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ZrO2-doped UO2 ceramic pellets have been identified as a high performance nuclear fuel in LWR due to its desirable properties. Mechanical properties such as hardness, elastic modulus and fracture toughness, as well as microstructure of ZrO2-doped UO2 ceramics containing 0, 10, 15 and 20 wt% ZrO2 were investigated by Vickers micro-indentation technique, X-ray diffraction and scanning electron microscopy, respectively. The lattice parameter of the ZrO2-doped UO2 ceramics linearly decreases as a function of the ZrO2 content and follows Vegard's law, indicating the formation of a complete solid solution between the ZrO2 and UO2 up to 20 wt% of the ZrO2 content. The average grain size was typically between 20 and 30 μm with no dopant additions and gradually decreased to about 10 μm for additions of 20 wt% ZrO2. Additionally, the addition of ZrO2 will significantly increase the hardness and elastic modulus of UO2 ceramic by approximately 16.4% and 14.5% with the ZrO2 contents increasing from 0 to 20 wt%, respectively. Furthermore, the fracture toughness of UO2 ceramic was considerably decreased by the addition of ZrO2 dopant, which is evidently contrary to the hardness and elastic modulus.

Uranium (U) is an important engineering material in nuclear energy industry. Its environmental instability, however, posed challenges on its applications. Due to the active property and similar electronic configuration, Cerium (Ce) can be used as a reference metal to simulate the protection of uranium from corrosion. In order to improve the corrosion resistance of Ce, ceramic CeN, and ceramic TiN and CeN/TiN bilayer composite film were prepared by dual ion beam sputtering deposition system (DIBD). The crystal structure of films was identified by X-ray diffraction (XRD) and the chemical states of CeN film were characterized by the X-ray photoelectron spectroscopy (XPS). The Surface morphology and structure changes in atmospheric environment were examined by scanning electron microscopy (SEM) and XRD, The corrosion behavior was studied by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The experimental results indicate the single film of CeN or TiN shows poor corrosion resistance, but the CeN/TiN bilayer composite film has better corrosion resistance. Based on the XPS, SEM and impedance analysis, we supposed that the loose of integrity and passivity breakdown on as deposited coating may be the corrosion inducement.

Luminescence sensing is an attractive approach for in-situ monitoring the health and integrity of multilayered surface protective coatings on highly active metals. In this work, we demonstrate the potential of erbium (Er) doped aluminum nitride (AlN) films to be applied as luminescence sensing layer in the Al/AlN multilayered coating system. The AlN:Er films were prepared by radio-frequency magnetron sputtering from Al targets doped with varied concentrations (from 0.5 at.% to 2.5 at.%) of Er. The chemical composition, surface morphology, crystal structure, chemical state and optical properties of the deposited AlN:Er films were investigated. We found that the crystal structure, surface roughness and optical properties of the AlN:Er films have strong correlations with the Er doping level. The as-deposited AlN:Er films showed the characteristic photoluminescence emission lines of the trivalent Er ions in the visible frequency range and quenching was observed when the Er doping level increased to 2.5 at.%. This work signifies the viability of AlN:Er film as luminescence sensing layer and the optimal Er doping concentration of the Al sputtering target is 2.0 at.% for practical applications.

Surface roughness is a vital issue when evaluating the coating performance. In this work, Ti/TiN multilayer coatings were deposited on U Nb substrates with varied roughness of Ra 0.8/1.6/3.2/6.3 μm, and then the effect of substrate roughness on the service performance of Ti/TiN multilayer coating was studied. The interface structure, adhesion and corrosion behavior have been investigated by scanning electron microscopy (SEM), scratch tests and electrochemical measurements, respectively. The results showed that coating adhesion increased with a decrease of substrate roughness, along with the improvement of its corrosion rate. Both adhesion and corrosion properties degraded dramatically as Ra exceeding 3.2 μm, which may be explained by the residual stress and interfacial defects. And the coating failure occurred by spallation and cracking, typically along the peaks of the coating profile. This study offered guidance to optimize the substrate roughness for the deposition of hard protective coatings, especially for nuclear industries.

