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The work investigates failure criteria and mean stress correction approaches for the fatigue lifetime prediction of two hyperelastic adhesives (a polyurethane, PU, and a silicon-modified polymer, SMP). Fatigue experiments are carried under constant amplitude cyclic loading at RT and 40°C/60% r.h with butt- and the thick-adherend-shear-test-joints at three stress ratios R = −1, 0.1 and 0.5. Three mean stress correction approaches are evaluated: Goodman (static strength based), Schütz (mean stress sensitivity based) and Kujawski & Ellyin (parameter optimisation based). Fatigue failure criteria considered are: Drucker-Prager (linear-relation with the hydrostatic stress), Beltrami (quadratic relationship with the hydrostatic stress), and a multivariable nominal shear and tensile stresses criterion (data-based). The comparison is based on prediction accuracy (R-squared of master SN curves) and complexity of parameter determination. The highest values of R-squared were obtained by the Kujawski and Ellyin correction, followed by Schütz and then Goodman. However, the complexity of parameter determination follows an opposite trend with Goodman being the lightest approach. Failure criteria yielded comparable results with the multivariable criterion having the advantage of not dealing with FEA, but being limited to joints with nearly uniform stress distribution. Finally, compared to Drucker-Prager, the Beltrami criterion had a more robust parameter determination. [ABSTRACT FROM AUTHOR]

Clinically serious congenital heart valve defects arise from improper growth and remodeling of endocardial cushions into leaflets. Genetic mutations have been extensively studied but explain less than 20% of cases. Mechanical forces generated by beating hearts drive valve development, but how these forces collectively determine valve growth and remodeling remains incompletely understood. Here, we decouple the influence of those forces on valve size and shape, and study the role of YAP pathway in determining the size and shape. The low oscillatory shear stress promotes YAP nuclear translocation in valvular endothelial cells (VEC), while the high unidirectional shear stress restricts YAP in cytoplasm. The hydrostatic compressive stress activated YAP in valvular interstitial cells (VIC), whereas the tensile stress deactivated YAP. YAP activation by small molecules promoted VIC proliferation and increased valve size. Whereas YAP inhibition enhanced the expression of cell-cell adhesions in VEC and affected valve shape. Finally, left atrial ligation was performed in chick embryonic hearts to manipulate the shear and hydrostatic stress in vivo. The restricted flow in the left ventricle induced a globular and hypoplastic left atrioventricular (AV) valves with an inhibited YAP expression. By contrast, the right AV valves with sustained YAP expression grew and elongated normally. This study establishes a simple yet elegant mechanobiological system by which transduction of local stresses regulates valve growth and remodeling. This system guides leaflets to grow into proper sizes and shapes with the ventricular development, without the need of a genetically prescribed timing mechanism. [ABSTRACT FROM AUTHOR]

The atomic mobility, microstructure, and local stress in the shear bands and necking regions of notched Cu50Zr50 metallic glasses are compared, and the intricate interplay among them is examined via molecular dynamics simulations. The virtual tensile tests show that the average atomic mobility inside the mature shear band is lower than that in the necked region, which is controlled by the corresponding atomic-level structure. The short-range topological orders revealed by the Voronoi tessellation of both the necked region and the shear band are, however, quite similar to each other, while the excess free volume of the former is significantly higher than that of the latter. This high free volume content contributes to the migration of atoms or clusters and, thus, expedites the local plastic deformation. Further investigations suggest that the free volume content correlates closely with the local hydrostatic stress, while the topological microstructure is rather insensitive to the hydrostatic stress, especially when the hydrostatic stress is low. In addition, it is also found that the atomic mobility of the central atom in a cluster correlates not only with the local packing environment, such as the free volume content or the hydrostatic stress, but also with the geometry of the cluster, such as its symmetry or coordination number. The findings are helpful in developing/selecting the constitutive models for the deformation of metallic glasses. [ABSTRACT FROM AUTHOR]

