Search Results (778)

The welded joints in steel bridges have a complicated structure, and their fatigue life is mainly determined by the real stress under the coupling effect of vehicle load stress, as well as weld-induced residual stress and its relaxation. Traditional fatigue analysis methods are inadequate for effectively accounting for weld-induced residual stress and its relaxation, resulting in a significant discrepancy between the predicted fatigue life and the actual fatigue cracking time. A fatigue damage assessment model of welded joints was developed in this study, considering weld-induced residual stress and its relaxation under vehicle load stress. A multi-scale finite element model (FEM) for vehicle-induced coupled analysis was established to investigate the weld-induced initial residual stress and its relaxation effect associated with cyclic bend fatigue due to vehicles. The fatigue damage assessment, considering the welding residual stress and its relaxation, was performed based on the S–N curve model from metal fatigue theory and Miner’s linear damage theory. Based on this, the impact of variations in traffic load on fatigue life was forecasted. The results show that (1) the state of tension or compression in vehicle load stress notably impacts the residual stress relaxation effect observed in welded joints, of which the relaxation magnitude of the von Mises stress amounts to 81.2% of the average vehicle load stress value under tensile stress working conditions; (2) the predicted life of deck-to-rib welded joints is 28.26 years, based on traffic data from Jiangyin Bridge, which is closer to the monitored fatigue cracking life when compared with the Eurocode 3 and AASHTO LRFD standards; and (3) when vehicle weight and traffic volume increase by 30%, the fatigue life significantly drops to just 9.25 and 12.13 years, receptively. Full article

An environmentally friendly and rational way of using wood waste is by introducing it into composite compositions. This paper investigates the use of wood chips from 10 to 60% for creating a thermal insulation composite. Prepared wood chips of various fractions were mixed with the sapropel/gypsum mixture. The composite with wood chips and a mixed sapropel/gypsum binder was hardened without thermal curing. Gypsum was added to absorb water from the sapropel and to give the composite its initial strength. Hydrated lime was used to improve the compressive stress of the binding material. The composite density varied from 400 to 1050 kg/m³, thermal conductivity varied from 0.0912 to 0.193 W/(m·K), and compressive stress varied from 0.2 to 7.9 MPa. The density of the composite and the studied properties depended on three factors: the ratio of sapropel to gypsum, the ratio of wood chips to binder, and the level of compaction. The content of sapropel/gypsum varied from 10 to 90%, the ratio of wood chips to binder varied from 0.5 to 1.5, and the compaction level varied from 16 to 40% according to the initial height of the mould. The main characteristics of the prepared composites with different sapropel/gypsum and wood chip ratios were determined. The density, compressive stress, and thermal conductivity results were statistically analysed. Full article

Hydraulic fracturing involving proppant injection is currently the most effective technology to stimulate tight, unconventional reservoirs. The conductivity offered by the created propped and unpropped fracture segments is directly linked to the well deliverability. The accurate modeling of the fracture network conductivity is key to well performance prediction. Unlike most previous studies that have focused on the single-fracture conductivity, a comprehensive fracture network conductivity model was developed by incorporating more complex rock and proppant deformation mechanisms and integrating the conductivity of different propped and unpropped fracture segments through hydraulic–electric analogies. Specifically, for propped fracture segments, the proppant pack permeability was described by simultaneously considering the viscous shear from fracture walls, stress sensitivity, and multiple- or single-proppant-layer placement, while the dynamic width was controlled through proppant pack compaction and proppant embedment. In unpropped fracture segments, as self-supported fracture surface deformation changes the fracture compressibility, the stress-dependent compressibility was utilized to depict the dynamic width. The developed propped and unpropped fracture conductivity models were separately verified against experimental measurement data. Through the hydraulic–electric analogies, a new partially propped fracture network conductivity model was established. For propped fracture segments, an increase in the proppant pack compressibility significantly reduced the fracture conductivity, particularly under high-stress conditions. A larger initial propped fracture aperture offered higher fracture conductivity under identical stress conditions. For single-layer propped fractures, a decrease in the fracture surface elastic modulus from 15 GPa to 10 GPa slightly reduced the fracture conductivity due to greater proppant embedment. For unpropped fractures, a larger compressibility reduction rate (lower fracture compressibility) led to better fracture conductivity maintenance. The fracture network conductivity was dominated by the unpropped fracture segment conductivity when the unpropped length reached 45.5% of the total fracture network length. Full article

