Stress Concentration State Research Articles

Failure Mechanism and Structural Safety Assessment of the Primary Support Structure of Soft Rock Tunnel: A Case Study

Based on a soft rock tunnel in a mountainous area of northwest Yunnan Province, a refined numerical model of different support forms considering structural interaction is established to comprehensively evaluate the structural failure mechanisms and mechanical responses of different support forms. Research shows that compared to steel rib structures, the axial force and bending moment of sprayed concrete structures is larger under different combinations of support structures. Compared with a single-layer support structure, the stress distribution at each position of the double-layer I-shaped steel structure is more uniform, and the sprayed concrete structure only experiences compression damage at the corner of the side wall. As the strength of sprayed concrete increases, the stress distribution of sprayed concrete at the arch waist becomes more uniform. The stress concentration state of sprayed concrete at the corner of the tunnel wall and the plastic yield state of the steel rib structure have also been improved. The higher the concrete strength, the lower the stress ratio on the steel rib’s inner and outer sides.

Influence of microstructure and geometric dimension on metal magnetic memory testing

Metal magnetic memory testing (MMMT) is a prominent NDT method for early damage diagnosis of ferromagnetic structures due to its significant advantages in stress concentration state evaluation. While it is still challengeable to apply the existing criteria from the laboratory to practical application, and some of the main reasons are the microstructure transformation and geometric dimensions for the ferromagnetic material itself. To explore the influence of microstructure and geometric dimension on MMMT, static tensile tests of specimens made of Optim 900QC steel with different microstructures and geometric dimensions have been carried out, and the MMMT signals of the specimens during the tests were collected and analyzed. It is found that the microstructure not only change the strength of the steel, but also affect the MMMT signal of the steel, and the bigger the grain is, the stronger the magnetic field is. Geometric dimension would also affect the MMMT signal, and thickness affects the MMMT signal the most, width less, and length the least. The results of the present work indicate that the microstructure and the geometric dimensions should be the same to fix the gap of damage determination criteria between the laboratory and the engineering application. When it is impossible to fabricate specimens with the same dimension with the engineering structure, set the same thickness would improve the accuracy of the criteria.

The numerical simulation of particulate reinforced composites by using a two-dimensional VCFEM formulated with plastic, thermal, and creep strain

The working environment of industrial fields where the Particulate Reinforced Composites (PRCs) are commonly used is complex. In these harsh environments, PRCs produce some nonlinear strains, such as plastic, thermal, and creep strains, which affect the performance of PRCs to some extent. In addition, the actual structure of PRCs contains a massive amount of inclusions, and the number of inclusions could reach the magnitude of millions or even tens of millions. For the large-scale numerical simulation of PRCs, ordinary displacement finite element requires a large number of meshes, which increases their computational complexity and unsolvability. For real PRCs with random particle distribution, homogenisation theory does not accurately capture the true state of stress concentration in PRCs. The Voronoi cell finite element method (VCFEM) can be a good solution to the drawbacks of the above two numerical simulation methods. The subject of the present work explores a modified complementary energy generalized function that takes the constitutive relationship of nonlinear strain into account, proposing a two-dimensional VCFEM, formulated with plastic, thermal, and creep strain. The simulation results are compared with the computed results of a commercial finite element software Marc to verify the validity and accuracy of the VCFEM. Results of the comparison show that the VCFEM has the advantages of simple meshing and faster computation under the same precision. The VCFEM, considering plastic, thermal, and creep strains proposed in this paper, lays the foundation for future large-scale numerical simulations of PRCs containing a large quantity of inclusion structures and can realistically capture the stress concentration state of PRCs. At the end of the article, the impact of the order of stress function on the calculation results is discussed.

Experimental Investigations on the Mechanical Performances of Auxetic Metal-Ceramic Hybrid Lattice under Quasi-Static Compression and Dynamic Ballistic Loading

In recent years, there have been increasing research interests in investigating the compression and ballistic responses of metal-ceramic hybrid structures, mainly making use of the synergistic effects of conventional metal honeycomb structures and infilled ceramic matrix materials. In this paper, a novel hybrid auxetic re-entrant metal-ceramic lattice is designed and manufactured to overcome the intrinsic conflicts between the strength and toughness of architected mechanical metamaterials, synergistic effects of auxetic re-entrant metal honeycombs and infilled ceramic materials are experimentally and numerically studied, and auxetic deformation features and failure modes are characterized with the digital image correlation (DIC) technique as well. It was found that (1) the infilled ceramic matrix of conventional honeycomb frames only endure longitudinal compression or impact loading along the external loading direction, while auxetic metal re-entrant honeycomb components endure both longitudinal and transverse loading due to the negative Poisson′s ratio effect and (2) the collaborative effects of infilled auxetics and the constraint frames’ hybrid structure dramatically moderate the stress concentration state and improve the impact resistance of single-phase ceramic materials. Our results indicate that the auxetic hybrid design exhibits promising industrial application potentials for blast protection engineering.

