Compressive Stress State Research Articles

The effects of initial stresses on nonlinear dynamic response of high arch dams subjected to far-field underwater explosion

Blast performance assessment of 300 m high arch dams is an important topic having been extensively studied in recent years. Because the arch dam is convex on the upstream face, the dam body is mainly in a compressive stress state under the action of upstream water pressure. However, due to the complexity of the problem, the initial stress is ignored in the blast response analysis of arch dams. For this purpose, the influence of initial stress on nonlinear dynamic response and failure modes of a 300 m high arch dam to the far-field underwater explosion has been studied in this paper. The acoustic-structural approach is employed to model the underwater explosion loading and the dam-reservoir-foundation interaction. The Concrete Damage Plasticity (CDP) model including the strain rate effect is used to model the concrete material behavior to blast loading. Damage development processes of high arch dams to the far-field underwater explosion are investigated. Nonlinear dynamic response characteristics of high arch dams with different initial stress states subjected to the far-field underwater explosion are discussed in terms of damage modes, residual displacement, and damage dissipated energy. The results show that the initial stress has a significant influence on the nonlinear dynamic and failure modes of high arch dams to the far-field underwater explosion. The initial stress state of high arch dams under hydrostatic pressure and deadweight should be the basic condition of blast response analysis.

In situx-ray diffraction study of dynamically compressedα-cristobalite using a dynamic diamond anvil cell

In this study we present results of the dynamic compression of $\ensuremath{\alpha}$-cristobalite up to a pressure of 106 GPa with the use of the dynamic diamond anvil cell. X-ray diffraction images were recorded at different ramp compression and decompression rates to investigate in situ the high-pressure phase transitions of $\ensuremath{\alpha}$-cristobalite. Our results suggest that the pressure onset of the phase transformation of $\ensuremath{\alpha}$-cristobalite to cristobalite II, cristobalite X-I, and ultimately to seifertite ($\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{PbO}}_{2}$ type ${\mathrm{SiO}}_{2}$) is dependent on the applied compression rates and stress conditions of the experiment. Increasing compression rates in general shift the studied phase transitions to higher pressures. Furthermore, our results indicate for single crystals under hydrostatic conditions a suppression of a phase transition from cristobalite X-I to seifertite at pressures of up to 82 GPa.

Secondary void interactions under shear in thin porous sheets

A compressive stress state decreases the mechanism of void growth in metals. However, in reality, void sheets are evidenced on the fracture surface of metallic shear-compression specimens. The prevailing stress state in the gauge is in contrast to the features observed on the fracture surface. This work examines the mechanical fields in the surroundings of a void under shear as a realization of the mechanical behavior of a ligament between coalescing voids. It is shown that the presence of secondary voids in the surroundings of one primary void reduces the average triaxiality in the cell and homogenizes its distribution; thus, it can be surmised to be mechanically favored. This paper examines this point from a micromechanical perspective utilizing the periodic unit cell method with a single void in its center as a reference case to be compared with other configurations of voids distributed in the cell. It is hypothesized that nucleated voids grow and promote nucleation and growth of voids around them – a mechanism to describe void sheet formation as a positive feedback loop process. The effect of extra secondary voids on the mechanical fields is examined and correlated to their size and location with respect to the existing primary void.

Optimal Design and Biomechanical Analysis of a Biomimetic Lightweight Design Plate for Distal Tibial Fractures: A Finite Element Analysis

