Minimum Stress Research Articles

Experimental study on bending fatigue strength of adhesively bonded joints

This paper focuses on the establishment of repair or strengthening method of steel structures by externally bonded patch plates. The bending static tests and bending fatigue tests of adhesively bonded joints were conducted and the fatigue life is evaluated. The Konishi E258R is used as an adhesive, and the bending fatigue tests were conducted, varying the applied stress ranges and the stress ratio, the ratio of minimum and maximum stress of fatigue test. The result indicates that the fatigue life of adhesively bonded joints can be evaluated in the function of principal stress ratio, the ratio of principal stress range against principal stress at debonding by the bending test, with constant linearity regardless of the stress ratio.

Comparative analysis between Graphene and Carbon Nanotube reinforced epoxy composite

Composite structures are very high in demand because of their high strength-to-weight ratio and directional strength. The arrangement of fibers in the matrix is the major design criterion for the composites. Past studies prove that the fiber reinforcement of carbon nano allotropes such as Carbon Nanotubes (CNTs) and Graphene fibers develop reliable and durable structures for different applications. In this work, three different composite laminate sheets are designed and modeled using finite element analysis (FEA) software. Carbon nano allotropes, CNTs, Graphene, and CNT-Graphene fibers are used as reinforcements in the resin epoxy matrix. The modeled laminate sheets are analyzed for deformation and stress distribution using Hashin’s Failure criteria. The FEA results indicate that the CNTs fiber-reinforced laminate shows minimum deformation (0.000064 mm) and experiences minimum stress (3.81 MPa) compared to Graphene and CNT-Graphene reinforced laminates. The results of FEA can be used as a research basis for further study of composites in the future.

Effect of gingival height of a titanium base on the biomechanical behavior of 2-piece custom implant abutments: A 3-dimensional nonlinear finite element study

Statement of problemTitanium base (TiBase) abutments to restore an implant-supported single crown are available in different gingival heights, but information on the biomechanical effects of the gingival heights is lacking. PurposeThe purpose of this nonlinear finite element analysis study was to evaluate the effects of TiBase gingival heights on the biomechanical behavior of custom zirconia (CustomZir) abutments and TiBase, including von Mises stress and maximum and minimum principal stress. Material and methodsTiBases with different gingival heights (0.5 mm, 1 mm, 1.5 mm, and 2 mm) with internal hexagon Morse taper connections were simulated in 3-dimensional models. The simulations (ANSYS Workbench 2020; ANSYS Inc) included the OsseoSpeed EV implant (Ø5.4 mm) (AstraTech; Dentsply Sirona), restoration, and surrounding bone in the mandibular first molar region. An occlusal force of 200 N was applied with a 2-mm horizontal offset toward the buccal side and a 30-degree inclination from the vertical axis. ResultsHigh-stress concentration was observed in the uppermost internal connection area on the buccal side and the antirotational part of the titanium abutment on the lingual side in all models. CustomZir abutments with a shorter gingival height exhibited larger concentrated areas of volume average stress von Mises stress and higher magnitude of maximum and minimum principal stress compared with a taller gingival height. ConclusionsA TiBase abutment with a taller gingival height reduced the fracture risk of a CustomZir abutment without increasing any mechanical risk.

DESIGN OF PATIENT SPECIFIC HIP PROSTHESIS BASED ON FINITE ELEMENT ANALYSIS: A COMPARATIVE STUDY

This study aims to develop a patient-specific hip implant for osteoarthritis conditions and to compare with intact and conventional implant. The femoral bone with head and shaft region was segmented from the pelvic griddle and converted into 3D model. The parameters such as femoral ball diameter, shaft length, acetabular cup diameter, and neck angle were measured from the segmented 3D model. In this study, designed part of hip implant was assembled together to form a customized hip implant. The von Mises stress was measured by means of Finite element analysis (FEA) method by applying various forces applied at the distal end of hip implant. The forces applied at hip implant were based on the assumption of 500 N force for standing, 2000 N force for walking, and 3000 N force for jogging condition. The minimum stress attained at the femur bone of custom-model is 1.32 MPa for 500 N loading condition, 5.3 MPa for 2000 N and 7.96 MPa for the maximum load of 3000 N. Thus the customized model experienced better stress distribution compared to conventional model under the maximum load of 3000 N. In pelvic region, the custom model attained a lower stress of 23% compared to conventional model. Thus, the study recommends the customized hip implants for the osteoarthritis conditions to avoid revision surgery.

