Deformation Behavior Research Articles

Experimental study on energy evolution and damage characteristics of unloading coal under cyclic loading

During deep coal mining, coal is often subjected to vertical cyclic loading and lateral unloading. Understanding the failure characteristics of coal under these combined mechanical conditions is essential for safe extraction. This study utilized the MTS816 system to conduct cyclic loading tests on unloaded coal samples under varying confining pressures. From an energy perspective and based on the obtained stress–strain curves, the energy evolution and damage characteristics of the coal samples were analyzed throughout the entire process from intact state to failure. A CT scanning system was used to examine the fracture distribution and failure characteristics of the post-test coal samples. Finally, based on energy dissipation principles, an energy-damage constitutive model was developed and validated with experimental data. The results show that, under cyclic loading, increases in stress level and confining pressure lead to greater input energy, enhancing the coal samples’ capacity to store and dissipate energy. The input energy is primarily stored as elastic energy, and this storage capacity is independent of confining pressure. At a given confining pressure, the damage variable increases with higher stress levels. With increasing stress levels under different confining pressures, the damage curves gradually diverge, showing significant differences in the damage variable, which reaches its maximum at the highest cyclic stress level. Under varying confining pressures, the two-dimensional/three-dimensional fractal dimensions and fracture volumes of the coal samples first increase and then decrease as confining pressure increases. The internal fractures in the tested coal samples consist of through cracks and micro-crack planes, with the degree of failure quantitatively characterized by fractal dimensions. The experimental loading stage curves show strong consistency with the energy-damage constitutive model curves, accurately describing the deformation behavior of unloaded coal samples under cyclic loading. These findings offer a valuable reference for controlling coal stability under cyclic loading conditions induced by mining activities.

Strain Rate Effect on Edge Formability of Complex-Phased Advanced High-Strength Steels in the Hole Expansion Test

Abstract This study aims to investigate the influence and mechanism of strain rate variation on the hole expansion ratio of complex phase advanced high-strength steels. Two different speeds of hole expansion tests were conducted within the quasi-static range, accompanied by a uniaxial tension test with a deformation mode similar to hole expansion for supplementary analysis. Finite element simulation was utilized to analyze the detailed deformation behavior within the material, particularly at the hole edge where cracks occur during hole expansion. The mechanical and fracture properties obtained from the uniaxial tension test were incorporated into the simulation, taking into account the anisotropy of the material to predict the precise location of crack initiation within the hole edge. To account for the strain rate effect on the experimentally determined hole expansion ratio, a ductile fracture model was introduced and its necessity was validated by considering the occurrence of material fracture before and after crack initiation. By utilizing 3D solid elements, considering material anisotropy, and applying the ductile fracture model, the simulation provided reasonable predictions for the hole expansion ratio, which exhibited variation with strain rate.

Investigation of processes in swelling soils depending on natural moisture content

The problem of soil swelling due to soaking is an important aspect of geotechnical analysis, especially in the design and construction of buried structures and foundations. In the conditions of Ukraine, where the creation of underground shelters is relevant, swelling soils create additional risks for the stability of structures. A separate threat is soil soaking, which can be caused by extinguishing fires during military operations. Studies of swelling clay soils have shown their orthotropy, which is manifested in a significant predominance of horizontal swelling over vertical - the difference can reach 60%. This causes not only an increase in the load on the foundation slabs, but also significant lateral pressure on buried structures. This feature of soils affects their deformation behavior, which must be taken into account when calculating the stability of buildings. Laboratory analysis of swelling confirmed the dependence of the degree of soil deformation on its initial humidity. A pattern has been observed, according to which soils with lower natural moisture content demonstrate a significantly higher swelling potential after soaking. This means that forecasting the moisture regime and preliminary analysis of hydrogeological conditions at the construction site are key factors in minimizing the risk of deformation and damage to structures. In the context of buried structures, it is necessary to consider not only vertical loads, but also possible lateral pressure on underground structures. This requires the implementation of additional structural measures aimed at strengthening the soils and increasing their stability. A thorough analysis of soil conditions, taking into account swelling factors and the use of effective methods of combating swelling will significantly reduce risks, ensure the durability of structures and their reliability of operation.

