Static Stress Changes Research Articles

A material model describing the nonlinear biaxial tensile behavior of fabric membrane in precise forming for inflated structures

Inflated membrane structures are increasingly used as formwork of concrete shells and in aerospace fields, which have high requirements for forming accuracy and therefore need to be analyzed for precise forming. The elastic modulus of membrane material is actually affected by stress ratio and strain state: during initial tension of membrane fabric, elastic modulus is small at low strain levels and changes quickly as stress state changes and strain grows. Therefore, a linear material constitutive model is inadequate for the precise forming analysis of an inflated structure with highly designed internal pressure, which leads to a high stress-strain state and various stress ratios. In this paper, a nonlinear material constitutive model is proposed by formulating the change of elastic modulus as a nonlinear function about strain state coupled to a linear function of stress ratio, considering the combined impacts of both factors. Biaxial tensile tests are performed to determine the material property and develop the nonlinear material model. The precise forming analysis results with the proposed material model and three commonly used membrane material models are discussed and compared based on an inflation experiment of a spherical inflated membrane structure. The simulation with the proposed model basically exhibits a nonlinear variation law and a variance of less than 0.2 % throughout the inflation, demonstrating that the proposed material model has superior accuracy in predicting the forming shape of the inflated membrane structure.

Mechanical Interplay in a Silicon-Graphite Composite Electrode Under High Current Density over Long-Term Operation

Silicon-Graphite (Si-Gr) composite electrodes, composed of heterogeneous active materials, combine the high capacity of silicon with the conductivity and stability of graphite. These anodes, known for their excellent electrochemical performance and cycle stability, are gaining attention as potential replacements for graphite electrodes [1]. However, their long-term cycle stability is not as good as graphite's, and the higher the silicon content, the quicker the degradation occurs. Thus, it is necessary to improve this aspect [2]. The primary reason for the degradation of cycle stability in Si-Gr composite electrodes is the repeated growth of the Solid Electrolyte Interphase (SEI) during the cycling process [3]. Initially, the SEI forms around the active material, and as the cycling progresses, it extends over the entire electrode [4, 5]. During this process, the electrochemical performance of the active materials within the electrode decreases, and occasionally recovers. This fluctuation is influenced by the repeated growth of the SEI. When the residual stress inside the electrode exceeds the yield point, electrode cracking occurs. Unintentionally, these cracks shorten the ion transport pathways, which in turn can restore the electrochemical performance [5]. However, this interpretation is based on the observed capacity reduction and changes in electrode structure ex-situ. Therefore, there are inherent limitations in clearly delineating the step-by-step processes that result in internal structural changes and crack formation within the electrode. Additional analysis is required to further investigate these phenomena.In this study, to understand the internal structural changes of the Si-Gr composite electrode during high-speed, long-term cycling, we monitored the changes in electrode stress states in-situ during 200 cycles at 1C [6]. Additionally, by varying the silicon ratio, we aimed to comprehend the role of each active material over the long term. In Figure 1a, the observed capacity and stress changes in the 6% Si electrode are overlaid values of the two active materials' reactions. To analyze the stress behavior characteristics due to the volumetric behavior of each active material, as shown in Figures 1b and 1c, the capacity and stress were differentiated to distinguish the contributions of each material (dQ/dV, dσ/dV). Additionally, by calculating the differential stress to differential capacity (dσ/dQ), as shown in Figure 1d, we understood the changes in the electrode structure at the reaction point of Silicon (Si1 for Li3.75Si → LixSi). By further analyzing these results and compiling data from different ratios of active materials, we aim to analyze the changes in internal electrode structure and the causes of crack formation in long-term, based on high-speed charging/discharging. Figure 1

Coulomb Stress Change of Active Fault in South Sumatra Region Revealed from Kerinci Earthquake October 06, 1995 Mw 6.7 and Sungai Penuh Earthquake October 01, 2009 Mw 6.6

