Average Stress State Research Articles

The Laboratory Study of Shear Strength of the Overconsolidated and Quasi - Overconsolidated Fine - Grained Soil

The paper presents results of laboratory shear strength test conducted on fine-grained soil samples with different grain size distribution and with different geological age and stress history. The Triaxial Isotopic Consolidation Undrained Tests (TXCIU) were performed under different consolidation stress in normal and overconsolidadion stress state on the samples with natural structure. Soil samples were selected from soil series of different age and geological origins: overconsolidated sensu stricto Miopliocene silty clay (siCl) and quasi overconsolidated Pleistocene clayey silt (clSi). Paper pointed out that overconsolidated sensu stricto and quasi overconsolidated fine-grained soil in same stress and environmental condition could show almost similar behaviour, and in other condition could behave significantly different. The correct evaluation of geotechnical parameters, the possibility of predicting their time-correct ability is only possible with appropriately recognized geological past and past processes that accompanied the soil formation.

30 nm X-ray focusing correlates oscillatory stress, texture and structural defect gradients across multilayered TiN-SiOx thin film

Resolving depth gradients of microstructure and residual stresses within individual sublayers of multilayered thin films and understanding their origin as well as their influence on functional properties are challenging tasks. In this work, a newly developed synchrotron focusing setup based on Multilayer Laue Lenses, providing an X-ray beam diameter of ∼30 nm, is used to characterize the cross-sectional properties of a 2.9 μm thick sculptured multilayered TiN-SiOx film, which consists of twelve ∼230 nm thick nano-crystalline TiN sublayers of zigzag columnar grain morphology separated by eleven amorphous SiOx sublayers, both prepared by oblique magnetron sputtering on a Si(100) substrate. The X-ray nano-diffraction analysis of the TiN sublayers reveals (i) an oscillatory variation of compressive residual stresses across the film ranging between −1.8 and −0.25 GPa, (ii) the presence of <100> fiber textures with three different fiber axis orientations, which abruptly change between the individual sublayers, and (iii) gradually decreasing densities of structural defects within every TiN sublayer, exhibiting a sawtooth-like depth-profile across the film. The near-substrate TiN sublayer shows the highest compressive stress reaching ∼ −1.8 GPa, a unique texture and the largest density of defects among all sublayers, which indicates diverse nucleation mechanisms of TiN on the native Si oxide and on the SiOx sublayers. The results demonstrate that the average stress state and the microstructure within the individual TiN sublayers can be influenced effectively by the applied deposition conditions, although self-organization processes during the sublayers' evolution give rise to the occurrence of qualitatively similar gradual trends, which differ in intensity and which were characterized also by complementary laboratory X-ray diffraction, as well as scanning and transmission electron microscopies. Finally, the presented results document that the novel X-ray nano-probe approach allows for the characterization of nano-scale gradients within individual microstructural features of polycrystalline thin films and pioneers the way for knowledge-based synthesis of thin films with predefined cross-sectional microstructure and property gradients.

Numerical Prediction of the Parameters of a Yield Criterion for Fibrous Composites

A numerical procedure is developed for predicting the parameters of a yield criterion for metal-matrix composites on the basis of known mechanical properties of the matrix and fibers. The macrocriterion of yielding is taken in the form of an invariant quadratic function of average stresses, with the use of a fourth-rank tensor. Transition to the plastic state is fixed using a FEM analysis of the distribution of stresses within the minimum representative volume of composite. A new procedure to simulate the periodicity conditions for the representative volume at a uniform average stress state of composite is proposed. A numerical analysis showed that the structure of yield function has to reflect the possibility of plastic deformations of composite in hydrostatic macroloading even if the matrix and fibers do not deform plastically at such a loading. For an orthogonally reinforced boron-aluminum composite, components of its yield tensor are determined, and convexity of the yield surface is established. Yield surfaces are constructed for different plane stress states of the composite.

