface is analyzed through digital simulation.Results show that the normal load of thermal elasto-plastic contact increases with the coefficient of linear expansion, scale factor, temperature difference, fractal dimension and fractal roughness. Normal contact stiffness of thermal elasto-plastic contact adds with increasing coefficient of linear expansion, scale factor, temperature difference, fractal dimension, decreases with fractal roughness. In certain cases, the affect of the linear expansion coefficient of the contact material can be ignored. The affect of temperature difference can not be ignored.4. Comparative analysis and applied research of the model. The difference between new normal fractal model of thermal elasto-plastic contact and traditional purely mechanical model is performed compared analysis. Moreover the actual application of the model is demonstrated and illustrated by example of cast iron and steel, which are common in internal combustion engine.Results show that the difference between the calculation results of no thermodynamics consideration model and new model developed in this thesis increases with increasing temperature difference. This model comprehensively considers the effects of such parameters as temperature difference, linear expansion coefficient, mechanical load scale factor, etc. Therefore this model can help calculate and analyze the real contact conditions between fixed contact surfaces when the temperature changes, and it shows better conformity with objective laws.5. Expanded fractal modeling research of thermal elasto-plastic contact. Based on normal fractal model of thermal elasto-plastic contact, the extended model is established for the example of fractal dimension. When the fractal dimension, as a single argument, is constant, proportional function, parabolic function along one dimension, one-dimensional models are established successively in order to calculate the normal load and normal stiffness of thermal elasto-plastic contact. Besides, one-dimensional modeling of multi-argument is conducted. When the fractal dimension is space plane, a secondary cone, elliptic paraboloid in the two-dimensional area, dimensional models are established successively.This research make theoretical models of thermal elasto-plastic contact expand from point model to line model, then to surface model. It can provide practical application with methodological guidance and theoretical basis.6. Study on thermoelastic contact of cylinder head-cylinder head gasket-engine block. A four-body contact simplified model of cylinder head-cylinder head gasket-engine block-bolts is introduced based on a four-valve diesel engine, and verified by results of the original machine model. Then under different bolt preload and thermal state, the contact state and deformation of cylinder head plate are calculated in order to study the influence of mechanical constraints and thermal state on thermo-elastic contact nonlinear problem that between cylinder head and cylinder head gasket. Finally, the difference of calculation results between the simulation value and the theoretical value is compared and analyzed, and suggestions are given.The study makes clear that, when preload maintains the rated value and burst pressure is exerted, contact stresses in rated thermal state condition of the parts on cylinder head plate that against the cylinder ring and around the screw, which is on exhaust valve side, are similar with those values in cold state, while nodal displacements are reduced, and in different thermal states, the rates of change among the nodal displacements around the screw and both contact stresses and nodal displacements that against the cylinder ring are all less than 2%; with few exceptions, contact stresses and nodal displacements of these parts of the cylinder head plate increase with the increases of the bolt preload. Cylinder head gasket sealing tests in normal temperature condition is roughly adequate to practical state. Contact surface roughness of components have a certain impact on contact state. There is a deviation between the calculated results by the finite element modeling of smooth surfaces and the actual situation.