Low Relative Density Research Articles

Interpretation of field tests using geo-statistics and Kriging to assess the deep vibratory compaction of the Dike A21, Diavik Diamond Mine

This article presents an interpretation and statistical analysis of heavy dynamic probing (DPH) tests carried out on the compacted core of Dike A21, of the Diavik Diamond Mine. Due to the inhomogeneous and inherently random nature of soil and the uncertainties associated with even well controlled field tests such as DPH tests, the evaluation of in situ properties (such as density) should take into consideration the natural variability of the soil. Aided by the large amount of relatively concentrated field data, this paper presents a methodology for not only evaluating the field tests statistically through Kriging, but also for judging the compaction effort, taking into consideration the variability. This paper documents the necessary normalization procedure, analyzing a number of external factors such as the temperature, time since compaction (aging) and the presence of nearby works that may affect the field data. It was shown that the appraisal of soil improvement measures should consider the presence of natural and unavoidable fluctuations of the fill material. The Kriging estimate combined with an averaging procedure and subsequent comparison with the expected variability, quantified by the variogram, was applied which enabled the assessment of areas with insufficient compaction. Additionally, the influence of aging of the fill was demonstrated, where a clear increase in the DPH tests between 0 and 14 days since compaction was observed. It was also found that over-compaction leads typically to a lower relative density. The authors note that this statistical analysis was performed following the control and acceptance of the compaction of the dike and as such had no impact on the acceptance of the dike construction.

Fabrication and experimental characterisation of a bistable tensegrity-like unit for lattice metamaterials

The peculiar nonlinear mechanical behaviour of tensegrity structures and their engineering applications have attracted considerable interest in the last two decades. However, the difficulties in their traditional fabrication and assembling methods represent current limitations to their widespread use. This paper presents a novel design and fabrication procedure for bistable tensegrity-like units. Starting from the classical triangular tensegrity prism and using stereolithography technology, a double tensegrity-like unit was designed and realised monolithically as a compliant mechanism. High repeatability of compression tests confirmed the activation of the designed bistable twisting mechanism in large displacements, proving that the bistability of a tensegrity-like unit with null selfstress and no cables can dependably be achieved. Numerical simulations showed that a reduced-order stick-and-spring model is able to provide predictions on the nonlinear mechanical behaviour of the unit in close agreement with experimental results. Low relative density and bistable characteristics make this type of tensegrity-like unit suitable to manufacture highly-customisable multistable metamaterials. The proposed procedure could be applied to transform and additively manufacture other types of tensegrity structures with different nonlinear responses into corresponding tensegrity-like versions. • Stereolithography fabrication of tensegrity-like units without pin-joints or cables. • Experimental investigation on the compressive behaviour of a tensegrity-like unit. • Bistable response of the designed unit confirmed by experimental results. • Design procedure suitable to lightweight metamaterials with nonlinear behaviour. • Tessellation of units for multistable metamaterials and control of compact waves.

Experimental Studies and Constitutive Modeling of Static Liquefaction Instability in Sand–Clay Mixtures

This paper examines static liquefaction phenomena in sand–clay mixtures from both experimental and theoretical perspectives. Consolidated undrained triaxial tests (CU tests) are implemented on a mixture of sand and clay to study the effects of initial relative density, confining pressure, and clay content on static liquefaction responses. By using the equivalent granular state parameter, a state-dependent hardening plasticity model is proposed to reproduce mechanical responses, as well as to replicate the observed static liquefaction phenomenon. In accordance with the second-order work theory, the criteria for predicting the potential instability and static liquefaction of sand–clay mixtures are obtained. The analyses show that (1) static liquefaction is prone to be promoted by low initial relative density and confining pressure; (2) even a small amount of clay particles can significantly increase the liquefaction susceptibility in loose sand–clay mixtures; (3) the proposed constitutive model can provide a satisfactory match between the test data and the simulations; and (4) even when the state of the soil is potentially unstable, soils can still remain stable unless second-order work vanishes, which indicates the inception of static liquefaction.