The integrity and reliability of surface protective coatings deposited on metal surface could be in-situ monitored via the attractive luminescence sensing technique. In this paper, we report the influence of substrate temperature on the properties of erbium (Er) doped aluminum nitride (AlN) film, which could be applied as a luminescent layer for monitoring the health of multilayered Al/AlN coating. The AlN:Er films were deposited via reactive radio-frequency magnetron sputtering, and the silicon substrate temperature was varied from non-intentional heating up to 400 &deg, C. The composition, morphology, crystalline structure, and dielectric function of the AlN:Er films deposited under these different substrate temperature conditions were studied. These properties of the AlN:Er films show strong correlation with the substrate temperature maintained during film fabrication. The obtained AlN:Er films, without further annealing, exhibited photoluminescence peaks of the Er3+ ions in the visible wavelength range and the strongest photoluminescence intensity was observed for the AlN:Er film deposited with the temperature of substrate kept at 300 &deg, C. The results demonstrated in this work offer guidance to optimize the substrate temperature for the deposition of AlN:Er film for future application of this sensing technique to thin metal components.

Depleted uranium (DU) is oxidized readily due to its chemical activities, which limits its applications in nuclear industry. TiN film has been applied widely due to its good mechanical properties and its excellent corrosion resistance. In this work, TiN protection films were deposited on DU by direct current magnetron sputtering (DCMS) and high power pulsed magnetron sputtering (HPPMS), respectively. The surface morphology and microstructures were investigated by atomic force microscope (AFM), scanning electron microscopy (SEM), and grazing incidence X-ray diffraction (GIXRD). The hardness and Young&rsquo, s modulus were determined by nano-Indenter. The wear behavior and adhesion was analyzed by pin-on-disc tests and scratch adhesion tests and the corrosion resistance was evaluated by electrochemical measurements. The results show that the TiN films that were deposited by HPPMS outperformed TiN film deposited by DCMS, with improvements on surface roughness, mechanical properties, wear behavior, adhesion strength, and corrosion resistance, thanks to its much denser columnar grain growth structure and preferred orientation of (111) plane with the lowest strain energy. Besides, the process of Ti interlayer deposition by HPPMS can enhance the film properties to an extent as compared to DCMS, which is attributed to the enhanced ion bombardment during the HPPMS.

Depleted uranium has been widely applied in nuclear energy fields. However, its poor corrosion and wear resistance restrict its applications. A titanium/titanium nitride (Ti/TiN) multilayer film was deposited on a uranium surface to improve its fretting wear resistance. Fretting wear tests were carried out using a pin-on-disc configuration. The fretting behaviors of uranium and the Ti/TiN film were investigated under different normal loads. With the normal load increasing, the mode of fretting wear gradually transformed from slip region (SR) to mixed fretting region (MFR) and then to partial slip region (PSR). It is illustrated that the normal load had an obvious effect on the fretting wear behavior. The friction coefficients of both uranium and Ti/TiN multilayer film decreased with the increase of the normal load. In SR, the main wear mechanisms were delamination and abrasion for uncoated uranium, and delamination and oxidation for uranium coated with the Ti/TiN multilayer film. Photoelectron spectroscopy (XPS) analysis also showed that the Ti/TiN coating was oxidized and formed TiO2 during fretting wear. The wear depth of naked uranium was much greater than that of coated uranium, which demonstrated that the Ti/TiN multilayer film could effectively improve the wear properties of uranium.

Experiments and heterogeneous simulations are combined to investigate cracking behavior of sheared asphalt mixture. A series of direct shear tests are performed at different temperatures with a self-manufactured direct shear test setup, and then a heterogeneous fracture modeling approach based on the random aggregate generation and packing algorithm and the bilinear cohesive crack model is utilized to simulate shear fracture processes. Two kinds of fracture, the single crack type for −20 and −10 °C, and the double crack type for 0 and 5 °C, are observed. Some internal mechanisms are analyzed based on numerical simulations.