We successfully highlight the correlation of Static Stress Change (ΔCFF) in the Kerimci earthquake October 06, 1995 Mw 6.7 and Sungai Penuh earthquake October 01, 2009 Mw 6.6 earthquakes to the seismicity conditions. The data used in this study are focal mechanism obtained from Global Centroid Moment Tensor (GCMT) and seismicity obtained from USGS with M ≥ 4 after the main earthquake with a time span of 11 years. ΔCFF obtained in the Mentawai Fault System area has increased coulomb stress. ΔCFF obtained in the Suliti Segment, Ketaun Segment, and Back arc Basin of Jambi experienced an increase in stress, which can indicate the potential for future earthquakes, but there was no increase in seismicity. Besides that, ΔCFF in areas that experienced a decrease in stress, experienced an increase in seismicity. This is caused by background seismicity in the area and several factors influence the results of the calculation of ΔCFF against seismicity. Simplicity in calculation causes difficulty in explaining seismicity, especially in the blue lobe. Moreover, the use of receiver fault mechanism produces a very large error in the complex regional stress field. The use of a constant friction coefficient also produces a very large error in the calculation.

Decoding Stress Patterns of the 2023 Türkiye‐Syria Earthquake Doublet

AbstractEarthquake interaction across multiple time scales can reveal complex stress evolution and rupture patterns. Here, we investigate the role of static stress change in the 2023 Mw 7.8 and 7.6 earthquake doublet along the East Anatolian Fault (EAF), using simulations of 19 historical earthquakes (M ≥ 6.1) and the 2023 earthquake doublet from 1822 to 2023. Focusing on six cascading sub‐events during the 2023 Kahramanmaraş earthquake doublet, we reveal how one sub‐event's stress alteration can impact the emergence and rupture of subsequent sub‐events. Our analysis unveils that the 2023 Mw 7.8 earthquake was delayed due to stress shadow effects from historical events, while the 2023 Mw 7.6 earthquake was accelerated as a result of stress increases from historical events and ultimately triggered by the 2023 Mw 7.8 earthquake. This study underscores the importance of grasping earthquake preparation, rupture initiation, propagation, and termination in the context of intricate fault systems worldwide. Based on these results, we draw attention to increased seismic hazards in the Elazig‐Bingol seismic gap of the EAF and the northern section of the Dead Sea Fault (DSF), necessitating increased monitoring and preparedness efforts.

Effect of Residual Stresses on the Fatigue Stress Range of a Pre-Deformed Stainless Steel AISI 316L Exposed to Combined Loading

AISI 316L austenitic stainless steel is utilized in various processing industries, due to its abrasion resistance, corrosion resistance, and excellent properties over a wide temperature range. The physical and mechanical properties of a material change during the manufacturing process and plastic deformation, e.g., bending. During the combined tensile and bending loading of a structural component, the stress state changes due to the residual stresses and the loading range. To characterize the component’s stress state, the billet was bent to induce residual stress, but a phase transformation to martensite also occurred. The bent billet was subjected to combined tensile–bending and fatigue loading. The experimentally measured the load vs. displacement of the bent billet was compared with the numerical simulations. The results showed that during fatigue loading of the bent billet, both the initial stress state at the critical point and the stress state during the dynamic loading itself must be considered. Analysis was demonstrated only for one single critical point on the surface of the bent billet. The residual stresses due to the phase transformation of austenite to martensite affected the range and ratio of stress. The model for the stress–strain behaviour of the material was established by comparing the experimentally and numerically obtained load vs. displacement curves. Based on the description of the stress–strain behaviour of the pre-deformed material, guidelines have been provided for reducing residual tensile stresses in pre-deformed structural components.

