Showing papers on "Stress field published in 2021"

Characteristics of evolution of mining-induced stress field in the longwall panel: insights from physical modeling

Abstract: The evolution of mining-induced stress field in longwall panel is closely related to the fracture field and the breaking characteristics of strata. Few laboratory experiments have been conducted to investigate the stress field. This study investigated its evolution by constructing a large-scale physical model according to the in situ conditions of the longwall panel. Theoretical analysis was used to reveal the mechanism of stress distribution in the overburden. The modelling results showed that: (1) The major principal stress field is arch-shaped, and the strata overlying both the solid zones and gob constitute a series of coordinated load-bearing structures. The stress increasing zone is like a macro stress arch. High stress is especially concentrated on both shoulders of the arch-shaped structure. The stress concentration of the solid zone in front of the gob is higher than the rear solid zone. (2) The characteristics of the vertical stress field in different regions are significantly different. Stress decreases in the zone above the gob and increases in solid zones on both sides of it. The mechanical analysis show that for a given stratum, the trajectories of principal stress are arch-shaped or inversely-arched, referred to as the “principal stress arch”, irrespective of its initial breaking or periodic breaking, and determines the fracture morphology. That is, the trajectories of tensile principal stress are inversely arched before the first breaking of the strata, and cause the breaking lines to resemble an inverted funnel. In case of periodic breaking, the breaking line forms an obtuse angle with the advancing direction of the panel. Good agreement was obtained between the results of physical modeling and the theoretical analysis.

Numerical study on strength and failure characteristics of rock samples with different hole defects

Abstract: In a rock mass, holes of various sizes and geometries naturally occur, which in turn can affect the mechanical properties of the rock mass. These defects often cause engineering problems in subsurface construction. In this study, PFC2D was used to perform uniaxial compression tests on a rock mass containing ten different types of hole defects to analyze their failure behavior and mechanical properties. Four failure modes were determined, and crack propagation and stress field evolution were studied. The results show that the hole defect reduces the uniaxial compressive strength, peak strain, and elastic modulus of a rock mass. Also, these defects accelerate the generation of cracks and promote the destruction of the rock. The failure modes can be classified as Y-type, inverted Y-type, upper left to lower right type, and upper right to lower left type. Before cracks are generated, the compressive stress concentration area is located on the left and right sides of the hole and distributed as a butterfly shape, and the tensile stress concentration area is located in the upper and lower parts of the hole. A zone where stress is decreasing is located near the tip of the tensile stress triangular area. The magnitude and concentration area of compressive and tensile stresses are greatly affected by various hole geometries. Finally, the maximum principal compressive stress decreases instantly after a crack coalesces. Overall, the hole shape has a noticeable influence on the stress distribution surrounding the hole, and a hole defect reduces the degree of failure of a rock mass.

Finite element modeling of mechanical behaviors of piezoelectric nanoplates with flexoelectric effects

Abstract: This paper uses the finite element method to simulate the mechanical, electric, and polarization behaviors of piezoelectric nanoplates resting on elastic foundations subjected to static loads, in which the flexoelectric effect is taken into consideration. The finite element formulations are established by employing a new type of shear deformation theory, which does not need any shear correction factors, but still accurately describes the stress field of the plate. The numerical results show clearly that the flexoelectric effect has a strong influence on the mechanical responses of the nanoplates. In particular, the normal stress distribution in the thickness direction is no longer linear when the flexoelectric coefficient is large enough, and this phenomenon differs completely from that of conventional plates. In addition, the distribution of the electric field and the polarization also depend on boundary conditions, which were not investigated in the published works.

Focal mechanisms and the stress field in the aftershock area of the 2018 Hokkaido Eastern Iburi earthquake ( M JMA = 6.7)

Abstract: The tectonic stress field was investigated in and around the aftershock area of the Hokkaido Eastern Iburi earthquake (MJMA = 6.7) occurred on 6 September 2018. We deployed 26 temporary seismic stations in the aftershock area for approximately 2 months and located 1785 aftershocks precisely. Among these aftershocks, 894 focal mechanism solutions were determined using the first-motion polarity of P wave from the temporary observation and the permanent seismic networks of Hokkaido University, Japan Meteorological Agency (JMA), and High Sensitivity Seismograph Network Japan (Hi-net). We found that (1) the reverse faulting and the strike-slip faulting are dominant in the aftershock area, (2) the average trend of P- and T-axes is 78° ± 33° and 352° ± 51°, respectively, and (3) the average plunge of P- and T-axes is 25° ± 16° and 44° ± 20°, respectively: the P-axis is close to be horizontal and the T-axis is more vertical than the average of the P-axes. We applied a stress inversion method to the focal mechanism solutions to estimate a stress field in the aftershock area. As a result, we found that the reverse fault type stress field is dominant in the aftershock area. An axis of the maximum principal stress (σ1) has the trend of 72° ± 7° and the dipping eastward of 19° ± 4° and an axis of the intermediate principal stress (σ2) has the trend of 131° ± 73° and the dipping southward of 10° ± 9°, indicating that both of σ1- and σ2-axes are close to be horizontal. An axis of the minimum principal stress (σ3) has the dipping westward of 67° ± 6° that is close to be vertical. The results strongly suggest that the reverse-fault-type stress field is predominant as an average over the aftershock area which is in the western boundary of the Hidaka Collision Zone. The average of the stress ratio R = (σ1 − σ2)/(σ1 − σ3) is 0.61 ± 0.13 in the whole aftershock area. Although not statistically significant, we suggest that R decreases systematically as the depth is getting deep, which is modeled by a quadratic polynomial of depth.

