[en] Design Basis Ground Motion is generally defined on rock outcrop because, in general safety related nuclear structures are founded on rock. For sites at which the bedrock is very deep, these safety related structures are founded on soil. Consequently, the behaviour of the foundation of structures resting on soil is very much different than on bedrock. Hence, seismic ground response analysis is required to develop a site-specific response spectrum for the design of important superstructures in the region. One-dimensional ground response analysis is a commonly used method to estimate the ground responses under earthquake excitation in both equivalent linear and non-linear domains for both low to high strains. To understand the above phenomena, tests on F-55 Ottawa sand have been performed in a large scale geotechnical laminar box (GLB) at the Buffalo State University, New York. In this study, these tests have been simulated using numerical procedures involving equivalent linear and nonlinear time history analysis (using hyperbolic stress strain curve) using the author’s in-house code which is limited to low to medium strains. The developed code, when tested for high strains, results in response accelerations that are lower than the experimental observations. The limitation of the model is that it predicts higher damping under larger strains. To match the damping under large strains, a modification is introduced in the developed nonlinear model which takes into account the above problem. With this modification, the predicted accelerations are in line with the experimental observation. Further, a study illustrating the performance of hyperbolic and multilinear backbone curves are studied based on the generation of the amplitude of high frequency harmonics in the response of nonlinear soil at high degree of nonlinearity. It is observed that the amplitude of odd harmonics is dependent on the number of parallel springs (or number of points) chosen for generating the multilinear stress strain curve and it is least (minimum) if a continuous (hyperbolic) backbone curve is chosen for conducting the nonlinear analysis of soil column. In addition, the results of the dynamic model tests are compared with the results from a plane strain finite difference program in terms of acceleration time history at the top and bottom accelerometer locations and it is found out that the numerical predictions are in reasonable agreement with the experimental observation using hyperbolic nonlinear stress-strain soil model. (author)

[en] Highlights: • Ni-CNT composites processed via dry milling / solution ball milling methods. • Ni-CNT composites exhibited significant improvement in the yield strength. • Ni-0.5CNT composites exhibited excellent combination of strength & ductility. • All possible Ni-CNT composites strengthening mechanisms were discussed. -- Abstract: The multi-walled carbon nanotubes (CNTs) reinforced nickel matrix nanocomposites (Ni-CNT) have been processed via ball milling followed by spark plasma sintering (SPS) process. The CNT content in these nanocomposites has been varied from 0.5 to 2 wt% (approximately from 2 to 8 vol%) to study their effect on the dispersion, microstructure, and mechanical behavior of these composites. Two Ni-CNT composite powders pre-mixing techniques have been employed, namely dry milling (DM) and solution ball milling (SBM), to investigate their effect on the dispersion of CNTs within a nickel matrix. The Ni-CNT powder was milled for different durations (1,2,6 and 12 hrs.) to investigate the milling effect on the grain size and the dispersion of CNTs in the nickel matrix. Ni-CNT nanocomposites exhibited improvement in microhardness and mechanical performance in comparison with pure nickel. Ni-1CNT-DM composites exhibited an excellent combination of the tensile yield strength of 455 MPa and around 13% elongation. This improvement in Ni-CNT nanocomposites is primarily attributed to the uniform dispersion of reinforcement within the nickel matrix, refined grain size, and strong nickel CNT interfacial bonding, which effectively transfers stress during tensile deformation. Various strengthening mechanisms associated with CNT-metal matrix composites have been discussed in detail. We have attempted to quantify the contribution of these strengthening mechanisms using micromechanical models.

[en] The graphene nanoplatelet (GNP) reinforced nickel matrix composites (Ni-GNP) have been processed using two different ball milling approaches, viz, dry ball milling (DM) and solution ball milling (SBM), followed by consolidation using spark plasma sintering (SPS) technique. The composites were reinforced with varying GNP concentration (0.5–2 wt%) and were milled for up to 12 hr to investigate the effect of premixing technique, milling duration, and GNP concentration on the grain size, microstructure, the dispersion of GNP in the nickel matrix, and mechanical behavior of these composites. Ni-GNP nanocomposites exhibited improved microhardness and tensile strength compared to pure nickel, primarily attributed to grain refinement and load transfer strengthening due to the uniform dispersion of these GNPs within the nickel matrix, promoting effective load transfer during tensile deformation. Ni-0.5GNP composites processed via dry milling followed by SPS exhibited the highest tensile yield strength of 586 MPa as compared to pure nickel and other Ni-GNP composites. The contribution of each strengthening mechanism in the overall improvement in yield strength of Ni-GNP composites has been qualitatively calculated/quantified and compared with experimentally obtained tensile properties.