Stress Concentration Zone Research Articles

Failure characteristics of overlying strata and mechanism of strong ground pressure during the large‐scale and continuous mining of deep multi working faces

AbstractIn this study, a three‐dimensional large‐scale numerical model is established to investigate the failure characteristics of overlying strata and mechanism of strong ground pressure induced by excavation disturbance from multiple working faces. The characteristics of overlying strata fractures, heights of the caving zone and the fracture zone, and evolution of the stress field are systematically analyzed. The numerical simulation results reveal that the height of the caving zone after mining is 8.1 m, and that of the fracture zone is 27.3 m under the conditions of gently inclined thin coal seams. These findings are consistent with the theoretical results. The fracture development process can be divided into three stages: extensive development of new fractures, partial compaction of fractures, and closure of numerous fractures. In the structure of the post‐mining overlying rock, four stress zones are identified as follows: two zones of stress concentration at both ends of the working face, respectively, a zone of relatively high stress at the middle of the working face with low overlying strata, and a zone of stress fully released at the middle of the working face with high overlying strata. Comprehensive analysis of the maximum vertical stress of the cross section and the stress of the working face indicates that the stress increases significantly when mining enters the gob square stage and the roof does not collapse timely.

Study on the Dynamic Evolution of Mining-Induced Stress and Displacement in the Floor Coal-Rock Induced by Protective Layer Mining

Protective layer mining is the most effective means to prevent and control coal and gas outbursts. In order to deeply understand the dynamic evolution law of mining stress and displacement of the bottom plate coal rock body in the process of protective layer mining, the effects of upper protective layer mining on stress variation and displacement deformation in the underlying coal seam were studied using the similar experiment and FLAC3D simulations. The results reveal that mining in the 82# coal seam notably alleviates pressure in the 9# coal seam below, with an average relief rate of 86.2%, demonstrated by the maximal strike expansion deformation rate of 11.3‰ in the 9# coal seam post-mining. Stress monitoring data indicates a stress concentration zone within 32 m ahead of the working face, and a pressure relief zone within 51 m behind it. The research provides a scientific foundation for pressure-relief gas extraction techniques, affirming the substantial impact of upper protective layer mining on alleviating pressure in underlying coal seams, enhancing safety, and optimizing mining efficiency.

Research on defect image inversion method based on multiple lift-off values magnetic memory signals

Metal magnetic memory (MMM) technique can detect macroscopic defects and stress concentration zones of ferromagnets, and it can identify the approximate positions of these flaws, while a little further information about defects characteristics can be provided. To promote study in this area, defects were modeled as located magnetic dipoles whose strength and locations should be determined, and the technique of truncated generalized inverse and reduced space inversion were used to image the dipole strength. Through the simulation experiment, we found that the inversion image quality was influenced by the lift-off value. When the magnetic field data for inversion was measured with a small lift-off value, the inner dipoles contributions were easily buried and the inversion image can only shown the surface dipole well. In contrast, if the magnetic field data was measured with a greater lift-off value, it was possible to inverse the deep dipoles while the image was somewhat out of focus. To overcome this limitation, we composed the magnetic field data measured under different lift-off values together for inversion. We used the same model to test this approach and applied it on real crack defects magnetic field data. It demonstrates that the new inversion approach shows better accuracy than the single lift-off value and it was possible to infer the location and size of defect by the evaluation of preset dipoles.

Interfacial microstructure and synergistic enhancement mechanism of symmetric gradient SiCp-reinforced aluminum matrix sandwich structure

Inspired by the biological sandwich structure to achieve excellent mechanical properties with both high strength and high ductility, the symmetric gradient silicon carbide particles (SiCp) reinforced aluminum (Al) composites, consisting of 10% SiCp/Al-3% SiCp/Al-Al-3% SiCp/Al-10% SiCp/Al, was fabricated using spark plasma sintering technology. The flat interface was achieved through hot rolling, significantly improving the bonding of interlayers. The differences in SiCp content among layers led to distinct evolution patterns in microstructure. In the pure Al layer, a notable continuous recrystallization mechanism was observed, while in the 3% SiCp/Al and 10% SiCp/Al layers, the recrystallization mechanism was nucleation-growth. The transmission electron microscope results indicated that the hindrance of dislocation motion at the interlayer interface and SiCp-Al interface enhanced the dislocation density, thereby improving its plastic deformation capability. On the other hand, the deflection and passivation of cracks at the interlayer interface significantly improves toughness. The differences intragranular strain among layers were most pronounced at the interlayer interfaces, leading to them becoming the stress concentration zone. Uncoordinated plastic deformation leads to sequential failure of layers, vertical cracks first occurred in the 10% SiCp/Al layer, then propagated towards the interlayer interface and the 3% SiCp/Al layer, respectively.

