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Density modulated thin films offer a compliant property that can reduce residual stress, which typically originate during the growth of thin films. Lower residual stress improves adhesion properties of the film with reduced buckling or delamination, and therefore leads to more durable coatings. In this study, finite element analysis (FEA) was employed to simulate the residual stresses developed in density modulated silicon (Si) thin films, which incorporate alternating low and high density layers. The main focus of this investigation is not developing new FEA algorithms but to verify the impact of density modulated layers quantitatively using computational methods. Hence, verification of a predicted stress reduction enhances the current understanding of the mechanics of density modulated layered thin films. FEA simulation results reveal that low density layers act compliant and result in significant reduction in film stress especially at the interface with the substrate. For example, maximum stress at the film/substrate interface, which is in the substrate, was reduced from 2897 MPa down to 2432 MPa by simply adding a 100 nm thick densitymodulated low-density Si layer in between a 300 lm thick Si wafer substrate and 1 lm thick conventional high density Si film, which makes the reduction percentage of the maximum stress about 16%. V

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A comprehensive methodology is developed to understand and characterize the fracture behavior of circumferentially cracked boiler tubes in this study. Weld overlay is applied on the coal-fired boiler tubes in order to prevent the degradation of corrosive and erosive environment that the boiler tubes are exposed to in the power plants. Finite element modeling and analysis are employed for all of the computations including steady-state and transient stress intensity factor (SIF) calculations in this study. Circumferential cracking has been one of the failure modes in waterwall boiler tubes, which results in high maintenance and replacement costs. Thermomechanical stresses and corrosive environment are basically the two remarkable contributors that bring about this failure mode. The former one is investigated and quantified in this study in order to explain the fracture behavior of weld overlay coatings during the power plant operation. Periodic soot blowing operations cause cyclic transient thermomechanical stresses on the weld overlay coating that results in crack propagation and fatigue failure. Three-dimensional fracture analysis of circumferentially cracked boiler tubes is examined using enriched finite element method in this study. Transient temperatures and thermomechanical stresses are computed using ANSYS for five different periodic crack spacing values (h), which are 2, 4, 6, 10, and 20 mm in the axial direction. 3D fracture analysis was performed, and stress intensity factors were computed using FRAC3D, which is Finite Element Analysis (FEA) software developed at Lehigh University. The maximum stress intensity factor is obtained at the deepest penetration of the crack in the model which has the largest periodic axial crack spacing, h = 20 mm. The stress intensity factors due to welding residuals decrease as the axial crack spacing, h, decreases. The FEA methodology developed in this research would provide the engineers with the ability to understand the fracture problem and predict component life and improve the reliability of the weld overlay coated boiler tubes utilized in the power plants.