Tensile Stress Distribution Research Articles

Flexural strength test and meso-mechanical evolution behavior of cement concrete based on discrete element method

Each damage of cement mixture is closely related to the heterogeneity and discontinuity of its internal materials. It is very important to analyze the mechanical evolution behavior of cement mixture with the theory of meso-mechanics to reveal the failure mechanism of cement mixture structure. In this paper, the microscopic parameters of the discrete element model are obtained in the compressive tests of concrete mixture cubes. The three-dimensional discrete element model of cement mixture was established by using these microscopic parameters. Under the action of a certain rate load, the generation and propagation process, stress distribution, transfer and displacement field evolution behavior of the meso-fracture between the cement mixture particles are analyzed. The results show that the error between the simulated discrete element flexural strength test and the actual standard test (flexural strength, micro-strain at the bottom of the specimen) is within 10%, which indicates that the discrete element method can well simulate the discontinuous and heterogeneous materials. When the external load acts, the horizontal and vertical displacement of the cementitious material is greater than that of the coarse aggregate, and the displacement angle of the coarse aggregate is greater than that of the cementitious material. When the horizontal tensile stress between the particles exceeds the bond strength of the material, the bond between the particles will break. There are few micro-cracks at the initial stage. In late stage, the micro-cracks penetrate each other and form larger cracks. Micro-cracks usually occur at the weak bonding position between coarse aggregate and cementitious material. The upper particle changes from compressive stress to tensile stress gradually with the duration of load. The bottom particles are in the non-uniform distribution of tensile stress and compressive stress, and the stress values between the particles are also in the non-uniform distribution.

Numerical investigation of the tensile strength of loess using discrete element method

The tensile strength of loess is important for the tension-related crack and failure in the Loess regions. While the previous work on the tensile strength of loess mainly focuses on the test method and its variation with different internal and external factors, the micro-mechanical characteristics of the tensile experiments are seldom studied. Taking the unconfined penetrated test (UP test) as example, the tensile strengths and internal stress distributions of undisturbed loess samples under the loading plate-sample ratios (d/D) ranging from 0.05 to 0.95 are numerically investigated using discrete element method. The internal stress distribution, failure pattern of sample and variation of internal crack are clearly reflected by the discrete element modelling. The DEM simulation results also show that: 1) the d/D in the range of 0.15 to 0.35 is recommended for measuring the tensile strength of undisturbed loess; 2) the tensile strength of undisturbed loess increases with d/D recommended above ;3) the failure of sample is mainly controlled by the tensile stress when the d/D is equal or lower than a critical d/D 0.50, while that is mainly controlled by the compressive stress when the d/D is larger than a critical d/D 0.50. This study shows the capability of discrete element method to simulate the UP test, thereby providing us a relatively reliable way to investigate the tensile strength of loess numerically and to explore the internal stress and crack distribution of soil sample.

Research on material removal mechanism and radial cracks during scribing single crystal gallium nitride

A diamond conical indenter with the cone angle of 60° and a tip arc radius of 5 μm was used to perform the gradual loading scribing experiments on gallium nitride (0001) crystal plane along the (Li et al., 2019; Wang et al., 2018; Yao et al., 2020; Fang and Zhang, 2013; Feng et al., 2009; Zhang et al., 2007; Jing et al., 2007; Yonenaga et al., 2018; Nowak et al., 1999; Shimada et al., 1998) [11-20] crystal orientation, and the material removal mechanism and machining properties of gallium nitride are studied. Single crystal gallium nitride will have the surface and sub-surface damage during machining, and the study of the stress field during scribing can effectively analyze and control the surface and sub-surface damage. Therefore, the stress field for scribing gallium nitride is acquired by superposing the boussinesq field, cerruti field and sliding blister field which have been led-in anisotropy parameters. Then on the basis of the distribution of the maximum tensile stress on the scribing surface, the nucleation and initial spread angle of radial cracks under our experimental conditions are analyzed, and for gallium nitride, the more severe the brittle spalling accompanying radial crack spread is, the greater the initial spread angle of radial cracks is.

