Direction Of Uniaxial Compression Research Articles

Anisotropy of elastic properties of SHS-synthesized porous titanium nickelide

Samples of porous NiTi were obtained by the method of self-propagating high-temperature synthesis. The mechanical characteristics of the porous ones were studied by quasi-static compression. When the samples of porous titanium nickelide were subject to quasi-static compression, the deformation was of an elastic-plastic nature. Three characteristic types of the fracture surface under quasi-static compression of the porous SHS – TiNi alloy were identified: 1) ductile fracture of the austenite phase in the form of a cup relief, 2) brittle fracture accompanied by the formation of cleavage steps, 3) large areas of plastic shear deformation, on which cups and cleavage facets were nucleated. To determine the anisotropy of the porous TiNi alloy properties, the volume of the porous sample was simulated, and estimated calculations were carried out. Based on the results of reconstructing 3D neutron high resolution tomography of the porous volume of titanium nickelide and the numerical parameters of the model porous medium, an algorithm was developed for obtaining a solid-state 3D model of the porous framework for using in finite element calculations. The studied porous titanium nickelide alloy, as well as spongy bone tissues, was shown to have orthotropic elastic properties conditioned by the geometric features of the porous framework. The effective moduli of elasticity and shear for the porous volume of the material were determined. The calculation results of the elastic moduli for the studied model of porous titanium nickelide numerically agree with the results obtained by compressing the samples of porous TiNi. The porous TiNi alloy under uniaxial compression was established to be destroyed under the action of tangential shear stresses at an angle of 45 degrees to the direction of uniaxial compression. The research was carried out with financial support of the Russian Science Foundation under the Grant No. 22-72-10037, https://rscf.ru/project/22-72-10037/.

A New Method to Study Contributions of Polymer Fibers and Water Respectively to the Hydrogel Stress under Tension and Compression Using 3D Micro-Fiber Network Model

The hydrogel can be regarded as a system comprising of a three-dimensional (3D) polymer network entrapping water in intermolecular space. Contributions of polymeric fibers and water to macrostresses of hydrogels are usually separated by fitting stress–strain relations from experiments. In this paper, we developed a new method to determine the two contributions by applying triaxial loads on a 3D micro-fiber network model, in which not only the uniaxial tension or uniaxial compression direction, but also the other two lateral directions are applied with external loads for keeping the unchanged volume of the fiber network model throughout the loading process. The lateral forces on the model can be regarded as the forces of water on the micro-fiber network at boundaries, since the lateral boundary conditions of hydrogels under uniaxial loading are completely free. With the calculation of the effective areas of polymeric fibers and water on the lateral surfaces, the hydrostatic pressure of water can be derived in each loading step, and further, the contributions of polymeric fibers and water to macrostresses of hydrogels in uniaxial loading direction can be determined quantitatively. The results show that in small deformation (the strain is less than about 10%), contributions of the water and polymeric fibers are of the same order of magnitude. In larger deformation, polymeric fibers are decisive in the contribution to stresses in tension, while water takes the dominant contribution in compression. Therefore, the difference in the contribution of water in tension and compression can explain the phenomenon of different initial elastic moduli of hydrogels in tension and compression. The method recommended in this paper can be used to learn the contributions of polymeric fibers and water in hydrogels with diverse fiber contents and even under multi-axial loading.

Anisotropic damage model for concrete and other quasi-brittle materials

A thermodynamically consistent formulation to model anisotropic damage for quasi-brittle materials is proposed. The model is based on proper expressions for the specific Gibbs free energy and the complementary form of the dissipation potential. Damaging of the material is described by a symmetric positive definite second order damage tensor. Especially, the failure surface is formulated in such a way that it will mimic the behaviour of the well known Ottosen’s four parameter failure surface. While testing the model against the experimental results found in literature, the results were in good agreement in uniaxial tensile and compressive loadings as well as in biaxial compression. Besides the correct failure stress states, the model predicts the correct failure modes of concrete: axial splitting along the direction of uniaxial compression and tensile damaging normal to the direction of tension.