The small-scale limits of electron beam melt additive manufactured Ti–6Al–4V octet-truss lattices

The emergence of powder-based additive manufacturing (AM) processes, such as electron beam melting (EBM), enables the one step manufacture of microarchitected metamaterials from topology optimized models. However, many applications are optimized by low relative density lattices with slender trusses whose diameter approaches small multiples of largest powder particles, potentially resulting in surface roughness. The thermal history experienced by alloy powders also modifies the alloy microstructure, and thus mechanical behavior, posing a significant challenge to metallic metamaterial designs and fabrication. We therefore build and characterize the multiscale structure and mechanical properties of EBM manufactured Ti–6Al–4V octet truss lattices with strut diameters approaching the particle diameter-imposed fabrication limit. We measure the dependence of their relative density, elastic modulus, and compressive strength on the fabrication process-controlled truss topology and microstructure, and compare them to identical smooth surface structures made from an annealed, wrought version of the same alloy built using a snap-fit assembly method. Micro-x-ray tomography confirmed that the lattice strut surfaces were covered with partially melted powder particles, resulting in about 29% of the lattice mass that inefficiently supported the applied loads. The use of a powder bed held at a temperature of 600–700 °C also resulted in a lamellar α/β phase microstructure with an elastic modulus, yield strength, and a ductility that were less than the equiaxed α/β microstructure of snap-fit assembled structures. However, the higher tangent modulus of the lamellar AM processed alloy resulted in significant strengthening of EBM lattices that failed by inelastic buckling during compression. The ability to increase the alloy tangent modulus during an EBM build process therefore provides a promising approach for increasing lattice compressive strength and therefore compensates for surface roughness induced losses.

Mechanically-grown morphogenesis of Voronoi-type materials: Computer design, 3D-printing and experiments

The present work introduces a novel and versatile computer-design and experimental strategy to obtain random Voronoi-type geometries, called M-Voronoi (from mechanically grown), with smooth void shapes and variable intervoid ligament sizes that can reach very low relative densities. This is achieved via a numerical, large strain, nonlinear elastic, void growth mechanical process. Originally small circular voids embedded in a cell of arbitrary shape (triangle, circle, rectangle, trapezoid) grow when subjected to displacement (Dirichlet) boundary conditions. The deformed voids evolve into smooth Voronoi-type geometrical shapes leading to macroscopic isotropy or anisotropy depending on the prescribed boundary conditions. The void growth process is a direct consequence of mass conservation and the incompressibility of the surrounding nonlinear elastic matrix phase and the final achieved relative density may be analytically estimated in terms of the determinant of the applied deformation gradient. In order to study the mechanical properties of the M-Voronoi materials, we focus on two-dimensional porous polymer square representative isotropic and anisotropic geometries in terms of void size and realization, which are 3D-printed and experimentally tested under uniaxial compression. For comparison, we also test random polydisperse porous materials with circular voids, standard eroded Voronoi geometries and hexagonal honeycombs. The first two are also isotropic while the latter are only isotropic in the linear elastic regime. We show that the randomness of the M-Voronoi geometry and their non-uniform intervoid ligament size leads to enhanced mechanical properties at large compressive strains with no apparent peak-stress and strong hardening well before densification. By comparing them with the hexagonal geometries, which tend to exhibit a peak-stress and a plateau-type response, we show that the hardening response of the M-Voronoi is mainly due to their geometrical characteristics and less due to the polymer hardening response. Anisotropic M-Voronoi are also produced and tested indicating that anisotropy only enhances the initial stiffness along the longitudinal direction but instead leads to lower buckling loads and hardening rates than the corresponding isotropic M-Voronoi in the nonlinear regime.

Evaluation of visual encounter surveys as a method for the rapid assessment of fish presence and relative density in high mountain lakes

Abstract Introduced fish are a widespread ecological threat in originally fishless high mountain lakes. However, basic distribution data are largely missing for most high mountain regions. Using time‐consuming standard methods (e.g. Nordic standard fishing nets) to assess fish distribution and relative densities at a relevant spatial scale can be impracticable, because of the large number of high mountain lakes. To overcome this problem, alternative rapid monitoring methods would be helpful. Visual encounter survey (VES) is a candidate method that enables observing fish from the shoreline. It takes only minutes to implement and is already widely used for amphibian monitoring in high mountain lakes and ponds. VES was evaluated as a method for monitoring introduced salmonids and cyprinids (the most widespread fish families) in 52 high mountain lakes. The probability of detecting both families by VES rapidly approaches 100% as the relative densities of fish increase, and false absences are restricted to populations living at low relative densities. VES also provides simple indications about fish relative densities, distinguishing between high‐density and low‐density populations. As VES usually does not enable fish species identifications, we propose VES as a useful method to describe large fish distribution inventories, not needing high taxonomic detail, but necessary for planning large‐scale conservation measures.