• Tubular specimens of AM 18NI300 maraging steel were subjected to multiaxial loading. • Fatigue life of AM 18Ni300 maraging steel is insensitive to axial-torsion phase shift. • Lifetime prediction model was built based on physics-constrained Gaussian process. • Inherent surface defects play a key role in fatigue of AM 18Ni300 maraging steel. The study aims to estimate the role of surface defects in the fatigue life of age-hardened and unmachined additively manufactured 18Ni300 maraging steel and to verify the effectiveness of stress-based life prediction models. Axial-torsional cyclic loading paths involving in-phase and out-of-phase cases are applied. Scanning electron microscopy and micro-computed tomography are performed to detect the fatigue crack initiation sites and paths. The effectiveness of the classic models and a physics-constrained Gaussian process for regression in fatigue life prediction is estimated. Results show that the intrinsic orientation of the surface defects affects the fatigue mechanism by increasing the material sensitivity to axial stress. Additionally, the steel is insensitive to the phase shift between the axial and torsional loading components. The hydrostatic stress and the integrated shear stress amplitudes over the critical plane are pointed as the most effective load-related quantities for life prediction. [ABSTRACT FROM AUTHOR]

[Display omitted] • The α" plates induced by hydrostatic pressure form self-accommodation morphologies via {1 1 1} twinning. • A peculiar hierarchical microstructure was induced in the Ti-30Zr-10Nb alloy by hydrostatic pressure. • The hierarchical microstructure effectively coordinates both isotropic macroscopic strains and local shear stresses. • The hydrostatic pressure facilitates the β to ω transformation but hinders the β to α" transformation. Hydrostatic pressure is a crucial tool to regulate the structure and properties of materials. However, the influence of hydrostatic pressure on the microstructure of metastable β-Ti alloys remains unclear. In this work, the microstructure and microhardness evolution of a metastable β-type Ti-30Zr-10Nb alloy under hydrostatic pressure (≤10 GPa) were investigated and compared with those under uniaxial pressure (≤1106 MPa). The results revealed that under hydrostatic pressure, Ti-30Zr-10Nb alloy formed a peculiar hierarchical microstructure composed of submicron-scaled α" plates with self-accommodating morphologies, nanoscale ω particles, and microscale shear bands containing nanoscale α" domains. The formation of a hierarchical microstructure effectively coordinates not only the isotropic macroscopic strains but also the local shear stresses under hydrostatic pressure. Compared to uniaxial pressure, the hydrostatic pressure facilitates the β to ω transformation, but hinders the β to α" transformation. The microhardness of the Ti3010 alloy reaches approximately 319 HV at a hydrostatic pressure of 10 GPa, which is 28 % higher than the highest microhardness obtained under uniaxial pressure. The current results highlight the promising potential of hydrostatic pressure in optimizing the mechanical properties of metastable β-Ti alloys. [ABSTRACT FROM AUTHOR]

Film adhesives provide an excellent method for joint manufacturing with minimum waste and highly accurate thickness. Here, the multiaxial behaviour in terms of elasticity, yielding and failure of a hot curing epoxy film adhesive was analytically and experimentally investigated. Butt, scarf and thick adherend shear test joints were tested under static conditions. The variation between normal and shear stress among the different joint types allowed the variation of the multiaxiality using the same experimental set-up with a uniaxial testing machine. A consistent set of elastic constants was obtained by carefully calibrated displacement sensors, proper consideration of adherend elasticity and of lateral strain restrictions. The presentation of the von Mises equivalent stress and the hydrostatic pressure at the point of yield and failure indicated that hydrostatic stresses have a large influence on the static strength of the film adhesive. Failure and yield conditions were formulated based on linear or exponential forms of Drucker-Prager type models. A strain-based failure hypothesis was formulated using the second invariant of the strain deviator, and the first invariant of the strain tensor. Results showed that high values of volumetric strain are detrimental to the strength of the adhesive joints. [ABSTRACT FROM AUTHOR]

