Nonlinear thermal stress/fracture analysis of multilayer structures using enriched finite elements

In this study, a two-dimensional finite element method was developed to study various mechanical, thermal and fracture mechanics problems encountered in thermal barrier coatings and overlay welding. This study consists of four main parts.; In the first part of this study, various edge and T-shaped crack problems encountered in thermal barrier coatings are solved using a special type of element. These elements are called "enriched finite elements" because they contain the 2-D crack tip strain singularity field. Both graded (FGM) and homogenous types of coatings are considered in the models. In these particular problems, large deformation, rotation and plasticity effects were neglected. The effect of an oxide layer and its shape on the fracture parameters is investigated in this section. At the end of this first part, numerical modeling of four point bending tests is also illustrated.; In the second part of the study, a 2-D large deformation and rotation formulation is developed and linked with the enriched crack tip elements. A complete finite element code is developed and various sample problems were solved and compared with available solutions in the literature. In a similar manner, a 2-D plasticity formulation is implemented into the code using an isotropic hardening model. Sample problems are solved and compared to available numerical and analytical results.; In the third part of the study, a 2-D transient thermal formulation is described and a heat transfer code is developed. Temperature dependence of the material properties and other types of nonlinearities were taken into account in the thermal code. Again, sample problems were solved and compared to the available solutions in the literature.; In the final part of this study, the previously developed and tested plasticity and heat transfer codes are used in solving overlay welding problems. Two specific cases are considered, overlay welding of plates and overlay welding of boiler tube panels. In these problems, the thermal and mechanical solutions are solved sequentially. First, the thermal analysis was performed to calculate the transient temperatures and then a nonlinear structural analysis was performed to calculate the residual stresses and strains. These calculations were performed for each weld bead deposited on the substrate. The calculations were cumulative, i.e., for the second, third, fourth..., weld beads, the results obtained from the previous weld were used as the initial condition for subsequent calculations. Temperatures, displacements, strains and stresses were calculated for each weld bead and a correlation was developed relating the number of weld passes to the amount of the distortion in the geometry.