Tritium permeation barrier (TPB) of Er 2 O 3 /SiC multilayer film was proposed to improve the tritium permeation resistance of 316L stainless steel. Surface and cross-section morphologies of film were observed by SEM and TEM, The Auger electron spectroscopy was used to investigate the depth profiles of Er, O, Si and C elements in Er 2 O 3 /SiC multilayer film. The chemical state of Er, O, Si and C elements were characterized by XPS. The deuterium permeation property is measured by using a quadruple mass spectrometer. The TEM cross-section microstructure and AES depth profiles shows that the multilayer film has alternative Er 2 O 3 and SiC layers and form a modulation structure. The 0.3-μm Er 2 O 3 /SiC multilayer film on 316L stainless steel reduces permeability by a factor up to almost 500, and higher PRFs can be obtained by optimizing the coating process. It is indicated that the Er 2 O 3 /SiC multilayer film shows the huge potential in improving the tritium permeation resistance.

A three-dimensional (3D) heterogeneous fracture modeling technology is presented to simulate complex crack evolution in quasi-brittle asphalt mixture. In this technology, the random aggregate generation and packing algorithm is employed to create 3D heterogeneous numerical model of asphalt mixture, and the cohesive elements with the tension/shear softening laws are inserted into both the mastic matrix and the aggregate–mastic interfaces as potential cracks. The nucleation and coalescence of microcracks, and inception and propagation of main macrocracks are carefully studied under uniaxial tension and temperature of −10 °C. The effects of the averaged coarse aggregate size and the cohesive fracture parameters on performance of asphalt mixture are also evaluated.

The Cross model is modified by introducing a long-term correction factor to predict the long-term mechanical behavior of compressed sand asphalt with the combination of the time–temperature superposition principle. A series of short-term experiments, including one-hour creep tests and constant strain-rate tests, are carried out to determine the Cross model parameters. Two long-term creep tests lasting more than 28 days are conducted at two different stress levels. One is for determining the long-term correction factor and the other for the model validation. It is found that the modified Cross model can predict the long-term viscoelastic behavior of sand asphalt better than the Cross model.

The random aggregate generation and packing algorithm is employed to model pre-cracked three-point bending beams of asphalt mixture which is treated as a two-phase material comprising randomly distributed coarse aggregates and asphalt mastic, and the cohesive elements are inserted inside mastic and into aggregate-mastic interfaces to simulate crack initiation and propagation. The mode I and mixed mode cracking behaviors are simulated through changing the beam location in the loading setup. The predicted results compare very well with the experimental ones, and then the influences of crack location and coarse aggregate distribution on cracking behavior are evaluated.

A numerical simulation framework is proposed to analyze damage evolution and crack propagation behaviors of heterogeneous asphalt mixture under low temperature. In this framework, asphalt mixture is considered as a two-phase composite consisting of coarse aggregates and asphalt mastic (namely a mix of fine aggregates and asphalt binder). The heterogeneous and random geometrical models of asphalt mixture are created with the parameterization method and the physical degeneration of asphalt mastic in fracture processes is characterized with the damage constitutive model with some experimentally determined parameters. The framework is validated by good agreement between the predictions and the experiments in crack path. Finally, the effects of crack location and coarse aggregate distribution on crack propagation and damage evolution in a pre-cracked three-point bending asphalt mixture beam are evaluated.

This paper develops a mesoscale finite element method for realistic modeling of complex cohesive fracture in asphalt mixture with a given gradation. A random aggregate generation and packing algorithm is employed to create 2D and 3D heterogeneous asphalt mixture specimens, and cohesive elements with tension/shear softening laws are inserted into both mastic and aggregate-mastic interfaces to simulate crack initiation and propagation. The nucleation and coalescence of microcracks and propagation of macrocracks in 2D and 3D specimens is realistically modeled in detail with a few important conclusions drawn. The effects of coarse aggregate distributions on performance of asphalt mixture are also evaluated.

