Stress Concentration Position Research Articles

Numerical Study on Failure Mechanisms of Deep Roadway Sidewalls with Different Height-Width Ratios and Lateral Pressures

Numerical Study on Failure Mechanisms of Deep Roadway Sidewalls with Different Height-Width Ratios and Lateral Pressures

Investigation on the failure mechanism of hard rock cavern subjected to adjacent structural plane based on physical model experiments

Structural planes present significant safety challenges in the construction of deep hard rock caverns. However, the influence mechanism of a single dominant adjacent structural plane remains inadequately explored. This study conducted a series of physical model experiments with non-penetrating structural planes positioned adjacent to the cavern under uniaxial step compression, utilizing acoustic emission (AE) and digital image correlation (DIC) to monitor the failure process simultaneously. The results indicate that adjacent structural plane can mitigate stress-induced failure, reducing peak strength and elastic modulus by 10–42% and 13–31%, respectively. In the absence of a structural plane or a non-intersecting plane, stress-induced failure intensifies. Conversely, an intersecting plane contributes to greater crack stability. Precursor AE signals emerge earlier in the combination system. The intact cavern system shows an escalating tensile fracture ratio with loading, whereas the combination systems experience a diminishing tensile fracture ratio. Finally, particle flow code (PFC) simulation revealed that the presence of a single structural plane alters the cavern’s force chains, influencing stress concentration position. This research reveals that the structural plane has the potential to alter the cavern failure mechanism, providing insights into the complex interaction between structural planes and caverns in deep engineering.

Effect of aspect ratio on mechanical anisotropy of lattice structures

Triply periodic minimal surface (TPMS) lattice structures with controllable mechanical properties and porous architecture are promising candidates for lightweight and energy-absorbing applications. In parametric structural design, the research on structural geometric characteristics, such as aspect ratio (R) and surface curvature, has mainly focused on fluid and heat transfer considerations. However, their impact on multi-directional mechanical properties has yet to be thoroughly investigated. This study, combining representative volume elements (RVE) based on periodic boundary conditions and theoretical surface morphological characteristics, investigates the mechanical properties and deformation behavior of lattice structures with different aspect ratios in multiple directions. The results indicate that the aspect ratio highly influences the deformation behavior of the lattice structure in different directions. The local curvature and force state of the lattice structure can be adjusted by aspect ratio to reduce local stresses and deformations in different loading directions. Compared with the simulation results of representative volume elements, the structural stress concentration and failure position can be predicted by the Gaussian curvature distribution. In addition, the elastic modulus of the structures in the [100] and [001] directions can also be adjusted by aspect ratio. Through experimental verification in the [001] direction, the structure with an aspect ratio of 2 can greatly enhance the elastic modulus (121%∼440%), maximum stress (10%∼183%), and energy absorption (33%∼85%). The significance of this work is to improve the understanding of the influence of geometric features on the mechanical properties and deformation behavior of lattice structures, which provides a new design approach for triply periodic minimal surface lattice structures in applications of impact protection and bone scaffold.

Fracture Analysis and Working Stress Calculation of Bearing Cage Used in Charging Pump in a Nuclear Power Plant

This paper presents an analysis of the fracture accident of a cylindrical roller bearing cage used in a charging pump in a nuclear power plant. The causes and mechanisms of bearing cage breakage were investigated by material failure analysis and simulation calculations. Macroscopic observation results confirmed that the cage fracture occurred at the stress concentration position. The microfracture morphology of the cage obtained from scanning electron microscopy showed a fatigue feature. The analysis of residual stress indicated large residual stress perpendicular to the fracture surface. The finite element calculation showed that when the bearing was moving in and out of the working area during operation, large working stress appeared at the stress concentration position. Working stress and residual stress acted together, approaching the fatigue limit of materials, and finally led to the cage fatigue fracture. The stress of the other two structural cages of the same type of bearing was also calculated, and no such large stress concentration was identified; thus, one plastic cage was temporarily used.

