Effective Principal Stress Research Articles

Ballistic Impact Analysis of Graphene Reinforced Kevlar-29 and Kevlar-129 Laminated Composites

Aircraft goes through various high-speed hindrances like bird attacks, etc. Kevlar is increasingly used in the aviation industry to overcome such hindrances. Kevlar, an organic fibre from the family of aromatic polyamides consists of Kevlar 29, Kevlar 129, Kevlar 49 and Kevlar 149. This paper focuses on the use of Kevlar 29 and Kevlar 129 due to its low stiffness in comparison with Kevlar 49 and Kevlar 149. When pure Kevlar 29 and Kevlar 129 with 20 layers each are impacted with a 9g bullet at 900m/s velocity, the bullet perforates the laminate completely. Hence, the Kevlar layers are reinforced with graphene nanoplates. The bullet impact simulation of the ‘single-layer Kevlar 29 reinforced with single-layer graphene nanosheet’ (K29G), ‘single layer Kevlar 129 reinforced with single-layer graphene nanosheet’ (K129G), ‘double layer Kevlar 29 reinforced with double-layer graphene nanosheets’ (KK29GG) and ‘double layer Kevlar 129 reinforced with double-layer graphene nanosheets’ (KK129GG) is carried out using LS-DYNA. Maximum principal stress, effective stress, absorbed energy and displacement of the composite laminates (K29G, K129G, KK29GG and KK129GG) are used to compare the performance of these four laminates in the targeted applications. Due to its higher maximum principal stress, effective stress and lower displacement, the composite laminate K29G proves to be the apt laminate for its usage in aircrafts.

Optimal Support Solution for a Soft Rock Roadway Based on the Drucker–Prager Yield Criteria

Through theoretical calculation, the stress and deformation of surrounding rock can be analyzed, providing guidance for the support design and optimization of soft rock roadways. In this paper, theoretical solutions for both the optimal support pressure and the allowable maximum displacement of surrounding rock are derived from the Drucker–Prager (DP) yield criteria and the steady creep criterion expressed by the third invariant of deviator stress. The DP criterion with different parameters is compared and analyzed with an engineering example. Then, based on the calculation results the effects of long-term strength, cohesion, and internal friction angle of soft rock on the maximum plastic zone radius and allowable maximum displacement of roadway are discussed. The results show that the optimal support solution of soft rock roadways based on the DP criteria can not only reasonably reflect the intermediate principal stress but can also be used to compare and discuss the influence of different DP criteria on the calculation results. The higher the long-term strength of the rock surrounding a roadway is, the smaller the optimal support force is and the larger the allowable maximum displacement is. When the calculated long-term strength of soft rock can ensure that the deformation of the roadway does not exceed the allowable maximum displacement, the roadway can maintain long-term stability without support. With an increase in the cohesion or internal friction angle of soft rock, the radius of the plastic zone decreases gradually and the allowable maximum displacement is reduced by degrees. The use of grouting and other means to improve the strength of surrounding rock can effectively reduce the roadway deformation and save support costs. This new theoretical solution can consider different intermediate principal stress effects and different DP strength criteria, enabling the parameters to become easier to determine. It has a wider range of applications, and the calculation results better demonstrate the strength potential of the surrounding rock.

Rus detachment in Dammam Dome, Eastern Saudi Arabia: a new soft-sediment structure as a ‘sensitive stress sensor’ for the Zagros collision

AbstractThis paper describes in detail hydroplastic structures, which are ‘odd’ kinematic indicators in the basal part of the Eocene Middle Rus Formation. Such structures were previously ignored or falsely interpreted. These hydroplastic structures are found in the massive limestone exposures on the King Fahd University of Petroleum and Minerals (KFUPM) campus. They occur in relation to a principal displacement zone along the boundary/interface between the Lower/Middle Rus, which is referred to as the Rus soft-sediment detachment. The structures are fist-sized vugs associated with carrot- or comet-trail imprints (VCT structures) which were previously translated calcite geodes that have been weathered out. VCT structures show transport/slip towards the NNW (345°) and are found on flat to low-dipping surfaces classified as Y, R and P shears with respect to the orientation of the Rus detachment. Palaeostress analysis indicates an Andersonian transtension stress regime, though it does not facilitate the activation of the Rus soft-sediment detachment. Detachment activity occurred due to the negative effective principal stress σ3′ and the abnormally low frictional coefficient caused by fluid pressure. The soft-sediment Rus detachment can be considered a ‘sensitive stress sensor’ for the Zagros collision since it indicates the Arabian platform’s instability in the wider area of the Dammam Dome during the Late Eocene. This instability is attributed to the inception of the Zagros collision, which was previously considered to occur during the Oligocene based on the well-established pre-Neogene unconformity.