Clay minerals are platelike particles that play a critical role in problems involving swelling, deformation, and failure. Fundamental understanding of these phenomena and the parameters that influence them requires studies at the nanoscale. The nanoscale mechanism of the sliding of clay sheets at different states of hydration and hydrostatic stress was studied here using molecular dynamics in an isobaric–isothermal ensemble. The hydration state lay in the range of crystalline swelling (0–400 mgwater/gclay), and the hydrostatic pressure was varied in the range 5–12 GPa. Under hydrostatic pressures as high as several gigpascals, the mobility and the molecular structure of water are comparable to those of supercooled water. Despite high hydrostatic stresses, the failure was located in the shear-dominated zone. Examination of the molecular structure of interlayer water, number of hydrogen bonds, and their configuration during shear loading showed evidence of stick-slip phenomena similar to those found in thin films. The number of hydrogen bonds between interlayer water molecules increased as the clay–water system approached the slip point. Hydration state and hydrostatic stress influenced the stress–strain behavior of system and the average shear stress. The hydration states, in which the maximum average shear stresses occurred, coincided with the formation of the second, third, and fifth water layers. The shear strength decreased with the increase of water content above 340 mgwater/gclay. The nanoscale cohesion and friction angle of the layers were calculated using the Mohr–Coulomb failure criterion and were found to be in good agreement with previous studies. [ABSTRACT FROM AUTHOR]

Joints enable the relative movement between the connected bones. The shape of the joint is important for the joint movements since they facilitate and smooth the relative displacement of the joint's parts. The process of how the joints obtain their final shape is yet not well understood. Former models have been developed in order to understand the joint morphogenesis leaning only on the mechanical environment; however, the obtained final anatomical shape does not match entirely with a realistic geometry. In this study, a computational model was developed with the aim of explaining how the morphogenesis of joints and shaping of ossification structures are achieved. For this model, both the mechanical and biochemical environments were considered. It was assumed that cartilage growth was controlled by cyclic hydrostatic stress and inhibited by octahedral shear stress. In addition, molecules such as PTHrP and Wnt promote chondrocyte proliferation and therefore cartilage growth. Moreover, the appearance of the primary and secondary ossification centers was also modeled, for which the osteogenic index and PTHrP–Ihh concentrations were taken into account. The obtained results from this model show a coherent final shape of an interphalangeal joint, which suggest that the mechanical and biochemical environments are crucial for the joint morphogenesis process. [ABSTRACT FROM AUTHOR]

Tension‐compression, tension‐tension, torsional, and 90° out‐of‐phase axial‐torsional fatigue tests were performed on a quenched and tempered U2 steel. All tests were conducted under force/torque control because macroscopic plastic strains were insignificant in the life range of interest (from 104 to 2 × 106 loading cycles). Stress‐based versions of the Fatemi‐Socie critical plane parameter and of the Smith‐Watson‐Topper parameter with a critical plane interpretation were evaluated using the experimental data. The Smith‐Watson‐Topper parameter was not able to correlate the test data. The Fatemi‐Socie method correlated most of the test data within factor‐of‐three boundaries. A modified Crossland invariant‐based parameter made of two interaction rules between the shear stress amplitude and the maximum hydrostatic stress, and of a definition of shear stress amplitude based on the maximum prismatic hull method, yielded fatigue life estimates in reasonable agreement with the experimental observations. [ABSTRACT FROM AUTHOR]

This paper reports the results of fracture testing of sandstone under constant minor principal stress (20 MPa) and various intermediate principal stresses. The results show that when the minor principal stress is constant, as the intermediate principal stress increases, the ratio of the octahedral shear stress (τoct) to the octahedral normal stress (σoct) decreases. The strength criterion of τoct/σoct = f(σ2) is obtained. This criterion reflects not only the hydrostatic stress and intermediate principal stress effects but also the Lode angle effect. This criterion reveals the reason why the rock strength increases and then decreases with increasing intermediate principal stress. The decreasing trend is fitted by linear, logarithmic and Boltzmann equations. The applicability of the three fitting equations for strength prediction and the π plane strength envelopes is analysed, and the results of the Boltzmann fitting equation are the best. The deformation characteristics of rock during the failure process are analysed. The changing process of the tangential deformation modulus of the rock is found to be divided into three stages during the loading process: an increasing stage, an initial decreasing stage and a rapidly decreasing stage. Based on an analysis of computed tomography (CT) images of the internal fractures of rock and photographs of the fracture surfaces, the internal fractures are found to be clear and smooth, and the shear stresses in the fracture surfaces are strengthened with increasing intermediate principal stress. The dominant shear stress in the process of failure is considered to cause these phenomena. [ABSTRACT FROM AUTHOR]