Sintered porous materials present challenges for any modeling approach applied to simulate their damage evolution because of their complex microstructure, which is crucial for the initialization and propagation of microcracks. This paper presents discrete element simulations of the damage evolution of a NiAl-based material reconstructed by micro-CT imaging. A novel geometry reconstruction procedure based on micro-CT images and the adapted advancing front algorithm fills the solid phase using well-connected irregular and highly dense sphere packing, which directly represents the microstructure of the porous material. Uniaxial compression experiments were performed to identify the behavior of the NiAl sample and validate the discrete element model. Discrete element simulations based on micro-CT imaging revealed a realistic representation of the damage evolution and stress–strain dependency. The stress and strain of the numerically obtained curve peak differed from the experimentally measured values by 0.1% and 4.2%, respectively. The analysis of damage evolution was performed according to the time variation rate of the broken bond count. Investigation of the stress–strain dependencies obtained by using different values of the compression strain rate showed that the performed simulations approached the quasi-static state and achieved the acceptable accuracy within the limits of the available computational resources. The proposed stress scaling technique allowed a seven times increase of the size of the time step, which reduced the computing time by seven times. Full article

This paper focuses on the stability issues of geological and engineering structures and conducts research from two perspectives: the mechanism of slope landslides under micro-seismic action and the cyclic failure behavior of concrete materials. In terms of slope stability, through the combination of model tests and theories, the cumulative effect of circulating micro-seismic waves on the internal damage of slopes was revealed. This research finds that the coupling of micro-vibration stress and static stress significantly intensifies the stress concentration on the slope, promotes the development of potential sliding surfaces and the extension of joints, and provides a scientific basis for the prediction of landslide disasters. This helps protect mountain ecosystems and reduce soil erosion and vegetation destruction. The number of cyclic loads has a power function attenuation relationship with the compressive strength of concrete. After 1200 cycles, the strength drops to 20.5 MPa (loss rate 48.8%), and the number of cracks increases from 2.7 per mm³ to 34.7 per mm³ (an increase of 11.8 times). Damage evolution is divided into three stages: linear growth, accelerated expansion, and critical failure. The influence of load amplitude on the number of cracks shows a threshold effect. A high amplitude (>0.5 g) significantly stimulates the propagation of intergranular cracks in the mortar matrix, and the proportion of intergranular cracks increases from 12% to 65%. Grey correlation analysis shows that the number of cycles dominates the strength attenuation (correlation degree 0.87), and the load amplitude regulates the crack initiation efficiency more significantly (correlation degree 0.91). These research results can optimize the design of concrete structures, enhance the durability of the project, and indirectly reduce the resource consumption and environmental burden caused by structural damage. Both studies are supported by numerical simulation and experimental verification, providing theoretical support for disaster prevention and control and sustainable engineering practices and contributing to ecological environment risk management and the development of green building materials. Full article

Grouting technology is a pivotal methodology for enhancing the mechanical properties of defective surrounding rock masses in tunnel engineering. Through uniaxial compression tests on intact, hole-containing, and grouted marble specimens, the influence of cement grout filling on the mechanical behavior of marble containing holes was investigated. Based on the experimental results, discrete element method (DEM) models were established for the three types of specimens, revealing the mesoscopic crack propagation mechanisms and stress distribution in potential stress concentration zones during failure. The experimental results demonstrated that the implementation of cement grouting enhanced the strength properties of the specimens by 22.38%. In terms of failure modes, the failure mode of the grouted specimens was similar to that of the intact specimens, and the filling material transformed the failure mode from tensile to shear failure. Numerical simulations revealed differences in microcrack evolution: cracks in the hole-containing specimens initiated near the upper and lower ends of the holes, while cracks in the grouted specimens originated around the filling material, with both types propagating axially. Microcracks in the grouted specimens initiated earlier, but the majority of microcracks in both types developed after peak stress and were predominantly tensile. The stress concentration coefficients for the intact, grouted, and hole-containing specimens were approximately 0.84, 2.25, and 2.96, respectively. The grouted specimens shortened the duration and alleviated the degree of stress concentration in the defect zones. This study elucidates the grouting reinforcement mechanisms in defective tunnel surrounding rock through a multiscale approach, providing theoretical underpinnings for optimizing tunnel support systems and preventing engineering hazards including collapse and rockburst. Full article

