Shear Stress Concentration Research Articles

Multiscale modeling of crystal plastic deformation of polycrystalline titanium at high temperatures

This paper presents a multiscale modeling framework based on microscale crystal plasticity theory and macroscale element-free methodology for computational simulation of polycrystalline metals. The element-free method employs an improved moving least squares (IMLS) shape function in the Ritz procedure for computational simulation. In the multiscale modeling scheme, the macroscale strain increments of polycrystalline metals are solved by the element-free method and transferred into the mesoscale grains through a scale transition law, and further transferred into the microscale slip systems through the Schmid’s law. The shear strain increments of all grains are then summed up and averaged to calculate the macroscale plastic strain increment. Functions of the temperature-related hardening parameters of the slip systems are determined by calibrating with experimental data. Then, tensile tests are performed on polycrystalline sheets to study their deformation behavior at elevated temperatures. The plasticity of polycrystalline sheets is found to be significantly affected by the grain size. The concentration of shear stress and additional hardening are occurred at the grain boundaries of soft and hard grains. Moreover, it is revealed that the yield strength of the polycrystalline sheets is greatly reduced as the hardening parameters and thermal softening factor decrease at elevated temperatures.

Experimental study on the interfacial shear stress of RC beams strengthened with prestressed BFRP rod

Abstract On the strengthening of concrete structures by externally bonding prestressed BFRP (basalt fiber reinforced polymer) rod, the release of the prestress may cause an interfacial shear stress concentrated at the prestressed BFRP ends which may cause BFRP prematurely debonding. In order to avoid this problem, the shear stress caused by BFRP prestress should not exceed the interfacial adhesive strength. In this study, the mechanical properties of the adhesive interface at the ends of the BFRP rod which is bonded to the concrete beams are evaluated by experimental studies considering different test methods such as the stretch adhesion test at both ends (double pull-pull test) and prestressed import test (single push-push test). Based on the existing research results, a modified formula is derived to predict the interfacial shear stresses, BFRP stresses and maximum achievable prestress level. The accuracy and validity of the proposed theoretical formula are confirmed by the experimental results. Moreover, an effective method of decreasing the interfacial shear stress is proposed for avoiding the possible premature failure near the BFRP ends caused by the shear stress concentration.

Elastic analysis of interfacial stresses in prestressedPFGM-RC hybrid beams

In this paper, the problem of interfacial stresses in damaged reinforced concrete beams strengthened with bonded prestressed functionally graded material plate and subjected to a uniformly distributed load, arbitrarily positioned single point load, or two symmetric point loads is developed using linear elastic theory. The adopted model takes into account the adherend shear deformations by assuming a linear shear stress through the depth of the damaged RC beam. This solution is intended for application to beams made of all kinds of materials bonded with a thin FGM plate. The results show that there exists a high concentration of both shear and normal stress at the ends of the functionally graded material plate, which might result in premature failure of the strengthening scheme at these locations. Finally, numerical comparisons between the existing solutions and the present new solution enable a clear appreciation of the effects of various parameters of the beams on the distributions of the interfacial stresses.

The Fundamental Regularities of the Evolution of Elastic Vortices Generated in the Surface Layers of Solids under Tangential Contact Loading

Conventionally discussed dynamic mechanisms of elastic strain energy redistribution in near-contact surface regions include P and S elastic wave pulses radiating from the contact surface. At the same time, the elastic strain energy can be transferred by localized vortex-like elastic waves (Rayleigh, Love, Stoneley wave, and so on). In the paper, we numerically studied the main features of the formation and propagation of localized vortex-like waves in the surface layers under the contact zone. The study was done using the numerical method of movable cellular automata. We showed that the initial phase of dynamic contact interaction with a nonzero tangential component of contact velocity is accompanied by the formation of a so-called elastic vortex. The elastic vortex is a fully dynamic object, which is characterized by shear stress concentration and propagates at the shear wave speed. We first revealed the ability of the elastic vortex to propagate toward the bulk of the material and transfer elastic strain energy deep into the surface layer in a localized manner. We analyzed the dependence of the direction of vortex propagation on the tangential contact velocity, contact pressure and Young’s modulus of the material. The results of the study are important for better understanding the dynamic mechanisms contributing to inelastic strain accumulation or gradual degradation of surface layers.