4D printing of polylactic acid (PLA)/PLA-thermoplastic polyurethane (TPU)-based metastructure: examining the mechanical, thermal, and shape memory properties

Abstract In the present day and age, increasing demand concerning the enhancement of the mechanical performance of Shape Memory Polymer (SMP) based structures has paved the way for developing newer metastructures of enhanced load-bearing, damping capacity, and durability. The present study focuses on developing SMP-based metastructures made of commercially available Polylactic Acid (PLA) and 30% by wt. of Thermoplastic Polyurethane (TPU) blended PLA. The designed metastructures are initially analyzed using numerical modeling to prevent lateral deformation, acute stress concentration zones, and row-wise collapse. Mechanical tests reveal that blending TPU with PLA enhances the material's flexibility and ductility, further improving the toughness and fracture resistance of the built metastructures. Loading-unloading and shape recovery tests (under compression mode) of the s-shape metastructure reveal that the PLA/TPU metastructure withstands ≅ 170 N load, less than neat PLA's ≅ 223 N due to TPU's flexibility. PLA/TPU endures 30 cycles, while PLA fails after the 9th cycle. In shape recovery plots, PLA/TPU metastructures exhibit a lower standard deviation (~0.32%) than PLA (~1.4%), attributed to the entropy decrease and cross-linkage disentanglements of PLA. Furthermore, a Dynamic Mechanical Analyzer (DMA) assesses glass transition temperature, energy storage capability, and dissipation in variation with the temperature. The nephograms of ABAQUS result divulge accurate fracture initiation locations of the metastructure unit cells, which involves implementing ductile damage behavior modeling by employing damage initiation and evolution parameters. Finally, assessing compression tests and shape recovery behavior results elucidates that these SMP-based metastructures are promising for load-bearing pallets in the transporting and packaging industries, providing superior damping and self-repairing capabilities during significant plastic deformations.

Effort of damage parameter in assessment of low cycle fatigue

A low-cycle fatigue (LCF) analysis is one of the main design stages for highly loaded structural elements used in various applications. For this analysis, it is necessary to determine the values of local stresses and deformations, taking into account both elastic and plastic regions in the zones of stress concentration. This study presents and assesses the engineering methods used for prediction of low-cycle fatigue in structural elements. For zones of stress (strain) concentration, the Neuber-Makhutov method for LCF, taking into account the type of material stress-strain diagrams, is employed. The concept of distributed damage, based on the main ideas of the continuum damage mechanics of Kachanov-Rabotnov, was used. An approach employing the damage parameter for assessment of damage accumulation in LCF in highly loaded areas of structural elements is presented.

Waste electric porcelain-based refractory bricks with significantly enhanced mechanical properties: preparation, characterization and mechanism

Using waste electric porcelain to produce refractory bricks is a viable technology. However, these bricks currently have lower mechanical strength compared to those made from mineral raw materials. This study optimized the proportions and sintering process to develop high-strength refractory bricks from waste electric porcelain. The impact of physical phase, crystallinity, sintering temperature, and fly ash content on the bricks properties was examined. The research found that low crystallinity in waste electric porcelain leads to disorganized growth of mullite nano whiskers at high temperatures, reducing mechanical strength due to insufficient support structures and stress concentration zones. The addition of fly ash promotes the formation of uniformly dispersed mullite columns through non-uniform nucleation, significantly improving mechanical properties by creating a collective tetrahedral structure. Bricks with 20 wt% fly ash sintered at 1440 °C achieved a flexural strength of 77.3 MPa, a compressive strength of 156 MPa, and an apparent porosity of 10.89%. High-temperature simulations indicate that these bricks exhibit good compressive strength and deformation resistance, with no adhesion observed. These high strength refractory bricks have promising potential for use in various high-temperature applications, including kiln linings, high-temperature flue, high-temperature support components and other industrial processes where durability and thermal stability are critical.