Fatigue investigation on asphalt mixture layers’ interface

In this paper, the interface shear fatigue performance is investigated based on a new fatigue test protocol, which includes both cyclic shear and normal static compressive load. Several asphalt structure types are tested at five temperatures (from −10°C to 50°C) and three normal stress levels (from 0 to 0.50 MPa). Based on fatigue tests at three different shear amplitude levels unique fatigue functions of the layers’ interface were identified. It was found that the interface shear fatigue performance is highly dependent on normal stress state and increases with increasing normal stress. This is a direct consequence of the enhanced friction, adhesion and interlock at the shear interface. Observing the fatigue lives at different test temperatures, the best resistance was observed at lowest temperature. From a practical viewpoint, it is highly recommended to perform the test without normal stress application, since this can better reflect the research and practice experience, while keeping the testing protocol to a reasonable level of complexity.

Mitochondrial expression and activity of P-glycoprotein under oxidative stress in outer blood-retinal barrier.

To investigate the role of oxidative stress in regulating the functional expression of P-glycoprotein (P-gp) in mitochondria of D407 cells. D407 cells were exposed to different ranges of concentrations of H2O2. The mitochondrial location of P-gp in the cells subjected to oxidative stress was detected by confocal analysis. Expression of P-gp in isolated mitochondria was assessed by Western blot. The pump activity of P-gp was evaluated by performing the efflux study on isolated mitochondria with Rhodamine 123 (Rho-123) alone and in the presence of P-gp inhibitor (Tariquidar) using flow cytometry analysis. The cells were pretreated with 10 mmol/L N-acetylcysteine (NAC) for 30min before exposing to H2O2, and analyzed the mitochondrial extracts by Western blot and flow cytometry. P-gp was co-localized in the mitochondria by confocal laser scanning microscopy, and it was also detected in the mitochondria of D407 cells using Western blot. Exposure to increasing concentrations of H2O2 led to gradually increased expression and location of P-gp in the mitochondria of cells. Rho-123 efflux assay showed higher uptake of Rho-123 on isolated mitochondria in the presence of Tariquidar both in normal and oxidative stress state. H2O2 up-regulated P-gp in D407 cells, which could be reversed by NAC treatment. H2O2 could up-regulate the functional expression of P-gp in mitochondria of D407 cells, while antioxidants might suppress oxidative-stress-induced over-expression of functional P-gp. It is indicative that limiting the mitochondrial P-gp transport in retinal pigment epithelium cells would be to improve the effect of mitochondria-targeted antioxidant therapy in age-related macular degeneration-like retinopathy.

The influence of a static constant normal stress level on the fatigue resistance of high strength spring steel

This paper presents a method to determine the optimal static compress normal stress level for alternate torsional loaded tabular bar in order to ensure longer lifetime. Numbers of cycles to failure of the springs, which have been subjected to static compress normal stress on different levels were obtained experimentally. The paper analysed the stress state for biaxial loaded tabular torsion springs for different theoretical criterions as Columg-Mohr, Drucker-Prager criterion, the Sines criterion, criterion Crossland and Dang Van fatigue criterion. The analysis results are compared with the experimentally obtained average values. Comparison between the experimentally obtained and calculated theoretical points of the stress state, indicating that the longest life expectancy reached the point closest to the area of the double-axle stress state endurance in all measures except the Drucker-Prager criterion. Also theoretical results, from mentioned criteria, show that with increasing constant compression stress at a constant alternating torsional stress, the points are shifted from the area of safe to the area of fatigue failure.

Deformation behavior of additively manufactured GP1 stainless steel

In-situ neutron diffraction measurements were performed during heat-treating and uniaxial loading of additively manufactured (AM) GP1 material. Although the measured chemical composition of the GP1 powder falls within the composition specifications of 17-4 PH steel, a fully martensitic alloy in the wrought condition, the crystal structure of the as-built GP1 material is fully austenitic. Chemical analysis of the as-built material shows high oxygen and nitrogen content, which then significantly decreased after heat-treating in a vacuum furnace at 650°C for one hour. Significant austenite-to-martensite phase transformation is observed during compressive and tensile loading of the as-built and heat-treated material with accompanied strengthening as martensite volume fraction increases. During loading, the initial average phase stress state in the martensite is hydrostatic compression independent of the loading direction. Preferred orientation transformation in austenite and applied load accommodation by variant selection in martensite are observed via measurements of the texture development.

Noncoding RNAs in protein clearance pathways: implications in neurodegenerative diseases.