Wave-induced Maintenance of Suspended Sediment Concentration during Slack in a Tidal Channel on a Sheltered Macro-tidal Flat, Gangwha Island, Korea

A field campaign was conducted to better understand the influence of wave action, in terms of turbulence and bed shear stress, on sediment resuspension and transport processes on a protected tidal flat. An H-frame was deployed in a tidal channel south of Gangwha Island for 6 tidal cycles during November 2006 with instrumentation including an Acoustic Doppler Velocimeter, an Acoustic Backscatter System, and an Optical Backscatter Sensor. During calm conditions, the current-induced shear was dominant and responsible for suspending sediments during the accelerating phases of flood and ebb. During the high-tide slack, both bed shear stress and suspended sediment concentration were reduced. The sediment flux was directed landward due to the scour-lag effect over a tidal cycle. On the other hand, when waves were stronger, the wave-induced turbulence appeared to keep sediments in suspension even during the high-tide slack, while the current-induced shear remained dominant during the accelerating phases of flood and ebb. The sediment flux under strong waves was directed offshore due to the sustained high suspended sediment concentration during the high-tide slack. Although strong waves can induce offshore sediment flux, infrequent events with strong waves are unlikely to alter the long-term accretion of the protected southern Gangwha tidal flats.

Numerical study of crack propagation in an indented rock specimen

Indentations were simulated to investigate the stress evolution characteristics of the indentation process using the discrete element method (DEM). The maximum principle stress and the shear stress were recorded by applying measurement circle logic. The results indicate that an increase in indentation force contributes to the concentration of shear and tensile stresses at the crack tips. The indentation force decreases because of the crack propagation, which is accompanied by stress dissipation at the crack tips. In addition, tensile and shear-tensile cracks, propagating in different modes, have been observed. The results show that the shear-tensile cracks are responsible for chip formation.

Improvement of mechanical strength of sintered Mo alloyed steel by optimization of sintering and cold-forging processes with densification

Powder metallurgy (P/M) materials have been expected to be spread in automotive industry. Generally, since sintered materials using P/M ones contain many pores and voids, mechanical properties of them are inferior to those of conventional wrought materials. To improve mechanical properties of the sintered materials, densification is effective. The aim of this study is to improve mechanical strength of sintered Mo-alloyed steel by optimizing conditions in sintering and cold-forging processes. Mo-alloyed steel powder was compacted. Then, pre-sintering (PS) using a vacuum sintering furnace was conducted. Subsequently, coldforging (CF) by a backward extrusion method was conducted to the pre-sintered specimen. Moreover, the cold-forged specimen was heat treated by carburizing, tempering and quenching (CQT). Afterwards, mechanical properties were investigated. As a result, it was found that the density of the PS specimen is required to be more than 7.4 Mg/m3 to strengthen the specimen by heat treatment after CF. Furthermore, density and the microstructure of the PS specimen are most important factors to make the high density and strength material by CF. At the CF load of 1200 kN, the maximum density ratio reached approximately 99% by the use of the PS specimen with proper density and microstructure. At the CF load of 900 kN, although density ratio was high like more than 97.8%, transverse rupture strength decreased sharply. Since densification caused high shear stress and stress concentration in the surface layer, microcracks occurred by the damages of inter-particle sintered connection of the surface layer. On the contrary, in case of the CF load of 1200 kN, ultra-densification of the surface layer occurred by a sufficient plastic flow. Such sufficient compressed specimens regenerated the sintered connections by high temperature heat treatment and thus the high strength densified material was obtained. These processes can be applicable to near net shape manufacturing without surface machining.

Analysis of stress concentrations in transversely loaded steel-fiber composites with nano-reinforced interphases

Continuous steel fiber polymer composites are susceptible to high stress concentrations under transverse loading. This is due to the high mismatch in elastic properties of steel fiber and polymeric matrix. In the present work, the effect of a nano-reinforced interphase on the stress concentrations is evaluated. Three types of nano-reinforcements are considered: 1) carbon nanotubes (CNTs) grown on fibers and 2) graphene nano-plates deposited on the fiber surface and 3) graphene nano-plates randomly distributed in the matrix. The study is carried out using finite element (FE) analysis with an intermediate homogenization step to obtain elastic properties of the nano-reinforced interphase. These properties are calculated using an inclusion based approach. Results indicate that the presence of the nano-reinforced interphase increases radial stress concentrations and those in the direction of the load. The suppression is only observed for shear stress concentrations in the case of the CNT interphase around the fibers.