In tunnel structures that traverse active fault zones, a vibration isolation layer is often installed between the primary support and the secondary lining. As a result, a three-layer flexible support structure composed of the initial support, damping layer, and secondary lining is formed. Currently, there is limited research on the mechanical behavior of interlayer interfaces. To address this, mechanical performance tests were conducted on composite specimens under compression-shear conditions, including foam concrete paired with C30 ordinary concrete (PC specimens) and foam concrete paired with Engineered Cementitious Composites (PE specimens). The interfacial shear mechanical properties under varying normal loads were analyzed. The results indicate that the shear mechanical properties of both PC and PE interfaces increase with rising normal stress. Under identical normal stress conditions, the PC interface exhibits higher shear strength, shear modulus, and shear-slip energy compared to the PE interface, but its failure displacement is smaller. When the normal stress increases from 0 MPa to 2 MPa, the interfacial shear strength of PC specimens increases by 1.6 times, while that of PE specimens increases by 2.7 times. The residual shear strength of the PC specimens and PE specimens increased by 6.1 times and 15.3 times, respectively. B Established the maximum shear strength formulas for PC specimens and PE specimens. These findings provide a scientific basis for the design of tunnel shock-absorbing layers and ductile linings. Full article

This study presents an analytical solution to examine the mechanical behavior of an incompressible, functionally graded hyperelastic cylinder under combined extension and torsion. The exp-exp strain energy density function characterizes the hyperelastic material, with parameters varying exponentially along the radial direction. To validate the solution, finite element simulations using a custom UHYPER in ABAQUS are performed. The analytical and numerical results show strong agreement across different stretch and twist levels. The stress distribution and maximum stress are significantly influenced by the exponential parameter governing material gradients. Unlike axial stretch, torsion induces a more intricate longitudinal stress distribution, with large twisting producing two extrema that shift toward the cylinder’s center and outer surface. Longitudinal stress primarily governs von Mises stress and strain energy density variations across the radial direction. A critical axial stretch is identified, below which torsion-induced axial force transitions to compression, elongating the cylinder during twisting. Beyond this stretch, the axial force shifts from tensile to compressive with increasing twist, causing initial shortening before further elongation. Full article

Gassy clay, commonly encountered in coastal areas as overconsolidated deposits, demonstrates distinct mechanical properties posing risks for submarine geohazards and engineering stability. Consolidated undrained triaxial tests combined with cyclic simple shear tests were performed on specimens with varying overconsolidation ratios (OCRs) and initial pore pressures, supplemented by SEM microstructural analysis. Triaxial results indicate that OCR controls the transitions between shear contraction and dilatancy, which govern both stress–strain responses and excess pore pressure development. Higher OCR with lower initial pore pressure increases stress path slope, raises undrained shear strength (s_u), reduces pore pressure generation, and induces negative pore pressure at elevated OCR. These effects originate from compressed gas bubbles and limited bubble flooding under overconsolidation, intensifying dilatancy during shear. Cyclic tests reveal gassy clay’s superior cyclic strength, slower pore pressure accumulation, reduced stiffness softening, and enhanced deformation resistance relative to saturated soils. Cyclic pore pressure amplitude increases with OCR, while peak cyclic strength and anti-softening capacity occur at OCR = 2, implying gas bubble interactions. Full article

