Stress Concentration Coefficient Research Articles

Structural improvement of a dredge pump based on numerical analysis to prolong its service life

Abstract Dredge pumps are prone to irreparable damage during offshore constructions: the flow passage components are rapidly worn out, and pump shafts often break. It seriously affects the construction efficiency and progress. For the case of high concentration conveying of medium coarse sand, the flow in a dredge pump was numerically studied to analyze the flow force on the impeller and the trajectories of sand particles in the impeller. A discrete phase model was introduced to predict the wear on the impeller, and the life of the pump shaft was predicted using S-N curves and the Rain-flow counting method. A new blade profile and an endwall profile were proposed to use to reduce the wear on the impeller and the structure of the shaft head were optimized to reduce the stress concentration coefficient. The numerical results showed that these measures can significantly improve the service life of the pump.

Dynamic inverse plane characteristics of circular holes in a strip-shaped medium under line source excitation

Based on the complex variable function method and the wave function expansion method, the dynamic anti-plane characteristics of circular holes in the strip domain under line source excitation were studied. The total wave field in the strip domain is constructed using the concept of infinite mirroring, which eliminates the mutual impact of the wave field under the action of the upper and lower limits of the strip domain. To determine the overall wave field, solve the wave field's unknown coefficients in accordance with the boundary conditions. The right number of mirror images is found using the dynamic stress distribution of a circular hole in the band-shaped domain as a specific calculation example. The effect of altering the incident frequency and band-shaped domain width on the dynamic stress concentration coefficient surrounding the circular hole is also explored. As a consequence of the findings, the dynamic stress concentration coefficient around the circular hole in the zone domain at low frequencies is substantially bigger than it is at medium and high frequencies. Under identical conditions, the dynamic stress concentration coefficient will exhibit periodic extreme values and converge as the distance increases between the lower boundary and the circular hole’s center.

Features of calculating the strength of gantry and ship cranes gear units in the probabilistic aspect

The performed study is the result of collecting statistical data on failures of port transshipment and floating cranes. It is found that many accidents of components and parts of crane mechanisms can be associated with an erroneous determination of design loads. In order to increase the strength reliability, numerous field tests were carried out in river and sea ports under various modes of transshipment for different types of cargo. Tests have shown that operational loads are random in nature, and their laws of distribution over time of individual crane mechanisms can be approximated by truncated normal Gauss laws, accounting for which at the stage of new design developments, as shown by the practice of performing strength fatigue calculations, is not possible. In the article, in order to use the obtained laws at the design stage, it is proposed to replace the representation of operational loads with initial moments that are common to the entire crane mechanism, and transfer individual characteristics consideration such as stress concentration coefficients, fatigue curve parameters, operating time or test base, to subsequent stages of machine components development. For practical testing of the operational loads representation by the normal Gauss laws, a critical analysis of their initial moments, which makes it possible to identify inconsistencies in the representations of variable loads proposed for strength calculation, is given. For the corrected truncated normal load distribution law, the initial moments, the value of the order of which corresponds to the formulas structure used in calculating fatigue resistance at variable load amplitudes (in particular, under alternating loading modes), are determined. The presentation of corrected normal distribution laws with initial moments complements the normative documents used in practice when performing strength calculations under variable loading conditions, significantly simplifies calculations and makes it possible to take it into account at the stage of new design developments in accordance with the formulas established in the current normative documents.

Dynamic anti-planar characteristics of circular holes in laminated structures under the action of line source

Based on the complex variable function method and the wave function expansion method, the dynamic anti-plane characteristics of the circular hole in the laminated structure under line source excitation were studied. To deal with the interface problem within the laminated structure, the idea of “fit” is adopted to construct continuity conditions that satisfy displacement and stress. The circular hole boundary stress-free condition is used, the wave field is constructed using the mirror method, and the unknown coefficients of the scattered wave field are solved through Fourier series expansion, and finally the total wave field in the laminated structure is obtained. Taking the dynamic stress distribution of a circular hole in a laminated structure as a specific calculation example, the effects of changes in line source position, layer thickness, and medium hard-soft ratio on the dynamic stress concentration coefficient around the circular hole are discussed. The results show that when the line source load is applied directly above the circular hole and the lower layer is narrow, the dynamic stress concentration coefficient will have a sudden change compared with other situations. When other conditions are the same, under low, medium, and high-frequency wave fields, the dynamic stress concentration caused by different soft-to-hard ratios shows completely different rules. These rules can be used to better detect defects in laminated structures.