COHESIVE FINITE ELEMENT SIMULATIONS OF CONTRACTION AND SHAPE EFFECTS ON CELL DE-ADHESION

Cohesive-interface-based finite element simulations were conducted to investigate the critical shear stress required for cell de-adhesion from extracellular substrates. The interface ligand–receptor bonds are modeled by a cohesive interface model with initial stiffness, interface strength, and fracture energy as the governing parameters. The ratio of the cell modulus to the interface stiffness defines a length scale. If this length is much less than the contact size, the de-adhesion process can be modeled by the linear elastic fracture mechanics, while the opposite limit leads to the concurrent sliding of the cell or, equivalently, debonding of all the interface ligand–receptor pairs. Since it generates additional shear-stress concentration along the interface, cell contraction generally reduces the critical de-adhesion stress. Cell de-adhesion is more prone to take place for three-dimensional irregular cell shapes because of the much easier failure in the anti-plane Mode III shear, as well as the additional stress concentration in these geometric irregularities.

Stress Evolution of Rock-Like Specimens Containing a Single Fracture Under Uniaxial Loading: a Numerical Study Based on Particle Flow Code

Stress concentrations are responsible for the crack propagation in rock and rock-like specimens. To investigate the stress evolution characteristics of the rock-like specimens containing a single prefabricated fracture, a numerical study based on the particle flow code was conducted. The numerical results indicate that the concentrated tensile stress and shear stress are responsible for the wing and secondary crack propagation. At the early compression stage, wing cracks tend to form by the concentrated tensile stress, whereas the secondary crack forms by the shear plane resulting from the shear stress concentration at later compression stage. Additionally, the increase in the inclination angle restrains the wing crack propagation. Moreover, in the propagation process of the typical wing and secondary cracks, sudden decreases of the tensile and shear stresses are observed. These decreases at the former monitor points are always accompanied with the sharp increases at the latter points. Furthermore, the results show that the wing and secondary cracks develop in different manners. The large axial strain gaps between adjacent monitor points for wing crack propagation indicates that the wing crack propagates in a stable and slow way, whereas the significantly reduced gaps indicate that the secondary crack propagates in a speedy and unstable way.

Shear band formation and wear mechanisms of Ti-6Al-4V powder metallurgy materials with different densities

Ti-6Al-4V powder metallurgy materials with different densities were prepared by hot isostatic pressing technology. Shear band formation and wear mechanisms of the powder metallurgy materials were studied. The wear mechanisms of the powder metallurgy material were revealed that the micro-void torn and accumulated built-up edge in sticking friction and sticking zones, respectively. The wear mechanism of tool rake face was varied when the material density deceased. The shear plane shifted to the residual micro-void, which made a difference on fluctuation of cutting force and caused the appearance of severe micro-fluctuation. The micro-fluctuation of cutting force was weakened with decreasing the material density. Therefore, occasional random shear model of the powder metallurgy material was verified, and the progressions of chip morphology characteristic parameters with material density were studied, the micro-void on improved shear stress concentration. It could be concluded that the shear band mechanism of serrated chip in machining of titanium alloy was a catastrophic shear.

Mechanism Analysis of the Use of LSPM for Reflection Crack Prevention of Semi-Rigid Base Asphalt Mixture

The damage mechanism of reflective cracking, particularly on semi-rigid asphalt pavement with large stone porous asphalt mixture (LSPM) is investigated in this study.An asphalt pavement model with multilayered base (i.e., LSPM and semi-rigid layer) using discrete element methodis developed.This study analyzes the influence of the inner structure of asphalt mixture on the stress field around the crack tip, which is under the vehicle load after the cracking of a semi-rigid layer. The focus is on the mechanism of cracking and the influence of the inner structure of the mixture, such as gradation, the geometric feature of coarse aggregate, and space.The crack expansion rate is also analyzed.Results show that, under vehicle load, the crack in the semi-rigid layer causes stress concentration, which decreases around the bottom of the LSPM.The load on the top of the crack tip produces stress level concentration, the unbalanced load produces shear stress concentration, and the stress decreases progressively from the crack tip to the surrounding area. In the LSPM, a large grain size of the coarse aggregate is better than a small grain size. Its overall stress level is low and the crack development rate is slow on the stress field of the crack tip. The LSPM can reduce stress concentration and prevent the development of reflective cracking effect.