A novel strategy is implemented to enhance the strength-ductility synergy in the Co-Cr-Ni-based medium-entropy alloy (MEA) by designing analogous harmonic structures (AHS), where the soft face-centered cubic (FCC) matrix is surrounded by hard multiphase constitutes. The high yield strength of 1028 MPa and ultimate tensile strength of 1558 MPa coupled with considerable uniform elongation of 20% are achieved for the AHS-MEA. Upon uniaxial tension, a large number of the nanograins appear in the core near the interface meanwhile the hard σ precipitates are penetrated by the mobile dislocations in the shell. A finite-element based model is developed to elucidate the intrinsic mechanisms underlying microstructure evolution during plastic deformation. Similar to the equal channel angular pressing/extrusion and high pressure torsion, the coexistence of large tensile hydrostatic stresses and shear stresses plays a crucial role in the grain refinement. The study provides new insights into the nanocrystalline formation mechanism and excellent strength-ductility synergy. [Display omitted] [ABSTRACT FROM AUTHOR]

The dynamic response of Cerium under high pressure and temperature is not clear due to the complexity of its phase diagram. Using large-scale atomistic simulations, we report the γ → α phase transition (PT) of Ce under shock loading. The PT behaviors predicted by present simulations are in reasonable agreement with previous experiments, thus confirm the validity of the interatomic potential. While the simulated wave velocities of elastic precursor are larger than experiments, the PT wave velocities agree well with the LASL data. In the T - P phase diagram, the Hugoniot states generally agree with the lower-pressure extrapolation of the higher-pressure experimental data, but differ significantly from each other orientations. By examining the ratio between the shear stress and hydrostatic stress, it is found that the anisotropic response of PT kinetics might be dependent on the variance of shear stress with lattice orientations. The present study suggests that the effect of shear stress usually ignored in shock wave experiments has to be treated seriously when studying the low-pressure γ → α PT of Ce. • The γ.→α phase transition of Cerium observed in our recent gas gun experiments was recovered by molecular dynamics simulations. • The simulated shock Hugoniot u s v.s. u p and T v.s. P relations show strong anisotropic dependence on lattice orientations. • The stress deviator cannot be ignored when considering the low-pressure phase transition behaviors of Ce under shock loading. [ABSTRACT FROM AUTHOR]

Summary: This paper presents analytical elastic‐plastic solutions for static stress loading analysis and quasi‐static expansion analysis of a cylindrical cavity in Tresca materials, considering biaxial far‐field stresses and shear stresses along the inner cavity wall. The two‐dimensional static stress solution is obtained by assuming that the plastic zone is statically determinate and using the complex variable theory in the elastic analysis. A rigorous conformal mapping function is constructed, which predicts that the elastic‐plastic boundary is in an elliptic shape under biaxial in situ stresses, and the range of the plastic zone extends with increasing internal shear stresses. The major axis of the elliptical elastic‐plastic boundary coincides with the direction of the maximum far‐field compression stress. Furthermore, considering the internal shear stresses, an analytical large‐strain displacement solution is derived for continuous cavity expansion analysis in a hydrostatic initial stress filed. Based on the derived analytical stress and displacement solutions, the influence of the internal shear stresses on the quasi‐static cavity expansion process is studied. It is shown that additional shear stresses could reduce the required normal expansion pressure to a certain degree, which partly explains the great reduction of the axial soil resistance due to rotations in rotating cone penetration tests. In addition, through additionally considering the potential influences of biaxial in situ stresses and shear stresses generated around the borehole during drillings, an improved cavity expansion approach for estimating the maximum allowable mud pressure of horizontal directional drillings (HDDs) in undrained clays is proposed and validated. [ABSTRACT FROM AUTHOR]

The present article analyzes the induced compressive, shear and tensile stresses due to a moving load on three distinctly characterized irregular orthotropic half-spaces, viz. functionally graded orthotropic viscoelastic half-space (Case-I), functionally graded initially stressed orthotropic elastic half-space (Case-II) and orthotropic magnetoelastic half-space (Case-III) under hydrostatic initial stress. The expressions for said induced stresses are deduced in closed form using analytical approach. The influences of various physical parameters, viz. maximum depth of irregularity, functionally gradedness, irregularity factor, initial stress, magnetoelastic coupling parameter, hydrostatic initial stress and frictional coefficient on induced stresses for concerned cases, have been investigated with a comparative analysis. To depict the outcomes numerically, the half-spaces comprised of Carbon fiber, Prepreg and T300/5208/graphite/epoxy material have been taken into account and the observations are highlighted. Moreover, some notable characteristics have been outlined and delineated through graphs. [ABSTRACT FROM AUTHOR]