Quasi-Static Compressive Behavior and Energy Absorption Performance of Polyether Imide Auxetic Structures Made by Fused Deposition Modeling

Auxetic structures have garnered considerable interest for being lightweight and exhibiting superior properties such as an excellent energy absorption capability. In this paper, re-entrant and missing rib square grid auxetic structures were additively manufactured via the fused deposition modeling technique using two types of polyether imide materials: ULTEM 9085 and ULTEM 1010. In-plane quasi-static compressive tests were carried out on the proposed structures at different relative densities to investigate the Poisson’s ratio, equivalent modulus, deformation behavior, and energy absorption performance. Finite element simulations of the compression process were conducted, which confirmed the deformation behavior observed in the experiments. It was found that the Poisson’s ratio and normalized equivalent Young’s modulus of ULTEM 9085 and ULTEM 1010 with the same geometries were very close, while the energy absorption of the ductile ULTEM 9085 was significantly higher than that of the brittle ULTEM 1010 structures. Furthermore, a linear correlation exists between the relative density and specific energy absorption of missing rib square grid structures within the investigated relative density range, whereas the relationship for re-entrant structures follows a power law. This study provides a better understanding of how material properties influence the deformation behavior and energy absorption characteristics of auxetic structures.

Differentiable physics-based system identification for robotic manipulation of elastoplastic materials

Robotic manipulation of volumetric elastoplastic deformable materials, from foods such as dough to construction materials like clay, is in its infancy, largely due to the difficulty of modelling and perception in a high-dimensional space. Simulating the dynamics of such materials is computationally expensive. It tends to suffer from inaccurately estimated physics parameters of the materials and the environment, impeding high-precision manipulation. Estimating such parameters from raw point clouds captured by optical cameras suffers further from heavy occlusions. To address this challenge, this work introduces a novel Differentiable Physics-based System Identification (DPSI) framework 1 that enables a robot arm to infer the physics parameters of elastoplastic materials and the environment using simple manipulation motions and incomplete 3D point clouds, aligning the simulation with the real world. Extensive experiments show that with only a single real-world interaction, the estimated parameters, Young’s modulus, Poisson’s ratio, yield stress and friction coefficients, can accurately simulate visually and physically realistic deformation behaviours induced by unseen and long-horizon manipulation motions. Additionally, the DPSI framework inherently provides physically intuitive interpretations for the parameters in contrast to black-box approaches such as deep neural networks.

Synergistic mechanisms of temperature and strain rate on plastic deformation in SLM 3D printed SS316L utilizing hot processing map analysis

The plastic deformation behavior of selective laser melting (SLM) 3D printed SS316L steel has been analyzed at the temperature range 25- 1000℃ (25 (room temperature), 200, 400, 600, 800 and 1000℃) and strain rate range 10−3-103s−1 (10−3, 10−2, 10−1, 100, 101, 102 and 103 s−1) under compressive loading environments. The flow stress vs. plastic strain results revealed that the flow stress was reduced 136.64% from room temperature to 1000℃ at 10−3s−1. Further, the flow stress was decreased 102.86% from room temperature to 1000℃ at 103s−1. The flow stress was increased 46.63% from 10−3s−1 to 103s−1 at room temperature. Moreover, the flow stress was increased 95.07% from 10−3s−1 to 103s−1 at 1000℃. The temperature and strain rate effect on strain rate sensitivity (m) has been observed for SLM 3D printed SS316L steel. Based on strain rate sensitivity (m), the power dissipation efficiency () and instability dimensionless parameter () map plot contours have been investigated under various hot working parameters for SLM 3D printed SS316L steel. Further, hot working processing maps have been generated by superimposing instability dimensionless parameters () map on the power dissipation efficiency () map for SLM 3D printed SS316L steel. The processing map was further related with investigated material microstructure to identify the hot processing safe and unsafe zone for SLM 3D printed SS316L. The unsafe instability region occurred at the low strain rate range (10−2 – 10−1 s−1), high strain rate range (102-103 s−1) and temperature range (200–400℃, and 800 − 100℃) for 0.02, 0.04, 0.06, 0.08 and 0.10 strain. Further, the remaining area was useful for hot workability. The Vicker’s hardness revealed that the hardness was decreased with 3.87%, 12.55%, 22.01%, 32.35%, and 43.70% at 2000C, 4000C, 6000C, 8000C and 10000C respectively with respect to room temperature hardness.