Understanding the High-Temperature Deformation Behaviors in Additively Manufactured Al6061+TiC Composites via In Situ Neutron Diffraction

Aluminum matrix composites (AMCs) are designed to enhance the performance of conventional aluminum alloys for engineering applications at both room and elevated temperatures. However, the dynamic phase-specific deformation behavior and load-sharing mechanisms of AMCs at elevated temperatures have not been extensively studied and remain unclear. Here, in situ neutron diffraction experiments are employed to reveal the phase-specific structure evolution of additively manufactured Al6061+TiC composites under compressive loading at 250 °C. It is found that the addition of a small amount of nano-size TiC significantly alters the deformation behavior and increases the strength at 250 °C in comparison to the as-printed Al6061. Unlike the two-stage behavior observed in Al6061, the Al6061+TiC composites exhibit three stages during compression triggered by changes in the interphase stress states. Further analysis of Bragg peak intensity and broadening reveals that the presence of TiC alters the dislocation activity during deformation at 250 °C by influencing dislocation slip planes and promoting dislocation accumulation. These findings provide direct experimental observations of the phase-specific dynamic process in AMCs under deformation at an elevated temperature. The revealed mechanisms provide insights for the future design and optimization of high-performance AMCs.

Investigation of the 2011 Yingjiang, Yunnan, China Ms. 5.8 Earthquake Sequence: Seismic Migration, Seismogenic Mechanism, and Hazard Implication

On March 10, 2011, an Ms. 5.8 earthquake struck Yingjiang City, western Yunnan, China, causing destructive damage. Due to the very sparse distribution of seismic stations on the southwestern border of China, its seismogenic structure and mechanism remain controversial. In this study, with the aid of machine-learning-based detection and location workflow and template matching technique, we detect 10,356 events ranging from December 1, 2010, to April 30, 2011. The high-precision earthquake catalog shows that the foreshocks initiated in the extensional stepover connecting the northeast and middle segments of the Dayingjiang fault and then bilaterally extended northeast and southwest, with migration fronts that can be simulated by fluid diffusion model with diffusivities of 0.8 m2/s and 0.19 m2/s, respectively. The mainshock occurred at the southwest end of the foreshock sequence and then probably activated the northwest-trending blind fault. In addition, we determine the full moment tensor solutions for the mainshock, six large foreshocks, and one aftershock, with magnitudes ranging from 3.03 to 5.80, in which the mainshock was characterized by an obvious negative isotropic (ISO) component. The static Coulomb failure stress change caused by five Mw ≥ 4.0 foreshocks on the mainshock fault plane is ∼24 kPa, reaching the typical static triggering threshold. Therefore, we suggest that both the fluid diffusion and stress perturbation contribute to triggering the mainshock. This study advances our understanding of the spatiotemporal evolution, seismogenic mechanism, and hazard implication for the Yingjiang Ms. 5.8 earthquake and provides additional evidence of natural fluid-triggered seismicity in western Yunnan.

Dynamic Monitoring of Steel Beam Stress Based on PMN-PT Sensor

Steel beams are widely used load-bearing components in bridge construction. They are prone to internal stress concentration under low-frequency vibrations caused by natural disasters and adverse loads, leading to microcracks and fractures, thereby accelerating the instability of steel components. Therefore, dynamic stress monitoring of steel beams under low-frequency vibrations is crucial to ensure structural safety. This study proposed an external stress sensor based on PMN-PT material. The sensor has the advantages of high sensitivity, comprehensive frequency response, and fast response speed. To verify the accuracy and feasibility of the sensor in actual engineering, the LETRY universal testing machine and drop hammer impact system were used to carry out stress monitoring tests and finite element simulations on scaled I-shaped steel beams with PMN-PT sensors attached. The results show that: (1) The PMN-PT sensor has exceptionally high sensitivity, maintained at 1.716~1.726 V/MPa in the frequency range of 0~1000 Hz. The sensor performance is much higher than that of PVDF sensors with the same adhesive layer thickness. (2) Under low-frequency random vibration, the sensor’s time domain and frequency domain output voltages are always consistent with the waveform of the applied load, which can reflect the changes in the structural stress state in real time. (3) Under the impact of a drop hammer, the sensor signal response delay is only 0.001 s, and the sensitivity linear fitting degree is above 0.9. (4) The simulation and experimental results are highly consistent, confirming the superior performance of the PMN-PT sensor, which can be effectively used for stress monitoring of steel structures in low-frequency vibration environments.