Because of large pronounced deformation in the compressive creep process of soft sand asphalt, it is necessary that both the true stress and true strain are measured. Accordingly, instead of fictitious creep compliance (FCC), true creep compliance (TCC) relating the true strain to the true stress is used to characterize the viscoelastic properties of sand asphalt in this paper. The relationship between TCC and FCC is described by a second-kind Volterra integral equation (VIE). A prediction method of long-term mechanical behavior of largely deformed sand asphalt with constant loading creep tests is proposed. In this method, the FCC master curves are constructed based on the time-temperature superposition principle, and then the linear VIE from the FCC to the TCC is solved with the collocation method, and the nonlinear VIE from the TCC to the FCC is solved with an iterative formula. Unconstrained compressive creep tests for 3,600 s were conducted on sand asphalt mixture samples in various temperature and nominal stress conditions. As an application example, the long-term creep behavior of sand asphalt at the given reference temperature is predicted with the proposed method.

Heterogeneous asphalt mixture is treated as a two-phase composite consisting of asphalt mastic, namely a mixture of asphalt and fine aggregates, and coarse aggregates in this paper. A novel three-dimensional (3D) random modeling frame for asphalt mixture is developed, and as its applications, some numerical samples involving various polyhedric coarse aggregates with given gradations are generated. Viscoelastic asphalt mastic is simply characterized by the generalized Maxwell model whose parameters are determined by fitting the uniaxial compressive creep experimental data of asphalt mastic. After validation, the 3D random numerical samples are used to perform a series of numerical experiments in order to evaluate the influences of coarse aggregate distribution, content, average size, size deviation, shape and loading rate on the mechanical behaviors of asphalt mixture quantitatively. Finally, some important conclusions are given.

A multiscale modeling frame combining macroscale and mesoscale is proposed in basis of the aggregate generation and packing algorithm so that the heterogeneity feature of asphalt mixture can be involved in the computational model. In this frame, the mesoscopic scale is used in some concerned local regions and the macroscopic scale is used in the other regions. As examples, a series of two-dimensional multiscale models of cracked three-point bending asphalt mixture beam are created and then the effects of crack location and aggregate distribution near the crack on cracking behaviors are evaluated. Finally, some important conclusions are concluded.

A three-dimensional aggregate generation and packing algorithm applicable for modeling asphalt mixture with high content of graded aggregates is presented in this paper. In the algorithm, arbitrary-shaped polyhedra are used to model aggregates, so that the effect of aggregate shape on the mechanical performance of asphalt mixture can be considered. The algorithm consists of two steps: Aggregate generation and packing. Polyhedra are created by extending triangular fundaments and treated as visualized aggregates after passing through convex control and sharpness judgment. After that, graded aggregates are taken out from the aggregate base and randomly packed in a given cylindrical or cubical region one by one equiprobably. Overlapping between nearby aggregates is avoided by the help of Boolean partition operation in ANSYS. Finally, some asphalt mixture samples with a given gradation are modeled as examples, and their effective elastic properties and creep behaviors under uniaxial compression are simulated.

Asphalt mixture is described as an elastic and viscous material. Hooke’s law and the Bailey-Norton formulation are respectively used to characterize its elastic and viscous behaviors. A numerical algorithm frame is constructed to analyze nonlinear and time dependent deformation behavior of asphalt mixture. The model parameters are determined by the uniaxial compressive creep experiments and the algorithm is validated by comparison between the numerical results and the rutting experiments. The algorithm is used to analyze rutting deformations in different types of asphalt mixture specimens under cycled wheel loads. Some preliminary studies on effects of aggregate gradation on the rutting deformation in asphalt mixture samples are conducted. Finally, some conclusions are given.