Characterizing Geometric Effects of Thermal Forming Mold Using Taguchi Method

Abstract The production of In-Mold Electronics (IME) involves applying heat and pressure to soften a polymer film with printed circuits, allowing it to conform to a mold shape. The geometrical features of the mold, such as draft angle, draw depth, and fillet radius, influence the axial strain distribution of the polymer film, which in turn affects the electrical performance of the printed circuits. This study investigates the deformation behavior of a 0.25 mm polycarbonate (PC) film during the thermal molding process, considering its temperature dependent viscosity. Two different molds configurations-one with a single protrusion and the other with dual protrusions were analyzed. The draft angles for the bump features were set at 25°, 30°, and 35°, while the fillet radii were 0.4 mm, 0.6 mm, and 0.8 mm, and the draw depths were 1 mm, 2 mm, and 3 mm. For the dual protrusions mold, the spacing between each protrusion was varied at 3 mm, 4 mm, and 5 mm. During the thermal forming process, the film temperature was raised to 200 °C, and a 3 kPa pressure was applied. Using Polyflow commercial software, the maximum axial strain in the PC film after forming was determined. The influence of mold geometry on strain distribution was then analyzed using Taguchi method, and analysis of variance (ANOVA) was conducted to quantify the contribution of each geometric factor. Results indicated that for the single protrusion mold, draw depth had the most significant impact on maximum tensile strain, followed by fillet radius and draft angle. In the dual protrusions mold, spacing had a minor influence compared to other geometric parameters. These findings highlight the importance of draw depth and fillet radius in thermal forming mold design for IME applications.

Chained Timoshenko Beam Constraint Model With Applications in Large Deflection Analysis of Compliant Mechanism

Abstract Accurately analyzing the large deformation behaviors of compliant mechanisms has always been a significant challenge in the design process. The classical Euler–Bernoulli beam theory serves as the primary theoretical basis for the large deformation analysis of compliant mechanisms. However, neglecting shear effects may reduce the accuracy of modeling compliant mechanisms. Inspired by the beam constraint model, this study takes a step further to develop a Timoshenko beam constraint model (TBCM) for initially curved beams to capture intermediate-range deflections under beam-end loading conditions. On this basis, the chained Timoshenko beam constraint model (CTBCM) is proposed for large deformation analysis and kinetostatic modeling of compliant mechanisms. The accuracy and feasibility of the proposed TBCM and CTBCM have been validated through modeling and analysis of curved beam mechanisms. Results indicate that TBCM and CTBCM are more accurate compared to the Euler beam constraint model (EBCM) and the chained Euler beam constraint model (CEBCM). Additionally, CTBCM has been found to offer computational advantages, as it requires fewer discrete elements to achieve convergence.