Detrimental Effect of Surface Mechanical Grinding on Stress Corrosion Behavior of Monel 400 Alloy in Hydrofluoric Acid Vapor

Surface finish or integrity is always an important problem threatening the safety service of components. In this study, the stress corrosion cracking (SCC) and crack initiation behavior of Monel 400 alloy with different surface working treatments (mechanical grinding TD specimen [the grinding direction is perpendicular to the loading direction] and PD specimen [the grinding direction is parallel to the loading direction]) and electrolytic polishing [EP specimen]) in hydrofluoric acid (HF) vapor were compared by conducting slow strain rate test (SSRT) and constant loading test (CLT). Although SSRT can evaluate the enhancement effect of mechanical grinding on SCC susceptibility, it cannot quantify its contribution to the crack initiation stage. The CLT shows the cracking initiation rate of different specimens in order of: EP specimen < TD specimen < PD specimen. For mechanical grinding specimens, local corrosion and cracks are inclined to nucleate at the groove shoulder zone rather groove bottom, which is mainly ascribed to the accumulation of higher corrosion degradation and higher defects at the groove shoulder. As the shoulder edge on the PD specimen is less constrained, it is easier to induce cracking under the synergistic action of stress concentration and preferential corrosion dissolution. The shoulder edge of the TD specimen is not a stress concentration position under tensile loading, and the microcracks formed by grinding are parallel to the loading direction, which is not conducive to the development of microcracks. Therefore, the crack initiation sensitivity of the TD specimen is lower than that of the PD specimen.

Crystal Plasticity Finite Element Modeling on High Temperature Low Cycle Fatigue of Ti2AlNb Alloy

Ti2AlNb alloy is a three-phase alloy, which consists of O phase, β phase and α2 phase. Because of the difference in the mechanical characteristics between phases, Ti2AlNb alloy often exhibits deformation heterogeneity. Based on EBSD images of the Ti2AlNb alloy, a crystal plasticity finite element model (CPFEM) was built to study the effects of O phase and β phase (dominant phases) on stress and strain distribution. Four types of fatigue experiments, and the Chaboche model with 1.2%~1.6% total strain range were conducted to verify the CPFEM. The simulation results showed that the phase boundary was the important position of stress concentration. The main reason for the stress concentration was the inconsistency deformation of grains which resulted from the different deformation abilities of the O and β phases.

A novel braiding method and impact behaviors of 3-D braided composite three-way integrated circular tubes

3-D braided composite three-way circular tubes are integrated seamless connected tubes used for high strength connecting joints. We developed a novel braiding method of ‘yarn-added, yarn realigned’ to prepare three-way tubular preforms. Low-velocity impact compressions (LVIC) along axial direction were conducted to find effects of branch length and braiding layers on impact damage with drop-weight tester, high-speed camera, Micro-CT and digital image correlation (DIC). It was found that the three-way braided tubes have uniform structure both at the branches and joint regions. There is shear band induced by the weakened interface appeared after the peak load and formed within 60 μs under impact. The ultimate strength was positively correlated with the impact energy and braiding layers, while negatively correlated with the branch tube length. The test results showed that the branch length affected the position of stress concentration, and the influence of braiding layers on the LVIC behaviors is more significant than that of the branch length.

Conformal PDMS films for strengthening flexibility and retaining absorption in pyramid ultra-thin c-Si solar cells

Ultra-thin c-Si solar cells with the advantages of flexibility, translucence and portability are weakened in the performance of flexibility due to the surface light trapping structure, which reduces its future application potential. In this work, we designed a conformal PDMS film on the Si surface with pyramid structure to greatly inhibit the stress concentration on the surface and enhance the flexibility of ultra-thin c-Si solar cells. The maximum decrease of the Si surface stress can reach 71.19%, from 232.07 N/m2 of the pyramid structure to 66.86 N/m2 of the conformal PDMS film. In this process, it is found that at the stress concentration position, the conformal PDMS film can change the included angle of stress directions from 180° to 90° for reducing the probability of Si wafer rupture. In addition, the refractive index change in the vertical direction of the conformal PDMS film is more moderate, which can achieve the reflection reduction to improve the light absorption from 91.36% to 94.17%. Using multi physical simulation to evaluate the mechanical and photoelectric properties, the ultra-thin c-Si solar cell with conformal PDMS film has the best comprehensive performance, with the maximum surface stress of 74.59 N/m2 and the power conversion efficiency (PCE) of 19.04%, which is reduced by 49.49% and increased by 4.27%.