The study of the compression characteristics of loess in seasonal regions involves analyzing the mechanical properties and mesoscale damage evolution of intact loess subjected to dry–wet freeze–thaw cycles. This study meticulously examines the evolution of the stress–strain curve at the macroscale and the pore structure at the mesoscale of loess by consolidation and drainage triaxial shear tests, as well as nuclear magnetic resonance (NMR), under varying numbers of dry–wet freeze–thaw cycles. Then, utilizing the Duncan–Chang model (D-C), the damage model for intact loess is derived based on the principles of equivalent strain and Weibull distribution, with testing to verify its applicability. The results indicate that the stress–strain curve of undisturbed loess exhibits significant strain softening during the initial stage of the freeze–thaw dry–wet cycle. As the number of cycles increases, the degree of strain softening weakens and gradually exhibits a strain-hardening morphology; the volume strain also changes from dilatancy to shear contraction. According to the internal pore test data analysis, the undisturbed loess contributes two components to shear strength: cementation and friction during the shear process. The cementation component of the aggregate is destroyed after stress application, resulting in a gradual enlargement of the pore area, evidenced by the change from tiny pores into larger- and medium-sized pores. After 10 cycles, the internal pore area of the sample expands by nearly 35%, indicating that the localized damage caused by the dry–wet freeze–thaw cycle controls the macroscopic mechanical properties. Finally, a damage constitutive model is developed based on the experimental phenomena and mechanism analysis, and the model’s validity is verified by comparing the experimental data with theoretical predictions. Full article

Quench–polish–quench (QPQ) nitro-carburizing of AISI 4140 steel in a salt bath was performed in this study. Nitro-carburizing in a salt bath enhanced the formation of Fe-nitride on the outer surface layer. Moreover, the oxidizing treatment formed a thin oxide layer decorated on the outermost part of the QPQ-treated sample. The dense compound layer formed after nitro-carburizing in a salt bath consisted of refined granular Fe₃N and transformed to Fe₂N after post-oxidation treatment. Micro-shot peening (MSP) was adopted before QPQ treatment to increase the treated steel’s fatigue performance. The results indicated that MSP slightly increased the thickness of the compound layer and harden depth, but it had little effect on improving the fatigue strength/life of the QPQ-treated sample (SP-QPQ) compared to the non-peened one (NP-QPQ). A deep compressive residual stress (CRS) field (about 200 μm) and a hard nitrided layer showed a noticeable improvement in the fatigue performance of the QPQ-treated ones relative to the 4140 substrates tempered at 570 °C. The ease of slipping or deforming on the substrate surface was responsible for its poor resistance to fatigue failure. The cracking and spalling of the brittle surface layer were the causes for the fatigue crack initiation and growth of all of the QPQ-treated samples fatigue-loaded at/above 875 MPa. It was noticed that fatigue crack initiation at the subsurface inclusions was more likely to occur in the SP-QPQ sample fatigue-loading at 850 MPa or slightly above the fatigue limit. Full article

This study experimentally and numerically investigated the overall buckling behavior of steel box column components. Two box section specimens were fabricated for axial compression tests. Prior to the tests, the material properties, initial geometric imperfections and residual stress were measured. In addition, an extended parameter analysis was conducted using a finite element model validated by experimental results to evaluate the impact of geometric defects and residual stresses on the bearing capacity of unstiffened and stiffened box section columns. A novel column curve was proposed based on massive datasets of parametric models. The short and long column specimens exhibited typical strength failure and buckling failure modes, respectively. The initial geometric imperfections and residual stresses slightly reduced the buckling strength from the models, with a maximum reduction in buckling strength owing to initial geometric imperfections of 5.2% and that owing to residual stresses of 6.52%. The unstiffened and stiffened box columns have the same stability coefficient when the slenderness ratio is the same. Additionally, the ultimate load capacity calculation formula for stiffened box columns proposed in this paper averages 2.20% higher than Class C curves in JTG D64-2015, lies between Japanese and U.S. codes, and demonstrates good accuracy. Full article