Insight into Thermal Stress Distribution and Required Reinforcement Reducing Early-Age Cracking in Mass Foundation Slabs.

The heat released during cement hydration results in temperature-induced non-uniform volume changes in concrete structures. As a consequence, tensile thermal stresses of significant values may occur. The level of these stresses can be lowered by using various technological measures during the construction process and a proper concrete mix composition. Nevertheless, the application of an appropriate reinforcement is a reliable method for controlling the width and spacing of possible cracks. The rules for calculating this reinforcement are not precisely detailed in the standards devoted to concrete structures. Additionally, the correct calculation of the reinforcement requires the identification of the tensile stress distribution in a mass slab. The presented study provides insight into stress distribution and relevant reinforcement for controlling early-age cracks of thermal origin. The existing standards and guidelines are discussed and clarified. The possible paths for calculating the reinforcement are proposed through the example of mass foundation slabs with different levels of external restraints. The results indicate a significant impact of the calculation method as well as the restraint conditions of the slab on the area of required reinforcement.

Mechanical Behavior of the Reinforced Retaining Wall Subjected to Static Load

To study the mechanical behavior and influence factors of the reinforced retaining wall under the static load, numerical simulation of the reinforced retaining wall is conducted by finite element analysis, and its mechanical behavior and influencing methods are studied in accordance with relevant theories. The results showed that the properties of back fill, reinforced spacing, reinforced stiffness, reinforced length, and panel stiffness all affect the mechanical behavior of retaining walls. According to the example calculations of different wall heights, the distribution of panel horizontal displacement and maximum tensile stress are analyzed. The gravel with good gradation has better durability and can reduce the amount of reinforcing steel; with the decrease of the reinforcement spacing, the deformation of the wall panel will become smaller, and the reinforcement effect will be improved; the length of reinforcement is not the longer the better, and the deformation of wall panel can be minimized at the suitable length; the larger the elastic modulus of the wall panel, the smaller the deformation of the wall panel will be.

Research on Dynamic Properties of Ancient Masonry Pagoda with Solid Structure in China

ABSTRACT In order to study the dynamic properties of the pagoda with solid structure when affected by an earthquake, take the Xuanzang Pagoda as an example which is one of the World Cultural Heritage Architecture Sites. An 1/8-scale model was constructed for a shaking table test. The principle properties of the structural damage were observed. The natural vibration period of the structure and the acceleration and displacement responses excited by the seismic waves were measured. The structure mode was calculated with the Finite Element Method. The fundamental mechanical parameters were determined with reference to the test result. The dynamic responses of the structure were calculated, and the simulation results were compared with the test. The relationship between the structural acceleration, the displacement response, and the height was analyzed. The distribution of the principle tensile stresses and the equivalent plastic strain was compared with the cracks in the structure. The calculated results of the acceleration and displacement amplification coefficient versus the height agreed with the test results. The dynamic amplification coefficient at the floors depended on the earthquake intensity, load direction, and seismic wave formation. For the cracks of masonry that had a certain influence on the structure dynamic response, the result values determined with the Finite Element Method differed a little from the test results. The research conclusions can provide references for the Xuanzang Pagoda to defend against seismic disasters.

Compression-Induced Tensile Mechanical Behaviors of the Crystalline Rock under Dynamic Loads.

Characterization of the tensile mechanical behaviors of rocks under dynamic loads is of great significance for the practical engineering. However, thus far, its micromechanics have rarely been studied. This paper micromechanically investigated the compression-induced tensile mechanical behaviors of the crystalline rock using the grain-based model (GBM) by universal distinct element code (UDEC). Results showed that the crystalline rock has the rate- and heterogeneity-dependency of tensile behaviors. Essentially, dynamic Brazilian tensile strength increased in a linear manner as the loading rate increased. With the size distribution and morphology of grain-scale heterogeneity weakened, it increased, and this trend was obviously enhanced as the loading rate increased. Additionally, the rate-dependent characteristic became strong with the grain heterogeneity weakened. The grain heterogeneity prominently affected the stress distribution inside the synthetic crystalline rock, especially in the mixed compression and tension zone. Due to heterogeneity, there were tensile stress concentrations (TSCs) in the sample which could favor microcracking and strength weakening of the sample. As the grain heterogeneity weakened or the loading rate increased, the magnitude of the TSC had a decreasing trend and there was a transition from the sharp TSC to the smooth tensile stress distribution zone. The progressive failure of the crystalline rock was notably influenced by the loading rate, which mainly represented the formation of the crushing zone adjacent to two loading points. Our results are meaningful for the practical engineering such as underground protection works from stress waves.