Study on overlying strata migration law of strip interval filling mining.

Aiming at the problem that it is difficult to popularize in Guizhou mining area due to high filling cost, taking 11071 working face of Panzhihua Coal Mine in Liupanshui as the research background, the idea and technology of strip interval filling mining are put forward. Through theoretical analysis and mechanical tests, the reasonable range of strip spacing distance and mechanical parameters of filling body were obtained. The FLAC3D simulation software is used to analyze the stress field, displacement field, and plastic zone of five kinds of design schemes, and the UDEC simulation software is used to carry out the filling mining of No.7 coal seam. The results show that the strip interval filling mining technology regards the empty roof area as a controllable underground spatial structure, and the single spatial stress field changes little. The load on the immediate roof is gradually transferred from the top of the empty roof area to the top of the filling body. The "filling body-direct roof" structure improves the self-bearing capacity of the immediate roof, and the overlying surrounding rock migration is controlled. With the increase of mining depth, it gradually tends to the original rock stress, and the control effect on surface subsidence is more significant. Finally, "filling 3 m interval 3 m" is determined as the optimal filling scheme. In the process of simulated filling mining, the peak stress in the stress concentration area of the front coal wall shows a trend of "increase-decrease-increase," and the peak stress curve of the immediate roof in the middle of the stope changes from "increase-decrease 'trend to' increase-decrease-increase-decrease" trend. The rock layer near the immediate roof is in a stress concentration state in the coal wall area on both sides, and the middle part is not obvious.

Enhanced stress concentration sensitivity of SiCp/Al composite with network architecture

For network architecture design, stress concentration sensitivity caused by particle shape may change, which is rarely studied. Here, the particle shape dependent stress concentration and its effect on the deformation, fracture, and mechanical properties were investigated. Three particle shapes including hexahedron, twenty-six face polyhedron, and sphere were utilized to generate different stress distribution states in the matrix. A numerical composite model showing network architecture (like grain boundary) was applied. A strong correlation between particle shape, stress concentration factor ( R SiC), and mechanical properties of network composite was built. The particle shape affected the load-bearing capability due to the stress concentration state generated at particle edges. Near the yield point, hexahedron particle wall parallel to the load direction (PaW) was more effective in carrying loads (∼1000 MPa) than that of twenty-six face polyhedron (750–1000 MPa) and sphere (600–1000 MPa) particles. In network composites reinforced by different shape particles, the main crack always initiated in perpendicular network walls (PeW), but propagated along different paths: in Al matrix for hexahedron particle, along macro-interface of SiC/Al–Al for twenty-six face polyhedron particle, and in PeW for sphere particle. Such crack propagation manners contributed to the different elongations of network composites by various particle shapes: sphere > twenty-six face polyhedron > hexahedron particles. Selection of round particle and adjustment of local volume fraction improved elongation with a sacrifice of modulus and strength.

Small–Scale Experimental Investigation of Fatigue Performance Improvement of Ship Hatch Corner with Shot Peening Treatments by Considering Residual Stress Relaxation

Ship hatch corner is a common structure in a ship and its fatigue problem has always been one of the focuses in ship engineering due to the long–term high–stress concentration state during the ship’s life. For investigating the fatigue life improvement of the ship hatch corner under different shot peening (SP) treatments, a series of fatigue tests, residual stress and surface topography measurements were conducted for SP specimens. Furthermore, the distributions of the surface residual stress are measured with varying numbers of cyclic loads, investigating the residual stress relaxation during cyclic loading. The results show that no matter which SP process parameters are used, the fatigue lives of the shot–peened ship hatch corner specimens are longer than those at unpeened specimens. The relaxation rate of the residual stress mainly depends on the maximum compressive residual stress (σRSmax) and the depth of the maximum compressive residual stress (δmax). The larger the values of σRSmax and δmax, the slower the relaxation rates of the residual stress field. The results imply that the effect of residual stress field and surface roughness should be considered comprehensively to improve the fatigue life of the ship hatch corner with SP treatment. The increase in peening intensity (PI) within a certain range can increase the depth of the compressive residual stress field (CRSF), so the fatigue performance of the ship hatch corner is improved. Once the PI exceeds a certain value, the surface damage caused by the increase in surface roughness will not be offset by the CRSF and the fatigue life cannot be improved optimally. This research provides an approach of fatigue performance enhancement for ship hatch corners in engineering application.

Research on Temperature Variation during Coal and Gas Outbursts: Implications for Outburst Prediction in Coal Mines.

