Location Of Maximum Stress Research Articles

The Influence of Materials Structure on the Main Features of the Fracture Process in Rocks: Discrete Elements Method and Laboratory Experiment

A computer model of fracture of the heterogeneous materials (including rocks) based on the Discrete Element Method (DEM) is proposed. We used the bonded particle model (BPM), various modifications of which are widely used in the study the fracture process. The material is modeled by a set of spherical particles (simulating polycrystalline grains) connected by bonds placed at the points of particle contacts (simulating grain boundaries). In BPM model, the initiation of cracks was determined by the bonds breakage, and their propagation is provided by the coalescence of many broken bonds. Computer experiments were carried out for the materials with different features (various grain mechanical properties and sizes, various mechanical properties of the grain boundaries), in order to find out the influence of these parameters on local stresses and the defect formation. Calculations were held in the MUSEN software. Cylindrical samples were filled with spherical particles of the same or different radii. The parameters of materials for grains and bonds (grain boundaries) were taken corresponding to granite, quartz, orthoclase, oligoclase, and glass. The sample was placed in a virtual press, in which the lower plate was stationary, and the upper plate moved towards the lower one at a constant velocity until the sample was destroyed. The calculation of the maximum local stresses showed that the homogeneity of material leads to greater space heterogeneity of local stresses and vice versa, heterogeneity contributes to their greater uniformity. Comparison with the results of laboratory experiments on rock deformation showed that the proposed model of polycrystalline materials realistically describes some features of their destruction when the main processes occur along the grain boundaries. These features include the brittle nature of homogeneous materials fracture and the presence of nonlinear elasticity (plasticity) for ones that were more heterogeneous. For heterogeneous materials, the model demonstrates a two-stage character of fracture process, when at the first stage the accumulation of defects occurs uniformly over the sample, and at the second stage – the formation and growth of the fracture site.

Impact of rail joints on the local stresses in crane runway girders

Vertically acting wheel loads from overhead bridge cranes and potential additional horizontal crane loads induce global stresses due to bi-axial bending and torsion and local stresses in crane runway girders which mainly affect the girder’s web. The latter typically represent one of the most decisive criteria for the fatigue design check of welded girders and are the focus of this paper. Crane rails with foot flange are typically used for moderate to high wheel loads. They are usually connected to the top flange with rail clips. Rails with foot flange are used with and without elastomeric bearing pads between the rail and the top flange of the girder. The local stresses in the webs of crane runway girders are strongly affected by discontinuities of the crane rails. Such discontinuities can originate from unplanned rail cracks as well as planned rail expansion joints. The current mechanical models for concentric as well as eccentric wheel loading in relevant design standards assume continuous crane rails. However, rail joints are expected to significantly increase the local stresses in the region beneath rail joints. The present paper focuses on the impact of rail joints on the local stresses in crane runway girders. The research activities comprise experimental tests as well as numerical parametric studies based on finite element calculations, both for rails with and without elastomeric bearing pads. The aim of the investigations is to develop a better understanding of the mechanical behaviour and to quantify the increase factor for maximum local stresses in order to avoid future fatigue damage due to an unexpected increase of local stresses at a rail joint.

Experimental and Analytical Study of Bond Stress–Slip Behavior at the CFRP-to-Concrete Interface

The application of externally bonded (EB) carbon fiber reinforced polymer (CFRP) systems for strengthening existing structures, such as RC beams, has been widely approved in the construction industry worldwide for its numerous benefits. The CFRP-to-concrete bond has a governing role in the reliability and effectiveness of EB-CFRP systems. Indeed, failure of the CFRP-to-concrete bond can lead to rupture of CFRP-strengthened structures. Hence, ongoing research into assessment of bond behavior at the CFRP-to-concrete interface helps to bring more insightful clarity to the use of EB-CFRP strengthening techniques. The aim of this study is to evaluate the bond behavior between CFRP and concrete by conducting a series of pullout tests. The parameters considered include CFRP type (sheet versus laminate), bonded length, and bonded CFRP width. The results show that using CFRP fabric sheets can contribute to higher bond load-carrying capacity and ductility than CFRP laminates. Furthermore, through the analyses of databases in the literature, a bilinear bond–slip model is proposed that takes into account the CFRP width factor. Through a comparison, it is shown that the proposed model performs well in terms of predicting the maximum local bond stress and CFRP slippage.