Cracking property and brittleness evaluation of granite under high-temperature true triaxial compression in geothermal systems

Cracking property and brittleness are critically important to the drillability of injection and production wells of enhanced geothermal systems. This paper is devoted to evaluating the cracking property and brittleness of the Gonghe granite under high-temperature true triaxial compression conditions through a series of laboratory tests. Thermal–mechanical coupled true triaxial compression tests were conducted on Gonghe granite samples under four representative temperatures (20 °C, 120 °C, 180 °C, and 240 °C) and three different minimum (5, 10, and 30 MPa) and intermediate (40, 60, and 75 MPa) principal stresses that correspond to the in-situ conditions of Gonghe geothermal reservoir. The strength, deformation, and cracking characteristics of the Gonghe granite are quantitatively evaluated from macro- to micro-scales based on the experimental results. Meanwhile, a novel brittleness index evaluation method considering both energy and failure surface roughness is proposed to accurately assess the brittleness strength of the Gonghe granite. We found that the strength of Gonghe granite is reduced by more than 20% when the temperature increases from room temperature to 240 °C. Higher temperature contributes to a smoother fracture surface and reduced brittleness of the Gonghe granite under thermal–mechanical coupled true triaxial compression. Our findings provide new insights for brittleness evaluation of the Gonghe granite formation and assist in efficient wells drilling design of enhanced geothermal systems.

Corneal Stress Distribution Evolves from Thickness-Driven in Normal Corneas to Curvature-Driven with Progression in Keratoconus

To introduce the novel parameter of Corneal Contribution to Stress (CCS) and compare stress distribution patterns between keratoconus (KCN) and normal corneas. Prospective, observational, cross-sectional study. The study included 66 eyes of 40 subjects diagnosed with KCN and 155 left eyes from 155 normal control (NRL) subjects. Tomography was obtained to calculate the newly proposed CCS, defined according to the hoop stress formula without intraocular pressure, R/2t, where R is the radius of curvature and t is the thickness. CCS maps were calculated from pachymetry and tangential curvature maps. Custom software identified the 2-mm-diameter zones of greatest curvature (Cspot-max), thinnest pachymetry (Pach-min), greatest stress (CCSmax), and lowest stress (CCSmin). Stress difference (CCSdiff) was calculated as CCSmax - CCSmin. Distances between Cspot-max vs. Pach-min, vs. CCSmax, and vs. CCSmin, as well as between Pach-min vs. CCSmax and vs. CCSmin, were calculated. t tests were performed between cohorts, and paired t tests were performed within cohorts. Univariate linear regression analyses were performed between parameters and distances. The significance threshold was P < 0.05. Corneal stress parameters, corneal features of maximum curvature, minimum thickness, and distances between corneal stress parameters and corneal features. CCSmax was significantly closer to Pach-min (0.79 ± 0.92) and Cspot-max (2.04 ± 0.85) than CCSmin (3.17 ± 0.38, 2.73 ± 1.53, respectively) in NRL, P < 0.0001, whereas CCSmin was significantly closer to Cspot-max (1.35 ± 1.43) than CCSmax (2.52 ± 0.72) in KCN, P < 0.0001. Cspot-max (severity) was significantly related to CCSdiff in KCN (P < 0.0001; R2= 0.5882) with a weak relationship in NRL (P < 0.0080, R2= 0.0451). Cspot-max was significantly related to the distance from Pach-min to CCSmax (P < 0.0001; R2= 0.3737) without significance in NRL (P= 0.8011). Corneal stress is driven by thickness in NRL, with greatest stress at thinnest pachymetry and greatest curvature. However, maximum stress moves away from thinnest pachymetry with progression in KCN, and minimum stress is associated with maximum curvature. Severity in KCN is significantly related to greater difference between maximum and minimum stress, consistent with the biomechanical cycle of decompensation. Proprietary or commercial disclosure may be found in the Footnotes and Disclosures at the end of this article.