Coal and gas outbursts are among the most severe disasters threatening the safety of coal mines around the world. They are dynamic phenomena characterized by large quantities of coal and gas ejected from working faces within a short time. Numerous researchers have conducted studies on outburst prediction, and a variety of indices have been developed to this end. However, these indices are usually empirical or based on local experience, and the accurate prediction of outbursts is not feasible due to the complicated mechanisms of outbursts. This study conducts outburst experiments using large-scale multifunctional equipment developed in the laboratory to develop a more robust outburst prediction method. In this study, the coal temperature during the outburst process was monitored using temperature sensors. The results show that the coal temperature decreased rapidly as the outburst progressed. Meanwhile, the coal temperature in locations far from the outburst mouth increased. The coal broken in the stress concentration state is the main factor causing the abnormal temperature rise. The discovery of these phenomena lays a theoretical foundation and provides an experimental basis for an effective outburst prediction method. An outburst prediction method based on monitoring temperature was proposed, and has a simpler and faster operation process and is not easily disturbed by coal mining activities. What is more, the critical values of coal temperature rises or temperature gradients can be flexibly adjusted according to the actual situations of different coal mines to predict outbursts more effectively and accurately.

Wear Resistance of Different Bionic Structure Manufactured by Laser Cladding on Ti6Al4V

In this study, the laser cladding system with an IPG YLS-6000 fiber laser was used, and the WC–Ti6Al4V powder reinforced composite coatings on Ti6Al4V titanium alloy with various bionic structures were innovatively fabricated. The microstructures and surface damage behavior of the coatings were characterized by scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction. Additionally, the wear resistance of different bionic structures was evaluated, which had not been comprehensively explored in the published literature. The results indicated that the un-melted WC particles in the coatings act as a hard reinforcement, avoiding serious wear of the coating. In addition, the hard coatings exhibit excellent deformation resistance and the soft substrate cushion the shear stress. So when the “Ratio”, which refers to the laser cladding area to sample area, is between 0.25 and 0.3, the sample has the highest wear resistance. Furthermore, the “Dot + Line” bionic structure has the best wear resistance compared with other structures. The separated line units and the addition of dot units can improve the stress concentration state of bionic structure are conducive to release the stress to the substrate under the cladding layer.

STUDY ON THE CHARACTERISTICS OF MAGNETIC MEMORY SIGNALS AT FERROMAGNETIC MATERIAL DEFECTS

In order to study the effect of defects on the surface magnetic field of specimen, a series of stepwise loading tensile tests were carried out on 30Cr alloy steel. Defects were artificially prefabricated in advance on the surface of specimens. The magnetic field signal was collected by a TSC-1M-4 magnetic detector and the characteristics of magnetic field signals were studied. It shows that the tangential magnetic field is more sensitive to the local yield of specimen. The surface magnetic memory signal changes significantly at the edge of defects. Thus, it is an feasible method to evaluate the state of stress concentration by surface magnetic memory signal.

Analytical estimation of stress distribution in interbedded layers and its implication to rockburst in strong layer

In-situ stress is one of the most important factors affecting the safety and stability of underground excavations. Past measurement results show that the magnitudes and directions of in- situ stresses vary substantially in different lithological formations. However, the relationship between stresses distribution and rock mechanical properties is still unclear. In this paper, the stress distribution in interbedded strong and weak layers was analytically studied based on linear elastic mechanics. Derived solutions were validated by comparison with Finite Element Method (FEM) results and field measurement results. Comparisons show that analytical results agree with the numerical results, and the maximum difference is less than 1%. The maximum difference between the analytical results and field measurements is 10.8%. Furthermore, the derived solution was used to evaluate the stress distribution in a geologically abnormal area in the Neelum-Jhelum (NJ) hydroplant project. Stress concentration state in strong layers changes as the strata change from sub-vertical to sub-horizontal, and the stress state in the sub-horizontal strata greatly favours rockburst according to the Turchaninov criterion. The change in stress concentration state might be the underlying reason for the severe rockburst in the abnormal structure area in the NJ project.

Study on Metal Magnetic Memory Testing Mechanism

Compared with traditional nondestructive testing technologies, the method of metal magnetic memory (MMM) is a more effective way in the earlier injury diagnosis of ferromagnetic materials, but the investigation of the MMM testing mechanism is limited. In order to study the mechanism of MMM method, in this article, the relationship between stress concentration state and spontaneous magnetic flux leakage (SMFL) signal is analyzed by quantum theory. The results demonstrate that the fundamental reason for SMFL is lattice distortion leaded by stress concentration, and the rising of stress causes the SMFL signal changing linearly. The theoretical calculation results in this article are in very good agreement with experimental results.