Progressive damage analysis in an open hole compression of FRP laminates including fiber kinking and pre-existing damage

Progressive damage and strength analysis in an open hole compression (OHC) test is crucial in designing components made of fiber reinforced plastics (FRPs) composites. In this work, a continuum damage mechanics based 3D progressive damage model (PDM) incorporating LaRC05 failure criteria is developed. It considers identification of fracture and kink-band plane, shear non-linearity, in-situ strengths, and mixed-mode fracture in the formulation. The proposed PDM is validated through single element tests and applied to analyze the damage patterns and strength during open hole compression (OHC) tests of three different layups with two different hole sizes and a pre-existing damage. It is observed that fiber kinking in 0∘ plies initiates at the location of maximum in-plane shear stress and propagates due to a combined action of longitudinal compression and in-plane shear stress. The final failure of 0∘ dominant and quasi-isotropic layup is governed by fiber kinking in 0∘ plies, whereas matrix cracking in ±45∘ plies is responsible for the final failure of the shear dominant layup. Furthermore, OHC strength of the layups reduces by 15%–20%, when the hole size increases from 6.35 mm to 9.525 mm. In the presence of a pre-existing kink or matrix damage, fiber kinking initiates at locations of the pre-existing damage. A small pre-existing damage reduces OHC strength of the layups up to 7%–11%. The results predicted by the model match well with the experimental results in the literature.

Design and Simulation of Autom0atic Feeding and Unloading System for Finishing RV Reducer Planetary Carrier

To realize the unmanned production of precision planetary cycloidal reducer planetary frame finishing, the design of this paper focuses on the robot loading and unloading system for RV reducer planetary frame finishing, and the structural dimensions of the end robot of the system are calculated in detail. The load-bearing parts of the truss affect the strength and stiffness of the whole truss system, and the ANSYS Workbench software is used to perform the static finite element analysis of the truss. The simulation results show that the maximum total deformation of the joist beam is about 0.403mm, which can effectively meet the overall loading and unloading accuracy requirements. The maximum local stress value of the joist is 5.5 MPa, the yield stress of Q235 is 235 MPa, the tensile strength is 375-460 Mpa, which meets the strength requirement, and the automatic loading and unloading system of the joist is determined to meet the design requirements.

Outer scaling of the mean momentum equation for turbulent boundary layers under adverse pressure gradient

A new scaling of the mean momentum equation is developed for the outer region of turbulent boundary layers (TBLs) under adverse pressure gradient (APG). The maximum Reynolds shear stress location, denoted as $y_{m}$ , is employed to determine the proper scales for the outer region of an APG TBL. An outer length scale is proposed as $\delta _e - y_{m}$ , where $\delta _e$ is the boundary layer thickness. An outer velocity scale for the mean streamwise velocity deficit is proposed as $U_e - U_{m}$ , where $U_e$ and $U_m$ are the mean streamwise velocities at the boundary layer edge and $y_{m}$ , respectively. An outer velocity scale for the mean wall-normal velocity deficit is proposed as $V_e - V_{m}$ , where $V_e$ and $V_{m}$ are the wall-normal velocities at $\delta _e$ and $y_{m}$ , respectively. The maximum Reynolds shear stress is found to scale as $(\delta _e - y_{m}) U_e \,{\rm d}U_e/{{\rm d}x}$ . The new outer scaling collapses well the experimental and numerical data on APG TBLs over a wide range of Reynolds numbers and strengths of pressure gradient. Approximations of the new scaling are developed for TBLs under strong APG and at high Reynolds numbers. The relationships between the new scales and previously proposed scales are discussed.

MRI-based mechanical analysis of carotid atherosclerotic plaque using a material-property-mapping approach: A material-property-mapping method for plaque stress analysis