The treatment of fractures of the distal tibia can be problematic due to the insubstantial soft-tissue covering this part of the anatomy. This study investigates a novel strategy for minimally invasive plate osteosynthesis of distal tibia fractures called bionic lightweight design plating. Following the structure of the animal trabecular bone, we utilized topological mathematical methods to redesign the material layout of the internal fixation device to fulfill the desired lightweight design within given boundary conditions. The results showed that this method can maintain the same stability of the construct as the original plate after a reduction in the original volume by 30%, and the differences in strain energy of plates and maximum node displacement of constructs between the constructs [RP construct vs. LP construct] were not statistically significant (p > 0.05). In the safety assessment of the constructs, the peak stress of plates between constructs was found to not be statistically significantly different under a doubled physiological load (p > 0.05). The average stress of the plates’ elements exceeding the allowable stress was analyzed, and no statistically significant differences were found between the two constructs under axial compression stress conditions (p > 0.05). The average stress of the plates’ elements in the redesigned plating construct under torsional stress conditions was 3.08% less than that of the locked plating construct (p < 0.05). Under the double physiological load condition, 89% of the elements of the plate in the redesigned plating construct and 85% of the elements of the plate in the locked plating construct were lower than the maximum safe stress of the plate, which was 410 MPa (secondary allowable stresses). That reminds us the topology optimization offer a possible way to improve the capacity of soft tissue protection while ensuring the safety of the RP construct by reducing the volume of the implants.

Critical analysis on the use of the shove test for investigating the shear-sliding behavior of brick masonry

The shove test (ASTM Standard C1531) is an experimental technique aimed at studying the shear-sliding behavior of brick masonry. It can be executed according to various testing methods that differ in the way the vertical compression load is applied and in the way bricks and/or joints are locally removed for inserting jacks. One of the most critical aspects is the correct evaluation of the compressive stress state on the sliding brick. The objective of the present paper is to investigate the capability of the shove test in determining the shear strength parameters of brick masonries and to highlight the main advantages and disadvantages of the various testing methods. To this aim, nonlinear numerical simulations of the shove test were performed by adopting a brick-to-brick modeling strategy. The 2D numerical model was calibrated and validated through comparisons with experimental results of triplet tests and shove tests. The numerical analyses allowed to understand the influence the different testing methods and the masonry mechanical properties, such as dilatancy, may have on the test results. Based on the numerical outcomes, correction factors were calibrated for the proper evaluation of the compressive stress state on the sliding brick. Improvements with regards to the experimental procedures, i.e. additional test phases and measurements, were also proposed to enhance the results interpretation.

The Effects of Nanoparticle Additives on Thermophysical Properties of Concrete Mixtures

In recent years, energy-retrofitting is becoming an imperative aim for existing buildings worldwide and increased interest has focused on the development of nanoparticle blended concretes with adequate mechanical properties and durability performance, through the optimization of concrete permeability and the incorporation of the proper nanoparticle type in the concrete matrix. In order to investigate the potential use of nanocomposites as dense barriers against the permeation of liquids into the concrete, three types of nanoparticles including Zinc Oxide (ZnO), Magnesium Oxide (MgO), and composite nanoparticles were used in the present study as partial replacement of cement. Besides, the effect of adding these nanoparticles on both pore structure and mechanical strengths of the concrete at different ages was determined, and scanning electron microscopy (SEM) images were then used to illustrate the uniformity dispersion of nanoparticles in cement paste. It was demonstrated that the addition of a small number of nanoparticles effectively enhances the mechanical properties of concrete and consequently reduces the extent of the water permeation front. Finally, the behavioral models using Genetic Algorithm (GA) programming were developed to describe the time-dependent behavioral characteristics of nanoparticle blended concrete samples in various compressive and tensile stress states at different ages.

Induced internal stresses and their relation to FLASH sintering of KNN ceramics

Direct link between FLASH sintering, structure, microstructure and higher residual compressive internal stress state compared to conventional sintering.

Evaluation of rolling contact fatigue crack propagation behaviors in railway wheel steels

Rolling contact fatigue cracks propagate in mode II or mixed mode of I and II owing to the cyclic shear stress under multiaxial compressive stress states at the subsurface. No standardized evaluation method for shear mode crack propagation has been established. A method to evaluate the rates and behaviors of rolling contact fatigue crack propagation was studied using twin disc type rolling contact fatigue tests on railway wheel steels with artificial defects and finite element analyses (FEA) simulating the tests. The rolling contact fatigue test results indicated that the cracks initiated from the leading side of defects propagated in the depth direction, whereas those initiated from the trailing side propagated in the surface direction. Calculation results of the stress intensity factor (SIF) indicated that the cracks initiated from the leading side of defects propagated in the mixed mode, whereas those initiated from the trailing side propagated in only mode II. In addition, the crack propagation rates in the trailing side of defects accelerated with increasing number of cycles. This was because of the increase in the shear mode equivalent SIF during the crack propagation. Thus, the results of FEA corresponded to those of rolling contact fatigue tests. Therefore, the relationship between the rolling contact fatigue crack propagation rate and the shear mode SIF range could be evaluated by combining twin disc type rolling contact fatigue tests using test specimen with artificial defects and FEA.