The effect of different abutment and restorative crown materials on stress distribution in single-unit implant-supported restorations: A 3D finite element stress analysis.

To evaluate the effect of restorative materials with or without resin content, modeled on zirconia and titanium abutment materials, on the stress distribution on the alveolar bone, implant, and prosthetic crowns with a 3D finite element stress analysis. Titanium and zirconia abutments were combined with three implant-supported crown materials (polymer infiltrated hybrid ceramic (PICN), lithium disilicate (LD), and zirconia-reinforced lithium silicate (ZLS)) to create six experimental groups. The 40×30×20mm alveolar bone, 3.75× 10mm implant, esthetic abutment, and maxillary first premolar crown bonded over the abutment were the components of the finite element models. On the lingual cusp of the crown, the 150 N occlusal loading was applied in the buccolingual direction at a 30° angle. Equivalent von Mises stress and maximum and minimum principal stresses were used for both the qualitative and quantitative evaluation of the stress distribution of the created models. The von Mises stress in implant and abutment did not differ according to the crown materials. The use of a zirconia abutment resulted in higher von Mises stress values in the abutment but lower stress values in the implant. The highest stress values were obtained in ZLS (196.65MPa) and LD (194.05MPa) crowns. The use of titanium abutments, regardless of crown materials, resulted in higher von Mises stress values in restorative crowns than in zirconia abutments. The principal stress values in alveolar bone showed similar distribution and concentration in all models. Changes in crown material did not affect stress distribution in the implant and peripheral bone. However, the zirconia esthetic abutment resulted in a lower stress concentration on the implant.

Updating the seismotectonic setting for the Gulf of Aqaba

The Gulf of Aqaba is known for its high seismic activity in Egypt and the Middle East. An inversion technique was applied to 113 earthquakes of magnitude 2.5 to 7.2 to distinct subsets of data based on tectonic regionalization to define the stress regime in the Gulf of Aqaba involving the Eilat basin, Aragonese basin, and Dakar basin. The stress inversion revealed two active stress patterns; an active strike-slip in the Eilat basin and a dominant extensional regime in the Dakar basin, whereas both strike-slip and extensional regimes coexist in the Aragonese basin. The stress pattern in the Eilat basin is consistent with the movement along the Dead Sea Transform Fault. In contrast, the extensional regime in the Dakar basin aligns with the extensional stress field throughout the northern Red Sea. The coexistence of two dominant types of stress regimes in the Aragonese basin is likely a result of the superposition of the two main neighbouring stress regimes: the strike-slip regime along the Gulf of Aqaba Dead Sea Transform Fault and the extensional stress state across the northern Red Sea. The orientations of the minimum principal stress in the three basins are almost similar, indicating ENE trending, nearly horizontal extension.

A wellbore-fracture coupled fracturing model to simulate non-uniform fracture propagation under multiple clusters of fracturing

Multiple clusters of fracturing within one stage have been widely used in the unconventional reservoir production. However, the non-uniform fracture propagation under multiple clusters of fracturing has not yet fully been understood, due to the fluid flow in the wellbore being neglected in the fracturing model. In view of this, a wellbore-fracture coupled fracturing model was established to simulate fracture non-uniform propagation under multiple clusters of fracturing. In this model, the governing equation was firstly given based on the pressure continuity and flow conservation, to obtain the flow distribution in each fracture. Then, equations of fracture propagation, fracture height growth and stress shadow were combined to determine the fracture geometric parameters. Substituting fracture geometric parameters into the governing equation, the wellbore-fracture coupled fracturing model was finally solved. The proposed model was validated by a ABAQUS cohesive elements numerical by comparing the fracture parameters, respectively considering conditions of wellbore-fracture coupling and non-wellbore-fracture coupling, within a maximum deviation that did not exceed 6.14%. Using the proposed model, the parametric analyses were performed, including energy-related parameters and wellbore-related parameters. For energy-related parameters, with the increase of surface energy density of rock and the minimum principal stress, the fracture length and height decreased, while the fracture width increased to some extent. In addition, with the increase of pumping rate of fracturing fluid, the fracture length, height and width increased significantly. For wellbore-related parameters, with the decrease of diameter of perforation and perforation number as well as the increase of wellbore roughness coefficient and viscosity of fracturing fluid and cluster distance, the flow distribution in each fracture and the injection pressure became more uniform and the non-uniform propagation of each fracture was reduced, but fracture dimensions were simultaneously decreased.