Coal Wall Instability Evolution Law of Large Mining Height Face in Steeply Dipping Hard Roof Soft Coal Seam

The coal wall of large mining height longwall fully mechanized face in steeply dipping coal seam has poor self-stability and high probability of instability, which is likely to induce secondary hazards such as support instability and flying gangue once catastrophic occurs, increasing the difficulty of surrounding rock control and production risk. Coal wall disaster prevention and control can improve the productivity of this type of face, and this is based on the sufficient understanding of damage patterns such as internal and external dispersion of coal wall pressure, deformation, and displacement; in order to clarify the coal wall dynamic failure law of large mining height face in steeply dipping coal seam, numerical calculation, field measurement, theoretical analysis, and other methods are comprehensively applied to the study. The results show that the coal wall bearing of large mining height face in steeply dipping coal seam is distributed asymmetrically, the stress concentration position in the upper and middle part of the face is closer to the coal wall than the lower part, the stress concentration degree is also larger than the lower part, and the stress concentration coefficients from upper to lower are 1.37, 1.50, and 1.29, respectively. Soft coal wall bearing is mainly dispersed in the shape of “X” in its interior, forming intertwined pressure increasing and unloading zones, and most of the pressure increasing zones are at the edge of the outer end of the pressure relief zone, compression-shear type is the main failure form of the coal, and the middle and upper part of the coal wall in the height direction is easy to break and cause disaster. The coal wall of steeply dipping face partitioning migration features is obvious, and vertical displacement within 3.77 m from the coal wall is lower > middle > upper; after 3.77 m, it is upper > middle > lower; horizontal displacement is middle > upper > lower; coal migration volume in the middle region of the face is largest; and the coal wall instability has high frequency, wide influence area, and alternating evolution law.

Loosening and fracture behavior of hybrid titanium-to-steel threaded connection under cyclic loading condition

Hybrid threaded connection involving titanium alloy and conventional steel components is introduced and studied in the face of lightweight metals application trend in modern machinery design. Pre-tightening procedure and loosening test were conducted based on specially designed specimen regarding cyclic pulling load condition. The loosening and fracture behavior were intentionally investigated through both mechanical analysis and related microstructure observation. The loosening evolution tendency along with the corresponding low cycle fatigue process was obtained, which was then further related with the examined structural mechanical responses and thread wear mechanism. Results have shown that in the early stage of fatigue process the pretension would experience a rapid and mass reduction accompanied with a high rate cyclic hardening response of structure. The cyclic load induced thread wear occurred mainly in the titanium nut component, which involved abrasive wear, fatigue wear and oxidative wear mechanism. Besides, the thread root turn closest to the bearing surface was demonstrated as major position of stress concentration and strain accumulation, making it critical source for loosening initiation and fatigue fracture, which should be properly optimized in both designing and fabricating process.

Research on Rock Burst Control Mechanism of Deep Buried Tunnel Using Surrounding Rock Modification Theory

In order to solve the rock burst disaster during the excavation of deep buried high stress hard rock tunnel, the method of changing the properties of surrounding rock by blasting is proposed in this paper. This method can effectively change the physical and mechanical properties of surrounding rock, reduce the storage capacity of surrounding rock corresponding to variable energy, and transfer the stress concentration position to the deep layer, which is an effective means to control rock burst. Taking the tunnel with rockburst tendency as an example, the stress distribution characteristics and displacement changes of surrounding rock under different modification depths are studied by using the numerical calculation method and elastic-plastic mechanics theory, and the effect is tested combined with microseismic data. The results show that the thickness of tunnel blasting loosening is closely related to the stress concentration of surrounding rock, and blasting loosening can effectively inhibit the occurrence and prevention of rock burst. With the increase of blasting depth, the maximum stress concentration decreases. The relevant research results can be used to reasonably design the blasting and loose surrounding rock reconstruction of high stress rock burst tunnel and improve the theoretical and practical basis.

Lightweight Design of Special Axle Hub Based on ANSYS Consider Different Working Conditions for Low Carbon

The lightweight design is considered to be of great significance for carbon reduction. Therefore, a lightweight design of a special axle hub is proposed based on ANSYS considering different working conditions. The structure of a special axle hub is identified. The ANSYS Workbench is used to mesh the hub and import the mechanical and physical parameters. Considering full load, emergency turning and braking, and the treatment of constraints and loads, the stress concentration position is obtained, and the lightweight design is carried out. The free vibration analysis and fatigue life prediction of the hub after lightweight design are verified. After the lightweight design, the optimized hub mass is reduced by 7.52%, and its structural stability and strength meet the demands of various working conditions. The application of a lightweight hub reduces CO2 emissions by 49875 kg during the life cycle of the vehicle. This study provides theoretical and methodological support for lightweight design of mechanical parts for low carbon.