A study on the preparation of oriented beta"-alumina ceramics using rod/flake-like boehmite as precursors and their properties

As a material for solid electrolytes and separators, beta"-Al2O3 is widely used in sodium beta alumina. In this study, a high fraction of beta"-phase for beta"-Al2O3 with preferred orientation was fabricated using the traditional solid-phase synthesis; and the effect of boehmite micro-structure, the aluminum source, on the phase, microstructure, mechanical properties, and conductivity properties of prepared ceramics was investigated. X-ray diffraction (XRD), and scanning electron microscopy (SEM) results illustrate that the C-axis of beta"-Al2O3 grains was along the direction of uniaxial compression. Above all, samples made of rod-shaped boehmite exhibited a better degree of orientation (0.21) and a higher fraction of beta" phase (96 %) than samples made of flake-shaped boehmite. Meanwhile, for samples fabricated with rod-shaped boehmite, the conductivity at 350 ℃ of the sample surface parallel to the uniaxial pressure was nearly 1.5 times (1.634E-1 S cm−1) greater than that of surface perpendicular to the uniaxial pressure.

Influence of inclination angles and confining pressures on mechanical behavior of rock materials containing a preexisting crack

SummaryTo deeply understand the cracking mechanical behavior of brittle rock materials, numerical simulations of a rock specimen containing a single preexisting crack were carried out by the expanded distinct element method (EDEM). Based on the analysis of crack tips and a comparison between stress‐ and strain‐based methods, the strain strength criterion was adopted in the numerical models to simulate the crack initiation and propagation processes under uniaxial and biaxial compression. The simulation results indicated that the crack inclination angle and confining pressure had a great influence on the tensile and shear properties, peak strength, and failure behaviors, which also showed a good agreement with the experimental results. If the specimen was under uniaxial compression, it was found that the initiation stress and peak strength first decreased and then increased with an increasing inclination angle α. Regardless of the size of α, tensile cracks initiated prior to shear cracks. If α was small (such as α ≤ 30°), the tensile cracks dominated the specimen failure, the wing cracks propagated towards the direction of uniaxial compression, and the propagation of shear cracks was inhibited by the high concentration of tensile stress. In contrast, if α was large (such as α ≥ 45°), mixed cracks dominated the specimen failure, and the external loading favored the further propagation of shear cracks. Analyzing the numerical results of the specimen with a 45° inclination angle under biaxial compression, it was revealed that lateral confinement had a significant influence on the initiation sequence and the mechanical properties of new cracks.

Experimental and modeling study of compressive creep in 3D-woven Ni-based superalloys

Micro-architectured Ni-based superalloy structures, with Ni-20Cr-3Ti-2Al (wt.%) composition and γ/γ′-microstructure, are created by a multi-step process: (i) non-crimp orthogonal 3D-weaving of ductile, 202 μm diameter Ni-20%Cr wires, (ii) gas-phase alloying with Al and Ti, (iii) simultaneous transient-liquid phase (TLP) bonding between wires and homogenization within wires via interdiffusion, (iv) solutionizing to create a single-phase solid solution, and (v) aging to precipitate the γ′ phase. The creep behavior of these 3D-woven γ/γ′ nickel-based superalloys is studied under uniaxial compression via experiments at 825 °C and via finite element (FE) analysis, using a 3D model of the woven structures obtained through X-ray micro-tomography. The creep strain rate for the woven Ni-based superalloy is higher than that for the bulk superalloy due to the lower solid volume fraction of the woven structure, while the creep exponents are identical. The compressive creep behavior is sensitive to the geometry of the woven structures: fewer wires perpendicular to the load and fewer bonds between wires cause lower creep resistance of the woven structure, due to a reduction in load transfer from the longitudinal wires (which are primarily load-bearing) and the perpendicular wires. Creep buckling of longitudinal wires drastically reduces creep resistance of the woven structure, confirming the importance of maintaining longitudinal wires vertical and parallel to the uniaxial compression direction. Finally, reducing wire cross-section, e.g., via oxidation, reduces creep resistance. The oxidation kinetics of the wire structures at 750, 825, and 900 °C displayed parabolic rate constants comparable to commercial Ni-based superalloys, but indicates that up to 35% of the wire cross-section is oxidized after 7 days at 825 °C, such that oxidation-resistant coatings are needed for long-term use in oxidative environment.