Effective phase transformation behavior of NiTi triply periodic minimal surface architectures

The effective behavior of shape memory alloy triply periodic minimal surface (TPMS) structures is investigated using finite element analysis and numerical homogenization. The TPMS unit cells considered are primitive, IW-P, gyroid, and diamond subjected to different loading conditions. Under uniaxial displacement-driven loading, the results show a dramatic increase in effective stress and martensite volume fraction with increased relative density of the TPMS unit cell. Comparison among the four types of TPMS unit cells shows that diamond has superior mechanical performance for the loading cases considered. Based on numerical homogenization results, the onset and subsequent thresholds of phase transformation are determined for all four unit cells considering multiaxial loading. At lower relative density, the loading surfaces corresponding to the onset of phase transformation were reasonably well represented by either von Mises or Hill’s criteria. The observed fit with the von Mises model degenerated with increased effective martensite volume fraction, while a proper fit was maintained with Hill’s criterion. Subsequent loading surfaces, corresponding to monotonically increasing martensite volume fraction, show a nonlinear hardening behavior, which seems to follow a similar trend regardless of geometry. The loading surfaces were found to reach asymptotic states with distinctly different features compared to their initial shapes.

Large deformation of shape-memory polymer-based lattice metamaterials

Through advanced computational analysis, this work demonstrates that metamaterial structures when made by shape-memory polymers (SMPs) can provide unique intelligent mechanical behavior. For the first time, geometrical effects, pertaining to low and ultra-low densities, on the thermomechanical behavior of SMP-based octet-truss lattice meta-structures are studied in this work. A reliable constitutive thermo-visco-hyperelastic model is applied to analyze the shape-memory behavior of several designed octet-truss lattices with ultra-low and low relative densities, ranging from 0.04 to 0.23. Different compressive strain values are tested to determine the effect of pre-straining. It is concluded that changing the relative density as a consequence of altering the diameter of struts in lattice meta-structures is a crucial factor affecting their shape-memory properties, deformation mechanism, and macro-scale mechanical properties compared to the bulk SMPs. The finite element results demonstrate that the relative density and pre-strain level provide two controlling parameters in programming the SMP-based lattice meta-structures and enhancing their stress recoverability.

Additively manufactured lattice structures with controlled transverse isotropy for orthopedic porous implants

Additively manufactured lattice structures enable the design of tissue scaffolds with tailored mechanical properties, which can be implemented in porous biomaterials. The adaptation of bone to physiological loads results in anisotropic bone tissue properties which are optimized for site-specific loads; therefore, some bone sites are stiffer and stronger along the principal load direction compared to other orientations. In this work, a semi-analytical model was developed for the design of transversely isotropic lattice structures that can mimic the anisotropy characteristics of different types of bone tissue. Several design possibilities were explored, and a particular unit cell, which was best suited for additive manufacturing was further analyzed. The design of the unit cell was parameterized and in-silico analysis was performed via Finite Element Analysis. The structures were manufactured additively in metal and tested under compressive loads in different orientations. Finite element analysis showed good correlation with the semi-analytical model, especially for elastic constants with low relative densities. The anisotropy measured experimentally showed a variable accuracy, highlighting the deviations from designs to additively manufactured parts. Overall, the proposed model enables to exploit the anisotropy of lattice structures to design lighter scaffolds with higher porosity and increased permeability by aligning the scaffold with the principal direction of the load.