The importance of noncoding genome has become more evident in recent years. Before genome sequencing, the most well studied portion of our genome was protein coding genes. Interestingly, this coding portion accounted only for 1.5% of the genome, the rest being the noncoding sequences. Noncoding RNAs (ncRNAs) are involved in normal cell physiology, stress, and disease states. A class of small ncRNAs and miRNAs has gained much importance because of its involvement in human diseases such as cancer. Involvement of long ncRNAs have also been acknowledged in other human diseases, especially in neurodegenerative diseases. Neurodegenerative diseases are characterized by the presence of abnormally folded proteins that are toxic to the cell. Several studies from model organisms suggest upregulation of pathways that clear this toxic protein may provide protection against neurodegeneration. In this review, I summarize the importance of ncRNAs in protein quality control system of cell that is implicated in this fatal group of neurodegenerative diseases.

The Effect of Quenching Temperature and Polishing Force on the Residual Stress and Deformation of Rack Bar Semi-Products

In the present study, the residual stress state and macroscopic deformation of automotive rack bar semi-products were examined. The rack bars were taken from the original production line after induction hardening, burnishing and polishing. Induction hardening was performed at 860°C, 880°C and 900°C and the polishing step was performed with standard and increased polishing force. Thus, the effect quenching temperature and polishing force are described. Non-destructive residual stress mapping was performed after which macroscopic deformation measurements were carried out on the same rods. It was shown that the variation of quenching temperature had no effect on the residual stress state and deformation of the components. The increased polishing force did not change the average stress state of the rods, however, it resulted in higher stress asymmetry and deformation of the rods. Stress asymmetry was shown to be the cause of bending of the rods.

Analysis of S Wave Propagation Through a Nonlinear Joint with the Continuously Yielding Model

Seismic wave propagation through joints that are embedded in a rock mass is a critical issue for aseismic issues of underground rock engineering. Few studies have investigated nonlinear joints with a continuously yielding model. In this paper, a time-domain recursive method (TDRM) for an S wave across a nonlinear Mohr–Coulomb (MC) slip model is extended to a continuously yielding (CY) model. Verification of the TDRM-based results is conducted by comparison with the simulated results via a built-in model of 3DEC code. Using parametric studies, the effect of normal stress level, amplitude of incident wave, initial joint shear stiffness, and joint spacing is discussed and interpreted for engineering applications because a proper in situ stress level (overburden depth) and acceptable quality of surrounding rock mass are beneficial for seismic stability issues of underground rock excavation. Comparison between the results from the MC model and the CY model is presented both for an idealized impulse excitation and a real ground motion record. Compared with the MC model, complex joint behaviors, such as tangential stiffness degradation, normal stress dependence, and the hysteresis effect, that occurred in the wave propagation can be described with the CY model. The MC model seems to underestimate the joint shear displacement in a high normal stress state and in a real ground motion excitation case.

Impact of a Compressive Stress on Water Sorption and Diffusion in Ionomer Membranes for Fuel Cells. A 1H NMR Study in Vapor-Equilibrated Nafion

The electrolyte membrane is exposed to high compression loads when operating in a proton exchange membrane fuel cell (PEMFC). It was reported in the literature that compression impacts the membrane properties when immersed in liquid water but little is known about the quantitative effects of stress on water sorption and its dynamical properties when the membrane is at equilibrium with a humid gas. In this work, we investigated the influence of a static normal stress on the properties of Nafion® by pulsed field gradient NMR (PFGNMR) with a NMR compatible compression device. The results demonstrated a reduction in water content due to compression at high relative humidity (≥ 90% RH) and much more moderate effects at low hydration (≤ 85% RH). The changes observed on the diffusion properties are more pronounced and related to both water loss and a reversible modification of the membrane structure effective at the micrometric scale.

A high-performance parametrized mixed finite element model for bending and vibration analyses of thick plates

Based on the parametrized mixed variational principle, a high-performance finite element model has been developed for bending and vibration analysis of thick plates. The proposed mixed variational formulation combines the concept of Reissner’s mixed variational theorem and Rong’s generalized mixed variational theorem. In addition to unknown parameters of the displacement fields, the transverse normal component of the stress tensor is also assumed as the independent field variable in the present variational formulation. The latter allows the fulfillment of the boundary conditions of the transverse normal stress at the top and bottom surfaces of the plate in a natural way. The variational constraint of the transverse normal strain–displacement relations is relaxed via the parametrized Lagrange multipliers method. The functional of the proposed variational formulation contains an arbitrary free parameter, called the splitting factor. Simple formulations have been proposed for selecting the splitting factor which leads to the results of higher precision. The numerical results obtained from the proposed mixed formulation have been compared with the results of three-dimensional (3D) theory of elasticity and other plate theories available in the literature. The comparison study shows that the proposed parametrized mixed plate theory is capable of achieving the accuracy of nearly exact 3D solutions.