Simulation of the Damage and Failure Characteristics of Coal under True Triaxial Static–Dynamic Loads

The damage evolution rules for coal under true triaxial combined static–dynamic loads are important to understand and reveal the mechanism of rockburst in coal mines. The numerical simulation was carried out using FLAC3D to explore the coupling effect of static and dynamic stress, especially the influence of dynamic load parameters. The results show that the increase in dynamic load amplitude not only makes the plastic zone grow exponentially and shortens the damage development time, but also decreases the residual strength of the coal sample, which leads to a much more violent dynamic failure. The stress wave propagation can be more stable in the coal sample with the increase in dynamic load frequency and the development time of the plastic zone also shows synchronous fluctuating features, and when the frequency is like the intrinsic frequency of the coal sample, the dynamic load has a significant stimulation effect. The static and dynamic loads have different damage effects on the coal sample under the true triaxial stress path, the weight decreases in descending order, namely the minimum principal stress, the maximum principal, the amplitude of the dynamic load and, lastly, the dynamic stress frequency, according to the entropy weight method analysis.

Numerical Investigation of the Effects of Stress Heterogeneity on the Propagation Behaviors of Hydraulic Fractures in a Shale Oil Reservoir

Minimum principal stress is a key factor governing the hydraulic fracturing behaviors in shale oil reservoirs. Due to the existence of stress heterogeneity, the hydraulic fracture growth and footprints can be affected, and the hydraulic fracturing efficacy can be consequently altered. This phenomenon is especially common during the development of shale oil reservoirs associated with continental sedimentary facies. This study uses a numerical workflow to analyze the effect of stress heterogeneity on hydraulic fracture growth. The numerical workflow consists of an open-source planar hydraulic fracturing model and a derived coupled flow and geomechanics model, which can address the effect of minimum principal stress heterogeneity on hydraulic fracturing. Two types of stress heterogeneity are considered: stress heterogeneity caused by legacy production in the horizontal direction and stress heterogeneity caused by high-stress interlayers in the vertical direction. Simulation results indicate that stress heterogeneity in the horizontal and vertical directions leads to asymmetric fracture growth horizontally and vertically. The corresponding fracture footprints and widths also become asymmetric accordingly. Thin interlayers cannot fully limit the fracture growth, and the fracture height growth can still penetrate through. When the high-stress interlayers are thick enough, the fracture cannot penetrate through them vertically, while the corresponding fracture growth is no longer highly sensitive to the thickness of the interlayer.

Using in-situ strain measurements to evaluate the accuracy of stress estimation procedures from fracture injection/shut-in tests

Fracture injection/shut-in tests are commonly used to measure the state of stress in the subsurface. Injection creates a hydraulic fracture (or in some cases, opens a preexisting fracture), and then the pressure after shut-in is monitored to identify fracture closure. Different interpretation procedures have been proposed for estimating closure, and the procedures sometimes yield significantly different results. In this study, direct, in-situ strain measurements are used to observe fracture reopening and closure. The tests were performed as part of the EGS Collab project, a mesoscale project performed at 1.25 and 1.5 km depth at the Sanford Underground Research Facility. The tests were instrumented with the SIMFIP tool, a double-packer probe with a high-resolution three-dimensional borehole displacement sensor. The measurements provide a direct observation of the fracture closure signature, enabling a high-fidelity estimate of the fracture closure stress (ie, the normal stress on the fracture). In two of the four tests, injection created an opening mode fracture, and so the closure stress can be interpreted as the minimum principal stress. In the other two tests, injection probably opened preexisting natural fractures, and so the closure stress can be interpreted as the normal stress on the fractures. The strain measurements are compared against different proposed methods for estimating closure stress from pressure transients. The shut-in transients are analyzed with two techniques that are widely used in the field of petroleum engineering – the ‘tangent’ method and the ‘compliance’ method. In three of the four tests, the tangent method significantly underestimates the closure stress. The compliance method is reasonably accurate in all four tests. Closure stress is also interpreted using two other commonly-used methods – ‘first deviation from linearity’ and the method of (Hayashi and Haimson, 1991). In comparison with the SIMFIP data, these methods tend to overestimate the closure stress, evidently because they identify closure from early-time transient effects, such as near-wellbore tortuosity. In two of the tests, microseismic imaging provides an independent estimate of the size of the fracture created by injection. When combined with a simple mass balance calculation, the SIMFIP stress measurements yield predictions of fracture size that are reasonably consistent with the estimates from microseismic. The calculations imply an apparent fracture toughness 2-3x higher than typical laboratory-derived values.