A new analysis model for borehole stability in gas drilling

Gas drilling is one of the main underbalanced drilling techniques, which has a significant effect on the ROP (rate of penetration) improvement in hard formation. However, because the circulating medium in drilling process is gas instead of liquid, the fluid column pressure of gas drilling cannot provide enough support force for the rock around borehole as that of conventional overbalanced drilling. Due to the combination influence of engineering and geological factors, the borehole rock is prone to instability during gas drilling, which can lead to rock blocks falling from the well wall and the formation of irregular borehole shape. To avoid the occurrence of complex underground accidents caused by too many falling rock blocks, such as the borehole instability and pipe-sticking, it is necessary for gas drilling to take into consideration the true triaxial stress state of the rock in the deep formation and the possibility of the borehole breakout. Based on the permitted breakout angle of borehole and true triaxial strength theory of rock, an analysis model of borehole stability has been developed for the engineering of gas drilling. The theory of linear elasticity, involved in this analysis model, is used to analyse the stress state of rock around a cylindrical hole under lower wellbore pressure. The failure of the rock around borehole is determined by the Mogi-Coulomb criterion, which considers the intermediate principal stress effect on rock material. The results show that the model is as simple and practical as the traditional model, which takes no account the effect of the intermediate principal stress or the permitted borehole breakout, and it can provide an accurate analysis for the borehole stability of gas drilling. Therefore, the results of this study, combined with the related work such as the prediction of water formation, can guide the adaptability evaluation for the engineering design of gas drilling.

Residual stress and osmotic swelling of the periodontal ligament

Osmotic swelling and residual stress are increasingly recognized as important factors in soft tissue biomechanics. Little attention has been given to residual stress in periodontal ligament (PDL) biomechanics despite itsrapid growth and remodeling potential. Those tissues that bear compressive loads, e.g., articular cartilage, intervertebral disk, have received much attention related to their capacities for osmotic swelling. To understand residual stress and osmotic swelling in the PDL, it must be asked (1) to what extent, if any, does the PDL exhibit residual stress and osmotic swelling, and (2) if so, whether residual stress and osmotic swelling are mechanically significant to the PDL's stress/strain behavior under external loading. Here, we incrementally built a series of computer models that were fit to uniaxial loading, osmotic swelling and residual stretch data. The models were validated with in vitro shear tests and in vivo tooth-tipping data. Residual stress and osmotic swelling models were used to analyze tension and compression stress (principal stress) effects in PDL specimens under external loads. Shear-to-failure experiments under osmotic conditions were performed and modeled to determine differences in mechanics and failure of swollen periodontal ligament. Significantly higher failure shear stresses in swollen PDL suggested that osmotic swelling reduced tension and thus had a strengthening effect. The in vivo model's first and third principal stresses were both higher with residual stress and osmotic swelling, but smooth stress gradients prevailed throughout the three-dimensional PDL anatomy. The addition of PDL stresses from residual stress and osmotic swelling represents a unique concept in dental biomechanics.