Failure simulations of open-hole and unnotched IM7/977-3 coupons subjected to quasi-static loading using Autodesk Helius PFA

Finite element simulations of three laminates in open-hole and unnotched configurations subjected to tension and compression quasi-static loading are investigated as part of the Damage Tolerant Design Principles program organized by the Air Force Research Laboratory. The coupons are made from unidirectional IM7/977-3 plies, which are a composite material composed of intermediate modulus carbon fibers and a toughened epoxy matrix. Blind simulations of coupon stiffness, nominal coupon stress at failure and damage evolution are benchmarked against experimental measurements and X-rays. The blind simulations are followed by a second round of simulations where the modeling strategy is modified to improve agreement between the simulations and experiments. In the present article, the commercial software Autodesk Helius PFA is used to model the non-linear response of the composite material. Within Helius PFA, failure is evaluated at the constituent level by extracting the fiber and matrix volume average stress state from the homogenized composite stress state. The relationships between the composite and constituents are developed using multicontinuum theory and a high-fidelity micromechanics model.

Development of Micro-shear Fatigue Test and its Application to Single Crystal of Pure Iron

In this study, a new fatigue testing method under shear and axial loading was developed for small specimens (3mm long and 0.3mm thick). The method was applied to a single crystal of pure iron and extra-low carbon steel in order to investigate the effect of shear and normal stress on cyclic slip activity. The specimen was sufficiently small to be prepared from a single crystal, aiming at a specific slip system in the shear loading direction. Moreover, the specimen had a single edge notch, which was designed to generate both pure shear stress and normal stress. Using this testing method, shear fatigue strength with a static normal stress on the slip system ▪[111] was evaluated, and the fatigue strength was compared with that of polycrystalline ferrite steel with static axial loading.

Effects of loading path on the fracture loci in a 3D space

Axi-symmetric and 3D unit cell analyses with continuous non-proportional loading paths are performed to investigate the path dependence of the fracture loci in a 3D space. The loading pattern utilized is the generalization of a number of non-proportional paths recorded in real tests. Failure of the unit cell is predicted when localization of plastic flow occurs, and the failure strains are plotted against the strain history averaged stress triaxiality and Lode parameter to construct fracture loci in a 3D space. The fracture locus with a non-proportional loading path deviates from that with a proportional loading path along the axis of stress triaxiality and becomes non-monotonic in high triaxiality regime. Meanwhile, such deviation occurs only when a certain level of triaxiality is reached. Agreement with the proportional locus as well as monotonicity maintains over a large range of stress triaxiality that covers most cases in reality, as long as the non-proportionality of the loading path is sufficiently low. This provides the rationale for utilizing the average triaxiality based fracture locus as an acceptable approximation in practice. Deviations of the non-proportional loci along the axis of Lode parameter are also observed. Further study on the Lode history dependence suggests using the final value of Lode parameter instead of the averaged one as the Lode axis in the fracture loci, which can alleviate the severity of path dependence for the loading patterns concerned. Based on these results, the effectiveness of the average stress state based fracture loci reported in the literature is discussed.

Effect of stress state on slow rupture propagation in synthetic fault gouges

Slow slip events (SSEs) in subduction zones are known to proceed so sluggishly that the associated slow ruptures do not generate any detectable radiating seismic waves. Moreover, they propagate at speeds at least four orders of magnitude slower than regular earthquakes. However, the underlying physics of slow slip generation has yet to be understood. Here, we carry out laboratory studies of unstable slip along simulated fault zones of lizardite/chrysotile (liz/ctl) and antigorite (i.e., low- and high-temperature serpentine phases, respectively) and olivine, under varying conditions of normal stress, with the aim of better understanding the influence of stress state on the process of slow rupture along the plate interface. During a single unstable slip, we clearly observe a slow rupture phase that is often followed by an unstable, high-speed rupture. We find that lower fault-zone friction coefficients (μ values from 0.7 down to 0.5) lead to increasing degree of the slow rupture mode, and also that the slow rupture velocities (V r = 0.07 to 5.43 m/s) are largely consistent with those of short-term SSEs observed in nature. Our findings suggest that the generation of SSEs is facilitated by conditions of low normal stress and low fault-zone strength along the plate interface, which may be weakened by metamorphic reactions that result in the production of hydrous phases (e.g., serpentine) and/or the direct involvement of fluid itself, leading to a reduction in effective normal stress.