The Metal Magnetic Memory Method and Its Application for Early Detection of Stress Zones in NPP Piping

The policy of each NPP is to ensure safe production of electrical energy as well as security of supply. It is known that stress concentration (SC) zones, in which corrosion, fatigue and brittle processes develop most intensively, are the main sources for the occurrence of failures in operational structures. In this paper the method of magnetic memory of metal (MMM method) is discussed. This method was developed in order to conduct an early diagnosis of the condition of the metal of the technological equipment. The method is new and has not yet been studied in depth. The article presents the essence of the method, equipment and scheme of scanning the object. The method was applied to perform a pipeline test of NPP equipment. The results are given and verified. The aim of the article is to present the application of the Metal magnetic memory method in practical terms for performing technical diagnostics of the equipment in NPPs.

An efficient multi-scale method for failure mechanism analysis of SiCf/Ti composites with experimental validation

This study focuses on developing and validating a high-precision concurrent multi-scale finite element model considering the progressive damage model (PDM) and the cohesive zone model (CZM) for predicting the damage evolution of SiCf/Ti composites. At the macro-scale, a constitutive model describing the anisotropic damage criterion of SiCf/Ti composites was implemented by means of a user-defined subroutine (VUMAT). Meanwhile, based on the submodelling, a representative volume element (RVE) model was developed to systematically analyze the failure modes of the SiC fibers, the matrix, and the interface, respectively. Specifically, the crack initiation and propagation in the stress concentration zones were further discussed. Simultaneously, experimental methods were employed to verify the feasibility of the current model. During the uniaxial compression, the brittle fracture of the SiC fibers and ductile fracture of the matrix are the predominant failure modes of SiCf/Ti composites, with fiber fracture being the dominant factor. The crack initiates at the fibers, propagates rapidly to the fiber-matrix interface, and then extends into the matrix. Moreover, the fracture locations are suscepticle to stress concentrations, leading to the crack initiation and propagation. The predicted results show that the local damage has a significant effect on the failure mechanism of SiCf/Ti composites and this multi-scale model provides a scientific reference for the design and optimization of composites.

Relaxation of residual stress in welded plates during long life fatigue loading

The presence of residual stress in welded components can have a significant impact on fatigue response. Residual stresses were assessed in thick welded A36 cantilever specimens (T-specimens) at the fatigue critical area. Testing involved subjecting the T-specimens to varying bending stresses and load cycles, leading to different levels of local plasticity at the weldtoe. Extensive X-ray diffraction (XRD) measurements were conducted to analyze the residual stress distribution near the weldtoe. Residual stress states were evaluated in both as-welded specimens and those subjected to fatigue loading with strain amplitudes up to 0.001 at the weldtoe stress concentration zone. The findings suggest that fatigue loading can induce changes in the welding-induced residual stress field, ranging from moderate to significant alterations. Therefore, fatigue life predictions should consider the potential evolution of residual stresses due to cyclic loading, including the effects of cyclic stress relaxation in fatigue life calculations.

Optimizing restorative procedure and material selection for pulpotomized primary molars: Mechanical characterization by 3D finite element analysis