Experimental and numerical investigations of the anisotropic deformation behavior of low-density polymeric foams

Low-density porous materials and foams have been widely used for a variety of applications, such as light structural components, impact energy absorption, thermal insulation and sound absorption. The macroscopic deformation of such materials is strongly dependent on their inherent micro-cellular structure. This study investigated the compressive anisotropic deformation behavior of low-density polymeric foam by using X-ray computed tomography (CT) and the finite element method (FEM) in order to understand both the microscopic and macroscopic deformation behavior. The foams used in this study have a closed cell structure, with pores that are elliptical in shape. Three different types of expansion ratios were employed. The porosities of these materials were 93.5, 95, and 96%. From the observations using the X-ray CT method, the averaged pore heights were 1 mm and the aspect ratios were 2, 2.5, and 2.25, respectively. The foam demonstrated anisotropic deformation, dependent on the uni-axial compression direction. It was found that the deformation rigidity in the longitudinal direction was larger than that in the transverse direction. By using the X-ray CT method in situ, the microscopic deformation behavior when subjected to compressive loading was observed. Deformation and collapse of pores was observed for both directions during the loading. In conjunction with this, FEM computations were carried out to elucidate how such pore geometry undergoes elastoplastic deformation and leads to macroscopic deformation behavior. The FEM-created three-dimensional spatial structures were based on elongated rhombic dodecahedrons. It is revealed that the FEM computation shows relatively good agreement with the experimental results. Thus, our experimental and computational models may be useful for microstructural design using anisotropic cellular materials.

The Mechanisms of Deformation and Damage of Mudstones: A Micro-scale Study Combining ESEM and DIC

Combining environmental scanning electron microscopy (ESEM) and digital image correlation techniques, the mechanical behaviour of mudstones is studied at the scale of their composite microstructure (that is, grains of carbonate and quartz embedded in a clay matrix). A specially designed apparatus is developed to allow in-situ uniaxial compression tests on samples with controlled humidity states in the ESEM chamber. As the mechanical behavior of mudstones is sensitive to water content, two tests on samples with contrasting water contents (3.8 and 7.4 %) are performed to identify the unified mechanisms of deformation and damage. We illustrate heterogeneous local strain fields that well correlate with the microstructure of mudstones. Three types of deformation bands involving different mechanisms have been classified: orthogonal (compaction of macro-pores and closure of pre-existing cracks), parallel (micro-cracking) and inclined (shear deformation) to the uniaxial compression direction. These deformation modes are activated at different stress levels, and they strongly interact: for instance, a high-strained shear band may result in tensile micro-cracks at its tip. We also illustrate damage phenomena, particularly at the inclusion-matrix interface, which is found to be a hazardous position for nucleation of micro-cracks.

On isotropic states in α-LiNH4SO4 crystals

The spectral and baric dependences of the birefringence of α-LiNH4SO4 crystals are studied. It is found that the birefringence is rather sensitive to uniaxial compressions. A baric shift of the isotropic point to longer or shorter wavelengths depending on the uniaxial compression direction is observed and a generalized temperature-spectral-baric diagram of the isotropic state is plotted. A possibility of the appearance of a pseudoisotropic state under the action of uniaxial pressure σz or simultaneous pressures σx = σz is shown.

Control of cooperative switching of microwrinkle orientations by nanopatterns

Microwrinkles are mechanically self-organized surface undulations with an intrinsic wavelength lambda(0) showing various stripe patterns. The average orientation of the stripes is repeatedly altered by changing, for example, the direction of uniaxial compression applied to the sample. Meanwhile, the stripe patterns can be modulated arbitrarily by the designed topographic patterns through mechanical coupling. Exploiting these results, here we study the switching of the microwrinkle orientations, that is, stripe patterns, under the influence of the artificial nanopatterns with a characteristic periodicity d. The switching is primarily driven by the rotation of the compression axis. The nanopattern-induced boundary condition disturbs the formation of the optimal stripe pattern having lambda(0). Thus a variety of modulated stripe patterns appear depending on both the relative value, lambda(0)/d (0.5-1.6), and the direction of the uniaxial compression. The relative length, lambda(0)/d, also affects the transition of the stripe patterns, showing different dynamics of the domain walls characterized by their creation, annihilation, and configurations. This results correlate with hysteresis of the spatially averaged stripe orientation for a rotation cycle of the compression direction. The results suggest for the general self-organized stripe patterns that we can tune the transition of the stripe pattern, in other words, the cooperative response of the coupled local stripe orientations, as well as the stripe patterns themselves, by exploiting the artificial patterns with proper designs.