Grain Rotation and Deformation Behavior in Cube-Textured Ni Polycrystalline Alloy Studied via In-Situ Tensile Testing and EBSD

The cube texture in alloys shows deterioration under plastic deformation. To further observe the evolution of orientation in individual grains during deformation, in-situ tensile testing was coupled with electron backscattered diffraction (EBSD). We found that the rotation of an individual grain is not only determined by its Schmid factor and size, but also by the condition of the adjacent grains. We demonstrated the interactions between grains using the EBSD data in different models, including the crystal orientation, the kernel average misorientation, the Schmid factor, the inverse pole figure, and the grain reference orientation deviation. A systematic three-factor coupled model involving the Schmid factor, grain size, and neighboring grain states is proposed. Furthermore, the mechanism by which small-sized grains induce the splitting of adjacent larger grains through the pinning effect has not been reported in highly textured polycrystalline materials to date. This characterization allows us to better understand the changes in grain shape and crystal lattice rotation, which can be used to characterize other polycrystalline alloys.

Mechanical Properties and Microscopic Mechanism of Granite Residual Soil Stabilized with Biopolymers

Granite residual soil exhibits a tendency to collapse and disintegrate upon exposure to water, displaying highly unstable mechanical properties. This makes it susceptible to landslides, mudslides, and other geological hazards. In this study, three common biopolymers, i.e., xanthan gum (XG), locust bean gum (LBG), and guar gum (GG), are employed to improve the strength and stability of granite residual soil. A series of experiments were conducted on biopolymer-modified granite residual soil, varying the types of biopolymers, their concentrations, and curing times, to examine their effects on the soil’s strength properties and failure characteristics. The microscopic structure and interaction mechanisms between the soil and biopolymers were analyzed using scanning electron microscopy and X-ray diffraction. The results indicate that guar gum-treated granite residual soil exhibited the highest unconfined compressive strength and shear strength. After adding 2.0% guar gum, the unconfined compressive strength and shear strength of the modified soil are 1.6 times and 1.58 times that of the untreated granite residual soil, respectively. Optimal strength improvements were observed when the biopolymer concentration ranged from 1.5% to 2%, with a curing time of 14 days. After treatment with xanthan gum, locust bean gum, and guar gum, the cohesion of the soil is 1.36 times, 1.34 times, and 1.55 times that of the untreated soil, respectively. The biopolymers enhanced soil bonding through cross-linking, thereby improving the soil’s mechanical properties. The gel-like substances formed by the reaction of biopolymers with water adhered to encapsulated soil particles, significantly altering the soil’s deformation behavior, toughness, and failure modes. Furthermore, interactions between soil minerals and functional groups of the biopolymers contributed to further enhancement of the soil’s mechanical properties. This study demonstrates the feasibility of using biopolymers to improve granite residual soil, offering theoretical insights into the underlying microscopic mechanisms that govern this improvement.

Study on the Effect of Pressure Rise Rate on the Burst Pressure and Failure Regularity of Conventional Domed Rupture Disk

Abstract Rupture disk is an important safety relief device whose bursting at set pressure is essential to protect pressure equipment. However, the existing standards and research still lack the system regularity study of the pressure rise rate on the burst pressure. And the range of the pressure rise rate is not sufficient. In this paper, a test platform with an adjustable pressure rise rate is developed, which can achieve 0–950 MPa/s pressure rise rate load. A series study of conventional domed rupture disk is conducted based on this platform. The results show that the burst pressure error is up to 44.69% with increasing pressure rise rate, significantly affecting the burst pressure. The change of burst pressure is generally an increasing trend. It is divided into three stages, showing a trend of first fast, then slow and finally fast. The reasons for the failure of the regularity of the rupture disk under different pressure rise rates are revealed from the three aspects of burst pressure, deformation behavior, and fracture morphology.