Shear deformation of fold structures in coal measure strata and coal-gas outbursts: Constraint and mechanism

The coal measure strata formed in the late Paleozoic era underwent tectonic movements on several occasions, leading to the development of a range of fold structures in the strata as a result of highly interlayered gliding and shearing deformation. In addition, brittle ductile tectonic coals were widely distributed in the reservoir, constituting one of the primary causes of coal-gas outbursts in Chinese mines. This indicates a strong connection between shear deformation and such outbursts. In this study, structural geology, gas geology, and rock mechanics are all taken into consideration to investigate the controlling effects of the fold structure formation process on coal thickness and tectonic coal formation. Numerical simulation, based on the stress test data, was deployed to identify the stress distribution law adjacent to fold structures under the modern stress field. Additionally, the mechanical mechanism underpinning coal-gas outbursts near the fold structure was determined by making a comparison with the distribution law relating to such outbursts. The results demonstrate that the formation and evolution of the fold structure not only form the material basis of outbursts but also control their power source. During the fold formation process, interbed sliding and shearing between strong and weak rock strata were caused by differences in the mechanical properties of the coal bed and rock layer, resulting not only in a change to the local thickness of the coal seam but also in its deformation and structural alteration. Interbed shearing and local stress concentration, caused by the coal gangue, led to coal damage and the development of layered tectonic coal of consistent thickness, simultaneously improving its ability to adsorb gas and providing the material basis for coal-gas outbursts. This reduced the seam’s capacity to resist such events. The conditions for these outbursts are created by the sudden desorption of excess gas as a result of formation pressure release during coal mining and the widespread distribution of tectonic coal. Under the modern tectonic stress field, the stress distribution characteristic is controlled by the fold structure shape; and because of the aforementioned differences in the mechanical properties of the coal bed and rock layer, the interlayer deformation is asynchronous. This causes shear stress concentration within a specific range of the anticline’s two wings. This concentration zone happens to be exactly aligned with that of coal-gas outbursts, meaning that shear stress concentration is considered to be both the power source for and main cause of the region’s outbursts.

Optimization study on roof break direction of gob-side entry retaining by roof break and filling in thick-layer soft rock layer

This paper proposes gob-side entry retaining by roof break and filling in thick-layer soft rock conditions based on the thick-layer soft rock roof strata migration law and the demand for non-pillar gob-side entry retaining projects. The functional expressions of main roof subsidence are derived for three break roof direction conditions: lateral deflection toward the roadway, lateral deflection toward the gob and vertically to the roof. These are derived according to the load-bearing boundary conditions of the main roadway roof stratum. It is concluded that the break roof angle is an important factor influencing the stability of gob-side entry retaining surrounding rock. This paper studies the stress distribution characteristics and plastic damage scope of gob-side entry retaining integrated coal seams, as well as the roof strata migration law and the supporting stability of caving structure filled on the break roof layer at the break roof angles of -5°, 0°, 5°, 10° and 15° are studied. The simulation results of numerical analysis indicate that, the stress concentration and plastic damage scope to the sides of gob-side entry retaining integrated coal at the break roof angle of 5° are reduced and shearing stress concentration of the caving filling body has been eliminated. The disturbance of coal mining to the roadway roof and loss of carrying capacity are mitigated. Field tests have been carried out on air-return roadway 5203 with the break roof angle of 5°. The monitoring indicates that the break roof filling section and compaction section are located at 0-45 m and 45-75 m behind the working face, respectively. The section from 75-100 m tends to be stable.

Morphology evolution and micro-mechanism of chip formation during high-speed machining

In this paper, the morphology and micro-mechanism of chip formation during high-speed machining aluminum alloy 7050-T7451 is investigated based on the combination of dislocation theory and plastic deformation theory. Experiments of quick stop stoppage for turning and special method (Buda) for milling process were carried out in order to obtain shear angle in different cutting speeds. The results show that effective flow stress and temperature in front edge zone is higher and more concentrated than that in other deformation zones. The shear front-lamellar structure was observed and analyzed in the front edge zone which influences the chip formation directly. The influence of cutting speed on chip formation was analyzed by simulation and experiments. Cutting speed is an important factor affecting the morphology evolution and chip formation. When the cutting speed is below 1500 m/min, the concentration of shear stress and the shear front-lamella structure of cutting deformation are more remarkable and easier for forming continuous ribbon chips. With the cutting speed increase, the ribbon chip transforms into serrated chip when a critical cutting speed (2500 m/min) is reached. Finally, microscopic mechanism of chip formation has been revealed and critical condition of the shear front—the layer structure formation—has been determined.