Cascade processes of induced and triggered earthquakes-Case study in the Weiyuan shale gas development area in Sichuan Basin, China

Identifying accurate seismogenic faults is critical for studying the mechanisms of induced earthquakes. On February 24th and 25th, 2019, three moderate earthquakes with magnitudes of MS 4.7, MS 4.3, and MS 4.9 occurred successively in the shale gas development area of Weiyuan, China. We utilized high-resolution three-dimensional (3D) seismic data to identify two pre-existing faults (F1 and F2) that were responsible for the three moderate earthquakes. InSAR data were used to validate the rationality of the two seismogenic faults. Furthermore, we analyzed the impact of fluid diffusion on fault F1 near the fracturing well and calculated the Coulomb failure stress (CFS) generated on fault F2 by the MS 4.7 and MS 4.3 earthquakes to analyze the interactions between these events. The results indicated that fluid diffusion caused by hydrofracturing induced the MS 4.3 and MS 4.7 earthquakes on F1. The static Coulomb stress changes from these two earthquakes subsequently triggered the larger MS 4.9 earthquake on F2. This study provides a case of a cascading process in which induced earthquake events triggered a more distant and higher-magnitude earthquake. This triggering scenario reminds us that earthquake-to-earthquake interactions may be more hazardous than a “typical” inducing mechanism and challenges current risk management practices.

Study on Geological Deformation of Supercritical CO2 Sequestration in Oil Shale after In Situ Pyrolysis

After the completion of in situ pyrolysis, oil shale can be used as a natural place for CO2 sequestration. However, the effects of chemical action and formation stress-state changes on the deformation of oil shale should be considered when CO2 is injected into oil shale after pyrolysis. In this study, combined with statistical damage mechanics, a transverse isotropic model of oil shale with coupled damage mechanisms was established by considering the decreased mechanical properties and the chemical damage caused by CO2 injection. The process of injecting supercritical CO2 into oil shale after pyrolysis was simulated by COMSOL6.0. The volume distribution of CO2 and the stress evolution in oil shale were analyzed. It is found that CO2 injection into oil shale after pyrolysis will not produce new force damage, and the force damage caused by the decrease in the mechanical properties of oil shale after pyrolysis can offset the ground uplift caused by CO2 injection to a certain extent. Under the combined action of chemical damage and mechanical damage, the uplift of a formation with a thickness of 200 m is only 10 cm. The injection of supercritical CO2 is beneficial for maintaining the stability of oil shale after in situ pyrolysis.

Hydro‐Mechanical Characterization of a Fractured Aquifer Using Groundwater Level Tidal Analysis: Effect of Pore Pressure and Seismic Dynamic Shear Stresses on Permeability Variations

AbstractGroundwater level tidal analysis is a powerful technique to monitor aquifer's permeability and hence its change over time. Earthquakes are known to affect aquifer's properties, in their vicinity through static stress changes but also further away through dynamic stresses. Most often changes are in the form of permeability increases, but sometimes decreases; the changes can be either permanent or transient. These observations are relatively well documented but the physical processes behind these changes are not well understood. By combining solid‐earth and barometric tidal groundwater level responses in a borehole in a coherent poroelastic theoretical framework, and a bi‐layer hydrogeological model, we recover a 15 years‐long time series evolution of aquifer transmissivity and shear modulus. This study showcases the full potential of the tidal analysis method, coupling pore pressure diffusion and rock deformation, at the frontier of hydrogeology and rock physics. This unprecedented measurement of permeability and shear modulus evolution by tidal analysis reveals, during interseismic period, high sensitivity of this shallow aquifer to effective stress, and thus to pore pressure. Thanks to additional finite element simulation of seismic wave propagation, we explore the different mechanisms affecting permeability and shear modulus in the studied fractured andesite aquifer. This study confirms the predominant role of seismic dynamic stresses, and more precisely of dynamic shear stresses, in the change of permeability following an earthquake.