Background and objectiveAtherosclerosis is a major underlying cause of cardiovascular conditions. In order to understand the biomechanics involved in the generation and rupture of atherosclerotic plaques, numerical analysis methods have been widely used. However, several factors limit the practical use of this information in a clinical setting. One of the key challenges in finite element analysis (FEA) is the reconstruction of the structure and the generation of a mesh. The complexity of the shapes associated with carotid plaques, including multiple components, makes the generation of meshes for biomechanical computation a difficult and in some cases, an impossible task. To address these challenges, in this study, we propose a novel material-property-mapping method for carotid atherosclerotic plaque stress analysis that aims to simplify the process. MethodsThe different carotid plaque components were identified and segmented using magnetic resonance imaging (MRI). For the mapping method, this information was used in conjunction with an in-house code, which provided the coordinates for each pixel/voxel and tissue type within a predetermined region of interest. These coordinates were utilized to assign specific material properties to each element in the volume mesh which provides a region of transition. The proposed method was subsequently compared to the traditional method, which involves creating a composed mesh for the arterial wall and plaque components, based on its location and size. ResultsThe comparison between the proposed material-property-mapping method and the traditional method was performed in 2D, 3D structural-only, and fluid-structure interaction (FSI) simulations in terms of stress, wall shear stress (WSS), time-averaged WSS (TAWSS), and oscillatory shear index (OSI). The stress contours from both methods were found to be similar, although the proposed method tended to produce lower local maximum stress values. The WSS contours were also in agreement between the two methods. The velocity contours generated by the proposed method were verified against phase-contrast magnetic resonance imaging (MRI) measurements, for a higher level of confidence. ConclusionThis study shows that a material-property-mapping method can effectively be used for analyzing the biomechanics of carotid plaques in a patient-specific manner. This approach has the potential to streamline the process of creating volume meshes for complex biological structures, such as carotid plaques, and to provide a more efficient and less labor-intensive method.

Studies on the elbow corrosion associated with the wall-wetting process in stratified and wave flows

ABSTRACT The flow-induced corrosion in gas-liquid flow usually occurs in the petrochemical and transportation industries, which has become a hot topic, especially for the elbow system. In this paper, a series of simulations using the VOF method and SST k-ω turbulence models were carried out to investigate the temporal and spatial distributions of the water wetting process in the elbow system. Combined with experiments, the relationship between the dynamic hydrophilic process and the corrosion mechanism was revealed. The results show that there is a stratified flow in the pipe at low speed. Wetness is always present in the inner wall of the elbow, and the corrosion rate reduces as the axial angle increases. As the velocity increases, the flow pattern changes to wave flow. Part of the water in the inner wall is entrained into the gas phase and thrown out onto the outer wall to form the water wetting. The location of maximum wall shear stress is transferred to outer wall, causing an increment of corrosion rate. The corrosion rate of the outer wall increases with the axial angle.

Formation of post-buckling shear mechanisms in stiffened web panels of slender steel plate girders

A series of validated finite element models are used to investigate the formation of mechanisms in stiffened slender web panels of steel plate girders under pure shear. The prototype girder is based on the seminal tests by Basler and has a web slenderness of 267. Six model cases are analyzed with variations in web panel aspect ratio (1, 1.5, and 3), steel yield strength (Grades 36 and 50), initial imperfection magnitude, and flange thickness. When loaded in shear, the web panels exhibit a 3-stage response: (1) elastic behavior, (2) web mechanism formation, and (3) panel mechanism. In Stage 1, the web initially exhibits an elastic in-plane shear response with no distinct buckling bifurcation. During this stage, the web’s initial out-of-plane imperfections become engaged in second-order bending along its compression diagonal. Concurrently, tension develops in the opposite diagonal as a non-uniform membrane stress field, with highest intensities at locations where second-order bending is low. In Stage 2, a web mechanism is formed when connected bands of thru-thickness von Mises yielding develop across the tension diagonal. These bands emerge at locations where tension membrane stresses interact with locations of maximum second-order bending stress in the buckled shape. The shear load at the end of web mechanism formation is recommended as a target for plastic limit state design because it marks a significant decrease in shear stiffness. In Stage 3, von Mises yielding continues to saturate the web panel, and the bounding flanges and transverse stiffeners become increasingly engaged in load redistribution from the plastified web. Any hardening increase in shear resistance during the panel mechanism stage is modest in magnitude (up to ∼10% for some cases in this study) and is heavily dependent on the sizing of the flanges and stiffeners to carry redistributed forces.