Mesozoic horizontal stress in the East Sichuan Fold-and-thrust Belt, South China: Insights for Lower Paleozoic shale gas retention

The Lower Paleozoic organic-rich shales are the primary targets of shale gas exploration in the East Sichuan Fold-and-thrust Belt (ESFB), South China. Horizontal compression during the Mesozoic orogeny caused the decollement of these shales and further influenced the retention of shale gas. In this study, seven sets of rock samples were recovered from outcrop sites in the ESFB, and the measured occurrences of the exposed strata in the ESFB were extracted from regional geological survey works. Acoustic emission tests under uniaxial compression were conducted on these outcrop samples. Magnitudes of the two principal horizontal effective stresses, SH-eff and Sh-eff, at the seven sampling sites, were acquired based on the Kaiser effect; magnitudes of the Mesozoic SH-eff experienced by the Lower Paleozoic shales in the different tectonic belts were then determined based on previous and newly measured paleo stress magnitudes, as well as their correlation with the measured dip angles of strata. Finally, the maximum depths of decollement (HD-max) of the shales were computed using the inferred paleo stress magnitudes. Measured SH-eff magnitudes of the outcrop samples ranged from 74.7 MPa to 117.0 MPa, and the ratios of SH-eff magnitudes to Sh-eff magnitudes fell between 1.10 and 1.55. The magnitudes of the paleo stress experienced by the Lower Cambrian shale were higher than those of the Lower Silurian shale due to a greater burial depth. The inferred maximum Mesozoic SH-eff magnitudes decreased from 180–200 MPa at the margin of the orogenic belt to 60–80 MPa at the margin of the basin, and the computed HD-max values decreased from more than 10 km to less than 5 km correspondingly. A lower magnitude of the Mesozoic horizontal stress and a shallower decollement contributed to better retention of the Lower Paleozoic shale gas inside the Sichuan Basin.

The critical state behaviour of granular material in triaxial and direct simple shear condition: A DEM approach

The critical state (CS), the anchor concept in the critical state soil mechanics (CSSM) framework, has been comprehensively evaluated in triaxial stress conditions, where all the principal stresses are known but the pure shear stresses in the shear directions are absent. Therefore, the evolution of first and second invariants of the stress tensor (σ′) i.e. the mean effective stress (p′) and deviatoric stress (q) along with volumetric strain (εv) is often used to identify the CS. However, the same does not apply for a direct simple shear (DSS) test, which is more representative of the real ground condition, as in most cases the minor principal stresses are not measured. This leads to the challenge of determining the CS line in the e-log(p') space and thus the concept of state parameter (ψ) cannot be used to characterise soil behaviour. Using DEM, this study evaluates CS for DSS, using evolution σ′ and εv, in the e-σ'N space, compare its location with triaxial CS lines in the e-log(p') space. The CS line for DSS in e-σ'N space was then used to define a modified state parameter, ψ, (ψm). It was found that both ψ and ψm showed good correlations with characteristic behaviour e.g. onset of liquefaction, phase transformation state, fabric anisotropy etc. Therefore, ψm maybe a good alternative of ψ for DSS test where the minor principal effective stresses are not measured to define a CSL in e-log(p') space.

Safety Factor of Unsaturated Soil Slopes considering the Intermediate Principal Stress Effect and Different Profiles of Matric Suction

Profiles of matric suction are critical for assessing the stability of unsaturated soil slopes, and the strength of unsaturated soils is affected by the intermediate principal stress. This study presents a theoretical formulation of safety factor for infinite unsaturated soil slopes under four different profiles of matric suction using the limit equilibrium method. The unified shear strength equation under plane strain conditions is adopted to capture the effect of intermediate principal stress on the strength of unsaturated soils. The proposed formulation of safety factor is found to have good comparability and broad applicability. The validity of the proposed formulation is demonstrated by comparing its predictions with the results of the extended shear strength method and the finite element method available in the literature. Parametric studies show that the effect of intermediate principal stress on the stability of unsaturated soil slopes is significant; the difference of safety factor among four suction profiles is pronounced, and the safety factor is highest for a linear suction profile. In addition, the safety factor changes with the infiltration depth in two stages, decreases with the slope angle, and increases with effective strength parameters. The results of this study are capable of providing beneficial guidance for optimization designs and disaster preventions of unsaturated soil slopes.