PurposeThis study aimed to assess the stress distribution in pulpotomized primary molars with different types of restorative materials using 3D-finite element analysis (FEA), and provide valuable insights into the selection and application of restorative materials, with the ultimate goal of reducing the risk of pulpotomy failure and protecting residual dental tissue. MethodsFour 3D models of pulpotomized primary molars with different restorative materials according to the material and its elastic modulus were analysed: resin composite, stainless steel crowns (SSCs), prefabricated zirconia crowns and endocrowns. The food layer was also designed before vertical and bucco-lingual forces were applied to simulate physiological masticatory conditions. The results were obtained by colorimetric graphs of the von Mises stresses (VMS) in the restoration and tooth remnant. The maximum shear stress on the bonding interfaces and pressure stress on the Mineral trioxide aggregate (MTA)-pulp interfaces were recorded. ResultsThe results of the 3D-FEA showed that all restorative materials generated stresses and strains on the tooth structure after pulpotomy. In the resin composite group, the marginal enamel exhibited the highest stress peaks. In the zirconia crown and SSC groups, there was a concentration of stress at the dentin-restoration margin. The shear stress concentrations were mainly at the adhesive margins, with lower levels around endocrowns compared to other groups. MTA in the resin composite group experienced more VMS than in the other group. The resin composite group also generated relatively higher pressure stress values at the MTA-pulp interface compared to the other groups. SignificanceIn the model of primary teeth following pulpotomy, the three types of restorations covering the occlusal surface can effectively reduce the stress on pulp capping materials under occlusal loads, thereby potentially decreasing the risk of pulpotomy failure. In addition, the group of endocrowns demonstrated reduced stress at the bonding interface and in the stress concentration zone near the dentist-restoration edge, making them more effective at protecting residual dental tissue.

Influence of curing conditions on the interface bonding properties of CFRP plate–steel

To understand the influence of different curing conditions on the interface bonding performance of CFRP-reinforced steel plates, tensile tests were conducted on epoxy resin dog bone specimens and CFRP-reinforced steel plate double lap specimens. The study explored the tensile performance of dog bone specimens under different curing temperature/time conditions and the mechanical properties of double lap specimens under different curing pressures and curing temperature/time conditions. The results indicate that increasing the curing temperature can shorten the curing time of dog bone specimens and also improve their elastic modulus and tensile strength. Elevating the curing pressure, curing temperature, and extending the curing time cause a transition in the failure mode of the double lap specimens from surface fiber stripping of the CFRP plate to a mixed failure dominated by surface fiber stripping and adhesive–steel interface debonding, and the bearing capacity, interface ultimate slip, and peak shear stress of the specimens significantly increase, with the shear stress concentration zone gradually expanding from the loading end towards the free end, simultaneously, the ductility of the specimen increased, and the failure process was delayed. The ultimate load, interface peak shear stress, and ultimate slip of the specimens cured at 90 °C are significantly higher compared to the control group cured at 20 °C and 0.01 MPa. However, with increasing curing time, the rate of growth shows a decreasing trend.

Study on the dynamic response and roadways stability during mining under the disturbance of hard roof break

Under the condition that the working face was directly covered with hard roof, the abrupt breaking of hard roof release significant amount of energy, thus prone to triggering dynamic disasters such as roadway instability or rockburst. This paper based on the engineering background of the Xieqiao Coal Mine's 11,618 working face, a numerical simulation method was put forward to study the dynamic response of roadway under the disturbance of hard roof breaking and proposed an evaluation index IC for roadway stability. Research indicates that the elastic energy released during the periodic weighting of the hard roof is higher than that released during the first weighting. Under the dynamic disturbance caused by hard roof breaking, the peak stresses of the roadway was slight decreased, accompanied by a significant increase in the range of stress concentration and plastic zone expansion. Roadway deformation patterns are significantly influenced by hard roof breaking, with noticeable increases in deformation on the roof and right side. During the period of hard roof breaking, the possibility of instability of the roadway increase significantly due to the disturbance caused by the dynamic load. The research results reveal the instability mechanism of roadway under the condition of hard roof, and provide a more reliable basis for evaluating the stability of roadway.

Localized corrosion and brittle fracture of X80 carbon steel under tensile stress induced by sulfate reducing bacteria

The microbiologically influenced corrosion (MIC) of X80 carbon steel under varying tensile stresses with the presence of sulfate reducing bacteria (SRB) was investigated. The stress loads affected the SRB sessile cell counts on the carbon steel, and exerted a significant influence on the rate of MIC to which the carbon steel was subjected. Stresses below 400 MPa elicited a modest promotion effect, whereas the stress of 550 MPa which was approaching the yield strength markedly expedited the corrosion of SRB on X80 carbon steel. Finite element simulation revealed that the elastic stress of 550 MPa led to plastic deformation in the stress concentration zone at the base of the corrosion pit, exacerbating the dissolution of the metal in pits and inducing secondary pitting corrosion. Additionally, this work confirmed that SRB promoted the permeation of hydrogen into the carbon steel matrix. The coexistence of high elastic stress and SRB rendered X80 carbon steel more prone to brittle fracture.