Study on damage evolution beneath spherical indenter through experiments and mechanism-based strain gradient crystal plastic simulations

Damage evolution of AA7075-T6 rolling plate with 2 mm thickness under highly compressive stress state, which is generated by spherical indentation, is studied through experiments and simulations. On the cross section of spherical deformation region, micro voids induced by crush of MgZn2 particles are observed by scanning electron microscopy (SEM) and some special characteristics are revealed including ductile tearing of the brittle particle, huddling between neighboring voids and an in-void crack, but no clue of void closure under such heavy compression. Through Electron Back-Scattered Diffraction (EBSD), strain gradient around void was found quite high (70 mm−1) and much higher than the theoretical strain gradient induced by spherical indentation (1.8 mm−1), which will result in a significant hardening effect and preventing the further shrink and closure of micro void. More insights are provided through mechanism-based strain gradient crystal plastic simulations which couples a polycrystalline, full-scale model, and a single crystal submodel. Simulation results verify the elevated strain gradient around microvoid and find strain gradient varies dramatically along the small range of void edge, which is validated by nanoindentation test around the edge of voids. Microvoid evolution controlled by strain gradient shows strong anisotropy on different cross sections. For the void at position D focused in present paper, its area on section perpendicular to Y axis (SecY) increases at the early stage of indentation, despite the compressive stress state. Influence of the orientations of adjacent grains is also studied. Void areas on SecZ and SecY are found more sensitive to the change of orientation of adjacent grain, however, the volume variation show much less sensitivity.

Flexural stress effect on mechanical and mechanochemical properties of soda lime silicate glass surface

AbstractAs a means to elucidate the mechanical stress effect on the durability of soda lime silicate (SLS) float glass, a thin glass plate under flexural stress was investigated with X‐ray photoelectron spectroscopy (XPS), specular reflectance infrared (SR‐IR) spectroscopy, nanoindentation, and tribo‐testing. A lab‐built four‐point bending rig was employed to create compressive or tensile stress (around 40 MPa) on the air‐side surface of SLS glass. XPS analysis showed that electric field‐induced sodium ion migration is greatly enhanced in both compressive and tensile stress surfaces. The SR‐IR analysis of the Si‐O‐Si stretch mode revealed that the structural distortion of the silicate network appears to be larger under compressive stress than tensile stress. The elastic and plastic responses of the SLS surface to nanoindentation were significantly altered under the flexural stress conditions even though the magnitude of the flexural stress was less than 0.7% of the applied indentation stress. Compared to the stress‐free surface, the resistance to mechanochemical wear at 90% relative humidity deteriorated under the compressive stress condition, while it just became more scattered under the tensile stress condition. Even though the applied flexural stress was very small, its impact on chemical and structural properties could be surprisingly large. Combining all results in this study and previously published works suggested that the changes observed in nanoindentation and mechanochemical wear behaviors may be associated with the strain in the Si‐O bonds of the silicate network.

Understanding asteroidal failure through quasi‐static compression testing and 3‐D digital image correlation of the Aba Panu (L3) chondrite