Features of assessing the stability of the open pit - overburden dump geotechnical system during the development of upland deposits in an open-pit manner

Abstract The main criterion for mining operations in the development of upland deposits by open-pit mining is the safety of their operation, and therefore the assessment of the stability of the geotechnical system is always relevant and requires a special approach and study. The article reveals the features of the assessment of the geotechnical system of the open pit - overburden dump of the a mountainous, consisting in studying the interaction of the geological and geomechanical environment. The factors of both the geological and geomechanical environment affecting the change in the natural stress state of the studied object are described. A scheme for evaluating a geotechnical system has been constructed, the essence of which is to study the elements of both the geological and geomechanical environment.

Integrating 1D and 3D geomechanical modeling to ensure safe hydrogen storage in bedded salt caverns: A comprehensive case study in canning salt, Western Australia

The viability of hydrogen storage in bedded salt caverns hinges on understanding the geomechanical challenges posed by the anisotropic stress states and complex geology of such environments. This study presents a comprehensive geomechanical analysis focusing on a proposed cavern within the Carribuddy Formation in Western Australia, characterized by its interbedded salt layers. This paper introduces a new geomechanical workflow, encompassing 1D and 3D modeling techniques to provide detailed changes of mechanical properties and stress state in interbedded salt formation allowing to identify the initial optimal operational pressures for underground hydrogen storage. Initial 1D models evaluated mechanical properties and in-situ stresses, while subsequent 3D simulations, enriched by data from neighboring wells, detailed the stress, strain, and displacement responses of the cavern walls to internal pressure changes. The analysis pinpointed an initial safe gas pressure range between 3000 and 4000 psi, attributing this margin to the robust characterization of the mechanical and in-situ stress of the formation. Our findings underscore the significance of high-resolution geomechanical modeling in identifying initial optimal operational pressures for hydrogen storage in salt caverns, ensuring both safety and structural integrity.

Investigating the potential influence of tectonic earthquakes on active volcanoes of Vanuatu

It is intuitive to think that an earthquake near a volcano could disrupt its equilibrium and potentially trigger an eruption. But this cause-and-effect link is far from obvious for active volcanoes with an unknown internal stress state and the complexity of its magma-hydrothermal processes. This phenomenon is clearer in continental-oceanic subduction zones where volcanoes generally have closed-vent systems, differentiated high-viscosity magma, and active hydrothermal systems. This phenomenon is less well known in oceanic-oceanic subduction zones, where volcanoes often have open-vent systems, low-viscosity mafic magma, and hypothetic hydrothermal systems. The Vanuatu oceanic-oceanic subduction is an ideal zone to perform such study due to a high-seismic rate and volcanoes with different characteristics. The Vanuatu volcanoes display both open- and closed-vent systems, low and relatively high viscosity magma that enhance different types of volcanic activities (such as lava lakes, strombolian eruptions, high-eruptive columns, phreatic activity), and potential active hydrothermal systems. We compiled and identified sixty-nine cases of earthquakes potentially triggering volcanic activities on Vanuatu volcanoes from 1913 to 2018. Our findings indicate that the triggered volcanic responses occur co-seismically or shortly (at most 2–3 months later) after the earthquake, that the activated volcanoes are mainly located at near-field distances of the potentially triggering earthquake, implying a strong influence of static stress changes. Using the value of the seismic density energy, we suggest that the mechanism of the Vanuatu volcanic responses is due to changes of the permeability within active hydrothermal systems at Lopevi and Ambrym volcanoes in addition to the well-established ones, at Ambae, Garet, and Yasur volcanoes.

Reforming toolpath effect on deformation mechanics in double-sided incremental forming

Incremental sheet metal forming is a die-less process known for its high flexibility, making it a suitable choice for fabricating low-batch, highly customized complex parts. In this paper, the deformation mechanism of double-sided incremental forming (DSIF) with single pass or reforming toolpaths and its effect on the deformation-induced martensite transformation kinetics are investigated by experiments and finite element (FE) simulations for a truncated pyramid part geometry for the first time. An FE model accounting for tool deflection is developed, considering the change in tools' position caused by the forces applied at the contact with the blank. This is done to enhance the simulation accuracy in comparison to experimental results. A comprehensive analysis is conducted with respect to the stress state change and strain accumulation during DSIF with the single pass and reforming toolpaths. The findings reveal that material points along the pyramid wall primarily deform by a combination of plane strain tension, plane strain compression, and shear, depending on the tools’ locations on the sheet surfaces in relation to the material point of interest. The reversal of the forming direction in the reforming toolpath causes material points to experience different stress states and more strain accumulation than the single pass case. Under the same forming temperature, this phenomenon results in a higher and closer martensite transformation on both forming and supporting surfaces of the final part when employing the reforming toolpath as opposed to the single pass case.