Cracking of composite fiber-reinforced concrete foundation slabs due to shrinkage

Industrial concrete floors made directly on the ground are concrete slabs reinforced with structural reinforcement and often additionally reinforced with dispersed reinforcement in the form of polypropylene or steel fibers. More often than not, these floors are not separated from the reinforced concrete slab; they form a monolithic whole with it. Due to high variable loads and additional pressure of groundwater, in the case of foundation slabs in underground garages, floors ought to be subject to precise static and strength calculations. However, the load from concrete shrinkage is often neglected, as it is considered to appear temporarily in concrete care phase and to disappear gradually when the slab is normally exploited. In the recipe of the concrete mix itself, the composition of aggregate, plasticizing additives, the amount of mixing water and the addition of dispersed reinforcement should be carefully selected in order to minimize concrete shrinkage. The quality of the flooring also has a huge impact upon the course of shrinkage and the distribution of tensile stresses in the floor. The article presents the main errors related to the appearance of excessive cracking caused by shrinkage stress.

Investigations of internal stresses in high-voltage devices with deep trenches

Deep trenches, as essential elements of silicon chips used in electronic high-power and high-frequency devices, are known as starting points for dislocation generation under the influence of internal mechanical stresses resulting mainly from the difference in the thermal expansion coefficients between silicon and silicon dioxide. Since the electrical insulation of the devices requires a sequence of mechanical, chemical, and high-temperature processes during the preparation of the deep trenches, including the formation of an amorphous SiO2 edge layer, the emergence of the internal stresses is hardly avoidable. The method of cross correlation backscattered electron diffraction in the scanning electron microscope is used here to quantitatively determine the magnitude and local distribution of internal stresses in silicon around the deep trenches after four different process steps. For this purpose, Kikuchi diffraction images are recorded of the wafer cross section areas along lines perpendicular and parallel to the deep trenches. After Fourier transformation, these images are cross correlated with the Fourier transform of the diffraction image from a stressfree reference sample site. The well-established numerical evaluation of cross correlation functions provides the complete distortion tensor for each measuring point of the line scan, from which the stress tensor can be calculated using Hooke's law. It is found that the in-plane normal stress component σ11 perpendicular to the long edges of the deep trench is larger than the other stress components. That means it essentially determines the magnitude of the von-Mises stress, which was determined as a general stress indicator for all measuring points, too. A characteristic feature is the local distribution of the stress component σ11 with maximum tensile stresses of some hundred megapascals at transition between Si and amorphous SiO2 on the long edges of the deep trench, and with even higher maximum compressive stresses immediately below the bottom of the deep trench. At a distance of about 2 μm from the edges of a single deep trench, all stress components decrease to negligibly small values so that steep stress gradients occur. The range and distribution of tensile and compressive stresses are in accordance with finite element simulations; however, the measured stresses are higher than expected for all investigated states so that dislocation formation seems to be possible. The influence of the electron acceleration voltage on the determination of the internal stresses is discussed as well.

Study on the Stability of the Geogrids-Reinforced Earth Slope under the Coupling Effect of Rainfall and Earthquake

This paper focuses on understanding the dynamic response problem of flexible wrapped reinforced Earth slope under the coupling effect of earthquake and rainfall; a numerical calculation model of reinforced Earth slope considering the coupling effect of earthquake and rainfall was established. The dynamic response, pore pressure, and tensile stress distribution of the reinforcement under the rainfall before earthquake, the rainfall after the earthquake, and earthquake-rainfall are studied. The results show that the coupling effect of earthquake and rainfall is an influential factor in the dynamic analysis of reinforced Earth slopes, the analysis of which should be paid attention to and researched in the future. The combination of geogrid and soil effectively improves the deformation of the slope and the overall stability, reduces the secondary disaster of the slope, and provides a reference for the seismic construction design of the reinforced Earth slope.