Influence of Mining Sequence of Branch on Stope Pressure Behaviour on Continuous Mining and Continuous Backfilling

Instability in coal pillars and filling bodies is a common occurrence during the mining process of continuous mining and continuous backfilling (CMCB). In view of this, combining numerical simulation, similarity simulation, and on-site testing approaches, backfill mining models were established in Flac3d5.01 software, similarity model test bench, and “two-stage”, “three-stage”, and “four-stage” mining sequences were conducted; the stress characteristics of coal pillar-filling body and the displacement evolution law of surrounding rock have been compared under three typical mining sequences. The results show that compared to two-stage mining sequence, three-stage and four-stage mining sequences provide sufficient time for the solidification of the filling body. The coal pillar exhibits better stability in the early stage of mining, but the stress concentration phenomenon is more significant in the later stage of mining. The stress concentration coefficient is the highest when the width of the coal pillar is 10 m. The integrity of the overburden is intact in different mining sequences, with only a small amount of separation and longitudinal cracks. Increasing the number of mining stages significantly reduces the roof subsidence, with the maximum roof subsidence in the three- and four-stage mining sequences being only 62.0% and 33.9% of that in the two-stage mining sequence. “Two stages”, “three stages”, and “four stages” of mining sequences are implemented in response to the requirements of weak and thick coal seam mining in Haoyuan Coal Mine and gangue disposal in Chahasu Coal Mine. Good engineering applications are achieved, enabling the realisation of safe, green, and efficient coal mining.

Enhancement of fatigue properties of selective laser melting fabricated TC4 alloy by multiple shot peening treatments

This study investigated the impact of multiple shot peening (SP) treatments on the surface morphology, microstructure, residual stress and fatigue performance of TC4 alloy manufactured by selective laser melting (SLM). The multiple SP treatments were conducted sequentially using cast steel beads, ceramic beads and glass beads at different peening intensities. In comparison to the conventional single SP process, multiple SP treatments reduced surface roughness and stress concentration coefficient while further increasing the residual compressive stress on the alloy surface. SP significantly enhanced the fatigue life of the alloy by 6.4 to 9.6 times, with a more pronounced improvement observed with multiple SP. Fatigue fracture analysis revealed that the untreated alloy showed multiple fatigue sources which all located on the surface, with surface printing defects acting as potential sites for crack initiation. After SP treatments, all fatigue cracks were initiated beneath the surface. The improved fatigue performance of SLM-formed TC4 alloy could be mainly attributed to the SP-induced surface microstructure refinement and residual compressive stresses. Furthermore, results indicated that, for subsurface crack initiation, the initial cracks primarily stemmed from the fracture of α/α' martensite in the primary β columnar grains.

Fatigue life analysis based on the groove angle reduction in the butt welding of excavator booms

This study analyzes the effect of changes in groove angle on the fatigue strength of the V-groove butt weld joint made of SM490A steel used in excavators. Butt-type specimens with different groove angles of 40° and 30° were manufactured by CO2 welding at a speed of 300 mm/min on a 13.5 mm thick steel plate. Four-point bending fatigue tests were conducted at room temperature with a frequency of 5 Hz and stress ratio of R = 0.1, and fatigue limits were set to 2 × 106 cycles. The high-cycle fatigue life of VG40 was approximately 7% higher than that of VG30. The stress concentration coefficient calculated using the effective notch stress method was 1.62 for the VG40, and 1.78 for the VG30, with an approximate difference of 9%. This decrease in fatigue life was caused by stress concentration due to the notch effect at the weld root, which was attributed to the adjacent backplate. Although the fatigue life decreased slightly owing to the improvement angle of the welded part, no difference was observed in microstructure and hardness. Therefore, it was confirmed that a sound weld was formed. The results of this study can be used as important basic data for determining the V-groove angle.

Quantitative study of high-frequency fatigue hole-edge stresses in nickel-based single-crystal superalloy

In this paper, the influence of air film holes on the stress distribution of nickel-based single-crystal superalloy blades is investigated, a simulated test piece is designed for the analysis and test of the stress distribution of air film holes, and a method of quantitative inverse inference of the stress at the edge of the air film holes is proposed. Finite element simulation of high circumferential vibration fatigue of the simulated test piece was carried out by ANSYS to obtain the gradient of stress distribution at the hole edge. The EBSD technique was used to measure the dimensions of the plastic zone in the region near the fracture, and the quantitative fatigue stresses were extrapolated using a crack tip plastic zone model with Tresca yield criterion. The compressive stress exerted by the femtosecond laser perforation method on the air film aperture wall was detected by the nanoindentation technique, and the residual stress was analysed using the S. Suresh computational model. The results of this study show that based solely on finite element simulation, it is not possible to completely simulate the effect of residual stresses caused by machining on fatigue fracture, which in turn leads to a lack of conformity between the results of finite element simulation and the results of the inverse plastic zone size extrapolation; however, the stress magnitude at the edge of the hole can be well computed by combining the stress concentration coefficients from finite element simulation with the stresses from the inverse plastic zone size extrapolation after taking into account the residual stresses from machining. The quantitative analysis results of this method have an error within 1.3 times the dispersion band compared to the test data.