AbstractA comprehensive understanding of the mechanical strength and failure mechanisms of asteroids is essential for the development of hazard mitigation strategies and in situ resource extraction. In this study, the Aba Panu (L3) ordinary chondrite meteorite is investigated to understand its failure response under compressive loading. Compression experiments were conducted on ten 1 cm cubes under quasi‐static conditions in constant displacement control mode at room temperature. Three‐dimensional (3‐D) digital image correlation (DIC) was used to measure the full‐field deformation and strain. These data were used to determine the elastic modulus and local strain distribution, and investigate the effects of the mineralogical and structural heterogeneity on the crack formation and growth sites. Aba Panu exhibits brittle failure during compression with a range of failure strength from 361.7 to 578.0 MPa. Axial splitting and multiple fracturing occur during the uniaxial compressive state of stress. Ultrasonic tests were used to calculate elastic moduli from sound speeds and compared with the results of L‐type ordinary chondrites from the literature. Characterization results from electron microprobe analysis identified different elements and areal distribution of mineral phases and pre‐existing cracks. In general, the DIC results did not show correlations between crack initiation and specific mineralogical or textural components in this meteorite, such as chondrules or metals/sulfide grains, suggesting that the pre‐existing microcracks and porosity control the Aba Panu failure mechanisms.

Core-shell heterostructure-enabled stress engineering in vanadium dioxide nanobeams

In strongly correlated materials (SCMs), especially for vanadium dioxide (VO2), manipulating physical properties through stress engineering is an important issue for the use of ultrafast metal-insulator transition (MIT) in device applications. Recent research efforts have mainly focused on modulation and related phenomena of physical properties by epitaxial and mechanical stresses in VO2 films or anisotropic nanocrystals. However, inhomogeneous stress in such planar and nanocrystal systems leads to complications induced by phase competitions or the creation of intermediate phases. Here, we demonstrate the core-shell heterostructures-enabled stress engineering on MIT, which provides accommodation of uniform axial stress and control of phase transition pathways in VO2 nanobeams. Specifically, the VO2 nanobeams with an amorphous Al2O3 shell undergo a simple and direct MIT at lower temperatures without intermediate phases, distinctly different from pristine nanobeams with coexisting phases. For the core-shell nanobeams, the VO2 core sustains a uniform compressive stress state along the nanobeam length caused by shell formation, which can be attributed to the different thermal behaviors coupled to the elastic modulus between the VO2 and shell. Our results can lead to better engineering of phase transitions in SCMs, providing the beneficial effect of suppressing inhomogeneities during the MIT process.

An improved Lubliner strength model based on Ottosen failure criterion for concrete under multiaxial stress state

AbstractThe Lubliner strength criterion (SC) can achieve desirable responses in the tension‐dominated and plane compressive stress states but overestimates the concrete strength under triaxial compression. Moreover, additional breakpoints without continuous derivatives occur along the hydrostatic pressure axis. The primary causes for these problems include the Lubliner SC constant γ parameter and breakpoints along the piecewise linear meridians. Therefore, a new Lubliner–Ottosen SC is developed with the parameter γ determined based on the Ottosen SC. The meridian contains smoothly connected straight and curved lines. Moreover, the deviatoric traces of the corresponding failure surface gradually change from curved triangles to approximate circles with increasing hydrostatic pressure. Finally, the proposed Lubliner–Ottosen SC is compared with other failure criteria and experimental results for concrete. Regardless of triaxial or nontriaxial compression, the new criterion can suitably represent the failure behavior of concrete in the various multiaxial stress states.

Effect of water on sandstone's fracture toughness and frictional parameters: Brittle strength constraints

Water presence causes a dramatic reduction of sandstone strength. Under compressive stress conditions, the strength of a rock sample is controlled by frictional parameters and the fracture toughness of the material. Here, we report fracture toughness, frictional and uniaxial compression tests performed on five sandstones under dry and water-saturated conditions, that provide new insight into the mechanical influence of water on sandstone strength. The mechanical data show that water saturation causes a reduction of i) the fracture toughness and fracture energy ranging from 6 to 35% and 21–52%, respectively; and ii) the static friction coefficient ranging from 0 to 19%. The results suggest that the water weakening in sandstones (with a reduction of the uniaxial compression strength ranging from 0 to 30%) is due to the reduction of the fracture toughness and of the static friction coefficient of the materials. The measured fracture toughness and frictional parameters are then introduced into two micro-mechanical models (a pore-emanating cracks model and a wing crack model) to predict the water weakening. It is shown that the models predict the water weakening relatively well with a general slight overestimation (10–20%). Finally, a parametric analysis on the wing crack model revealed that a sandstone's absolute strength can be estimated by means of combined physical and mechanical parameter measurements.