Relationship between monotonic and cyclic behavior of saturated soft clay in undrained triaxial compression tests

Cyclic loading–induced deformation of soil is a common problem in engineering practice. In current practice, however, monotonic triaxial tests are more commonly used due to the availability of apparatus and ease of operation. Thus, it would be very useful and practical if monotonic triaxial tests could be used to evaluate the behavior of soil under cyclic loading. This study aims to find an explicit relationship between monotonic and cyclic behavior of saturated soft clay. Six monotonic and nine cyclic triaxial compression tests were conducted on undisturbed saturated soft clay under an undrained condition. The test results showed that the monotonic and cyclic tests shared the same stress–strain surface in a three-dimensional p′–q–εa space (p′, effective mean principal stress; q, deviatoric stress; εa, axial strain). It was also found that it is possible to evaluate the effective stress states of cyclic tests at two specific numbers of cycles, using corresponding monotonic tests. Based on these two findings, a simple procedure is proposed to predict the peak axial strain for the saturated soft clay under different cyclic loadings based on the monotonic tests and only one cyclic test, which was further verified against additional test data from the previous literature.

Design of a small-scale wind turbine blade of glass fibre/epoxy composite for electrification of rural villages in Ethiopia: a case study

This paper targeted the design and stress comparison of a small-scale wind turbine blade which was constructed for the specific environmental conditions of the small rural village in Ethiopia. Firstly, the blade was designed from the characteristic of aerodynamic performance. Secondly, modelling and stress analysis of the windmills was conducted which is very significant for the further design and application of wind turbines. The material was epoxy-GFRP composite. The orthotropic mechanical properties of woven fibre epoxy polymer composite were created (ASTM D1683) with help of Autodesk Helius Composite V.16 software. The tensile and flexural testing specimens were fabricated by D3039M-17 and D7264M-15, respectively. Load, displacement, and modulus were investigated using UTM. Based on the parameters of the airfoil, the three-dimension and the assembly models of blades were mounted with the SOLIDWORK-V.16. Then the models were imported into ANSYS-V.15 WORKBENCH software for analysis. The displacement, Von Misses, and principal stress effects were determined.

Experimental Study on the Dynamic Modulus and Damping Ratio of Compacted Loess under Circular Dynamic Stress Paths

Dynamic loads such as earthquakes and traffic will simultaneously generate vertical dynamic stress and horizontal shear stress in the foundation soil. When the vertical dynamic stress amplitude is twice the horizontal shear dynamic stress amplitude, and the phase difference between them is 90°, a circular dynamic stress path is formed in the τzθd ~ (σzd − σθd)/2 stress coordinate system. To simulate the stress state of soil in the area of the circular dynamic stress path caused by bidirectional dynamic stress coupling, a series of tests of compacted loess under the action of a circular dynamic stress path were carried out using a hollow cylindrical torsion shear apparatus. The effects of the mean principal stress, dry density, and deviatoric stress ratio (the ratio of deviator stress to average principal stress) on the dynamic modulus and damping ratio of compacted loess were mainly studied. The test results show that, under the action of the circular dynamic stress path, the larger the mean principal stress is, the larger the dynamic compression modulus and dynamic shear modulus are. The dynamic compression modulus increases obviously with increasing dry density, but the dynamic shear modulus increases only slightly. When the deviator stress ratio increases from 0 to 0.4, the dynamic compression modulus and dynamic shear modulus increase to a certain extent. In addition, the greater the dry density and deviatoric stress ratio are, the greater the initial dynamic compression modulus and initial dynamic shear modulus of the compacted loess. The dynamic compression damping ratio of compacted loess increases with increasing mean principal stress, but the dynamic shear damping ratio decreases with increasing mean principal stress. Dry density basically has no effect on the dynamic compression damping ratio and dynamic shear damping ratio of compacted loess. When the dynamic strain exceeds 1%, the greater the deviatoric stress ratio is, the smaller the dynamic compression damping ratio and the dynamic shear damping ratio are. The research results can provide reference for the study of dynamic modulus and damping ratio of loess under special stress paths.