Abstract This paper provides a simplified procedure for analysing the interaction between fully-grouted bolts and rock mass for circular tunnels under a hydrostatic stress field. In this procedure, the relative movement between the rock mass and the bolt is accounted for by considering the shearing stiffness at interface. Further, the elastic elongation of the bolt is considered. Two cases of elastic-brittle-plastic and elastic-perfectly-plastic rock mass behaviours are considered. The rock-bolt interaction is modelled at the initial and final states. At the initial state, immediately after the bolt is installed, a finite difference solution to obtain stresses and strains in the rock mass is proposed. At the final state, after the tunnel excavation advances and the fictitious support pressure becomes zero, another finite difference solution for computing stresses along the bolt and rock mass displacements is presented. To obtain the final converged results, an iterative process is performed by repeating the above solution at the final stage. The simplified procedure is validated by comparison with the commercial finite difference software, FLAC3D. A series of parametric studies is conducted to assess the sensitivity of the bolt and rock behaviours to variations in interface shear stiffness, the stiffness of the end plate and the assumed post-peak behaviour of the rock mass. The results indicate that the post-peak behaviour of the rock mass has an important effect on the bolt performance and, in turn, it should be a key design consideration. It is also shown that, under squeezing conditions, the failure mode of the bolt is influenced by the stiffness of the end plate. [ABSTRACT FROM AUTHOR]

In this article, a bio-mimetic approach to the generative design of contact interfaces with uniform pressure distribution is explored. During the morphogenesis process, biological joints grow depending on the states of stress generating a shape adapted to the mechanical loads. This adaptation is driven by two main rules: shear stress inhibits growth, whereas cyclic hydrostatic compressive stress promotes it. In this work, we demonstrate that the stress-dependent growth rules of synovial joint morphogenesis can be applied to generate contact interfaces with uniform pressure in engineering applications. For that, we present a mathematical model that comprises a contact formulation and a bio-inspired growth function; the model is solved numerically using finite element methods. We analyse the impact of the growth rules of synovial joint development on the contact pressure distribution of two-dimensional contact interfaces. We study the parameter space of the bio-inspired growth function to fine-tune the model parameters. The model is tested in several cases with different boundary conditions and material properties. The results show that the proposed generative design process leads to contact interfaces that provide uniform contact pressure. A quantitative metric of the uniformity of the contact pressure is also defined. This metric indicates that the bio-inspired process generates geometries as good as those obtained with other methods present in the literature without the need of an initial shape close to the final one. Thus, this work demonstrates that synovial joint morphogenesis can be adapted for generative design in engineering. • Synovial-joint-morphogenesis-inspired generative design for contact profiles. • The effect of growth rules on the evolution of uniform contact pressure. • Stress-adaptive design to provide contact profiles with uniform contact pressure. [ABSTRACT FROM AUTHOR]

The microstructure of a TiPt high temperature shape memory alloy induced by high pressure torsion (HPT) was studied using synchrotron radiation experiments and transmission electron microscopy. Upon the severe plastic deformation, the parent B19 martensite is strongly refined and highly strained. While neither strong chemical disorder nor amorphization are encountered, the occurrence of crystal reflections is observed that can be indexed in terms of the L1 0 phase and long range ordered stacking structures. The occurrence of the L1 0 phase might be triggered by the high hydrostatic stress as well as shear stress superimposed during the HPT. The austenite and martensite start and finish temperatures of TiPt are not affected by the HPT since upon heating, recrystallization occurs prior to the onset of the reverse martensite to austenite transformation. [Display omitted] • High-pressure torsion of TiPt yields a strongly fragmented and strained martensite. • No evidence for amorphization or a reverse B19 to B2 transformation is obtained. • Severe plastic deformation induces a L1 0 phase by high hydrostatic and shear stress. • Recrystallization occurs before the reverse martensite to austenite transformation. [ABSTRACT FROM AUTHOR]