Dislocation behavior in KCl crystal under uniaxial compression: Molecular dynamics simulation

Molecular dynamics simulations of dislocations behavior in KCl crystal under uniaxial compression along the [100], [110], and [111] axis directions were performed to discuss the elastoplastic transition under shock compression. The simulation results showed that the dislocation moved along about 45 deg to the uniaxial compression direction through the displacements of atomic lines around the dislocation core. Under uniaxial compression along the [111] axis direction, two extra half-(011) planes were inserted as dislocations in KCl crystal. The minimum stress for dislocation motion under the uniaxial compression along the [111] axis direction was much larger than those under the uniaxial compressions along the [100] and [110] axis directions. It was suggested that the large minimum stress for dislocation motion along the [111] axis direction was caused by the existence of two edge dislocations which were inserted as extra half-(011) planes. This anisotropic behavior was consistent with the experimental measurement of Hugoniot-elastic limit.

Elastic properties of monolithic porous blocks of compressed expanded graphite related to their specific surface area and pore diameter

Highly porous monolithic blocks of compressed expanded graphite (CEG) with various apparent densities ranging from 0.026 to 0.276 g cm −3, produced of initial expanded graphite of an apparent density of 4 g l −1, were characterized using a gas helium pycnometer, a mercury porosimetry and physical adsorption of N 2 gas at 77 K. The dynamic elastic module of CEG cubes were determined by means of measurements of the velocity of the longitudinal ultrasonic wave (frequency of 100 kHz) propagated along three mutually orthogonal directions: parallel and perpendicular to the bedding plane (or the uniaxial compression direction). CEG was found to be characterized by macropores and mesopores of various diameters. The most probable sizes of pores are 10, 2.4, and 0.9 μm, 4 and 2.5 nm. The elastic properties were related to the porous structure of CEG—represented by the volume of open and closed pores, specific surface area S BET and pore diameter.

Highly oriented polydiacetylene monolayer formed at the air—water interface

Spectral and structural investigation of soluble polydiacetylene monolayer at the air—water interface is carried out using in situ polarized spectroscopy. The coil-like polydiacetylene that is adsorbed two-dimensionally on the water surface is distinguishable from the usual coil form in a solution and film in terms of the position of the absorption peak and the spectral feature. Through the coil-to-rod transition induced by increasing pressure, the linear conjugated backbone of the rigid rod polydiacetylene is gradually packed and remarkably oriented perpendicular to the direction of uniaxial compression. This orientation mechanism is discussed in relation to the preparation of monolayer and the nature of the rigid linear polymer and its coil-to-rod transition.

Deformation of polycrystalline salt in compression and in shear at 250–350°C

Abstract Dry synthetic polycrystalline salt (NaCl) has been deformed in uniaxial compression and in shear in order to gain insight into the influence of deformation mode on the development of crystallographic preferred orientation, microstructural evolution and mechanical behaviour (flow strength). The experiments were carried out between 250 and 350°C, at strain rates between 10 −5 and 10 −7 s −1 . Under these conditions halite deforms by climb-controlled dislocation creep. In our samples, the presence of deformed grains containing cellular networks of subgrains, and the development of texture are consistent with such mechanisms. The weaker {110} <110> slip plane appears to align parallel to the shear plane and perpendicular to the uniaxial compression direction. In the sheared samples, a <111> maximum is observed parallel to the shear direction. The textures obtained agree favourably with texture simulations. When the mechanical behaviour seen in uniaxial compression and shear is compared in terms of equivalent stress and strain, as defined in the von Mises theory of isotropic plasticity, the uniaxially deformed samples appear to be stronger. From a detailed consideration of the data, it is inferred that the difference in flow strength is largely due to anisotropy resulting from texture development. The results suggest that the assumptions underlying the von Mises theory of isotropic plasticity are not applicable in the present case, and that the associated flow rule does not offer an accurate method of generalizing creep laws for salt under the conditions investigated.

Structural and mineralogical evaluation of an experimentally produced impact crater in granite

A crater 30 cm in diameter and 4.4 cm in depth was produced upon impact of an aluminium sphere with a homogeneous granite target. The volume excavated was 748 cm3, the mass ejected 1933 g. The crater geometry is compared with previous laboratory experiments. Mineralogical investigations revealed that shock induced, microscopic fracturing is lowest in the direction of uniaxial compression, followed by a 45° profile. Due to reflections of stress waves at the free surface, the horizontal profile displayed the highest fracture index. Kinking of biotite was very common in samples close to the crater walls (≈ 50 kb). However it faded out at a distance which corresponds to approximately 10 kb. This seems to be the lower pressure limit for the formation of kink bands under shock conditions.