Simulation of Quasi-Static Crack Propagation by Adaptive Finite Element Method

Abstract: The finite element method (FEM) is a widely used technique in research, including but not restricted to the growth of cracks in engineering applications. However, failure to use fine meshes poses problems in modeling the singular stress field around the crack tip in the singular element region. This work aims at using the original source code program by Visual FORTRAN language to predict the crack propagation and fatigue lifetime using the adaptive dens mesh finite element method. This developed program involves the adaptive mesh generator according to the advancing front method as well as both the pre-processing and post-processing for the crack growth simulation under linear elastic fracture mechanics theory. The stress state at a crack tip is characterized by the stress intensity factor associated with the rate of crack growth. The quarter-point singular elements are constructed around the crack tip to accurately represent the singularity of this region. Under linear elastic fracture mechanics (LEFM) with an assumption in various configurations, the Paris law model was employed to evaluate mixed-mode fatigue life for two specimens under constant amplitude loading. The framework includes a progressive analysis of the stress intensity factors (SIFs), the direction of crack growth, and the estimation of fatigue life. The results of the analysis are consistent with other experimental and numerical studies in the literature for the prediction of the fatigue crack growth trajectories as well as the calculation of stress intensity factors.

Experimental and Numerical Analysis of Mode I Fracture Process of Rock by Semi-Circular Bend Specimen

Abstract: The semi-circular bend (SCB) specimen is widely used to measure fracture toughness of brittle materials such as rock. In this work, the stress field, fracture process zone (FPZ), and crack propagation velocity of SCB specimen are analyzed during the fracture process of rock specimens. The FPZ of specimen is obtained by experimental and numerical methods under a three-point bend test. The stress concentration zones of specimen present a heart shape at peak load points. FPZ forms before macro fracture occurs. The macro fracture form inside FPZ in a post-peak region of a load–displacement curve. The crack propagation process of specimen include two stages, namely the rapid crack initial development stage, and the final crack splitting stage. The maximum crack propagation velocity of specimen is about 267 m/s, and the average crack propagation velocity is about 111 m/s.

Discrete element method simulation of the growth and efficiency of multiple hydraulic fractures simultaneously-induced from two horizontal wells

Abstract: Multiple hydraulic fracturing treatment from horizontal wells has been widely used to enhance the productivity of unconventional hydrocarbons. Interaction among the fractures can cause the reorientation of fractures and lower the stimulation efficiency. We perform discrete element method (DEM) simulations to assess the influence of completion scheme on the trajectory and efficiency of fractures simultaneously-induced from two horizontal wells. Simulation of a single hydraulic fracture yields reasonable agreement with the theoretical solution regarding the fracture pattern and stress alteration and thus confirms the validity of the model. Stress shadowing effect caused by the opening of early fractures alters the state of local stress and thus leads to the appearance of dominant fractures and the reorientation of other fractures located in the altered stress field. Influences of the fracturing scheme including the distance between wells, the spacing between injection points, and arrangement of injection points are examined. The distribution uniformity index reveals that the effect of distance between wells on improving the efficiency of treatment is conditionally dependent on the spacing between injection points. Under the conditions considered in this study, the distance between wells enhance the efficiency only when the spacing reaches 40 m. Similarly, enlarging the spacing between injection points does not affect the distribution uniformity index when the distance between wells is 40 m, but effectively mitigate the stress shadowing effect and promotes the fracturing efficiency when the distance exceeds 60 m. Arrangement of injection point plays a negligible role on the fracturing efficiency compared with the spacing among them.

Thermo-mechanical modelling of stress relief heat treatments after laser-based powder bed fusion

Abstract: Laser-based powder bed fusion, due to its layer-by-layer nature, results in a unique stress profile in a part after the primary production process. The residual stresses are typically tensile near the top, while they are compressive near the bottom of the part. When it is removed without proper precautions, the part will bend excessively. In order to alleviate this deformation, a stress relief heat treatment can be applied. In this paper, such a stress relaxation heat treatment is modelled to investigate the effect of the post-processing parameters. The model uses an Arrhenius-type creep equation to simulate the influence of the heat treatment temperature and dwell time on the stress field in a relatively simple cantilever beam produced in Ti-6Al-4V. Via validation of the simulations, the effect of the heat treatment is shown to be represented accurately. The validated model is used to predict the deformation that results from the residual stresses after heat treating the part under various conditions. The results from the simulations ultimately allow choosing the optimal heat treatment conditions to obtain a given reduction in the residual stress level, while reducing the need for extensive experimental investigations.