Spatial variation in stress orientation in and around Türkiye: rupture propagation across the stress regime transition in the 2023 Mw 7.8 Kahramanmaraş earthquake

SUMMARY On 6 February 2023, the Mw 7.8 Kahramanmaraş, Türkiye, earthquake occurred on the East Anatolian Fault Zone (EAFZ). This study examined the spatial variation of the stress field in and around Türkiye, particularly along the EAFZ, better to understand the rupture process of this event. We first combined focal mechanisms around Türkiye, created a data set consisting of 2984 focal mechanisms, and conducted stress tensor inversion. The results showed that the maximum compressional axis near the EAFZ was oriented north–south and slightly varied along the strike. Moreover, the relative magnitude of north–south compressional stress gradually increased from south to north, and the stress regime changes from a normal fault regime to a strike-slip fault regime. The static stress change caused by the 2023 Mw 7.8 and 7.7 events does not explain this lateral pattern, implying that this stress regime transition existed before the main shock. This suggests that shear stress on the EAFZ was low in this southern segment because it was unfavourably oriented to the regional stress field. Dynamic stress changes due to rupture propagation and dynamic weakening may have triggered the slip at the southern segment under low background shear stress. Previous studies have reported that the Mw 7.8 main shock rupture started at a splay fault, first propagated through the central and northern segments and then backpropagated with a time delay towards the southern segment, where it caused a significant but relatively small slip. The pre-existing along-strike shear stress variation on the fault may have contributed to the smaller and delayed coseismic slip in the southern segment than in the central and northern segments. The main shock rupture possibly caused stress rotation locally near the central segment where the magnitudes of the vertical and north–south compressional stresses were almost equal.

Evaluation for long-term skid resistance of ultra-thin asphalt overlay based on texture characteristics

To evaluate the durability of ultra-thin asphalt overlays in terms of skid resistance accurately, the long-term degradation trends in texture characteristics need to be carried out comprehensively. In this paper, three types of ultra-thin asphalt overlays with different gradations were selected. The long-term abrasion test was conducted, with friction coefficient and texture characteristics tested in each abrasion cycle. Additionally, the stress state changes at the interface of the tire and mixtures were also obtained. Finally, the reliability of texture characteristics was validated based on the correlation between different indicators. The results show that the friction coefficient and texture characteristic show a two-stage decay pattern during the abrasion process, with a significant decrease in stress between 0.75 and 2.5 MPa at the interface of tire and mixtures. The surface mean texture depth (SMTD), root-mean-square deviation of surface (Sq), peak density of surface (Spd), and spatial fluctuations of texture (Δα) were correlated with British pendulum number (BPN) strongly, and the Sq and Δα were recommended as reliable evaluation parameters for the long-term skid resistance of ultra-thin asphalt overlays. This study provides theoretical guidance for the optimal design and field monitoring of ultra-thin asphalt overlays from the perspective of surface texture characteristics.