Computational and experimental assessment of the fatigue strength dissipation characteristics of power lugs

Methods for experimental and computational assessment of the dissipation indicators of the endurance limit and cyclic durability of power lugs of machines and mechanisms are presented. Based on a generalization of the results of fatigue tests, the dependences of the standard deviation of the logarithm of the number of cycles before failure of lugs made of high-strength steels on the maximum amplitudes of cyclic stresses were constructed. Formulas are obtained for calculating the coefficient of variation of the theoretical stress concentration coefficient of the lugs from production deviations of their dimensions in plan and from the dispersion of the gap between the bolt and the hole. Based on these formulas, the values of the coefficient of variation of the theoretical stress concentration coefficient for typical lugs are obtained and given.

The stress state of a plate with several holes under the action of axial tensile load

The purpose of the research is to develop a numerical method for determining the stress state of plates with several holes. The research approach is based on the development of the superposition method for singular solutions. The study objective is to solve a practical problem of determining the stress concentration coefficients in a plate with two holes of the same diameter. The study uses the numerical method of elasticity theory in the form of a superposition of four special solutions, each of which lies entirely within the corresponding hole contour. The resulting contour integrals in the displacement formula are replaced by their approximate values using the M-point trapezoidal formula or the Gaussian quadrature formula. Equating to zero the expression in variations of unknown constants (the principle of minimum potential strain energy), we have obtained a system of linear algebraic equations 4MN of unknown distribution densities. After determining the constants using known formulas, the stress-strain state of the plate with holes can be calculated. The error of the developed method is no more than 0.6% compared to the known solution.

Comprehensive Fatigue Analysis and Practical Design Methodology for Weld Toes in the Sphere of Welded Hollow Spherical Joints in Grid Structures

This paper addresses the issue of fatigue in grid structures, a topic of interest in engineering and academia. The goal is to establish a practical fatigue design calculation method for weld toes in welded hollow spherical joints (WHSJs). The study focuses on commonly used steel tube-WHSJs in grid structures, conducting 25 constant amplitude and four variable amplitude fatigue tests on tube–sphere joints (TSJs) to derive corresponding S-N curves. Using ANSYS, the hot spot stress concentration coefficient Kh at the weld toes in 22 TSJs was calculated, resulting in a numerical solution for Kh ranging from 2.0550 to 4.8600. Based on this, fatigue design methods were established using nominal stress amplitude and hot spot stress amplitude as fundamental parameters. Within a fatigue design reference period of two million cycles, the allowable nominal stress amplitude for TSJs is 22 N/mm2, and the allowable hot spot stress amplitude is 66 N/mm2. The study also conducted macroscopic and microscopic analyses on fatigue fractures of TSJs, revealing that the weld toe in the sphere of TSJs is the primary site for fatigue crack initiation. This research provides practical calculation methods for fatigue design in WHSJ grid structures, contributing to their broader application.

Stress and Couple Stress Concentration and Plastic Rotation Analysis of Local Micro-structure of Flat Plate

In classical elasticity, the analytical solution of the stress concentration coefficient at the edge of a circular plate hole is 3, but the experimental result is less than 3, and the fatigue theory still has no theoretical answer to the mechanism of fatigue. For the problem that the stress concentration coefficient is less than 3, the stress and couple stress concentration at the hole edge of a circular plate hole is calculated using generalized elasticity. On this basis, the strain-elastic and rotation-plastic theory is put forward, and an example of a flat wedge structure is used to demonstrate the plastic curvature of the contact boundary of different materials to explain the mechanism of fatigue. Through calculation, it is found that there is a stress concentration at the edge of the hole and a couple of stress concentrations, and the stress concentration coefficient decreases with the decrease of the hole diameter. For flat wedge structures, the plastic curvature is mainly concentrated on the contact surface of different materials, and the plastic curvature will increase obviously when the boundary changes. This paper provides a new idea for the development of fatigue theory.