Wheel impact test by deep learning: prediction of location and magnitude of maximum stress

For ensuring vehicle safety, the impact performance of wheels during wheel development must be ensured through a wheel impact test. However, manufacturing and testing a real wheel requires a significant time and money because developing an optimal wheel design requires numerous iterative processes to modify the wheel design and verify the safety performance. Accordingly, wheel impact tests have been replaced by computer simulations such as finite element analysis (FEA); however, it still incurs high computational costs for modeling and analysis, and requires FEA experts. In this study, we present an aluminum road wheel impact performance prediction model based on deep learning that replaces computationally expensive and time-consuming 3D FEA. For this purpose, 2D disk-view wheel image data, 3D wheel voxel data, and barrier mass values used for the wheel impact test were utilized as the inputs to predict the magnitude of the maximum von Mises stress, corresponding location, and the stress distribution of the 2D disk-view. The input data were first compressed into a latent space with a 3D convolutional variational autoencoder (cVAE) and 2D convolutional autoencoder (cAE). Subsequently, the fully connected layers were used to predict the impact performance, and a decoder was used to predict the stress distribution heatmap of the 2D disk-view. The proposed model can replace the impact test in the early wheel-development stage by predicting the impact performance in real-time and can be used without domain knowledge. The time required for the wheel development process can be reduced using this mechanism.

Strain-rate-dependent tensile response of an alumina ceramic: Experiments and modeling

The strain-rate-dependent tensile response of a commercial alumina ceramic (CeramTec 98% alumina) is investigated by experimental and modeling methods. The experiments at different loading rates are carried out on a standard MTS machine and a modified split-Hopkinson pressure bar system with flattened Brazilian disk specimens. High-speed imaging coupled with digital image correlation (DIC) is used to measure the strain fields, and this enables us to capture the fracture process and the corresponding stress field based on theoretical considerations. In the dynamic tests, it is verified that multiple cracks appear simultaneously around the locations of maximum tensile stress and strain. Next, a matching approach based on theoretical models (i.e., the uniform and sinusoidal load models) is proposed to synchronize the stress and strain history curves in time, and the matching results show the tensile cracks are often generated prior to the peak stress as visualized in ultra-high-speed camera images. This peak stress corresponds to the failure of the sample structure, which is different from the material tensile strength as an inherent material property. In this study, we use the term “overloading” to describe the structural failure of the material. The difference between the peak stress and material tensile strength is associated with the time it takes for the crack to propagate, interact, and span the structure during the loading process, which is termed as “time-dependent structural failure”. The strain-rate-dependent tensile strength of the alumina ceramics is computed with a correction method, and the tensile strength is defined as the tensile stress when the central crack first appears in the ultra-high-speed camera images. Then, the fracture surfaces of the alumina fragments are examined by Scanning electron microscopy to explore the fracture mechanism in the failure process. Finally, a strain-rate-dependent tensile strength model is proposed to describe the tensile strength of the CeramTec 98% alumina and other alumina ceramics in the literature.

Shape optimization method for strength design problem of microstructures in a multiscale structure

AbstractIn this paper, we propose a solution to a shape optimization problem for the strength design of periodic microstructures in multiscale structures. Two maximum stress minimization problems are addressed: minimization of maximum microstructural stress and minimization of maximum macrostructural stress. The homogenization method is used to bridge the macrostructure and the microstructure and to calculate local microstructural stress. By replacing the maximum stress value with a Kreisselmeier‐Steinhauser function, the difficulty of nondifferentiability of maximum stress is avoided. Each strength design problem is formulated as a distributed parameter optimization problem subject to an area constraint including the whole microstructure. The shape gradient functions for both problems are derived using Lagrange's undetermined multiplier method, the material derivative method, and the adjoint variable method. The H1 gradient method is used to determine the unit cell shapes of the microstructure, while reducing the objective function and maintaining smooth design boundaries. In the numerical examples, the optimal shapes obtained for minimization of the maximum local stress of the microstructure and the macrostructure are compared and discussed. The results confirm the effectiveness of the microstructure shape optimization method for the two strength design problems of multiscale structures.