Numerical Study of Damage to Rock Surrounding an Underground Coal Roadway Excavation

In coal mines, underground roadways are required to transport coal and personnel. Such tunnels can become unstable and hazardous. This study simulates deformation and damage in the rock surrounding a shallow coal seam roadway using particle flow code. A numerical model of particle flow in the surrounding rock was constructed based on field survey and drilling data. Microcharacteristic indices, including stress, displacement, and microcrack fields, were used to study deformation and damage characteristics and mechanisms in the surrounding rocks. The results show that the stress within the rock changed gradually from a vertical stress to a circumferential stress pattern. Stress release led to self-stabilizing diamond-shaped and X-shaped tensile stress distribution patterns after the excavation of the roadway. Cracking increased and eventually formed cut-through cracks as the concentrated stress transferred to greater depths at the sides, forming shear and triangular-shaped failure regions. Overall, the roof and floor were relatively stable, whereas the sidewalls gradually failed. These results provide a reference for the control of rock surrounding roadways in coal mines.

Pullout Behavior of Sensor-Enabled Geobelts in Weathered Rock Material–Granulated Rubber Mixtures

As lightweight materials, granulated rubber–weathered rock material (WRM) mixtures could be utilized as backfills in geosynthetics reinforcement structures to reduce deformations. However, the deformation behaviour of geosynthetics in granulated rubber–WRM mixtures is not clear. In this paper, pullout tests considering different normal pressures (30 kPa, 50 kPa and 100 kPa) and different granulated rubber contents (10%, 15% and 30%) are performed. Sensor-enable geobelts (SEGB) are employed, which can realize distributed measurements on the strains of SEGB inside soil. Two constitutive models—bilinear model and hyperbolic model are established based on tensile tests and direct shear tests to capture the full stress–strain curves and the interfacial shear responses, respectively, and are employed in the derivation of load transfer equation for pullout process. Numerical results of load transfer equation give distributions of tensile forces, strains, displacements and shear stresses. Test results validate the established constitutive models. Rubber content has an optimal value for the best geobelt–soil interaction. The deformations of geobelts in pullout process start from the front end, but decrease rapidly in soil. Under high normal pressures, the front part of geobelts exhibit quasi-plasticity. The shear stress distributes more uniformly under low normal pressure.

Numerical Analysis of Multi-pass Cold Spinning of Superalloy of Cylindrical Part

In view of the poor plasticity and easy cracking of superalloy at room temperature, the multi-pass cold spinning of cylindrical part was studied. Firstly, the tensile test of superalloy GH3030 is carried out, and the true stress-strain curve is obtained. Then, the numerical models of 250 and 87.5 diameter-thickness ratio blank are established, and the stress and strain field, wall thickness distribution and tool force distribution of the cylindrical part are analyzed and compared. The wrinkle model of 250 diameter-thickness ratio blank was analyzed, and the experiment of 87.5 diameter-thickness ratio blank was carried out. The results show that the wall thickness of the cylindrical part has metal accumulation at the flange, and the thickness of the waist is very small, which is easy to crack under the repeated forming of the roller; the wall thickness distribution of the large diameter-thickness cylindrical part is more uniform, but it has a great challenge to the equipment force and the equipment stiffness. The main reason for the flange wrinkle is staggered distribution of the tangential tensile and compressive stress of the whole flange.

Microstructure, Mechanical Properties, and Residual Stress Distribution of AISI 316L Stainless Steel Part Fabricated by Laser Metal Deposition.

