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[en] Zirconium alloys are typically used in nuclear pressurized water reactors (PWR) as fuel cladding tubes due to their chemical stability and their mechanical strength at operating temperatures (≈300 °C). However, the corrosion of Zr-based cladding tubes is one of the factors limiting the burn-off rate in PWRs. It is commonly accepted that the corrosion kinetics involves a periodic succession of growth where the oxide thickness varies parabolically with time. As the oxide thickens, a cracking structure forms. The oxide appears striated with periodic layers of cracks running parallel to the metal/oxide interface. This cracking structure has been experimentally related to the periodicity of the oxide growth. In the present work, a finite-element study is used to investigate the development of stresses in the oxide under the combined influence of molar volume expansion during oxide formation, metal/oxide interface geometry and metallic substrate creep. The generation of tensile stresses capable of initiating the cracks that are observed experimentally is explored
[en] Highlights: • A new analytical model for multi-material composites subject to rotary wear is developed for efficient wear predictions. • Numerical predictions of key wear characteristics for Aluminum/Epoxy composites agree well with the experimental findings. • Potential applications of the wear model for the design of composites subject to dry abrasive rotary wear are discussed. Due to the prevalence of sliding interfaces in mechanical assemblies, fast and reliable wear prediction capabilities are imperative for system design and analysis. This study investigates the rotary wear of multi-material composite systems that have thrust washer geometries. An analytical rotary wear model is developed to predict the rotary wear performance based on Archard's wear law and a Pasternak elastic foundation model. Numerical methods are used to track the evolution of key wear parameters including surface profile, contact pressure distribution, volume loss and composite wear rate during both run-in and steady-state wear regimes. A direct method is also developed to determine the steady-state characteristics from just the initial conditions and configurations of a given composite system. Optimal designs and design guidelines for several wear objectives are identified. Initial optimal material distributions for target steady-state surface profiles are determined. In addition, the steady-state composite wear rate is minimized to reduce material loss for bi-material systems with prescribed volume fractions. It is found that the optimal material configuration for this objective is counterintuitive. Wear tests are conducted to evaluate the proposed models and optimal design solutions. Results obtained from the wear models agree well with the experimental measurements.
[en] Highlights: • An efficient modeling method for PCM solidification with fins was developed. • A finned heat pipe structure was optimized with negligible computational cost. • Suggestions to economically weld fins on a heat pipe are given. - Abstract: Phase Change Materials (PCMs) are gaining importance in energy storage applications. However, many PCMs are poor thermal conductors and thus can benefit from the optimal use of appropriate fins. This work introduces a PCM-fin structure optimization framework. Typically, the non-linear solidification process increases the complexity associated with solving the mathematical equations for the PCM-fin structure optimization problem, making it computationally expensive. In this paper a modeling approach called Layered Thermal Resistance (LTR) model is extended and developed in 2D cylindrical geometry in order to enable efficient PCM-fin structure optimization. The finned LTR model represents the nonlinear transient solidification process by analytic equations. This significantly reduces the computational cost associated with optimization. A finned heat pipe structure modeled by the finned LTR approach is optimized based on minimizing cost while meeting operational requirements. The optimal results imply that thinner fins result in lower system cost and that there is a thickness limit for the fins to be economically welded on a heat pipe. The finned LTR model also gives the optimal cost of material usage for a large scale latent thermal energy storage system in terms of dollars per kilowatt and it was found that the system cost is slightly lower by using carbon-steel as the construction material for the heat pipes and fins than by using Al 6061.