Designing Barium Titanate Ceramics with High Energy Storage and Mechanical Properties

Designing the fine-grained ceramics with high recoverable energy storage density and excellent mechanical performance, still presents great challenges. Because the ceramics with large pore size and low relative density were obtained by the traditional sintering method, they always exhibit small breakdown strength and poor mechanical properties, which limits the miniaturization and operation of devices in harsh environments. In this paper, we designed the barium titanate ceramics (BT) ceramics with grain size of 252 nm and relative density of 0.92 can be obtained via co-sintering of two sizes of BT particles at 1000 °C for 10 h. The cubic phase BT particles with 80 nm are doped in the tetragonal phase BT particles with 200 nm to obtain tetragonal phase fine-grained BT ceramics. When doped with 80 nm BT particles, the sintering of 200 nm BT particles is promoted. The enhanced sinterability is due to the phase transformation and metastability, which act as sintering aids. The obtained BT ceramics with nanodomains, high energy storage and mechanical properties. The discharge energy density is of 0.81 J/cm3, and the Vickers hardness is 1.75 GPa.

Poro-elastic and poro-elasto-plastic modeling of sandy seabed under wave action

The seabed could be liquefied due to the wave-induced accumulation of residual excess pore water pressure (EPWP), resulting in the instability of offshore and marine structures. Thus, it is of importance to consider both the buildup of EPWP and the associated deformation behavior of the seabed. In this study, a dynamic poro-elasto-plastic seabed model, integrated with a state-dependent plasticity model, is developed in COMSOL Multiphysics. The numerical model of seabed is validated against the analytical solutions and model tests. The elliptical trajectories of soil particle at different depths of the poro-elastic seabed under the progressive wave are illustrated. The ratio of the amplitude of vertical displacement to that of horizontal displacement generally decreases as the depth increases for the poro-elastic seabed. In the poro-elasto-plastic seabed, however, the soil particles tend to move following the direction of the wave propagation. Moreover, the soil particles at the shallow part of the seabed go downwards before the initial liquefaction, while the soil particles at the lower position initially go upwards and then turn to go downwards when the initial liquefaction occurs. Particularly, the surface seabed with lower relative density is more prone to lose the stability, resulting in a dramatic upward vertical displacement.

Effective stiffness, wave propagation, and yield surface attributes of Menger sponge-like pre-fractal topologies

In the current work, the effective stiffness, long-wavelength attributes, and yield limits of Menger sponge-like pre-fractal topologies are investigated for the first time. Three types of Menger sponge-like structures with circular (MC), rhombic (MR), and square (MS) cavities are considered up to their third-level fractal topology. For each level, the elastic, shear, and bulk moduli, as well as the Poisson's ratio values are computed. Moreover, third-level Menger sponge-like fractal structures are additively manufactured and experimentally tested. A clear difference between the elastic stiffness and yield strength of the different Menger sponge-like structures is numerically and experimentally identified. It is observed that the cavity type considerably affects the scaling of elastic stiffness attributes upon increasing fractal level. The MS and MC topologies relate to lower relative density structures compared to the MR topology for all fractal levels investigated. The percentage relative density reductions induced by the fractal topology are a multiple of the corresponding decrease in the magnitude of the propagating shear and longitudinal phase velocities. Furthermore, all Menger sponge levels and inner cavity types yield analogous wave propagation directional dependencies, though with different phase velocity magnitudes. It is also found that increasing fractal topologies diminish the yield strength limits of the Menger fractal structures, with the factor associating the normal and shear loading yield strength of two successive Menger topology levels to be lower, but comparable in magnitude with the fractal dimension number. The fractal level evolution of the yield surface is well-captured by the extended Hill's criterion both for bi-axial and axisymmetric loads. The corresponding coefficients are identified for first, second, and third-level MS structures.

Thermal shock resistance enhancement of auxetic honeycomb layer considering multi-cracking and temperature-dependent material properties

This paper assesses the thermal shock behaviors of the auxetic honeycomb layer (HL) through consideration of crack propagation and new crack initiation. All material properties are temperature-dependent. To calculate the thermal stress field with no crack, the finite difference approach is used. On this basis, a theoretical model for thermal stress intensity factor and thermal stress between cracks is developed. The model combines the equivalent fracture toughness and equivalent tensile fracture strength as the failure criteria. The critical temperature for crack propagation and new crack initiation in the HL is determined. The results reveal that the thermal shock resistance (TSR) of the HL is overestimated if the temperature-dependent material properties (TDMPs) are not taken into account. Multiple cracking can dramatically improve the TSR of the HL. Compared to the non-auxetic HL, the TSR of the auxetic HL is significantly enhanced. Furthermore, the HL with lower relative density generally possesses higher TSR, and the corresponding physical mechanisms are revealed.