In this paper, AISI 316L stainless steel part is obtained by laser metal deposition additive manufacturing method. The microstructure of the part was observed and analyzed by an optical microscope. The tensile mechanical properties and residual stress distribution of the part were tested by tensile test and the contour method. The results show that the bulk structure is mainly columnar crystal and equiaxed crystal, and the latter layer of laser metal deposition will form a remelted zone and heat-affected zone in the former deposition zone. Tensile test results show that the tensile strength of tensile specimens parallel to laser scanning direction and perpendicular to laser scanning direction is basically the same, but the elongation of the specimens perpendicular to the laser scanning direction is relatively better. The main reason is the different distribution characteristics of columnar crystals and equiaxed crystals in the two directions. Relatively large deformation occurs on the cut surface of the specimen after low-speed wire cut. The residual stress test results indicate that tensile stress is formed in the upper part and it reaches 315 MPa at the top surface. And compressive stress is formed at the part/substrate interface and the substrate.

Internal resonance and stress distribution of pipes conveying fluid in supercritical regime

Internal resonance and stress distribution of pipes conveying fluid at the supercritical flow around curves have been firstly investigated, with the goal of improving the mechanical fatigue properties of such pipes. Axial pre-pressure as a way of regulating the onset and growth of 1:3 internal resonance has been examined. Based on the direct multi-scale method and the Galerkin method, an approach has been proposed to analyze the nonlinear gyroscopic elastic system with time-dependent nonlinear perturbations. The tensile, bending, and resultant vibratory stress distributions of the pipe system have been determined in the presence of 1:3 internal resonance. Internal resonance degrading fatigue properties of the pipes has been highlighted. The numerical simulations support the analytical results. Then the analytical frequency responses are employed to identify factors that govern the internal resonance and stress distribution and to offer explanations as to why improving fatigue life is accompanied by suppression in the internal resonance. The boundary for incurring internal resonance suggests that appropriate axial pre-pressure should be modified in the presence of 1:3 internal resonance at different fluid velocities.

Three‐dimensional discrete element simulation of indirect tensile behaviour of a transversely isotropic rock

SummaryThis paper presents the development of a three‐dimensional discrete element model using flat‐joint and smooth‐joint contact models to investigate the effect of anisotropy on the tensile behaviour of slate, a transversely isotropic rock, under Brazilian testing from both macro and microscales. The effect of anisotropy is further realised by exploring the influence of foliation orientations (β and ψ) on the tensile strength, fracture pattern, microcracking and stress distribution of the transversely isotropic rock. The variation of tensile strength with foliation orientation is presented. The cross‐weak‐plane fracture growth observed in laboratory is reproduced, and the criterion for which to form is also given from the aspect of foliation orientation. Furthermore, the proportional variations of microcracks well account for the effects of foliation orientation on the tensile strength and failure pattern. Finally, it is found that the existence of weak planes increases both the heterogeneity and the anisotropy of stress distributions within the transversely isotropic rock, with the degree of influence varying with the foliation orientation.

Investigation of surface integrity in laser-assisted machining of nickel based superalloy

While laser-assisted machining can significantly improve the machinability of nickel-based superalloy, the mechanism of surface integrity evolution and its influence on the material functional performance is still not clear. The present study gives a comprehensive investigation on the surface integrity of laser-assisted milling (LAMill) process with an in-depth study of the mechanism of chip formation, microstructural and mechanical alternations, supported by key outcomes from the two constitutive processes, conventional milling (CMill) and single laser scanning (LS). Although the high thermal affected layer in LAMill process has been removed through the cutting chips, a significant bending effect has been found in both the LAMill and LS workpiece. More interestingly, a combined impact of the residual stress from LS and CMill has been found on LAMill workpiece while a lattice evolution has been revealed regarding both the thermal and mechanical influence. Specifically, inadequate fatigue performance on LAMill and LS workpiece has been found due to the high thermal effect in the superficial layer regarding the residual tensile stress distribution and microstructure variation. While LAMill is generally considered as a promising machining method with improved machinability of difficult-to-cut materials, this research shows a poor workpiece functional performance (fatigue) and justifies its application prospect.