Surrounding Rock Deformation Mechanism and Control Technology for the Roadway in the Fault-Disturbed Zone under Special-Shaped Coal Pillars

In order to explore the impact of residual special-shaped coal pillars and fault disturbances on the lower layered roadway, this study takes the short-distance coal seam mining in Luwa Coal Mine as the engineering background to explore the surrounding rock deformation mechanism along the mining roadway in the fault-disturbed zone under special-shaped coal pillars, it presents the roadway surrounding rock control technology and it conducts on-site industrial test verification. The study shows that the abutment pressures on the floor of special-shaped coal pillars are distributed as “three peaks and two ridges”. The part beneath coal pillars is mainly disturbed by vertical stresses, while the part below the coal pillar edge is co-affected by vertical stresses and shearing stresses, generating a stress concentration coefficient ranging from 1.26 to 1.38 in the lower coal seam. According to the superposed effects of special-shaped coal pillars and fault disturbance on the mining roadway, the mining roadway is divided into the lower section of goaf, the section crossing the coal pillar edge, the lower section of coal pillars, and the section obliquely crossing the coal pillar edge. According to the above sections, the segmental control strategies of “improving stress distribution on surrounding rock + reinforcing support on special sections” are proposed. A joint control technology of large-diameter drilling hole pressure relief and special section anchor cable reinforcement support was adopted to carry out on-site industrial testing and monitoring. Overall, the convergence rate on the roadway surrounding rock is controlled within 5%, and the deformation of roadway surrounding rock is under effective control.

Research on Representative Volume Element Fex-Cy High-Temperature Mechanical Model Based on Response Surface Analysis

In this research, a multi-scale representative volume element method is introduced that combines the temperature and stress fields to analyze the force field distribution around microcracks in low-carbon steel using a combination of molecular dynamics and finite element analysis. Initially, an orthogonal experimental design was used to design the molecular dynamics simulation experiments. Next, a nano-level uniaxial tensile test model for mild steel was established based on the experimental design, and the uniaxial tensile behavior of low-carbon steel was investigated using molecular dynamics. Lastly, mathematical models of the modulus of elasticity E and yield strength Q of mild steel at a high temperature were obtained statistically using the response surface methodology. Meanwhile, a finite element model with a coupled temperature–stress field was established to investigate the force field distribution around the microscopic defects, and the microscopic crack stress concentration coefficient K was revised. The results indicate that regardless of the location of microcracks within the structure, the stress distribution due to size effects should be considered under high-temperature loading.

Minor and Major Strain: Equations of Equilibrium of a Plane Domain with an Angular Cutout in the Boundary

Large values and gradients of stress and strain, triggering concentrated stress and strain, arise in angular areas of a structure. The strain action, leading to the finite loss of contact between structural elements, also triggers concentrated stress. The loss of contact reaches an irregular point and a line on the boundary. The theoretical analysis of the stress–strain state (SSS) of areas with angular cutouts in the boundary under the action of discontinuous strain is reduced to the study of singular solutions to the homogeneous problem of elasticity theory with power-related features. The calculation of stress concentration coefficients in the domain of a singular solution to the elastic problem makes no sense. It is experimentally proven that the area located near the vertex of an angular cutout in the boundary features substantial strain and rotations, and it corresponds to higher values of the first and second derivatives of displacements along the radius in cases of sufficiently small radii in the neighborhood of an irregular boundary point. As far as these areas are concerned, it is necessary to consider the plane problem of the elasticity theory, taking into account the geometric nonlinearity under the action of strain, to analyze the effect of relationships between strain orders, rotations, and strain on the form of the equation of equilibrium. The purpose of this work is to analyze the effect of relationships between strain orders, rotations, and strain on the form of the equilibrium equation in the polar system of coordinates for a V-shaped area under the action of temperature-induced strain, taking into account geometric non-linearity and physical linearity.

Modification of the model for calculating the long SiC fibre-reinforced metal matrix composites transverse strength and study for interfacial influences

The transverse strength of long SiC fibre-reinforced metal matrix composites is superior to its longitudinal strength owing to interface factors. To accurately predict the transverse strength of long SiC fibre-reinforced metal matrix composites, a micro-mechanics representative volume element (RVE) model was developed; periodic boundary conditions were applied to the model to ensure its displacement and stress continuity. Taking into account the influence of the stress concentration coefficient of the matrix, the failure strength of long SiC fibre-reinforced metal matrix composites under transverse tensile loads and compressive loads were calculated, where the error between the calculation results of the model and the test results was found to be large. A novel calculation method based on the interfacial cohesion model is proposed herein, to improve the accuracy of the RVE model. It was found that the accuracy of the corrected model calculation has been improved through a comparison with the experimental values. The stress/strain relationship between interfaces of different strengths under tensile and compressive loads was analysed, the failure index of interface strength was extracted, and the relationship between the influence of interface strength on failure was determined.