Sound absorption properties of spinodoid structures with spatially varying properties

Creating lightweight structures with tailored noise reduction at low-frequencies remains a major engineering challenge. Open-celled porous structures with low relative densities offer a possible solution to this problem. Spinodoids—structures with microstructural architectures based on spinodal decomposition—have been recently shown to provide robust and tunable mechanical properties. Here, we study the sound absorption properties of spinodoids with spatially varying properties such as relative density, spinodoid type, and wavenumbers. We generate the sample geometries using MATLAB and print them using low-cost additive manufacturing methods. The printed samples were then tested using a normal impedance tube setup to characterize their absorption coefficients. Our results show that spatially gradient spinodoids provide an attractive solution for designing multifunctional structures with application-specific acoustic properties.

Numerical study on the warm compaction and solid-state sintering of TiC/316L composite powders from particulate scale

Powder compaction and sintering are critical stages of powder metallurgy in manufacturing high performance particle reinforced metal matrix composites. In this paper, particulate scale numerical investigations on the warm compaction and solid-state sintering of TiC/316L composite powders with different particle size ratios (PSRs, R316L/RTiC) and TiC contents were performed using multi-particle finite element method (MPFEM) in two dimensions. The effects of PSR and TiC content, compaction pressure and temperature, and sintering temperature on the compaction and sintering processes were comprehensively analyzed. On this basis, a variety of macro- and microscopic analyses were conducted to further identify the densification dynamics and mechanisms. The results indicate that green compacts with higher relative density and more uniform stress distribution can be readily achieved by warm compaction. The large-scale plastic deformation of 316L particles caused by the cooperative actions of temperature and pressure is the main densification mechanism in warm compaction. During solid-state sintering, the improvement of relative density is mainly achieved by the vanishing or shrinkage of residual pores along with the growth of sintering necks, accompanied by the deformation of 316L particles. Large displacements of particles mainly occur in contacting areas of adjacent particles. Moreover, the equivalent stress in 316L particles is smaller than that in TiC particles. With the increase of PSR and TiC content, lower relative density of green compacts and sintered parts were obtained and more irregular morphologies of 316L particles can be observed.

High-performance B4C matrix composites fabricated from the B4C–Si–CeO2 system via reactive hot pressing

Boron carbide (B4C) matrix composites had the advantages of high hardness, high melting point and low density. However, due to the low relative density and poor fracture toughness of B4C, its comprehensive properties were limited in engineering applications. In this work, in order to improve the comprehensive properties of B4C composites, B4C–SiC–SiB6–CeB6 composites were designed and fabricated via reactive hot pressing at 2050 °C and 20 MPa with B4C matrix and novel additives (Double doping of Si and CeO2) as raw materials. The effects of additive CeO2 content on the microstructures and mechanical properties of composite were investigated, and reaction mechanisms of B4C, Si and CeO2 at different temperatures were studied in detail. The work showed that liquid phase Si and SiB6 greatly improved the densification of composites. CeB6 played an indispensable role in the formation of SiC–SiB6 agglomerate structure, increasing strength and supplementing toughness. When the content of CeO2 was 6 wt%, the relative density, hardness, flexural strength and fracture toughness reached to 99.7%, 34.9 GPa, 461.46 MPa and 5.57 MPa m1/2, respectively. Our strategy benefited from the formation of two liquid phases and SiC–SiB6 agglomerate structure, showing great potential in promoting sintering and improving fracture toughness.

Extremely stiff and lightweight auxetic metamaterial designs enabled by asymmetric strut cross-sections