Analysis and Prediction of Deformation of Shield Tunnel Under the Influence of Random Damages Based on Deep Learning

Shield tunnels in operation are often affected by complex geological conditions, environmental factors, and structural aging, leading to cumulative damage in the segments and, consequently, increased deformation that compromises structural safety. To investigate the deformation behavior of tunnel linings under random damage conditions, this study integrates finite element numerical simulation with deep learning techniques to analyze and predict the deformation of shield tunnel segments. First, a refined three-dimensional finite element model was established, and a random damage modeling method was developed to simulate the deformation evolution of tunnel segments under different damage ratios. Additionally, a statistical analysis was conducted to assess the uncertainty in deformation caused by random damage. Furthermore, this study introduces a convolutional neural network (CNN) surrogate model to enable the rapid prediction of shield tunnel deformation under random damage conditions. The results indicate that as the damage ratio increases, both the mean deformation and its variability progressively rise, leading to increased deformation instability, demonstrating the cumulative effect of damage on segment deformation. Moreover, the 1D-CNN surrogate model was trained using finite element computation results, and predictions on the test dataset showed excellent agreement with FEM calculations. The surrogate model achieved a correlation coefficient (R2) exceeding 0.95 and an RMSE below 0.016 mm, confirming its ability to accurately predict the deformation of tunnel segments across different damage conditions. To the best of our knowledge, the finite-element–deep-learning hybrid approach proposed in this study provides a valuable theoretical foundation for predicting the deformation of in-service shield tunnels and assessing structural safety, offering scientific guidance for tunnel safety evaluation and damage repair strategies.

Analysis of Spot-Welded Stiffened Plates under Compression Loading for Automotive Applications

Abstract This research focuses on improving the structural performance and safety of automotive components through the optimization of spot-welded stiffened structures. Experimental investigations were conducted alongside Finite Element Analysis (FEA) to evaluate the strength and deformation behaviour of spot-welded assemblies under various loading conditions. The findings of this study offer valuable insights into enhancing the crashworthiness and reliability of automotive structures, thereby contributing to vehicular safety. Keywords: Spot welding, stiffened plates, crashworthiness, finite element analysis, modal analysis, compression loading, LS-DYNA, Hyper Mesh.

Response of Giant Unilamellar Vesicles (GUVs) in Acoustic Field

AbstractLipid membranes are important components of all living organisms. Giant unilamellar vesicles (GUVs) with diameters ranging from several to a few hundred micrometers provide a unique opportunity to study physical chemistry of biomembranes.In this paper, the response behaviors of GUVs under acoustic radiation force is investigated. The temperature‐mediated deformation of GUVs and GUVs deformation with binary lipid composition under acoustic field were studied in detail. We believe that the deformation behavior of GUVs under acoustic radiation force will provide a new platform for membrane mechanical properties research and can be used in biotechnology.

Evolution of LPSO Phase in Hot Deformation of as‐Forged Mg‐9Gd‐3Y‐2Zn‐0.3Zr Alloy and its Effect on Microstructure and Recrystallization Mechanism

In this article, the hot deformation behavior of as‐forged Mg‐9Gd‐3Y‐2Zn‐0.3Zr alloy is investigated by using Gleeble 3500 thermomechanical simulation under 350–500 °C, strain rates of 0.01–10 s−1, and 60% deformation. The evolution of long period stacking ordered (LPSO) phases and their effects on microstructure and dynamic recrystallization (DRX) mechanisms are systematically explored. The results show that the flow behavior is strain‐rate‐sensitive, transitioning from dynamic recovery (<0.1 s−1) to DRX (>0.1 s−1). The LPSO phases flatten and fragment during deformation, with fragmentation increasing at higher strain rates (0.01 → 10 s−1), causing volume fraction to first decrease (12.5% → 8.6%) then rise (17.2%). At 1 s−1, higher temperatures (400 → 500 °C) lead to LPSO aggregation, with volume fraction peaking at 14.1%. LPSO phases promote DRX via particle‐stimulated nucleation and kink‐induced continuous DRX. Optimal grain uniformity (2.21 μm) occurs at 400 °C/1 s−1 with 8.6% LPSO. The findings reveal LPSO's role in microstructure modulation, aiding high‐strength magnesium alloy design.