Bearing Properties and Stability Analysis of the Slope Protection Framework Using Recycled Railway Sleepers

The slope protection framework developed using recycled railway sleepers offers a novel sustainable solution for slope protection. However, this has been inadequately reported, and its force and deformation, its protective effect, and the bonding characteristics between sleepers are still unclear. The slope protection framework project of a recycled railway sleeper embankment slope on the Beijing–Tongliao railway was numerically analyzed using three typical recycled railway sleeper slope protection structures. The bearing properties and the slope stability of rectangular, rhombic, and herringbone framework structures were determined. The results show that the stress state, stress level, and failure mode of the three types of slope protection structures are similar on average. The slope protection skeleton’s stress concentration position and failure area are all concentrated at the sleeper connection node at the slope base. The rectangular and rhombic framework structures have better stability than the herringbone framework. This study proposes applying a slope protection framework constructed entirely using recycled railway sleepers. Furthermore, it allows for proper disposal of recycled railway sleepers and a reduction in stone mining.

Analysis of the influence of groundwater seepage on the deformation of deep foundation pit with suspended impervious curtain

Taking the deep foundation pit of a metro station with rich water environment as the research background, a three-dimensional model of seepage-stress coupling is established. Numerical simulation is used to study the deformation process and influencing factors of deep foundation pit dewatering and excavation of the station, and the influence of dfferent pre-dewatering conditions and different depths of the water-stop curtain on the deformation of the foundation pit is analyzed. The results show that during the dewatering and excavation of foundation pits, seepage-stress has an obvious coupling effect in dewatering and excavation. The stress concentration position of retaining structure and the maximum displacement of lateral displacement are about 0.5–0.6 excavation depth of pile body. With the depth of precipitation increases, the seepage effect in the soil layer is obviously enhanced, which is not conducive to the stability of the foundation pit.

Experimental and theoretical research on relation between magnetic and mechanical property of structural I-steel column

As an ignored physical property, the metal magnetic memory signal has the potential for non-destructive testing in the future. To get the variation characteristic of magnetic signals under different loads, the normal components of surface spontaneous magnetic signals Hp(y) of the Q235 I-steel column were measured online. In this paper, the variation law of Hp(y) with stress was investigated in the position of the point of contraflexure and stress concentration. The physical mechanism between its magnetic property and mechanical property was established at the elastic and plastic stages. Experimental results show that Hp(y) reaches the extreme value in the position of stress concentration, where the magnetic field gradient K presents the zero-crossing phenomenon along with the peak–peak value phenomenon on the left and right sides. In the position of the point of contraflexure, when the Hp(y) curve reaches the extreme value, the corresponding equivalent uniaxial stress σeq is 54 MPa, which is in good agreement with the Jiles–Atherton (J–A) model. Characteristic parameters Kmax, ζ, and η can reflect the extent of damage of the specimen. In the position of stress concentration, Hp(y) values decrease sharply at the early plastic stage. The analysis shows that the distribution regularity of Hp(y) at the elastic stage is in good agreement with the J–A model. The modified J–A model is consistent with the experimental results at the plastic stage.

Nanoindentation-induced deformation behaviors of tetrahedral amorphous carbon film deposited by cathodic vacuum arc with different substrate bias voltages

The diamond-like carbon (DLC) thin films were deposited using a direct cathode vacuum arc (DCVA) technology at a variety of substrate bias voltages from 0 to −50 V. The cross-sectional morphologies, compositions, chemical state, and surface roughness of the prepared DLC films were investigated and highly controlled by the bias voltage. The detailed microstructure of the DLC films and interfaces were observed by transmission electron microscope (TEM). The mechanical properties of the DLC films were obtained by nano-indentation test and significantly depended on the applied bias voltages. The deformation mechanisms and crack initiation/propagation of the DLC films at different substrate bias voltage (SBV) in the nano-indentation process were analyzed by TEM and clarified by evolution models. There were three forms to release the energy induced by indenter for the DLC films with an amorphous structure, as follows: (i) the plastic deformation stemmed from the flow of a large number of C atoms; (ii) the formation and slip of the shear bands; (iii) the cracking at the position of stress concentration.