Coherence analysis of the crack strain field in coal rock with borehole-crack composite defects

Cracks in the coal mass surrounding boreholes for gas extraction will significantly impact the efficiency of gas extraction. The primary objective of this research is to explore the effects of original cracks and borehole grouting on cracks propagation in the coal mass around the boreholes. Accordingly, this research employs coal mass with borehole, borehole-cracks composite coal mass, and borehole-cracks-grouting coal mass as experimental subjects. The digital image correlation (DIC) platform of the coal deformation and damage was used to carry out the damage experiment of the coal mass around the boreholes. The Matlab GUI crack detection system was used to extract the crack skeleton, and time domain characteristics of strain and strain rate coherence coefficients in the area near the cracks before and after grouting were obtained according to the Pearson product-moment coherence analysis method. The research findings obtained in this article are as follows: (1) The tips of the prefabricated fractures act as stress concentration zones, where the further extension of wing cracks severely degrades the continuity and uniformity of the coal mass. The injection of ultrafine cement significantly enhances the overall deformation resistance of the coal mass. However, the local stiffness under specific stress conditions shows limited improvement due to the presence of micro-damage around the prefabricated cracks. (2) The expansion of the cracks around the borehole and the wing cracks at the tips of the prefabricated cracks is primarily due to tensile forces. As the cracks extend to the top of the specimen, they transition to a mixed mode of tensile-shear, with the extension of Type II wing cracks being faster and more unstable compared to Type I. (3) The correlation coefficient between multiple crack fields and their highly significant characteristics indicate that crack propagation influences each other. This influence diminishes as the specimen progressively damage. (4) The mechanical properties of the specimen show a strong correlation with the average interference coefficient R̅ of the cracks. Better mechanical properties of the coal rock are associated with higher geometric similarity in the strain and strain rate curves across multiple strain fields, the cracks tend to propagate synchronously.

Non-Contact Magnetometric Diagnostics of Pipelines Using the Metal Magnetic Memory Method

The non-contact magnetometric diagnostics (NCMD) of buried pipelines (oil and gas pipelines, heat lines, water supply lines, etc.) has been developed by Energodiagnostika Co. Ltd. since 2000 based on the 25-year experience of the metal magnetic memory (MMM) method application (ISO 24497-1:2007(E)). NCMD is based on measurement along the pipeline route of the Earth’s magnetic field intensity (HEarth) distortions caused by variation of the pipe metal magnetization in stress concentration zones (SCZs) and in zones of developing corrosion-fatigue damages. The paper describes the experience of complex diagnostics of buried pipelines based on NCMD, the MMM method and conventional methods of non-destructive testing. Based on NCMD results, the most strained pipeline sections are determined for their opening and additional control by the MMM method, ultrasonic testing and other NDT methods. On opened pipeline sections the MMM method is used to determine the presence of defects and the mechanical properties of the metal by hardness parameters. Inadmissible defects are removed, and the actual mechanical properties (yield strength and ultimate strength) are taken into account in check strength calculations. The inspection should provide the answer to the following question: “Where and when should a damage or an accident be expected?”. If this task is solved, then the possibility of timely replacement or repair of a potentially hazardous section is provided. Application of NCMD in combination with additional inspection of pipelines (UT, eddy current, etc.) in prospect holes determined by NCMD is aimed exactly at solution of this problem. The paper presents the experience of NCMD of buried polyurethane foam insulated pipelines in order to detect girth welded joints with developing damage.