Deformation mechanisms and fluid conditions of mélange shear zones associated with seamount subduction

Lithologic heterogeneity and the presence of fluids have been linked to seamount subduction and collocated with slow earthquakes. However, the deformation mechanisms and fluid conditions associated with seamount subduction remain poorly understood. The exhumed Chichibu accretionary complex on Amami-Oshima Island preserves mélange shear zones composed of mudstone-dominated mélange and basalt–limestone mélange deformed under sub-greenschist facies metamorphism. The mudstone-dominated mélange contains sandstone, siliceous mudstone, and basalt lenses in an illitic matrix. The basalt–limestone mélange contains micritic limestone and basalt lenses in a chloritic matrix derived from the mixing of limestone and basalt at the foot of a seamount. The basalt–limestone mélange overlies the mudstone-dominated mélange, possibly representing a submarine landslide from the seamount onto trench-fill terrigenous sediments. The asymmetric S–C fabrics in both mélanges show top-to-SE shear consistent with megathrust-related shear. Quartz-filled shear and extension veins in the mudstone-dominated mélange indicate brittle failure at near-lithostatic fluid pressure and low differential stress. Microstructural observations show that deformation in the mudstone-dominated mélange was accommodated by dislocation creep of quartz and combined quartz pressure solution with frictional sliding of illite, whereas the basalt-limestone mélange was accommodated by frictional sliding of chlorite and dislocation creep of coarse-grained calcite, with possible pressure solution creep and diffusion creep of fine-grained calcite. The mélange shear zones formed in association with seamount subduction record temporal changes in deformation mechanisms, fluid pressure, and stress state during megathrust shear with brittle failure under elevated fluid pressure, potentially linking tremor generation near subducting seamounts.

Stress sensitivity mechanism of pores in unconsolidated sandstone reservoir using NMR technology

Unconsolidated sandstone reservoirs exhibit low cementation strengths. With change in the effective stress state of the reservoir, stress sensitivity damage is induced. Consequently, the crude oil productivity is affected and the overall recovery efficiency of the reservoir decreases. Hence, a systematic investigation of the stress sensitivity of unconsolidated sandstone is crucial for the efficient development of unconsolidated sandstone reservoirs. This study primarily employed nuclear magnetic resonance (NMR) to conduct stress-sensitivity experiments on unconsolidated sandstone core samples. It quantitatively characterized the dynamic changes in the damage index of pore sensitivity in unconsolidated sandstone cores under different confining pressures. This research elucidated the stress sensitivity characteristics of unconsolidated sandstone cores during the process of increasing confining pressure. The results of the laboratory study indicated a positive correlation between the experimental confining pressure and stress sensitivity index of the core samples. During the initial stages of increasing confining pressure, the stress sensitivity index increased. With continued increase in the confining pressure, the stress sensitivity damage index for smaller pore throats ranged as 4.18–30.08%, whereas the stress sensitivity damage index for medium pore throats ranged as 4.15–28.45%. Stress sensitivity primarily occurred in smaller and medium pore throats during a progressive increase in the confining pressure. The degree of stress sensitivity was positively correlated with the scale of reservoir pore-throat development, with highly permeable reservoirs experiencing a greater extent of stress sensitivity damage. The statistical results provide crucial support for optimizing mining field production systems, effectively controlling reservoir damage, and achieving the efficient development of unconsolidated sandstone reservoirs.

Dynamic stress analysis of rock joints under railway loading

The proper estimation of stresses generated by the passage of a train is fundamentally important to the serviceability and longevity of railways, and yet very limited knowledge is available where the track substructure is built on a jointed rock mass. The present study introduces an analytical solution for estimating the ground stresses arising from moving wheel loads, causing a change in the three-dimensional stress state in the track formation, in relation to the stress variation with depth and along the longitudinal track section – that is, the direction of train passage. Based on 21 case histories, an array of field measurements and numerical simulations covering a wide range of freight tonnage, train speeds and different formation conditions were considered to validate the proposed analytical solution. The proposed methodology (analytical solution) was then applied to a jointed rock subgrade to determine the normal and shear stresses acting along a specific discontinuity plane. The main analytical outcome demonstrates that the orthogonal vertical and shear stresses present different and phase-shifted history plots for homogeneous ground conditions with principal stresses rotation. However, conversely for a jointed subgrade, the normal and shear stresses along the discontinuity have the same history plot pattern and are in phase. As a practical guide, the results from this study would help to define which cyclic loads should be applied in laboratory tests to simulate realistic traffic patterns of trains travelling over a jointed rock subgrade.