The microstructure of a TiPt high temperature shape memory alloy induced by high pressure torsion (HPT) was studied using synchrotron radiation experiments and transmission electron microscopy. Upon the severe plastic deformation, the parent B19 martensite is strongly refined and highly strained. While neither strong chemical disorder nor amorphization are encountered, the occurrence of crystal reflections is observed that can be indexed in terms of the L1 0 phase and long range ordered stacking structures. The occurrence of the L1 0 phase might be triggered by the high hydrostatic stress as well as shear stress superimposed during the HPT. The austenite and martensite start and finish temperatures of TiPt are not affected by the HPT since upon heating, recrystallization occurs prior to the onset of the reverse martensite to austenite transformation. [Display omitted] • High-pressure torsion of TiPt yields a strongly fragmented and strained martensite. • No evidence for amorphization or a reverse B19 to B2 transformation is obtained. • Severe plastic deformation induces a L1 0 phase by high hydrostatic and shear stress. • Recrystallization occurs before the reverse martensite to austenite transformation. [ABSTRACT FROM AUTHOR]

This article delves to study the effect of corrugated boundary surfaces on the propagation of horizontally-polarized shear waves (SH-waves) in a magnetoelastic transversely isotropic layer under a hydrostatic state of stress lying over an elastic half-space under gravity. A dispersion equation has been derived in closed-form and is found to be in good agreement to the classical Love-wave equation. The effect of magnetoelasticity, hydrostatic state of stress, gravity, corrugation, position parameter, and undulation on the phase velocity of the SH-wave has been identified. Numerical computation along with graphical demonstration has been carried out for cadmium, magnesium, and zinc materials of hexagonal symmetry to highlight some significant facts. [ABSTRACT FROM PUBLISHER]

A crystal-plasticity-based damage model that incorporates material length-scale through use of the slip-plane lattice incompatibility is developed with attention to the physical basis for the evolution of damage in a "bulk" shear deformation and without resort to ad hoc measures of shear deformation. To incorporate the physics of the shear damage process recently found by Kweon et al. (2010, "Experimental Characterization of Damage Processes in Aluminum AA2024-O," ASME J. Eng. Mater. Technol., 132(3), p. 031008), the development of tensile hydrostatic stress in grains due to grain-to-grain interaction, two existing theories, crystal plasticity, and the void growth equation by Cocks and Ashby (1982, "On Creep Fracture by Void Growth," Prog. Mater. Sci., 27(3-4), pp. 189-244) is combined to make the model in this study. The effect of the void volume increase onto the constitutive behavior is incorporated by adding the deformation gradient due to the void volume growth into a multiplicatively decomposed kinematics map. Simulations with the proposed model reveal the physics of shear and reproduce the accelerated damage in the shear deformation in lab experiments and industrial processes: the gradient of hydrostatic stress along with the development of macroscopic normal stress (hydrostatic stress) components amplifies the development of the local hydrostatic stress in grains under tensile hydrostatic stress. [ABSTRACT FROM AUTHOR]

Mechanical stimuli play a significant role in the process of endochondral growth. Thus far, approaches to understand the endochondral mechanical growth rate have been limited to the use of approximated location and geometry of the growth plate. Furthermore, growth has been simulated based on the average deflection of the growth plate or of the femoral neck. It has also been reported in the literature that the growth plate lies parallel to one of the principal stresses acting on it, to reduce the shear between epiphysis and diaphysis. Hence the current study objectives were (1) to evaluate the significance of a subject-specific finite element model of the femur and growth plate compared to a simplified growth plate model and (2) to explore the different growth direction models to better understand proximal femoral growth mechanisms. A subject-specific finite element model of an able-bodied 7-year old child was developed. The muscle forces and hip contact force were computed for one gait cycle and applied to a finite element model to determine the specific growth rate. Proximal femoral growth was simulated for two different growth direction models: femoral neck deflection direction and principal stress direction. The principal stress direction model captured the expected tendency for decreasing the neck shaft angle and femoral anteversion for both growth plate models. The results of this study suggest that the subject-specific geometry and consideration of the principal stress direction as growth direction may be a more realistic approach for correct prediction of proximal femoral growth morphology. [ABSTRACT FROM AUTHOR]