Earth Pressure of Three‐Dimensional Stress States under Different Strength Criteria and Its Application

To solve the Earth pressure problems in practical engineering, such as retaining walls and foundation pits, we derive active and passive Earth pressure formulas in accordance with the relationship between intermediate principal stress and excavation under three‐dimensional stress states. The formulas are derived on the basis of the Mohr–Coulomb, spatially mobilized plane (SMP), SMP, Lade–Duncan, axisymmetric compression‐ (AC‐) SMP strength, and generalized Mises (Gen‐Mises) criteria and then extended to clay. We also compare the calculated Earth pressure with the measured data. Results indicate that the Earth pressure considering medium principal stress contribution under a three‐dimensional stress state is consistent with the actual engineering. The calculated active Earth pressure in the Mohr–Coulomb strength criterion is larger, and the passive Earth pressure is smaller than the practical one because the intermediate principal stress effect is not considered. The calculated results of the SMP, SMP, Lade–Duncan, AC‐SMP strength, and Gen‐Mises criteria are close to the measured data, among which the result of the Gen‐Mises criterion is closer. The Earth pressure calculated using the Lade–Duncan criterion is no longer appropriate to describe the Earth pressure under medium principal stress condition in this study. The results of this study have theoretical significance for retaining structure design under a three‐dimensional stress state.

Fracture Reactivation Modeling in a Depleted Reservoir

Injection-induced fracture reactivation during hydraulic fracturing processes in shale gas development as well as coal bed methane (CBM) and other unconventional oil and gas recovery is widely investigated because of potential permeability enhancement impacts. Less attention is paid to induced fracture reactivation during oil and gas production and its impacts on reservoir permeability, despite its relatively common occurrence. During production, a reservoir tends to shrink as effective stresses increase, and the deviatoric effective stresses also increase. These changes in the principal effective stresses may cause Coulomb fracture slip in existing natural fractures, depending on their strength, orientation, and initial stress conditions. In this work, an extended finite element model with contact constraints is used to investigate different fracture slip scenarios induced by general reservoir pressure depletion. The numerical experiments assess the effect of Young’s modulus, the crack orientation, and the frictional coefficient of the crack surface on the distribution of stress and displacement after some reservoir depletion. Results show that the crack orientation significantly affects the state of stress and displacement, particularly in the vicinity of the crack. Slip can only occur in permitted directions, as determined by the magnitudes of the principal stresses and the frictional coefficient. Lastly, a larger frictional coefficient (i.e., a rougher natural fracture surface) makes the crack less prone to shear slip.

Prediction of fracture initiation pressure in multiple failure hydraulic fracturing modes: Three-dimensional stress model considering borehole deformation

Hydraulic fracturing technology has been widely used as the main method of exploiting coalbed methane (CBM). The core of this technology is the accurate prediction of the fracture initiation pressure. As the exploitation depth increases, the influence of the in-situ stress obviously increases. Because the existing model of calculating the fracture initiation pressure ignores the influence of in-situ stress on the extraction borehole (CBM well) deformation, and the fracture initiation failure mode is single, the fracture initiation pressure of hydraulic fracturing cannot be accurately predicted in the field. In this study, a new model for predicting the fracture initiation pressure in hydraulic fracturing under a three-dimensional stress state was established. For the first time, the deformation of a circular borehole is related to the in-situ stress, and the size of the elliptical borehole can be accurately obtained after deformation. Eleven fracture initiation failure modes for the hydraulic fracturing of coal seams are proposed. The fracture initiation pressure of each failure mode was calculated in MATLAB, and the fracture initiation pressure of the shear failure of the ρz plane (a) was the smallest. The effects of the horizontal maximum principal stress, horizontal minimum principal stress, vertical intermediate principal stress, coal seam dip angle, horizontal azimuth of borehole, and gas pressure on the fracturing pressure were investigated using the proposed model. The coal seam dip angle exerted the greatest influence, and a new hydraulic fracturing construction procedure is proposed: the pump pressure should be gradually increased from the minimum initiation pressure according to the fracture initiation pressure gradient required by each failure mode. A field hydraulic fracturing test was conducted, and the experimental results are in good agreement with the theoretical calculation results. The maximum error was 5.5%, which validates the proposed theoretical model. The accuracy of the proposed theoretical model in the calculation of the fracture initiation pressure increased by 364.4% compared with the classical hydraulic fracturing theory. The results obtained by this study can provide more precise guidance to hydraulic fracturing projects.