Preliminary Failure Analyses of Loaded Hot Water Bottles

Hot water bottles are widely utilised for their therapeutic advantages, such as relieving muscle tension and imparting warmth. However, the increasing frequency and potential risks associated with bursting or failure necessitate a detailed examination of the contributing factors as their failure is not fully understood in a scientific manner. With the apparent lack of analysis of hot water bottles in the literature, this study employs, for the first time, a dual methodology involving finite-element (FE) analysis conducted in ABAQUS and experimental validation to systematically investigate the underlying mechanisms leading to failure incidents. Through FE modelling and analysis, the stress and strain distribution within typical hot water bottles is modelled under compression loading conditions, facilitating the identification of vulnerable areas prone to failure. Experimental validation encompasses uniaxial loading compression tests on distinct specimens, generating load–displacement curves that elucidate material responses to compressive forces and highlight variations in load-bearing capacities. The study explores diverse failure modes, attributing them to stress concentration at geometric transitions and contact regions. Stress–strain curves contribute valuable insights into material characteristics, with ultimate stress values as crucial indicators of resistance to deformation and rupture. The FE analysis simulation results visualise deformation patterns and stress concentration zones. The findings illustrate that the highest stress concentration areas exist in the internal boundary of hot water bottles near the neck and cap region. This is experimentally confirmed through the bursting failures of four samples, with three failures occurring in this specific region. The findings support the guidance that users should avoid sleeping with a hot water bottle as it may fail under compression if they lay on top of it. Meanwhile, this result guides manufacturers to strengthen the weak areas of hot water bottles around the nicks and edges. This study significantly enhances our understanding of hot water bottle mechanics, thereby guiding design practice to improve overall performance and user safety. In summary, hot water bottles are commonly used but have not been investigated scientifically regarding external loading conditions and their related failure, as the current study has achieved. Identifying the weak points through experiment and simulation directs manufacturers towards required improvements in particular regions, such as the bottleneck and edge reinforcement during the design and manufacturing phases.

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

The dynamic evolution of powder flow and wall normal stress in different flow pattern silos

Major safety accidents involving silos in industrial processes are closely associated with abnormal wall stresses. The effects of different flow patterns and particle velocity distribution on wall stresses are investigated during silo discharge. The existence of particle velocity retardation layer near the wall boundary in the mass flow was observed by tracer particles and high-speed camera. The silo discharge undergoes a transformation from funnel flow to mass flow at a hopper half angle of 30°. As the outlet diameter increases, the time point of the flow pattern transformation becomes more and more later. The evolution of particle velocity in the central of the funnel flow is related to the outlet velocity wave propagation and the V-shaped surface expansion. The relationship between stress fluctuations and velocity characteristics is established. The flow channel simultaneously expands upwards as the velocity wave propagates and generates a periodic fan-shaped velocity wave at the top. The formation of stress concentration zone at the bin/hopper transition was observed.

Числовий аналіз впливу стрічкових включень на концентрацію напружень в тонких циліндричних і конічних оболонках з прямокутними отворами

Thin-walled plate-shell structures are widely used in various branches of technology and the national economy, in particular in the aerospace industry, the oil and gas industry, power engineering, construction, etc. The continuity of such structures is often disrupted by various inhomogeneities in the form of openings, inclusions, recesses, cracks, etc., which are local stress concentrators. Reducing the concentration of the stresses that develop in the vicinity of such structural inhomogeneities is an important problem in deformable solid mechanics. In particular, a pressing problem in the design of new equipment in modern mechanical engineering is a significant reduction in material consumption and an increase in the service life of cast parts taking into account the use of new materials and technologies. Such parts are responsible for the competitiveness of new equipment for various industries. This paper presents the results of a numerical simulation and analysis of the stress and strain field of thin-walled cylindrical and truncated conical shells with rectangular openings and tape inclusions around them. The material of the inclusions has properties that differ from those of the base material of the shells. The effect of the geometrical and mechanical characteristics of the inclusions on the parameters of the stress and strain field in the vicinity of the openings was studied by varying the elastic modulus of the inclusion material and the inclusion width. For definiteness, the inclusions were assumed to be homogeneous and located in the shell plane. The stress and strain intensity distributions in the zones of local stress concentration were obtained. The numerical results for shells of both shapes were compared with the corresponding results for shells with a circular opening. The study showed that the presence of a “soft” homogeneous tape inclusion helps in reducing the stress concentration around rectangular openings by ~ (21 – 54) % depending on the width of the inclusion and its elastic modulus, both in cylindrical and in conical shells. Unlike shells with a circular opening, in this case the presence of inclusions does not cause the mechanical effect of shifting the stress concentration zone from the contour of the opening to the interface between the materials.