Determining the present-day in-situ stresses of tight-oil sandstones by conventional logs: An approach in the Triassic Yanchang Formation, southern Ordos Basin

The present-day in-situ stresses affect the drilling design, well pattern deployment, well completion modification, hydraulic fracturing and water injection of tight-oil sandstones. The measurement data of these stresses are commonly unavailable because of their high costs and limited core samples, therefore employing conventional logs for these stress determination is imperative for tight-oil sandstones. Firstly, the suitable calculation models for the present-day in-situ stress calculation by conventional logs were selected according to the geological characteristics of the sixth member of the Yanchang Formation (Chang 6) in Heshui area of the southern Ordos Basin, China. Then, the dynamic rock mechanical parameters were determined by conventional logs, and corrected by the static rock mechanical parameters obtained from the triaxial rock mechanical tests. Moreover, the pore fluid pressure was determined by the empirical formula method. Finally, the maximum and minimum horizontal compressive stresses (σH and σh), and the vertical stress (σv) of six wells were calculated according to the selected models of these stresses, respectively. The present-day in-situ stresses, determined by the proposed method in the paper, were verified by those obtained from acoustic emission tests and finite-element numerical simulations with the relative errors of less than 10%. The results show that the magnitudes of σH, σh and the horizontal differential stress (σH−h) in the study area mainly range from 32 to 43 MPa, 23 to 37 MPa and 5 to 8 MPa, respectively. The magnitude of the three-dimensional present-day in-situ stress increases with the increase of depth. The average gradients of σH, σv and σh are 0.018, 0.014 and 0.015 MPa/m, respectively, that is σH&gt;σv&gt;σh. In this stress state, the hydraulic fractures, with a trend of little expansion towards multiple directions, are commonly developed at a small angle intersecting with the direction of σH in the study area.

Crack mode-changing stress level in porous rocks under polyaxial stress conditions

As the global push towards clean energy intensifies, the demand for critical minerals has driven deep excavation in hard rock formations, posing significant challenges related to rockburst and spalling. Spalling refers to explosion-like rock fractures under high geo-stresses. Despite several successful studies and practical models, the mechanisms governing spalling propagation under polyaxial stress states remain inadequately understood, particularly in weaker and high-porosity rocks. This study introduces a novel Crack Mode-Changing Stress (CMCS) concept, which defines the minimum principal stress required to change the crack mode from shear to tensile failures when rock spalls. The concept was validated using cubed sandstone samples containing centric circular holes subjected to a range of loading conditions including uniaxial, biaxial, generalized triaxial compression, generalized triaxial tensile, and true triaxial loading stress states. Our results highlight the significance of the out-of-plane minor principal stress on the crack initiation threshold and the CMCS, emphasizing the need for careful consideration when designing openings in highly stressed environments.