High-power, short-duration, laser-driven, shock compression and recovery experiments on [001] silicon unveiled remarkable structural changes above a pressure threshold. Two distinct amorphous regions were identified: (a) a bulk amorphous layer close to the surface and (b) amorphous bands initially aligned with {111} slip planes. Further increase of the laser energy leads to the re-crystallization of amorphous silicon into nanocrystals with high concentration of nano-twins. This amorphization is produced by the combined effect of high magnitude hydrostatic and shear stresses under dynamic shock compression. Shock-induced defects play a very important role in the onset of amorphization. Calculations of the free energy changes with pressure and shear, using the Patel-Cohen methodology, are in agreement with the experimental results. Molecular dynamics simulation corroborates the amorphization, showing that it is initiated by the nucleation and propagation of partial dislocations. The nucleation of amorphization is analyzed qualitatively by classical nucleation theory. [ABSTRACT FROM AUTHOR]

In the shear deformation of powder beds beyond the quasi-static regime the shear stress is dependent on the strain rate. Extensive work has been reported on the rapid chute flow of large granules but the intermediate regime has not been widely addressed particularly in the case of cohesive powders. However in industrial powder processes the powder flow is often in the intermediate regime. In the present work an attempt is made to investigate the sensitivity of the stresses in an assembly of cohesive spherical particles to the strain rate in ball indentation using the Distinct Element Method. This technique has recently been proposed as a quick and easy way to assess the flowability of cohesive powders. It is shown that the hardness, deviatoric and hydrostatic stresses within a bed, subjected to ball indentation on its free surface, are dependent on the indentation strain rate. These stresses are almost constant up to a dimensionless strain rate of unity, consistent with trends from traditional methods of shear cell testing, though fluctuations begin to increase from a dimensionless strain rate of 0.5. For dimensionless strain rates greater than unity, these stresses increase, with the increase in hardness being the most substantial. These trends correlate well with those established in the literature for the Couette device. However the quantitative value of the strain rate boundary of the regimes differs, due to differences in the geometry of shear deformation bands. Nevertheless, this shows the capability of the indentation technique in capturing the dynamics of cohesive powder flow. [ABSTRACT FROM AUTHOR]

The behavior of multiwall carbon nanotubes under a high pressure (up to 55 GPa) combined with shear deformation was studied by experimental and theoretical methods. The unexpectedly high stability of the nanotubes' structure under high stresses was observed. After the pressure was released, we observed that the nanotubes had restored their shapes. Atomistic simulations show that the hydrostatic and shear stresses affect the nanotubes' structure in a different way. It was found that the shear stress load in the multiwall nanotubes' outer walls can induce their connection and formation of an amorphized sp3-hybridized region but internal core keeps the tubular structure. [ABSTRACT FROM AUTHOR]

Highlights: [•] Formulation of gradient fatigue criteria to capture gradient and loading effects. [•] Gradients of the shear stress amplitude and hydrostatic stress are simultaneously introduced. [•] Gradient versions of Crossland and Dang Van criteria are provided. [•] Experimental observations of gradient and loading effects are reproduced. [ABSTRACT FROM AUTHOR]

Highlights: [•] The bct-5 Si has a great future in the fabrication of opto-electronic devices. [•] bct-5 Si can be generated by scratching the {100} surface along [110] direction. [•] Hydrostatic stress plays a key role in the initiation and development of bct-5 Si. [•] Residual octahedral shear stress of 6–8GPa is required to have stable bct-5 Si. [Copyright &y& Elsevier]

Due to the complex nature of the heterogeneous microstructures of short fiber composites, computationally predicting their mechanical behavior, especially past the elastic regime and into the damage initiation and progression regimes, is very challenging. Matrix cracking has notably been difficult to predict because it can propagate in different manners, such as fiber mediated interfacial cracking and conoidal cracking, which have not been well understood. Therefore, this work couples in-situ X-ray micro tomography experiments with a finite element simulation of the exact microstructure to enable a sub-fiber 3D microstructural study by tracking damage propagation and computing the local stresses and strains in the microstructure. Here we show the role of shear stress in interfacial cracking (and how it differs from debonding), the role of hydrostatic stress in conoidal cracking, and the role of environmental damage in longitudinal fiber breakage. In doing so, this work gives insight into the stress states resulting in non-linear damage propagation in thermoplastic fiber composites. [Display omitted] [ABSTRACT FROM AUTHOR]