Transient Response of Dynamic Stress Concentration around a Circular Opening: Incident SH Wave

The present study aims to investigate the transient response of stress concentration around a circular opening. The study focuses on the composition of the shockwave, which consists of SH waves of multiple frequencies. The wave equation, expressed by the displacement function, is transformed into the Helmholtz equation through the Fourier transform method. The spectral function can be obtained by employing analytic continuation and Fourier transform of the incident wave field. An analytical expression for the dynamic stress around the aperture can be derived using the wave function expansion method and by considering the boundary conditions. The influence of the aperture on the transient response is discussed based on the distribution of the dynamic stress concentration coefficient and stress peak coefficient under different aperture sizes. The results show that the peak of the dynamic stress concentration coefficient changes with the aperture. In contrast, the stress peak coefficient is primarily concentrated in the early stages of the transient response. Furthermore, it is observed that larger radii can induce alternating stress in the material, which may lead to fatigue failure. This strategy provides a solution for addressing similar challenges.

Influence of Key Strata on the Evolution Law of Mining-Induced Stress in the Working Face under Deep and Large-Scale Mining

When there are multiple key strata in the overburden of a deep coal seam and the surface subsidence coefficient after mining is small, this indicates that the overlying key strata fail to break completely after mining. On this occasion, stress is easily concentrated in the working face, which in turn leads to the occurrence of dynamic disasters such as rock bursts. This study adopted a comprehensive analysis method of field monitoring and numerical simulations to explore the influence of the key stratum on the evolution law of mining-induced stress in the working face. A distributed optical fiber sensor (DOFS) and a surface subsidence GNSS monitoring system were arranged inside and at the mouth of the ground observation borehole, respectively. According to the monitoring results of strain obtained from the DOFS, the height of the broken stratum inside the overlying strata was obtained and according to the monitoring results of surface subsidence, the surface subsidence coefficient was proven to be less than 0.1, indicating that the high key stratum is not broken completely, but enters a state of bending subsidence instead. In order to reveal the influence of the key stratum on the mining-induced stress of the working face, two 3DEC numerical models with and without the key stratum were established for a comparative analysis. As the numerical simulation results show, when there are multiple key strata in the overburden, the stress influence range and the stress concentration coefficient of the coal seam after mining are relatively large. The study revealed the working mechanism of rock burst accidents after large-scale mining and predicted the potential area with a rock burst risk after mining of the working face, which was verified by field investigations. The research results are of great guiding significance for the identification of the working mechanism of rock bursts in deep mining condition and for their prevention and control.

МЕТОДЫ ОЦЕНКИ КОНЦЕНТРАЦИЙ МЕХАНИЧЕСКИХ НАПРЯЖЕНИЙ

Methods of estimation of mechanical stress concentrations by conducting a computer experiment on models of welds with defects having zones with increased stress concentration in the form of non-welding of the root of the weld are considered. The paper notes that a full-scale study of the stress state with the determination of stress concentration coefficients on the equipment itself is difficult due to the duration of the development of degradation processes, wear and aging, therefore, their conduct is associated with risk and high time and material costs. Analytical research methods using mathematical models and application packages are increasingly being used

Experimental investigation of perforated nanocomposites under thermal cycles for use in the shell of sports buildings

One of the most important applications of composites is their use in structures with periodic thermal loading. Sports buildings are one of the most appropriate options for using these composites due to the need for a balanced temperature. In this research, composite samples of polymeric base reinforced with nanoparticles, structure, and effects of thermal cycling and circular holes on it have been investigated. The nanoparticles used were multi-walled carbon nanotubes and the weight percentage was 0.1%. In this research, the behavior of perforated nanocomposite samples with a diameter of 2 and 4 mm holes under thermal cycles was investigated. By performing a tensile test on the samples, the fracture toughness and modulus of elasticity due to the number of different heating cycles and the ratio of the hole diameter to the width for different samples were checked and compared. According to the results, as the ratio of diameter to width increases, the coefficient of stress concentration also increases. Also, the results show that the fracture strength increases during the thermal cycle. Fracture strength increases between 9% and 25% after 360 thermal cycles. It seems that one reason for this increase in tensile strength is the sintering of glass/epoxy nanocomposite during the thermal cycle.