Deep grouting rock engineering is faced with the dual influence of high temperature and dynamic load, which has become a hot issue in geotechnical engineering. This study analyzes the mechanical responses and failure properties of deep-grouted fractured rock under real-time coupling of temperature and dynamic loads through the high-temperature-split Hopkinson pressure bar (HT-SHPB), high-speed imaging, and scanning electron microscopy (SEM) tests. Key findings reveal that (1) the dynamic compressive strength of grouted fractured rock exhibits significant temperature dependency, where the strength increases with the increase of temperature, which has been verified by relevant references. From indoor temperature to 100 °C, the dynamic strength increases moderately, while a pronounced increase is observed between 100 °C and 300 °C. (2) In contrast, the dynamic peak strain demonstrates a two-stage evolution, which sharply rises from indoor temperature to 100 °C, followed by a slowly rise from 100 °C to 300 °C. (3) Macroscopically, impact fractures preferentially initiate as parallel lines at the extremities of pre-existing grouted fractures, consistent with stress concentration patterns under dynamic loading. Microscopic analysis reveals that grouting materials effectively suppress micro-crack generation and propagation at 300 °C, attributed to thermally enhanced cementation and pore-filling effects, explaining the variation of the macroscopic dynamic strength with temperature from the microscopic point of view. Full article

During the formation of deep rock bodies, such as hot dry rock, they are frequently exposed to high temperatures and repeated stress perturbations. The prolonged interaction of these two factors is a potential cause of deep underground rock instability. To investigate the effects of high temperature and cyclic loading–unloading modes on rock mechanical properties, cyclic tests were conducted on granite under real-time high-temperature conditions using a multifunctional high-temperature testing machine. By comparing uniaxial compression test results with scanning electron microscopy (SEM) observations, the following was found: (1) The uniaxial compressive strength and elastic modulus of granite under real-time high-temperature conditions initially increase and then decrease as the temperature rises, while the peak strain consistently increases with temperature. (2) Under both cyclic loading–unloading modes, the mechanical properties of granite first improve and then deteriorate as the temperature increases. (3) As the temperature rises, microcracks in granite under both cyclic loading–unloading methods evolve from intracrystalline to intergranular cracks. The fracture surfaces of granite exhibit a significant increase in fracture severity, along with a noticeable rise in both the number and width of cracks. Crack propagation and crystal integrity degradation are more severe and complex in specimens subjected to variable lower limit cyclic loading–unloading than in those under constant-limit cyclic loading–unloading. These findings are of significant theoretical value for studying rock stability under simultaneous high-temperature and cyclic stress conditions. Full article

Laser shock peening (LSP) is an effective method to improve the fatigue property of metallic materials, and a thorough understanding of its strengthening mechanism is crucial for technology application. In this study, the LSP and fatigue tests of TC4 titanium alloy have been carried out. Combined with the structural characterization and the crystal plasticity finite element (CPFE) simulation, the relationship of stress distribution, microstructure evolution and fatigue performance caused by LSP is revealed. The results indicate that the material’s fatigue life initially increases and subsequently declines with the rising pulse energy. At the optimal pulse energy condition, the laser-shocked specimen demonstrates a 126% increase in fatigue life relative to the untreated specimen, which is accompanied by the higher residual compressive stress along the depth. Meanwhile, the grains become more refined with a uniform size change gradient, and the β phase content drops from 4.1% to 2.2%. Notably, regions with <$\stackrel{-}{1}$2$\stackrel{\mathrm{-}}{1}$0> crystal orientation can be selectively achieved. With the favorable <$\stackrel{\mathrm{-}}{1}$2$\stackrel{\mathrm{-}}{1}$0> slip direction orthogonal to the applied fatigue loading axis, the generation and propagation of dislocations are effectively constrained, thereby significantly enhancing the material’s fatigue performance. The stress distribution and fatigue life in models with different grain sizes and phase contents are further analyzed by the CPFE method, showing good consistency with the experimental results. Theoretically, the excessively high pulse energy causes the transient temperature (1769 °C) to surpass the melting point (1660 °C), which can affect the recrystallization structure and stress distribution. Full article