A novel approach to optimize material distributions of rotating functionally graded circular disk under minimum and prescribed stresses

This study focuses on the optimization of a rotating disk composed of functionally graded material (FGM), which is subjected to both inertial and thermal loads. The novelty of this study lies in the fact that the optimum material distribution for achieving minimum and prescribed stress profiles in the FGM disk is not limited to the priori assumed functional variation, unlike the case in the majority of existing literature. An optimization model is developed to solve the inverse problem and obtain the volume fraction or material distribution of the FGM disk corresponding to the minimum and prescribed stress profiles. First, the problem is formulated utilizing second-order differential equations in accordance with theories of two-dimensional thermoelasticity. Then, the second-order governing differential equation, with the design variables being the volume fractions across the disk, is used in an objective function. Finally, the determination of the global minimum value of the objective function is achieved by the differential evolution method through the utilization of a standard finite element approach for solving the differential equation. Thus, the volume fraction distribution of the disk corresponding to the objective function is obtained. The present investigation pertains to two distinct forms of the objective function, specifically, an objective function for minimum stress and another for prescribed stress. In addition, a mathematical model of the direct problem is developed to analyze the minimum stress profile of the FGM disk, with the optimum material distribution derived from the optimization model provided as input. The models are validated through a comparison of their results with those found in existing literature. The numerical results of this study ensure that it is feasible to design an FGM disk with optimum material distribution, achieving the minimum and prescribed stress profile within the disk. It is also revealed that the optimum material distribution is significantly influenced by the stress profile, temperature field, angular speed, and radial thickness of the disk.

Numerical investigation of force and deflection of nanoneedle penetration into cell using finite element approach: Parameter study and experimental validation of results.

This paper aims to develop a numerical methodology to investigate the penetration process of nanoneedles into cells and the corresponding force and indentation length. The finite element approach via the explicit dynamic method handles convergence difficulties in the nonlinear phenomenon. The cell is modeled as an isotropic elastic hemiellipsoidal shell with a thickness of 200 nm, which represents the lipid membrane and actin cortex, encapsulating cytoplasm that is regarded as an Eulerian body because of its fluid-type behavior. Nanoneedles with diameters 400, 200, and 50 nm are considered for model development based on available experimental data. The Von Mises strain failure criterion is used for rupture detection. A parameter study using 1, 2.5, 5, 7.5, and 10 kPa shows that Young's modulus of the HeLa cell membrane is about 5 kPa. Moreover, a failure strain of 1.2 chosen among 0.2, 0.4, 0.6, 0.8, 1, and 1.2 matches best the experimental data. In addition, a diameter study shows that the relations between force-diameter and indentation length-diameter are linear and polynomial, respectively. Furthermore, regarding the experimental data and by using contour of minimum principal stress around needle and an analytical equation for calculation of buckling force of a woven fabric, we proposed that for a given cell, membrane structural stability-a function of the coupled effect of Young's modulus and actin meshwork size-contributes directly to needle insertion success rate for that type of cell.

Effects of different treatment modalities on biomechanical behavior of maxillary incisors with external invasive cervical resorption at different progression levels.

This study aimed to evaluate the biomechanical behavior of maxillary incisors with external invasive cervical resorption (EICR) at different progression levels after receiving different modes of treatment under occlusal forces using finite element analysis (FEA). Three-dimensional (3D) models of intact maxillary central incisors were constructed and modified to include EICR cavities with different progression levels in the buccal cervical areas. The EICR cavities confined to dentin were repaired using Biodentine™ (Septodont Ltd., Saint Maur des Fausse ́s, France), resin composite, or glass ionomer cement (GIC) . Additionally, EICR cavities with pulp invasion requiring direct pulp capping were simulated as repaired using Biodentine only or 1 mm thick Biodentine and either resin composite or GIC for the rest of the cavity. Moreover, models with root canal treatment and EICR defects repaired using Biodentine, resin composites, or GIC were also generated. A force of 240 N was applied to the incisal edge. The principal stresses in the dentin were evaluated. GIC showed more favorable results than the other materials in EICR cavities confined to the dentin. However, Biodentine alone resulted in more favorable minimum principal stresses (Pmin ) compared to other materials in EICR cavities with close pulp proximity. Exceptionally, the models localized in the coronal third of the root with a circumferential extension of the cavity >90° showed more favorable results for GIC. The presence of root canal treatment had no significant effect on stress values. Based on this FEA study the use of GIC in EICR lesions confined to the dentin is recommended. However, Biodentine may be a better option for restoring EICR lesions close to the pulp with or without root canal treatment. Except when the circumferential extension of the cavity is >90°, the use of GIC may be more advantageous.