An investigation of Hertzian contact in soft materials using photoelastic tomography

Hertzian contact of a rigid sphere and a highly deformable soft solid is investigated using integrated photoelasticity. The experiments are performed by pressing a styrene sphere of 15 mm diameter against a 44 × 44 × 47 mm3 cuboid made of 5% wt. gelatin, inside a circular polariscope, and with a range of forces. The emerging light rays are processed by considering that the retardation of each ray carries the cumulative effect of traversing the contact-induced axisymmetric stress field. Then, assuming Hertz's theory is valid, the retardation is analytically calculated for each ray and compared to the experimental one. Furthermore, a finite element model of the process introduces the effect of finite displacements and strains. Beyond the qualitative comparison of the retardation fields, the experimental, theoretical, and numerical results are quantitatively compared in terms of the maximum equivalent stress, surface displacement, and contact radius dimensions. A favorable agreement is found at lower force levels, where the assumptions of Hertz theory hold, whereas deviations are observed at higher force levels. A major discovery of this work is that, at the maximum equivalent stress location, all three components of principal stress can be determined experimentally and show satisfactory agreement with theoretical and numerical ones in our measurement range. This provides valuable insight into Hertzian contact problems since the maximum equivalent stress controls the initiation of plastic deformation or failure. The measured displacement and contact radii also reasonably agree with the theoretical and numerical ones. Finally, the limitations that arise due to the linearization of this problem are explored.

Failure Mechanisms of Ba0.5Sr0.5Co0.8Fe0.2O3-δ Membranes after Pilot Module Operation.

The step from the testing of oxygen transport membranes on a lab scale to long-term operation on a large scale is a challenge. In a previous study, membrane failure was observed at defined positions of one end of the cooled tubular Ba0.5Sr0.5Co0.8Fe0.2O3-δ membranes after an emergency shutdown. To understand the failure mechanisms, strength degradation and transient stress distribution were investigated by brittle-ring tests and finite element simulations, respectively. A 15% decrease in the characteristic strength of 162 MPa was proven after aging at 850 °C and was attributed to grain coarsening. The reduction in characteristic strength after thermal shock ranged from 5 to 90% depending on the cooling rates, and from 5 to 40% after the first and 20th soft thermal cycling. Simulations indicated the chemical strains induced by a 10-bar feed air and 50 mbar permeate pressure, which caused tensile stresses of up to 70 MPa at the outer surface. These stresses relaxed to 43 MPa by creep within a 1000 h operation. A remaining local stress maximum seemed to be responsible for the fracture. It evolved near the experimentally observed fracture position during a 1000 h permeation and exceeded the temperature and time-dependent strength. The maximum stress was formed by a chemical strain at temperatures above 500 °C but effective creep relaxation needed temperatures above 750 °C.

Finite Element Analysis and Experimental Validation of the Anterior Cruciate Ligament and Implications for the Injury Mechanism.

This study aimed to establish a finite element model that vividly reflected the anterior cruciate ligament (ACL) geometry and investigated the ACL stress distribution under different loading conditions. The ACL’s three-dimensional finite element model was based on a human cadaveric knee. Simulations of three loading conditions (134 N anterior tibial load, 5 Nm external tibial torque, 5 Nm internal tibial torque) on the knee model were performed. Experiments were performed on a knee specimen using a robotic universal force/moment sensor testing system to validate the model. The simulation results of the established model were in good agreement with the experimental results. Under the anterior tibial load, the highest maximal principal stresses (14.884 MPa) were localized at the femoral insertion of the ACL. Under the external and internal tibial torque, the highest maximal principal stresses (0.815 MPa and 0.933 MPa, respectively) were mainly concentrated in the mid-substance of the ACL and near the tibial insertion site, respectively. Combining the location of maximum stress and the location of common clinical ACL rupture, the most dangerous load during ACL injury may be the anterior tibial load. ACL injuries were more frequently loaded by external tibial than internal tibial torque.

Surface tension effect on the sliding inception between an elastic sphere and a rigid flat

This paper develops an analytical model to characterize the sliding inception of elastic spherical contact accounting for surface tension effect. The distribution of von Mises equivalent stress on the contact interface under pure normal loading is first formulated, and thereby, the maximum allowable local shear stress is determined based on the concept of interface yielding failure. Then, the maximum tangential load that the contact interface can bear, and the friction coefficient at the sliding inception are obtained, which are functions of the normalized interference, the Poisson's ratio, and a surface tension-related parameter. The results show that the role of surface tension in the state of sliding inception primarily gives credit to its influence on the change of contact area. Based on this observation, explicit expressions of the maximum tangential load and the static friction coefficient are derived. It is found that both the maximum tangential load and the static friction coefficient decrease with the surface tension parameter increasing. For a given material, the smaller the sphere is, the lower the friction coefficient is predicted, which is in agreement with relevant experimental results.