Negative Poisson’s ratio has been shown to enhance packing efficiency, impact absorption, indentation resistance, fracture resistance and acoustics attenuation in auxetic materials, making them useful for applications such as protective cases and stents. However, these benefits are usually realized at the expense of a low specific modulus and poor loading efficiencies, as auxetic structures rely on ‘soft’ strut bending and/ or joint rotation deformation modes to reach perceptible levels of negative Poisson’s ratios. Here, a general 3D Anti-Tetrachiral (3ATC) structure was analyzed and shown that, in the limit of low relative density (i.e lightweight structure), the trade-off between specific relative stiffness and auxeticity is linear and the maximum attainable specific relative stiffness is 1/3. If the strut cross-section is symmetric, the maximum attainable Poisson’s ratio is -0.5, while that for an asymmetric cross-section depends on the specific geometry of the cross-section. Importantly, our analysis shows that the non-zero product of inertia for asymmetric cross-sections can lead to an additional twist of the joint, thereby increasing the auxeticity of the 3ATC structure. This allows the 3ATC lattice to exhibit a large specific relative stiffness for a given Poisson’s ratio, which can be more than an order of magnitude higher when compared to other auxetic designs in the literature. Our analysis was validated by finite element simulations and experiments on 3ATC lattices with strut cross-sections that were square (symmetric) and L-shaped (asymmetric).

Numerical Investigation of Ti6Al4V Gradient Lattice Structures with Tailored Mechanical Response

Open porous lattice structures have found a wide range of applications as lightweight load‐bearing and energy absorbent structures in various fields. A common requirement for any application is the preliminary assessment of the mechanical response of the proposed architecture. Herein, uniform and continuous functionally graded lattice structures (FGLSs) made of titanium alloy, Ti–6Al–4V, are designed using cubic and pillar octahedron unit cells at overall porosities of 60%, 75%, and 85%. The lattice morphology is modulated using axial, dense‐in, and dense‐out gradient strategies. The mechanical performance of the structures is studied using numerical simulations implementing damage initiation and evolution under quasi‐static compression. The proposed models could properly simulate the mechanical properties and failure behavior of the lattice structures. The designed FGLSs displays a crushing behavior starting from the lower relative density layers toward the higher relative density ones. Cubic and pillar octahedron‐based structures exhibit stretch‐ and bending‐dominated deformation behaviors, respectively. The power‐law analysis verified by the Gibson–Ashby model is used to assess the mechanical response of the FGLSs. The effect of the gradient strategies on the mechanical properties as well as their adaptability for orthopedic implants is discussed in detail.

Selective Electron Beam Melting (SEBM) of Pure Tungsten: Metallurgical Defects, Microstructure, Texture and Mechanical Properties.

Effects of processing parameters on the metallurgical defects, microstructure, texture, and mechanical properties of pure tungsten samples fabricated by selective electron beam melting are investigated. SEBM-fabricated bulk tungsten samples with features of lack of fusion, sufficient fusion, and over-melting are examined. For samples upon sufficient fusion, an ultimate compressive strength of 1.76 GPa is achieved at the volumetric energy density of 900 J/mm3–1000 J/mm3. The excellent compressive strength is higher and the associated volumetric energy density is significantly lower than corresponding reported values in the literature. The average relative density of SEBM-fabricated samples is 98.93%. No microcracks, but only pores with diameters of few tens of micrometers, are found in SEBM-ed tungsten samples of sufficient fusion. Properties of samples by SEBM and selective laser melting (SLM) have also been compared. It is found that SLM-fabricated samples exhibit inevitable microcracks, and have a significantly lower ultimate compressive strength and a slightly lower relative density of 98.51% in comparison with SEBM-ed samples.

A comparative study of microsatellites among crocodiles and development of genomic resources for the critically endangered Indian gharial.

Next-generation sequencing has allowed us to explore new methods, where comparative and population genomics can be used simultaneously. Keeping this in mind, we surveyed and analyzed the frequency and distribution of microsatellites in the Indian gharial (Gavialis gangeticus) and compared it with American alligator (Alligator mississippiensis) and saltwater crocodile (Crocodylus porosus) to enrich them with genomic resources. TheIndian gharial has a low frequency, relative abundance (RA), and relative density (RD) of microsatellites as compared to other crocodilians. RA and RD were positively correlated with the GC content of genomic and transcriptomic sequences. The genomic sequences were dominated by dinucleotide repeats, whereas the transcriptomic sequences had an excess of trinucleotide repeats. Motif conservation studies among the three crocodilians revealed conservation of 69.2% of motifs. Species-specific unique motifs identified in this study could be used as molecular probes for species identification. A total of 67,311 primers were designed in all three species to enrich the crocodilians with genomic resources. The genomic resources developed in this study could accelerate diversity analysis within its individuals to design a proper mating plan to reduce inbreeding stress and further improve the species.