Influence of interfacial features and strain rate on the mechanical properties and plasticity mechanisms of lamellar (γ+α₂) TiAl alloys: a crystal plasticity finite element study

ABSTRACT This work aims to characterize the mechanical properties and plasticity behaviors of (γ+α2) TiAl alloys under compressive loading using crystal plasticity finite element method (CPFEM). The analysis incorporates variations in α2-phase content (10%–20%) and lamellar orientations (0°, 45°, and 90°) at strain rates of 5 × 10− 3 s− 1, 5 × 10− 1 s− 1, and 5 × 101 s− 1. The results demonstrate that lamellar orientations of 45° and 90° correspond to the minimum and maximum yield stresses, respectively. Moreover, a higher α₂-phase fraction increases yield strength by enhancing the mechanical stability of lamellar interfaces, making slip initiation more difficult. Yield strength also increases with strain rate, demonstrating strain rate sensitivity. Additionally, stress distribution analysis reveals that stress concentrations predominantly occur in the α₂-phase and at triple junction interfaces, while γ-phase and bilayer interfaces exhibit lower sensitivity. Furthermore, slip system activity monitoring reveals that strain rate, lamellar orientation, and α₂-phase content dictate the deformation mechanisms in the γ-phase, causing a transition between twinning and ordinary slip. Conversely, the α₂-phase consistently deforms via pyramidal slip. T These findings offer valuable insights into the deformation behavior of dual-phase TiAl alloys and provide guidance for enhancing their mechanical performance in high-stress applications.

Atomic simulation on the effect of twin boundary angles on the mechanical properties of TiAl alloys under supersonic fine particles bombardment

Abstract TiAl alloys have been widely adopted in aerospace due to their exceptional mechanical properties. However, surface defects such as cracks and voids that develop during service significantly compromise component longevity. To enhance surface mechanical performance, this study employs Supersonic Fine Particle Bombardment (SFPB) to treat TiAl alloys with varying twin boundary angles, systematically investigating post-treatment deformation mechanisms, tensile properties, and indentation characteristics. Findings demonstrate that twin boundary angles significantly influences the mechanical properties of SFPB-treated TiAl alloys. The 0° specimen exhibits the highest yield strength, while the 30° specimen achieves maximum hardness. Surface analysis reveals that the 30° specimen undergoes the least severe plastic deformation following SFPB treatment. The tensile deformation mechanisms involve: dislocation motion, dislocation-twin boundary interactions, and vacancy evolution. The 60° specimen fractures along the twin boundaries, where the formation of secondary twins and stacking fault tetrahedrons during deformation effectively retards crack propagation. In contrast, other specimens fracture at the surface. Nanoindentation analysis confirms that dislocation-twin boundary reactions dominate the deformation behavior across SFPB-treated specimens. These results demonstrate that controlled adjustment of twin boundary angles provides an effective approach for tailoring the mechanical properties of TiAl alloys processed by SFPB, offering valuable theoretical guidance for practical engineering applications.

Evaluation of Hydrogen Embrittlement Sensitivity with Respect to Crystallographic Orientation of Single Crystal Ni Base Alloys

As demand for hydrogen, a clean energy source, has increased, hydrogen embrittlement caused by hydrogen penetration into materials has became a major challenge. This study evaluated the effect of crystallographic orientation on hydrogen embrittlement sensitivity, using single-crystal Ni-based superalloys that are used in hydrogen gas turbines. The two crystallographic orientations selected for testing were near the <110> and <111> directions. Specimens were first exposed to a 250oC, 10 MPa hydrogen environment for 4 days to allow hydrogen precharging. The hydrogen content within the specimen was measured via thermal desorption spectroscopy. To evaluate hydrogen embrittlement sensitivity, mechanical deformation was conducted by tensile loading. The hydrogen content in the <111> specimen was found to be approximately 16% higher than in the <110> specimen, but the hydrogen embrittlement sensitivity was determined to be higher in the <110>, at 74.0%. Both crystallographic orientations exhibited brittle characteristics regardless of hydrogen penetration and fractures were observed along two common dislocation slip directions. Deformation behavior was evaluated by comparing the crystal orientation of the microstructure before deformation with the change in crystal orientation due to deformation by misorientation. The deformation behavior in the <110> direction was relatively characterized by localized deformation regions with high misorientation, while the <111> direction showed more widespread deformation with lower misorientation. Upon hydrogen penetration, misorientation increased regardless of crystallographic orientation, and it was determined that the <110> direction, with its localized misorientation deformation behavior, was more sensitive to hydrogen embrittlement due to local stress concentration.