Effect of Stress on Spinodal Decomposition in Binary Alloys: Atomistic Modeling and Atom Probe Tomography

Self-organizing nanostructure evolution through spinodal decomposition is a critical phenomenon determining the properties of many materials. Here, we study the influence of stress on the morphology of the nanostructure in binary alloys using atomistic modeling and atom probe tomography. The atomistic modeling is based on the quasi-particle approach, and it is compared to quantitative three-dimensional (3-D) atom mapping results. It is found that the magnitude of the stress and the crystallographic direction of the applied stress directly affect the development of spinodal decomposition and the nanostructure morphology. The modulated nanostructure of the binary bcc alloy system is quantified by a characteristic wavelength, lambda . From modeling the tensile stress effect on the A-35 at. pct B system, we find that lambda _{001}< , lambda _{111}< , lambda _{101}< , lambda _{112} and the same trend are observed in the experimental measurements on an Fe-35 at. pct Cr alloy. Furthermore, the effect of applied compressive and shear stress states differs from the effect of the applied tensile stress regarding morphological anisotropy.

(Keynote) A Proposed Solution to Li Dendrite Penetration Into Solid Electrolytes

The use of lithium (Li) or sodium (Na) metal anodes together with highly ion-conductive solid electrolytes (SEs) could provide batteries with a step improvement in volumetric and gravimetric energy densities. Unfortunately, these SEs face significant technical challenges, in large part because Li and Na dendrites can penetrate through SEs, leading to short circuits. The ability of such a soft material (Li or Na metal) to penetrate through ceramic is surprising from the point of view of models widely used in the Li-battery field. We introduce a concept, new to the battery field, for preventing penetration of lithium dendrites through SEs by putting the SE surfaces into a state of residual compressive stress. For a sufficiently high compressive stress, cracks have difficulty forming, and cracks that do form are forced to close, inhibiting dendrite penetration. This approach is widely used to solve commercially important stress corrosion cracking problems in metals and static fatigue problems in ceramics and glasses (e.g., Gorilla Glass). However, the technique will not be useful for SEs if the Li-ion transport rate through a SE is substantially reduced when the SE is under compression. Our molecular dynamics calculations for Li-ion transport through a common SE demonstrate that the introduction of even very high residual compressive stresses (10 GPa) has only a modest effect on Li-ion transport kinetics, suggesting that this approach is viable and capable of providing a new paradigm for developing high-performance and mechanically stable SEs. We discuss several possible mechanisms for establishing high residual compressive stresses, including ion implantation and ion exchange. Preliminary results are highly promising.

Influence of residual stresses in hard tool coatings on the cutting performance

During the hard machining of powder metallurgical high-speed steel, finely dispersed carbides in the steel expose the tools to both thermal and mechanical load. This can influence the cutting performance and cause damage to the tools such as premature abrasive and adhesive wear. Thin hard coatings like TiAlN deposited by physical vapor deposition are widely used in order to improve the tool performance. High power pulsed magnetron sputtering (HPPMS) results in technical benefits, such as a more homogeneous coating thickness distribution on the tools compared to direct current magnetron sputtering (dcMS). The advantages of HPPMS can be combined with the high deposition rates of dcMS leading to a higher economic efficiency conducting a dcMS/HPPMS hybrid process. Adding silicon to the coating system TiAlCrN results in TiAlCrSiN leading to a nanocomposite coating architecture with improved mechanical properties. The influence of the residual stresses on the mechanical properties and on the roughing performance of nanocomposite coatings is of high interest and was therefore investigated in the present study. Four different TiAlCrSiN hybrid coatings deposited with four different substrate bias potentials were examined for this purpose. The residual stresses and the mechanical properties including the resistance against crack formation as well as the compound properties of the coatings on cemented carbide were investigated. Finally, the roughing performance of the coated cemented carbide tools were tested by milling the powder metallurgical high-speed steel HS6-5-3C. For the coatings investigated, it can be concluded that a compressive residual stress state of approx. -2 GPa ≤ σ ≤ −3 GPa leads to the highest resistance against crack formation, and thus to the best cutting performance.