Cryogenic Fracture Proliferation from Boreholes under Stresses

Cryogenic fracturing has been explored in recent years as a waterless fracturing method for well stimulation to avoid issues encountered in water-based hydraulic fracturing. Cryogenic stimulation using liquid nitrogen applies large thermal gradients on reservoir rocks to induce fractures. This study investigates the initiation and proliferation of cryogenic fractures from boreholes under external stress on specimens. We flowed liquid nitrogen through boreholes drilled through the center of transparent PMMA cylinders under uniaxial stress and monitored fracture proliferation, temperatures, and borehole pressures. Our results show that the effect of stress resembles that of hydraulic fractures such that fractures propagate more in the direction of the stress. Under loading perpendicular to the borehole axis, a cloud of annular and longitudinal fractures extends more in the direction of loading. Under loading parallel to the borehole axis, longitudinal fractures dominate, and annular fractures become more suppressed and more sparsely distributed than those of unconfined specimens. Even if fractures are driven to initiate against the influence of stress, such as those from a boundary edge of a high stress concentration, they gradually deflect in the direction of stress, similar to hydraulic fractures from perforation holes that curve toward a direction perpendicular to the minimum stress direction.

Numerical simulation of deflagration fracturing in shale gas reservoirs considering the effect of stress wave impact and gas drive

Methane deflagration fracturing is a new reservoir stimulation method that serves the efficient development of shale gas reservoirs. However, the propagation law of deflagration fractures is still unclear. In this paper, a numerical model considering the effect of stress wave impact and gas drive of deflagration fracturing was established based on the continuum–discontinuum element method (CDEM). The correctness of the numerical model was verified by comparing it with a laboratory experiment, the steady and unsteady analytical solutions of gas flow, and the approximate solution of fracture propagation. Then, numerical simulations of methane deflagration fracturing in vertical wells and horizontal wells under different factors were carried out to analyze the fracture mechanism. The results indicate that deflagration fracturing in vertical wells can break through the stress concentration around the borehole; the initial radial fractures are formed under the action of stress wave impact and then propagate substantially under the driving action of high-pressure gas. The in-situ stress difference affects the deflagration fracture propagation and makes the half-fracture length in the direction of maximum principal stress larger than that in the direction of minimum principal stress. The more significant the stress difference is, the more noticeable this deviation will be. When the deflagration peak pressure is high, the reservoir burst degree is large, which is conducive to enlarging the stimulation range of deflagration fracturing. Staged deflagration fracturing in horizontal wells can form 5–8 obvious fractures perpendicular to the horizontal borehole in each explosion section. A large cluster spacing and explosion section length are conducive to expanding the stimulation scope. Moreover, the propagation of deflagration fractures will be induced by the natural fractures, and the natural fracture with a considerable length or a slight angle between the dip angle and the propagation direction of deflagration fractures is more likely to be activated.

Effect on Surface Properties of H13 Mold Steel Cladding Layer by Scanning Strategy

Laser cladding technology is used to clad the surface layer of H13 mold steel with Ni60A metal powder coating to solve the failure problem. The study used JMatPro software to extract and fit the thermophysical property parameters of the substrate and the clad material, and then used ANSYS APDL software to qualitatively analyze the distribution of melt pool morphology, nodal temperature versus time course curve and residual stress magnitude during the laser cladding process. Based on the results of the minimum residual stress in the cladding, reasonable scan paths were derived for the preparation of metal coatings on the surface layer of the die steel. The results show that the maximum peak temperature of the cladding process is 2515°C using short path scanning. The cladding layer can form a good metallurgical bond with the substrate at this temperature, with a stress of 406.68 MPa in the scanning direction and 284.45 MPa perpendicular to the scanning direction, which is significantly smaller than the residual stresses of other scanning methods. The residual stress values for the different strategies are from largest to smallest: spiral scan &gt; block scan &gt; long path scan &gt; short path scan.