Strengthening Mechanism of Cold-Rolled Co-Based Super Alloy for Diaphragm Valves under Room Temperature Tensile Loading

Cold-rolled Co-Cr-Ni alloy (Elgiloy) and Co-Ni-Cr alloy (SPRON 510) can be considered for diaphragm valve materials in gas valve systems. In this study, the effect of direct aging (DA) heat treatment on the tensile properties of Co-based alloys was investigated and the influence of microstructural changes on the tensile behavior was examined. The Co-Cr-Ni Elgiloy alloy exhibited a hexagonal close packed (HCP) phase formation through strain induced martensite transformation (SIMT) in both as-rolled and DA conditions, promoting finer grains due to increased sub-structured and recrystallized grains. In contrast, the Co-Ni-Cr SPRON 510 alloy showed a face centered cubic (FCC) single phase with a higher precipitates fraction and dislocation density. Both Co-based alloys showed improved hardness and strength after DA treatment, with the Co-Cr-Ni alloy demonstrating higher tensile strength and the Co-Ni-Cr alloy exhibiting higher yield strength and elongation. The Co-Cr-Ni alloy revealed fine shallow dimples and cleavage fracture mode, while Co-Ni-Cr showed larger dimples and river patterns on tensile fracture surfaces. The Co-Cr-Ni alloy’s deformation behavior was dominated by active SIMT due to low stacking fault energy (SFE), while the Co-Ni-Cr alloy primarily formed deformation twins due to higher SFE. The high tensile strength of the Co-Cr-Ni alloy was dominated by hardening effects due to a relatively fine grain size and SIMT behavior, while the high yield strength of the Co-Ni-Cr alloy was dominated by deformation twin, dislocation density, Taylor factor and high precipitate fraction.

Evolution of Reversible and Irreversible Deformations of Compact Sandstone Under Different Modes of Triaxial Loading

Abstract Uniaxial or standard triaxial tests, which are the most frequently used in geotechnical practice, do not always simulate in-situ conditions of rocks well. Therefore, the deformations of rocks are now investigated even along alternative loading paths in the stress space. However, these alternatives are mainly used to study the conditions of the ultimate failure of rocks. Their deformational behavior along the alternative paths in the anticipated quasi-elastic stress region and corresponding stiffness moduli are rarely analyzed. Our recent study of the Brenna sandstone therefore compared deformational responses to monotonic loading along selected alternative paths in this stress region. The present new study aimed to evaluate the role of irreversible processes in deformation behavior under the different triaxial loading modes. The experimental methods included the following tests: cyclic isotropic compression, cyclic triaxial pure shear, and cyclic conventional triaxial compression. Irreversible deformation processes representing gradual breakage of rock were confirmed throughout the hypothetical quasi-elastic region. These processes, which may include damage to inter-grain connections or permanent displacements at grain contacts in cracks, are dependent on the stress path. The results also indicate a slight material anisotropy associated with stratification. The reversible and irreversible strains along several alternative triaxial stress paths were estimated. The study’s results are significant for predicting the deformation response of sedimentary rocks to in situ stress. Identification of the extent of the irreversible processes is crucial for understanding the mechanical behavior of these rocks during different time evolutions of in-situ stress or conditions of repeated loading/unloading.