Uplift and lateral buckling failure mechanisms of offshore pipes buried in normally consolidated clay

Buried offshore pipelines are crucial for the swift transportation of oils and similar fluidized materials from the oil rigs to the processing center or vice versa. In this study, a 2-D finite element analysis is performed to understand the pipeline-soil interaction, the failure mechanism of soil and the variation of capacity factors with different normalised soil properties during upward and lateral buckling of a buried offshore pipeline for no tension (NT) and full tension (FT) condition. The pipeline is installed in a normally consolidated clay bed with linearly varying undrained shear strength. The effect of embedment depth (H), unit weight of soil (γ'), undrained shear strength (Su), and roughness and tensile capacity of the pipe-soil interface on the failure mechanism and capacity factors of pipes are also studied. The current numerical model is validated first against a past numerical study, which showed a good agreement between the result obtained from the current study and the results obtained from the past study (Maitra et al., 2016). Then the failure mechanism and capacity factors are obtained, and their variation for different combinations of embedment depth and unit weight of soil are demonstrated. Both the uplift and lateral capacity factors are observed to be influenced by embedment depth and unit weight of soil. It is observed that, depending on the influencing parameters, both the uplift and lateral capacity factors can be critical for the buckling of the pipeline. Several displacement diagrams are presented to show the failure mechanisms clearly, while, the stress contours are given to observe the behavior of principal effective stresses during different failure mechanisms.

A Unified Constitutive Model for Rock Based on Newly Modified GZZ Criterion

This paper proposes a unified constitutive model for rock based on the newly modified generalized Zhang-Zhu (GZZ) criterion. The constitutive model adopts a non-associated plastic flow rule and a continuous potential function that takes the three effective principal stresses into account. To reflect strain-softening, strain-hardening, and elastic-perfectly plastic behavior of rock in a unified way, a general expression is proposed to model the post-failure behavior of rock using the deviatoric plastic shear strain as the fundamental variable. The proposed constitutive model has been successfully implemented in a 3D finite-difference code and validated using it to simulate the true triaxial test of two types of rocks and comparing the simulation results with the experimental data. Finally, a 3D numerical model based on the proposed constitutive model is constructed to simulate a highway rock tunnel during construction. The results show that the predicted displacements of the rock tunnel are in good agreement with the field measurements.

Boundary Effect and Dynamic Response Study for the Penetration of Rigid Projectiles into Thick, Finite-Radius, Metallic Targets

This study uses the unified strength theory to analyse the elastoplastic stage and plastic stage of a linear strain-hardening target material while considering the effects of the intermediate principal stress and the free lateral boundaries of the target. In this investigation, analytical solutions of the radial stress in the cavity wall are obtained, and a unified penetration model of the target material is built. On this basis, penetration resistance formulas and penetration depth formulas for rigid projectiles with various nose shapes penetrating into thick, finite-radius, metallic targets are deduced, the solutions of which are obtained by utilizing the Simpson method. Accordingly, the proposed method offers a broader scope of application and higher accuracy than previous methods. Through this method, a series of analytical solutions based on different criteria can be obtained, and the penetration depth ranges of targets under different striking velocities can be effectively predicted. Moreover, penetration processes under different conditions are numerically simulated using the software ANSYS/LS-DYNA to study the motion law of the projectiles and the dynamic response of the targets. From the theoretical and numerical approaches, a list of influencing factors for terminal ballistic effects are analysed, including the strength criterion differences, the strength parameter b, the striking velocity v 0 , the projectile nose shape, and the target radius-to-projectile radius ratio rt/a. The results indicate that, as b changes from 1 to 0, the penetration depth Dmax increases by 22.45%. Additionally, Dmax increases by 40.76% when rt/a changes from 16 to 4; hence, it cannot be calculated as an unlimited-size target anymore when rt/a ≤ 16. In weapons field tests, the radius of the metallic target can be conservatively designed to be greater than 28 times the projectile radius to ignore the effect from the free lateral boundaries of the target.