Potassium dihydrogen phosphate (KDP) is an important material in the optics industry due to its excellent physical and optical properties. It is also a difficult-to-process material due to its brittleness and sensitivity to environment and external stresses such as contact sliding stresses, temperature variation and humidity change. This paper presents the first investigation into the nano-abrasion wear mechanisms of KDP using molecular dynamics (MD) analysis. Nanoscratching simulations were conducted with various scratching depths in various crystallographic orientations. It was found that the deformation mechanisms of KDP under nano-abrasion are greatly affected by the anisotropy of the material. The nano-abrasion on the (001) surface experiences the least resistance of material removal. The elastic recovery on the (100) surface is greater than that on the (001) surface. The coefficient of friction increases with increasing the scratching depth, and reaches the highest on the (001) surface. Different phase transformations emerge under contact stresses. The study revealed that the phase transformations are initiated under corresponding critical hydrostatic stresses with high shear stress. • The first investigation into the nano-abrasion wear mechanisms of KDP using molecular dynamics (MD) analysis was presented. • Nanoscratching simulations were conducted with various scratching depths in various crystallographic orientations. • The deformation mechanisms of KDP under nano-abrasion are greatly affected by the anisotropy of the material. • The phase transformations are initiated under corresponding critical hydrostatic stresses with high shear stress. [ABSTRACT FROM AUTHOR]

Molecular dynamics (MD) simulations were applied to study the fundamental mechanism of nanoscale grinding with a modeled tool trajectory of straight lines. Nevertheless, these models ignore curvature changes of actual tool paths, which need optimization to facilitate understanding of the underlying science of the machining processes. In this work, a three-dimensional MD model considering the effect of tool paths was employed to investigate distributions of stresses including hydrostatic stress, von Mises stress, normal and shear stresses during atomic grinding. Simulation results showed that average values of the stresses are greatly influenced by the radius of the tool trajectory and the grinding depth. Besides the averaged stresses, plane stress distribution was also analyzed, which was obtained by intercepting stresses on the internal planes of the workpiece. For the case of a grinding depth of 25 Å and an arc radius 40 Å, snapshots of the stresses on the X–Y, X–Z and Y–Z planes showed internal stress concentration. The results show that phase transformation occurred from α- silicon to β- silicon in the region with hydrostatic stress over 8 GPa. Moreover, lateral snapshots of the three-dimensional stress distribution are comprehensively discussed. It can be deduced from MD simulations of stress distribution in monocrystalline silicon with the designed new model that a curved tool trajectory leads to asymmetric distribution and concentration of stress during atomic-scale grinding. The analysis of stress distribution with varying curve geometries and cutting depths can aid fundamental mechanism development in nanomanufacturing and provide theoretical support for ultraprecision grinding. [ABSTRACT FROM AUTHOR]

Micro/nano-structured surfaces often present many interesting behaviors and lend themselves to advanced functions. To further improve the polishing performance of diamond abrasives, large scale MD (molecular dynamics) simulations are employed to investigate the ultra-precision mechanical polishing of monocrystalline silicon with laser-fabricated nano-structured diamond abrasive. The material removal mechanism during polishing using structured diamond abrasive is studied in detail. The effects of the structured depth, structured width, structured factor, and structured pattern on the material deformation are studied in detail by analyzing the surface morphologies of the polished surface, normal stresses, shear stress, polishing temperature, atomic coordination numbers, von Mises stress, hydrostatic stress, polishing force, and dislocation extraction algorithm (DXA). The simulation results show that a bigger structured width abrasive, smaller structured depth abrasive，higher structured factor abrasive, or rectangle pattern abrasive would cause a larger material removal rate. Moreover, the abrasive with a smaller structured width, smaller structured depth, or higher structured factor tends to polish silicon materials in a more ductile mode. The abrasive with a bigger structured width has a lower hydrostatic stress, better polished surface, and less defect atoms. However, the abrasive with a bigger structured width, smaller structured depth, or higher structured factor results in a larger normal polishing force. Polishing using an abrasive with a higher structured factor leads to more polishing heat. In addition, the abrasive with 33.3% structured factor polishing can reduce the tangential polishing forces. [ABSTRACT FROM AUTHOR]