Influence of Driving Direction on the Stability of a Group of Headings Located in a Field of High Horizontal Stresses in the Polish Underground Copper Mines

Abstract: This paper investigates the problem of stability in a group of headings driven in high horizontal stress fields in the copper ore mines of the Legnica-Glogow Copper Belt (LGCB). The headings are protected with the roof bolting system. This problem is of high importance due to special safety regulations which apply in mining workings serving as airways and haulageways. The analysis was performed for a group of four headings driven in the geological and mining conditions of the Polkowice-Sieroszowice mine. The stability of the headings was evaluated with the use of Finite Element Method (FEM). The parameters of the rocks used in the numerical modeling have been determined on the basis of the Hoek–Brown classification, with the use of the RocLab 1.0 software. The parameters of the stress field have been identified on the basis of in situ measurements, which were performed in the Polkowice-Sieroszowice mine in 2012. The measurements were carried out with the use of the overcoring method, which is a stress relief method. A CSIRO HI probe was used as the measuring device. The tests were carried out on three measuring points, on which six successful tests were performed. The measurements confirmed the presence of high horizontal stresses in the rock mass. Numerical modeling was performed using the Phase2 v.8.0 software, in a triaxial stress state and in a plane strain state. The rock mass was described with an elastic-plastic model with softening. Numerical analyses were based on the Mohr–Coulomb failure criterion. It was assumed that the optimal measure of the stability of the group of headings is the range of the formed zone of yielded rock mass in the excavation roof. Numerical simulations have shown that the direction of driving the headings in the field of increased horizontal stresses may be of key importance for the stability of the headings in LGOM mines. The greatest extent of the yielded rock mass zone in the excavation roof occurred when the group of headings was driven in the direction perpendicular to the direction of the maximum horizontal stress component σH. The obtained results served to provide an example of the application of a roof bolting system to protect headings driven in unfavorable conditions in a high horizontal stress field.

Simulation of temperature field and stress field of selective laser melting of multi-layer metal powder

Abstract: In this study, the temperature field and the stress field of a multilayer metal powder were investigated using the point exposure scanning mode. A three-dimensional thermo-structural coupling model was established by using the finite element method, and the multi-layer multi-track SLM process was simulated. The model considered the latent heat of phase change, physical parameters with temperature change, convective heat transfer, and so on. The temperature, molten pool and stress changes at different positions were analyzed, and finally the residual stress distribution of the model was obtained. The residual stress distribution on the upper surface of the SLM fabricating part under point exposure scanning had strong volatility, which was different from continuous exposure scanning. The maximum residual stress appeared at the end of the first track in the first layer. The reliability of the numerical simulation was verified by the size of the molten pool in the experiment.

Adaptive Finite Element–Discrete Element Analysis for the Stress Shadow Effects and Fracture Interaction Behaviours in Three-Dimensional Multistage Hydrofracturing Considering Varying Perforation Cluster Spaces and Fracturing Scenarios of Horizontal Wells

Abstract: Optimization of complex fracture networks improves the fracturing effects and enhances production in multistage hydrofracturing technology. To understand the controlling mechanisms of multistage hydrofracturing in unconventional tight reservoirs, some governing issues, such as hydro-mechanical coupling, stress shadow effects, propagation interaction behaviours of three-dimensional (3D) multiple fractures, and 3D multistage hydrofracturing, should be addressed. However, the characterization of perforation cluster spaces and fracturing scenarios of horizontal wells, which significantly affect the evolution of the stress field and 3D morphology of the fracture network, is a challenge. In this study, to overcome the drawbacks of the traditional finite-element method in simulating 3D fracture propagation, the adaptive finite element–discrete element method is used. This method uses a local remeshing and coarsening strategy to ensure the accuracy of solutions, reliability of the fracture propagation path, and computational efficiency. The study proposes 3D engineering-scale numerical models, considering the crucial hydro-mechanical coupling and fracturing fluid leak-off, to simulate 3D multistage hydrofracturing and fracture interaction behaviours. The numerical results show that the stress shadow effects and fracture interaction behaviours become more intense once the spaces between different propagating fractures become thinner due to superposition and reduction effects in fracturing-induced shear stress variation areas. The alternate fracturing can reduce the stress shadow effects through adjusting the sequence of perforation clusters that are activated and injected with fracturing fluid. When the perforation cluster spaces become narrow, the alternate fracturing scenario can yield more fracturing fracture areas and improve the fracturing effects as compared to sequential and simultaneous fracturing.

Stress inversion in a gelatin box: testing eruptive vent location forecasts with analog models.

Abstract: Assessing volcanic hazard in regions of distributed volcanism is challenging because of the uncertain location of future vents. A statistical-mechanical strategy to forecast future vent locations w...