Research On Functionally Graded Material For Fracture Of Collinear Cracks And Crack Orientation Angle And Thermal Stress Problem

Functionally graded materials (FGMs) are generally multi-phasecomposites with continuously varying theromechanical properties. Theyare primarily used as coatings and interfacial zones and they tend toreduce residual stresses and provide protection against severe thermaland mechanical environments. The newly developed FGMs have at-tracted considerable research interests as candidate materials for struc-tural applications ranging from aerospace to automobile to manufac-turing. Recently, the concept of FGMs has been extended from theinitial high-temperature components to the piezoelectric components.Although FGMs have many good comprehensive properties, the inho-mogeneity of FGMs can sometimes lead to the failure of materials underthe high temperature and mechanical loads, especially dynamic loading.In order to guide the microstructure selection and the design and per-formance assessment of components made of FGMs, it is necessary toanalyze the knowledge of crack growth and propagation, especially theirfracture behaviors.The fracture behaviors of various functionally graded structuresare investigated under diferent loads in this thesis. Firstly, the inter- face crack problems in functionally graded media are analyzed underantiplane shear loads. A standard integral-transform techniques andfinite Fourier transform techniques are employed to reduce the singlecrack problem and periodic cracks problem to the solution of an integralequation with Cauchy-type or Hilbert-type singular kernel, respectively.Then, using Laplace and Fourier transform and a very powerful sin-gular integral equation (SIE) method, the collinear cracks in a bi-FGMscoating-substrate structure is studied under static fractue problem or dy-namic fracture problem. The bi-FGMs coating-substrate structure withinternal and surface crack and crack crossing the interface configurationsare considered. For the case of crack crossing the interface, we evaluatedand proved the singularites of the crack tips by using complex potentialtheory, and obtained the expression of stress intensity factor(SIF).Some crack problems of an arbitrarily oriented crack in FGMs havebeen solved during the past ten years, but most of these studies limitedto static analyses. In this paper, the problem of an arbitrarily orientedcrack in a FGMs is investigated under dynamic loading. The resultsdemonstrated that the dynamic stress intensity factor (DSIF) is sensitiveto the nonhomogeneity constant and crack orientation angle.Finally, considering a new boundary condition, it is assumed thatinterface crack is partially insulated the temperature drop across thecrack surfaces is the result of the thermal resistant due to the heat con-duction through the crack region. The governing equations of the tem-perature fields and displacement fields for bi-FGMs coating-substrate structure are converted into a system of singular integral equations. Theinfluences of material properties (i.e. the elastic module, the thermal ex-pansion coefcient and the thermal conductivities) gradient parametersand dimensionless thermal resistant on the temperature distribution andthe thermal stress intensity factors (TSIF) are figured.With the advent of composite materials technology, more complexmaterial microstructures are being introduced, and more mechanics is-sues such as inhomogeneity and nonlinearity come into play. This paperwork is an attempt to meet these challenges by making contributionsto both mechanics modeling and applied mathematics. Results in thisstudy have wide-ranging applications, such as thermal barrier coatingson turbine components, combustion chambers, and many more applica-tions where the material is macroscopically nonhomogeneous.