A simulation investigation on elliptical vibration cutting of single-crystal silicon

In recent years, elliptical vibration cutting (EVC) has been proven as a promising technique to improve the cutting performance of brittle materials like silicon. However, the influence of vibration parameters on the machined surface quality has not been explored clearly. In this paper, molecular dynamics simulation was carried out to explore the machinable mechanism of silicon by EVC. Firstly, the difference in material removal performance between ordinary cutting (OC) and EVC was discussed. Then, the influence of two critical parameters in EVC, i.e., the vibration amplitude in the depth of cut (DOC) direction and the nominal DOC, on the machined surface quality was explored in detail. The simulation results demonstrate that the vibration amplitude in the DOC direction has a significant influence on the compressive stress and tensile stress distribution, which affects the phase transition and chip pulling-up motion in the EVC process. Based on the simulation results and previous experimental achievements, the high-quality surface can be obtained when the amplitude ratio in the nominal cutting/DOC direction is about 3.5. Furthermore, when the nominal DOC is increased, although the variation of tensile stress is quite small, the increased chip pulling-up distance is responsible for brittle fracture on the machined surface.

3D DIC-assisted residual stress measurement in 316 LVM steel processed by HE and HPT

A method has been developed for determining residual stress based on displacement fields near drilled holes analyzed using 3D digital image correlation. Finite element modeling was used to determine corrections for analytical equations describing displacement fields near the blind holes, which made it possible to determine the residual stress distribution over a wide range of hole depth-to-hole diameter ratios and various areas of displacement field measurements using inverse method iterative calculations. The proposed method eliminates many drawbacks of traditional procedure based on strain gauges as hole eccentricity sensitivity and requirement of the relatively large span between holes. The method and testing setup, build-up of generally available components, were used to determine the residual stress distribution for 316 LVM samples processed by two methods from the large deformation group: hydrostatic extrusion (HE) and high-pressure torsion (HPT), by drilling 1.75 and 0.58-mm-diameter blind holes, respectively. In the case of the measurements performed on the surface of a HE-processed 16 mm bar cut along its diameter, a gradual change was revealed—from a compressive to a tensile residual stress distribution (from ~ − 300 MPa in the center to 400 MPa in 4 mm distance from the edge) in the longitudinal direction, with near-zero values in the radial direction. Moreover, the method was also adapted to perform measurements on the outside surface of the bar, which gave results consistent with those taken along the radius profile (~ 600 MPa longitudinal stress). Measurements on the top surface of a cylinder 10 mm in diameter and 1 mm high processed by HPT showed a high compressive residual stress in the center and a dominant shear component for the holes drilled at different distances from the center.

Multi-Body Dynamics Simulation of Hoisting Wire Rope and Its Stress Analysis

As a key component of the hoisting system of the crane, the steel wire rope will bear a variety of loading actions such as stretching, bending, vibration and impact in the process of traction hoisting. Therefore, it is important to determinate the dynamic characteristics of the steel wire rope under complex loads and understand the stress-strain state to predict the risk of hoisting operation in advance. This article takes the bridge crane as the engineering background, first, a dynamic model of a steel wire rope lifting system based on ADAMS/Cable was established, and the dynamic stress spectrum of the steel wire rope during the lifting process was calculated and obtained. Secondly, by establishing the geometric model and finite element model of the wire rope, the tensile stress and wire displacement distribution of the wire rope and the contact stress between the wire rope and the pulley and the wires inside the wire rope are analyzed during the lifting process of the crane. The final results show that the instantaneous acceleration of the steel wire rope increases the maximum tensile stress of the steel wire rope by 37% compared with the stable lifting stage at the instant of starting the steel wire rope, causes an increase in the stress amplitude of the wire rope cross section, and the lifting process of the steel wire rope is accompanied by unstable vibration loads. The analysis found that the outermost cross-section of the steel wire rope's outer strand was subjected to the greatest stress, and its local maximum tensile stress amplitude was increased by 56% compared to the stable lifting stage. The contact stress generated by the contact between the steel wire rope and the pulley causes contact wear on the external and internal strands of the steel wire rope, and promotes fatigue fracture of the steel wire rope.