Numerical Simulation Study on Failure of Graphite Heat Exchanger with Orifice Block

Phenolic impregnated graphite is a new kind of material with higher corrosion resistance and thermal conductivity. In the chemical industry, it is often used to make containers and pipes containing strong acid and alkali medium. It is the characteristic of phenolic resin impregnated graphite heat exchanger that the corrosion resistance and heat transfer efficiency of the heat exchanger are greatly improved. In this paper, a failure case of graphite heat exchanger in a chemical enterprise in Shanghai was analyzed by numerical simulation. According to the actual working conditions, the temperature field and flow field in the porous graphite heat exchanger were numerically simulated. The numerical simulation results of temperature field showed that the graphite block would be subject to thermal shock when the equipment was started and shut down, resulting in the increase of temperature difference of graphite block In addition, the pre tightening force of the flange of graphite heat exchanger was simulated by numerical simulation, and it was found that the position of stress concentration coincided with the crack position of actual failure, and the improvement suggestions were put forward for the structural design of graphite block.

Diaphragm Splicing Deviation in Steel Bridge Deck: Effect on Fatigue Performance and Its Preventive Measures

Asymmetric scallop cutouts on the sides of a U-rib caused by diaphragm splicing deviation were detected in a bridge. To study the possible effect of this phenomenon on fatigue performance, the stresses of the asymmetric scallop cutout and the diaphragm-to-rib weld were tested, and the influence was compared with that of the symmetric case. This and numerical simulation showed that, for the side of the U-rib with the smaller-radius scallop cutout, the position of stress concentration transferred from the scallop cutout to the diaphragm-to-rib weld, and the stress value of the weld was greater than that of the scallop cutout, resulting in more fatigue damage to the weld. To reduce the adverse effect caused by the asymmetric scallop cutouts, an application experiment with a hammer peening was carried out, and the stresses before and after hammering were monitored. The results showed that hammer peening could improve the fatigue performance by inducing compressive stress.

Structural optimization of the heavy haul wagon body based on MPSO-BP algorithm

Fatigue cracks usually occur at the welded joints of the heavy haul wagon body. To effectively limit the occurrence of such problems at the design stage, this study moves from the structural design strategy to the structural optimization method. First, when analyzing the results of the finite element simulation, the new structural design strategy not only considers the maximum stress, which can reflect the stress concentration position, but also focuses on the stress distributions at the key welded joints. Second, in order to further reduce the stress at the key positions, the study presents an optimization method that integrates an orthogonal experimental design, BP network, and MPSO algorithm. Finally, an 80 t gondola car body is optimized from the whole structure to the local area by the proposed method. Further, it is also verified that the structural optimization method of the heavy haul wagon body is reasonable and feasible.

Safety performance of self-expandable NiTi alloy stent

In order to evaluate the safety performance of self-expandable NiTi alloy stents systematically, the dynamic safety factor drawn up by International Organization for Standardization, was used to quantitatively reflect the safety performance of stents. Based on the constitutive model of super-elastic memory alloy material in Abaqus and uniaxial tensile test data of NiTi alloy tube, finite element method and experiments on accelerated fatigue life were carried out to simulate the self-expansion process and the shape change process under the action of high and low blood pressure for three L-type stents of Φ8×30 mm, Φ10×30 mm, Φ12×30 mm. By analyzing the changes of stress and strain of self-expanding NiTi alloy stent, the maximum stress and strain, stress concentration position, fatigue strength and possible failure modes were studied, thus the dynamic safety factor of stent was calculated. The results showed that the maximum stress and plastic strain of the stent increased with the increase of grip pressure, but the maximum stress and strain distribution area of the stent had no significant change, which were all concentrated in the inner arc between the support and the connector. The dynamic safety factors of the three stents were 1.31, 1.23 and 1.14, respectively, which indicates that the three stents have better safety and reliability, and can meet the fatigue life requirements of more than 10 years, and safety performance of the three stents decreases with the increase of stent's original diameter.