Modelling resilient modulus seasonal variation of silty sand subgrade soils with matric suction control

The resilient modulus of unbound materials is an important parameter in the mechanistic design of pavements. Although unbound layers are frequently in a partially saturated state, a total stress approach is conventionally used in modeling the material behaviour, and therefore pore pressure effects are not considered. In fine-grained unbound materials, the saturation state can affect their mechanical behaviour due to pore pressure effects. In this study a modified test procedure and a predictive resilient modulus model that takes into account the subgrade soil matric suction as a stress state variable is presented. Two different silty sand subgrade materials were tested in unsaturated conditions using a series of repeated load triaxial tests under controlled pore suction conditions to study its influence on the resilient modulus. The test data were further used to obtain the resilient modulus model regression parameters that account for moisture content variations through the matric suction parameter. Generally, the prediction model could effectively capture the resilient modulus behaviour of the subgrades with respect to changes in the normal stress state and the matric suction. Given the completeness of this method, this prediction model is recommended as an improved approach in capturing the moisture content effects on the material stiffness properties.

Squeeze behaviors of magnetic powders between two parallel plates

The dynamic compressive and static normal stresses of magnetic powders (MPs) with the constant volume squeezed between two parallel plates were experimentally studied. The compressive stress increased in a power law as the gap distance decreased, with an exponent range of −0.73 (0.04 T) to −2.63 (0.77 T). The values of the scale factor to normalize the compressive curves were mainly dominated by the applied magnetic field. The compressive behavior of the MPs showed a less significant velocity effect and initial gap distance effect than those of the magnetorheological (MR) and electrorheological (ER) fluids because of the absence of a host liquid. The compressive stress was generated by the serious particle aggregation, which was induced by a magnetic field and friction during compression. This study is designed to further the understanding of the behavior of smart ER/MR materials and the application of MP actuators in squeeze mode.

Comparative micromechanical assessment of adobe and clay brick masonry assemblages based on experimental data sets

Many existing structures worldwide are made of adobe masonry (AM) or clay brick masonry (CBM). In this study, the influence of different mechanical properties for bricks and mortar on global behaviour of AM and CBM is investigated through micromodelling and homogenisation. Firstly, a large number of experimental data on mechanical properties of bricks and mortar was collected and analysed for both AM and CBM. Micromechanical analysis was then carried out accounting for statistics and robust empirical models, which were derived for strengths and elastic moduli of bricks and mortar through an experimental data analysis. Masonry was modelled as a periodic composite made of two components representing bricks and mortar. Finite element homogenisation analysis was performed on running bond AM and CBM assemblages to determine their critical curves and homogenised properties. Each critical curve provides the masonry average stresses corresponding to initiation of damage in masonry, which was assumed to be subject to a plane average stress state. In the plane of normal and shear average stresses, critical curves are similar to Mohr–Coulomb failure curves with nonlinear cap in compression. Those curves show that initiation of damage is very sensitive to the difference between Young’s moduli of brick and mortar.

Analysis of stress states in compression stage of high pressure torsion using slab analysis method and finite element method

High pressure torsion (HPT) is useful for achieving substantial grain refinement to ultrafine grained/nanocrystalline states in bulk metallic solids. Most publications that analyzed the HPT process used experimental and numerical simulation approaches, whereas theoretical stress analyses for the HPT process are rare. Because of the key role of compression stage for the deformation of HPT, this paper aims to conduct a theoretical analysis and to establish a practical formula for stress and forming parameters of HPT process using the slab analysis method. Three equations were obtained via equations derivation to describe the normal stress states corresponding to the three zones of plastic deformation for HPT process as stick zone, drag zone and slip zone. As to the compression stage of HPT, the stress distribution results using the finite element method agree well with those using the slab analysis method. There are drag and stick zones on the contact surface of the HPT sample, as verified by the finite element method (FEM) and slab analysis method.