Critical support pressure of shield tunnel face in soft-hard mixed strata

Support pressure is a key factor to the stability of excavation face during shield tunneling, which further imposes a safety issue on surrounding buildings and facilities. However, several studies have been reported on critical support pressure in mixed strata regarding twin-tunnel excavation. This paper aims to obtain the critical support pressure based on minimizing the influence on the surrounding environment induced by shield tunneling in soft-hard mixed strata. A series of three-dimensional finite element models are established to analyze soil and lining responding with different support pressure caused by twin tunnels excavation. The results reveal that the ground surface settlement and lateral displacement of soil are sensitive to the change of support pressure, especially under clay-dominated conditions. Increasing support pressure slightly changes the lining movement and location of maximum stresses of the existing tunnel due to adjacent tunnel excavating. According to this study, the upper and the lower critical support pressures are 1.43 and 1.06 times of lateral earth stress at tunnel crown, respectively, which are also compared with the limit support pressure and measured results from a shield tunnel in the literature. The critical support pressure has great significance for the stability of the excavation face and minimizing the influence on the surrounding environment caused by shield tunnel excavation.

Finite-difference analysis of stresses of a non-uniform functionally graded material circular disk rotating in the magneto-thermal environment: An equal mass study

The stress field of a functionally graded material rotating disk is studied for different cases of non-uniform thickness variation in the magneto-thermal environment. Three different cases of thickness variation are considered by assuming the variation of non-uniform thickness profiles to be linear, rational, and exponential functions of radius. The mass of the functionally graded material disk is considered equal in all cases of uniform/non-uniform thickness variation. The finite-difference method is used to obtain the numerical results for an Al/Al2O3 functionally graded material disk of fixed-free boundary conditions. The resultant thermo-elastic analysis has shown that the decrease in outer end thickness significantly increases circumferential stress at that end. In the absence of a magnetic field, for the disk with thin outer end thickness minimum stress intensity can be found with linearly varying thickness profile, and high intensity of circumferential stress in case rational and exponential variation of thickness profiles can significantly be reduced with an optimum magnetic field. The transient stress fields and the effect of material properties are also analyzed in detail. All the analyses showed that along with affecting the magnitude, the presence of the magnetic field changes the location and nature of maximum stress in the disk. Finally, results are compared with the finite-element method and available analytical results to verify the analysis.

Numerical analysis of delamination degradation of epoxy-impregnated superconducting coils wound with REBCO tapes caused by thermal stress

REBa2CuOy (REBCO) superconducting wires are expected to be used in coil applications owing to their excellent magnetic field properties. However, their weak delamination strength may deteriorate the superconducting characteristics of an epoxy-impregnated REBCO coil during cooling. In general, degradation is unlikely to occur if the outer-to-inner diameter ratio is less than 1.4; however, degradation has been reported in single pancake coils with an inner diameter of 560 mm and an outer diameter of 670 mm. A thermal stress analysis study is conducted to investigate the cause of this problem. Considering the non-uniformity of the epoxy resin thickness at the top and bottom of the coil, coil warping during cooling is simulated. When an FRP reinforcement plate is attached to the coil to suppress warpage, a high radial stress generated in the coil owing to the thermal contraction difference between the coil and FRP plate. The maximum local radial stress exerting in the delamination direction is 30 MPa, which is believed to degrade the superconducting characteristics of the REBCO single pancake coils.

Relationship between subsurface stress and wear particle size in sliding contacts during running-in

The results of an experiment set using a pin-on-disk rig on samplers made of ST37 disk and bearing steel pin are reported to characterize the wear rate of the specimen, weight loss, friction coefficient behavior, and wear particle size during running-in. Using the equations of subsurface stress for spherical contact, the location and value of maximum subsurface stress in the running-in period in three dimensions is predicted. The results indicate that there exists a relation between the size of the detached particles and the location at which the maximum subsurface shear stress occurs. Based on wear particles’ SEM images, there is a direct relationship between the size of the wear particles with the applied load, applied speed, and the initial surface roughness. It is shown that increasing the load and speed results in a rise in the friction coefficient, which affects the magnitude and location of the maximum subsurface shear stress. The latter is responsible for the nucleation and propagation of subsurface cracks.