Study on the Mechanism of Backfill and Surrounding Rock of Open Stope during Subsequent Backfill Mining

In order to analyze the mechanism of backfill and surrounding rock of open stope during subsequent backfill mining, take M1 ore body mining in a mine as engineering background, and simulate the mechanism of surrounding rock and backfill by using FLAC3D numerical software. Results show that the backfill can effectively alleviate or transfer the stress concentration state of the room floor and pillar, apparently restrain the displacement of the cavity plate and pillar, and improve the plasticity distribution range in a limited degree.

The quantitative interpretation by measurement using the magnetic memory method (MMM)-based on density functional theory

The paper demonstrates the relationship between a local stress concentration state and the self magnetic leakage field (SMLF) measured by the metal magnetic memory (MMM) effect using a first-principle calculation based on the density functional theory (DFT). In order to verify the theoretical approach, blasting experiments with X70 steel pipes were performed to document the reliability of the investigation methods. Interestingly concerning the theoretical approach is the linear change of the local SMLF signal with the increase of stress which is in very good agreement with the experimental observations. A new research program for quantitative interpretation of the MMM effect was initiated.

First-principles caculation and experimental study of metal magnetic memory effects

In order to investigate the mechanism and regular pattern of metal magnetic memory (MMM) signal, from the angle of electron spin, the magnetomechanical model of MMM is set up, and the relationship between stress concentration state and self magnetic flux leakage (SMFL) signal is calculated by the plane wave and pseudo-potential method based on the density functional theory. The research results show that the fundamental reason for SMFL is lattice distortion induced by loads, and the theoretical calculations are in very good agreement with the experimental observations. The present work is helpful for testing the mechanism of MMM.

Source Ruptures and Deep Processes of Great Earthquakes and Short‐Term and Impending Earthquake Prediction

AbstractResponse to tectonic stress, micro ruptures will be produced in media within and surrounding a seismic source. With steady building up of stress, these ruptures will grow to become a rupture chain of some scale, indicative of a critical state of stress concentration. At this moment, the accumulated giant energy in the source is released suddenly in seismic waves propagating outward, producing a series of destructions and large faults on the surface. After a major shock occurs, using a proper model, seismic records at near and far stations as well as calculation of focal mechanisms, we can determine the rupture length and rupture processes of the seismic source. Observations show that during the generation and occurrence of a major or large earthquake, ruptures occur in the source as well as on the surface, which are of lengths several, tens or even thousand kilometers. It has been found that responses and traces of ruptures occurred before some earthquakes. Therefore it is possible to detect the initial micro ruptures and the dynamic processes of rupture chain generation through observations on the surface or in deep wells. This is probably an important approach to realize short‐term and impeding earthquake prediction.

20419 Javaを用いたコースティックス法測定システムの開発 : 応力拡大係数の測定への応用(材料力学(2))

The caustics method is a non-contact measurement method using a laser ray. This method has been applied to stress concentration state such as a crack tip to measure the stress intensity factors. This method has a lot of advantage: for example, it is applicable to a continuous measurement of a dynamic change of stress distribution; it is easy to set up the optical apparatus for measurements. For using the caustics method, the theoretical simulation is necessary to evaluate the stress intensity factors from the caustic images. So, in this paper, simulation program of caustic image based on the theory is developed. "Java" is used as the programming language, thus this program can be used through the following web site without limitation of time and area.

Strength of a joint under stress concentration

formulate strength conditions in the presence of stress concentration. In this work, using the sectioning method, which is well known in engineering mechanics, we examine the strength of a joint in composite bodies with exponential strengthening of materials in the presence of stress concentration. The use of the sectioning method in the linear mechanics of cracks is covered in [3]. Let a composite body be made of two dissimilar materials. The stress and strain intensities are related by the exponential dependence or0 = kr n. For both materials, the values of the parameter m are considered equal and the values of k different. We assume that at the end of the contact surface of the composite body there is a reentrant angular "cut" with stress concentration at the tip. Assume that we know a solution of the corresponding problem ignoring the stress concentration caused by the "cut." This stressed state will be called nominal. Further, using this solution for the neighborhood of the angular point, we should find the strength condition for the end of the joint with the stress concentration state. 1. Twisting. We consider a composite bar of constant cross section made of materials strengthening by an exponential law. The bar has a reentrant angle at the end of the contact surface and is twisted by the moments M applied at the end cross sections. Initial Relations. We assume that the nominal solution, i.e., without an angular cut, is specified in the polar coordinate system p~a (Fig. 1). We denote stresses by Tpi(p, ~a) and T~i(p,~a), and displacements by Wi(p, ~a). Here and below, the subscript i = 1 and 2 denotes the values of the constituent materials. Longitudinal shear strains occur in the vicinity of the angular point r = 0. According to [2], this solution is representable as 7"0i -= kir(A-1)m f~xi , 7"ri --__ Akir(A-1)rn fixi '