Photonics and fracture toughness of heterogeneous composite materials

Fracture toughness measures the resistance of a material to fracture. This fundamental property is used in diverse engineering designs including mechanical, civil, materials, electronics and chemical engineering applications. In spite of the advancements made in the past 40 years, the evaluation of this remains challenging for extremely heterogeneous materials such as composite concretes. By taking advantage of the optical properties of a thin birefringent coating on the surface of opaque, notched composite concrete beams, here we sense the evolution of the maximum shear stress distribution on the beams under loading. The location of the maximum deviator stress is tracked ahead of the crack tip on the experimental concrete samples under the ultimate load, and hence the effective crack length is characterised. Using this, the fracture toughness of a number of heterogeneous composite beams is evaluated and the results compare favourably well with other conventional methods using combined experimental and numerical/analytical approaches. Finally a new model, correlating the optically measured shear stress concentration factor and flexural strength with the fracture toughness of concretes is proposed. The current photonics-based study could be vital in evaluating the fracture toughness of even opaque and complex heterogeneous materials more effectively in future.

Two types of electric field enhancements by infinitely many circular conductors arranged closely in two parallel lines

In stiff fiber-reinforced composites, high shear stress concentrations occur in narrow regions between neighboring fibers. Electric fields and conductors correspond to shear stresses and cross-sections of fibers respectively in the anti-plane shear model. Due to material failure of composites, there have been intensive studies so far to estimate an electric field in between only a finite number of conductors. Indeed, the composites contain a large number of stiff fibers, and the concentration can be strongly enhanced by some combinations of inclusions. Two types of enhancements in parallel and perpendicular directions by a combination of infinitely many inclusions in a line are the main subjects of this paper. We thus consider the electric fields in between a infinite number of perfectly conducting unit disks arranged closely and regularly in two parallel lines. Asymptotes and optimal blow-up rates for the fields in two kinds of narrow regions are obtained in terms of the distances between conductors. In particular, very strong enhancement in the parallel direction is exhibited to have the blow-up rate substantially different from the existing result in the case of finite inclusions.

The mechanical and thermal responses of colliding oxide-coated aluminum nanoparticles

The aggregation and fracture of oxide-coated metal nanoparticles have a significant influence on their physical and chemical behaviors in synthesis, combustion, or detonation. How does the dynamic loading caused by the impact between nanoparticles affect them? Motivated by this issue, we carried out molecular dynamic simulations of two colliding aluminum nanoparticles to investigate their mechanical and thermal properties and response at impact velocities of 200 m/s, 600 m/s, 1000 m/s, and 2000 m/s. At the relatively low impact velocities (equal to or less than 1000 m/s), it was observed that the particles are mildly deformed and adhere to each other, but the shells do not undergo fracture under the dynamic loading. The metal core and oxide shell behave elastically at 200 m/s and elasto-plastically at 600 m/s. A concentration of dynamic volumetric stress appears but no concentration of shear stress and no formation of a hot spot. Due to the low intensity of the loading and the efficient propagation of the stress wave, the shells fail to fracture. At an impact velocity of 2000 m/s, the impact region of the shell undergoes ductile fracture, and the two particles undergo sintering and form a new particle re-coated with oxide, resulting from the concentration of dynamic shear stress and the formation of a hot spot. At all impact velocities in our simulations, the impact between the nanoparticles improves the aggregation but has a little effect on the fracture of the oxide shell.

Pull-out behavior of straight steel fibers from asphalt binder

The interaction between a straight steel fiber and the surrounding asphalt matrix is investigated through single fiber pull-out tests and numerical simulations. Based on experimental observations from 240 pull-out tests for various temperatures, displacement rates, and fiber dimensions, pull-out failure modes are classified into three types: matrix, interface, and mixed failure modes. The experimental data suggests that there is a critical shear stress beyond which fiber-binder interface debonding will occur. Detailed finite element analyses employing a nonlinear viscoelastic constitutive model for asphalt are carried out to explain the observed test results. The numerical simulations show that the stress distribution along the fiber surface varies substantially with temperature and fiber dimensions. At −20°C, the concentration of interfacial shear stress becomes significant as the embedded aspect ratio of fibers becomes higher, and causes a decrease in the value of average interfacial shear stress at the onset of the interface debonding. The simulations also showed that this stress concentration decreases at 0°C as the contribution of viscous behavior becomes more significant. The simulation results confirm that the shear bond strength between steel fiber and asphalt binder is 6.9MPa, and it is independent of temperature, loading rate, and fiber dimensions within the ranges used in the study.

Analysis of Early Crack at Bottom of Cement Concrete Pavements Slab

During hardening behavior of concrete, shrinkage is induced. While, new poured pavements slab is subject to constraint by subsurface structure which always arouses tensile and shear stress concentration at end-bottom of slab. Employed finite element method, this paper aimed at stress analysis of slab bottom during hardening, and derived the disciplinarian of nonloaded crack formation at bottom of slab.