Tarik Saleh

In this Letter, the time reversal mirror is used to focus elastic energy at a prescribed location and to analyze the amplitude dependence of the focus signal, thus providing the nonlinearity of the medium. By varying the frequency content of the focused waveforms, the technique can be used to probe the surface, by penetrating to a depth defined by the wavelength of the focused waves. The validity of this concept is shown in the presence of gradual and distributed damage in concrete by comparing actual results with a reference nonlinear measurement and X ray tomography images.

Developed in the late 1980s, Nonlinear Resonant Ultrasound Spectroscopy (NRUS) has been widely employed in the field of material characterization. Most of the studies assume the measured amplitude to be proportional to the strain amplitude which drives nonlinear phenomena. In 1D resonant bar experiments, the configuration for which NRUS was initially developed, this assumption holds. However, it is not true for samples of general shape which exhibit several resonance mode shapes. This paper proposes a methodology based on linear resonant ultrasound spectroscopy, numerical simulations and nonlinear resonant ultrasound spectroscopy to provide quantitative values of nonlinear elastic moduli taking into account the 3D nature of the samples. In the context of license renewal in the field of nuclear energy, this study aims at providing some quantitative information related to the degree of micro-cracking of concrete and cement based materials in the presence of thermal damage. The resonance based method is validated as regard with concrete microstructure evolution during thermal exposure.

Developed in the late 1980s, Nonlinear Resonant Ultrasound Spectroscopy (NRUS) has been widely employed in the field of material characterization. Most of the studies assume the measured amplitude to be proportional to the strain amplitude which drives nonlinear phenomena. In 1D resonant bar experiments, the configuration for which NRUS was initially developed, this assumption holds. However, it is not true for samples of general shape which exhibit several resonance mode shapes. This paper proposes a methodology based on linear resonant ultrasound spectroscopy, numerical simulations and nonlinear resonant ultrasound spectroscopy to provide quantitative values of nonlinear elastic moduli taking into account the 3D nature of the samples. In the context of license renewal in the field of nuclear energy, this study aims at providing some quantitative information related to the degree of micro-cracking of concrete and cement based materials in the presence of thermal damage. The resonance based method is validated as regard with concrete microstructure evolution during thermal exposure.

Pre-irradiated thermodynamic and microstructural properties of nuclear fuels form the necessary set of data against which to gauge fuel performance and irradiation damage evolution. This paper summarizes recent efforts in mixed-oxide and minor actinide-bearing mixed-oxide ceramic fuels fabrication and characterization at Los Alamos National Laboratory. Ceramic fuels (U_1-x-y-zPu_xAm_yNp_z)O₂ fabricated in the compositional ranges of 0.19≤x≤0.3 Pu, 0≤y≤0.05 Am, and 0≤z≤0.03 Np exhibited a uniform crystalline face-centered cubic phase with an average grain size of 14μm; however, electron microprobe analysis revealed segregation of NpO₂ in minor actinide-bearing fuels. Immersion density and porosity analysis demonstrated an average density of 92.4% theoretical for mixed-oxide fuels and an average density of 89.5% theoretical density for minor actinide-bearing mixed-oxide fuels. Examined fuels exhibited mean thermal expansion value of 12.56×10⁻⁶/°C-1 for temperature range (100°C<T<1500°C) and ambient temperature Young's modulus and Poisson's ratio of 169 GPa and of 0.327, respectively. Internal dissipation as determined from mechanical resonances of these ceramic fuels has shown promise as a tool to gauge microstructural integrity and to interrogate fundamental properties.

ULTIMET® alloy, a cobalt-based superalloy with good corrosion and wear resistant properties, exhibits a deformation-induced phase transformation from the face-centered-cubic (FCC) phase to the hexagonal-close-packed (HCP) phase. The HCP phase formation during monotonic tensile loading was investigated using in-situ neutron diffraction. The HCP phase is first observed at a stress level of 810 MPa, which is well beyond macroscopic yielding. Strain analysis is performed on the FCC phase diffraction data in order to relate the lattice-strain development with the evolution of the new HCP phase. A method of calculating the effective macroscopic stress associated with the measured lattice strains is presented here. The effective stress can then be compared to the applied macroscopic stress in order to draw conclusions about the load-partitioning behavior of the material as a new phase develops.