2D Thermoelastic Contact Instability Of Functionally Graded Materials

When two contacted elastic bodies are heat flowed or sliding against each other,the thermal contact resistance at the interface or the frictional heat generation can cause the thermoelastic distortion,which modifies the content of the contact area and the contact pressure distribution.In turn,the varying contact pressure distribution also impacts the thermoelastic distortion.This feedback process is generally cause the system to be unstable and is known as the thermoelastic instability(TEI),which can be classified to two different categories.The first category is described in the static contact system,called as static TEI.The second category is found in sliding contact system,called as the frictionally-excited TEI which involving the frictional heat.What's more,the interaction between the static TEI and the frictionally-excited TEI is extremely dangerous for the sliding system,called as coupled TEI.The behavior of the TEI is of considerable importance in many industrial settings,and has a grave responsibility for hot spots,which is known as one of the most dangerous appearances in the brake disk of trains,cars and airplanes.Many numerical and experimental results have shown that functionally graded materials(FGMs)used as interfacial zones or coatings can reduce the mitigate stress concentration,thermal stresses and magnitude of residual,and increase the fracture toughness.In this thesis the two-dimensional(2D)TEI of FGM are studied,the stability boundaries for these different TEI caterories are given,and possibility of the improvement of the stability in the static/sliding system by using the FGM is discussed.The main contents and conclusions include:(1)The 2D static TEI between an FGM layer and a homogeneous layer is investigated using the perturbation method.The thermoelastic properties of the FGM layer are assumed to be exponential along the thickness direction.The thermoelastic stability behaviors are discussed in detail.The results imply that the FGMs can be used to improve the thermoelastic contact stability of the static systems.(2)The 2D coupled TEI of the FGM coating with arbitrary varying properties is investigated.The homogeneous multi-layered model,perturbation method and transfer matrix method are used to deduce the characteristic equation of the coupled TEI problem,which is then solved to obtain the relationship between the critical heat flux and critical sliding speed.It is shown that an appropriate gradient type of the FGM coating can adjust the coupled TEI of the out-of-plane sliding systems,and the effect of the graded thermal diffusivity coefficient on the stability cannot be ingnore normally.(3)Then,the 2D TEI of the FGM in sliding system is investigated.Assuming the properties of FGM varying as the exponential function or arbitrarily along the thickness direction,the frictionally-excited TEI and coupled TEI of FGM in sliding systems are studied,respectively.It is shown that the stability boundaries for the sliding system is much more sensitive to the varying thermal properties than the elastic properties;the effect of the gradint index on the critical speed and critical friction heat is close related to the direction of the heat flux.The present investigation is a positive contribution to the theory of TEI of FGMs,also a significant to optimal design and practical applications of FGM for the purpose of improving the TEI,i.e.,the brakes in the trains.