Monotonic and cyclic characteristics of K0-Consolidated saturated soft clay under a stress path involving a variable confining pressure

In certain field conditions such as offshore projects under wave loads or embankments under traffic loads, both the vertical and horizontal stresses are variable. However, previous investigations rarely considered the variation in horizontal stress. To better understand the characteristics of natural saturated soft clay, a series of monotonic and cyclic triaxial tests with a K0-consolidation state were carried out under a variable confining pressure (VCP) stress path. The development of axial strain, pore water pressure and effective stress path is analysed. The results show that with the increase in η (the ratio of the variation in the mean effective principal stress to that of the deviatoric stress), the undrained shear strength (qf) decreases continuously. The pore water pressure generation is slightly improved under a stress path with increasing confining pressure. Based on the test results, a unified formula was established to predict the pore water pressure under VCP stress paths. The unique p–q–e relationship of normally consolidated clay in monotonic VCP triaxial tests was also demonstrated. Under VCP stress paths, the amplitude of the pore pressure increases, and the effective stress path tilts more sharply to the right. Moreover, a unified formula was established that can provide a good reference for predicting effective stress paths under cyclic VCP triaxial tests.

Behavior of high-performance concrete under multiaxial tensile-compressive loading

In this study, an experimental investigation was conducted to determine the triaxial tensile-compressive-compressive (T–C–C) behavior of three different high-performance concrete (HPC) groups with uniaxial compressive strength ranging from 44 to 90 MPa, using servo-hydraulic actuators. In the experimental program, seventeen stress ratios for each batch of HPC were designed, ultimate strength, principal stress–strain curves under T–C–C were measured and recorded, as well as crack patterns. The test results revealed that the uniaxial compressive strength and the stress ratios had a negative effect on the normalized ultimate strength and deformation of HPCs under T–C–C loading. An increase in the minimum principal stress (tensile stress) significantly decreased the ultimate strength, stiffness, and ductility of the HPC samples. Compared with the minimum principal stress effect, intermediate principal stress effect is not obvious for HPCs at different stress ratios. With increasing uniaxial compressive strength, the obtained normalized ultimate strength and deformability of HPCs decrease. Tensile brittle failure mode was observed for the HPC samples under triaxial T–C–C loading. Finally, a modified four-parameter failure criterion was established for HPC based on the experimental results, regarding the effect of uniaxial compressive strength.

Fracturing and Porosity Channeling in Fluid Overpressure Zones in the Shallow Earth’s Crust

At the time of energy transition, it is important to be able to predict the effects of fluid overpressures in different geological scenarios as these can lead to the development of hydrofractures and dilating high-porosity zones. In order to develop an understanding of the complexity of the resulting effective stress fields, fracture and failure patterns, and potential fluid drainage, we study the process with a dynamic hydromechanical numerical model. The model simulates the evolution of fluid pressure buildup, fracturing, and the dynamic interaction between solid and fluid. Three different scenarios are explored: fluid pressure buildup in a sedimentary basin, in a vertical zone, and in a horizontal layer that may be partly offset by a fault. Our results show that the geometry of the area where fluid pressure is successively increased has a first-order control on the developing pattern of porosity changes, on fracturing, and on the absolute fluid pressures that sustained without failure. If the fluid overpressure develops in the whole model, the effective differential and mean stress approach zero and the vertical and horizontal effective principal stresses flip in orientation. The resulting fractures develop under high lithostatic fluid overpressure and are aligned semihorizontally, and consequently, a hydraulic breccia forms. If the area of high fluid pressure buildup is confined in a vertical zone, the effective mean stress decreases while the differential stress remains almost constant and failure takes place in extensional and shear modes at a much lower fluid overpressure. A horizontal fluid pressurized layer that is offset shows a complex system of effective stress evolution with the layer fracturing initially at the location of the offset followed by hydraulic breccia development within the layer. All simulations show a phase transition in the porosity where an initially random porosity reduces its symmetry and forms a static porosity wave with an internal dilating zone and the presence of dynamic porosity channels within this zone. Our results show that patterns of fractures, hence fluid release, that form due to high fluid overpressures can only be successfully predicted if the geometry of the geological system is known, including the fluid overpressure source and the position of seals and faults that offset source layers and seals.