Investigation of the role of crown crack in cohesive soil slope and its effect on slope stability based on the extended finite element method

Tensile cracks in soil slopes, especially developing at the crown, have been increasingly recognized as the signal of slope metastability. In this paper, the role of crown cracks in natural soil slopes was investigated and its effect on stability was studied. A numerical modeling of slope and simulation of tensile behavior of soil, based on the Extended Finite Element Method (XFEM), was used. A numerical soil tensile test was applied to validate the use of XFEM on tensile behavior of soil before the simulation. Slope failure was simulated by using the Strength Reduction Method, which determines the potential slip surface of slope. The simulation results indicate forming of crown crack in natural soil slopes when the plastic zone starts penetrating. Therefore, it is reasonable to consider the crown crack as the signal of slope metastability. A sensitivity analysis shows the effect of cracking on slope stability if cracks are at the position of the tension zone. The stress variation analysis, from the surface slip deformation, reveals that the slope is at a state of compressive stress. When plastic zone starts penetrating, the upper part of slope generates tension zone, but the extent of tension zone is restricted by the slope failure. This suggests why tensile cracks are difficult to form and stretch in the deep part of the slope. The implementation of XFEM on slope stability analysis can be used for assessing the tensile strength of soil and forecasting the time of slope failure related disaster.

Atomistic simulation of microstructure evolution of NiTi single crystals in bending deformation

In order to explore the microstructure evolution of the equiatomic NiTi shape memory alloy during the bending deformation, molecular dynamics simulations based on the 2NN-MEAM potential were respectively carried out in the compressive zone, tensile-compressive zone (identified as “combined zone”) and tensile zone, which were divided according to the force analysis. Positive and negative strain rates or velocities with linear variations were applied in different zones during the bending deformation. The results show that the simulated lath martensites and dislocations during the bending deformation are highly consistent with the results of bending experiment. In the bending simulation, the martensitic transformation occurs in different degrees in the above three zones. The microstructure evolution under compressive stress conditions is different from that of tensile stress conditions, including stress-induced martensitic transformation, dislocation density and distribution, the formation of highly stressed austenites and Frenkel defect pairs. Additionally, during the stress-induced martensitic transformation, the morphology and structure of martensites strongly depend on the deformation methods, strain rate and Ni/Ti atomic ratio. The current simulation work reveals lots of micro information that cannot be observed and captured by experiments on an atomic scale, which is expected to grasp the information of the stress-induced martensitic transformation and the resultant defect structures during the bending deformation.

Study on the bearing capacity and chloride ion resistance of RC structures under multi-factor corrosive environment and continuous load

In this paper, the bearing capacity and chloride resistance of reinforced concrete (RC) structures under multi-factor corrosive environment and continuous load are investigated, and the correlation between the bearing capacity and chloride resistance is also studied. First, different corrosion environments and compressive stress conditions were simulated by freeze-thaw (F-T) cycles test, carbonization test and continuous loading test. Among them, carbonization test and F-T cycles test constitute corrosion cycle test. Then, according to the test results of bearing capacity, under the action of corrosion cycle test, the bearing capacity of RC beam decreases with the increase of corrosion cycles. The bearing capacity of RC beams have a tendency to rise first and then drop with the increase of compressive stress under the multi-factor corrosive environment. In addition, according to the test results of chloride ion erosion, the equation for calculating the corrosion resistance of chloride ion is established, and the resistance to chloride ion erosion increased with the increase of corrosion cycle. With the increase of compressive stress, the ability to resist chloride ion erosion displays the tendency rising up at the beginning and declining in late. Finally, through grey correlation analysis, the correlation between corrosion cycles and yield load is greater than the correlation between chloride ion erosion and yield load, while the correlation with ultimate load is opposite. Based on the results of correlation analysis, the bearing capacity decay equation is established.