Mechanics of Time-Dependent Materials最新文献

This paper investigates the high-cycle fatigue behavior of an additively manufactured Inconel-type Ni-based superalloy, GH3625, at 900 °C. Stress-life responses were measured for specimens with different build orientations, and the fatigue mechanisms were examined using transmission electron microscopy, electron backscatter diffraction, and fracture-surface analysis. The results show that the fatigue performance is orientation dependent. At 900 °C and a stress ratio of 0.1, the transverse specimens exhibit a higher fatigue limit than the longitudinal specimens. Under high stress levels, fatigue failure is associated with low dislocation density, limited dislocation interaction, and rapid crack propagation. Under lower stress levels, a higher dislocation density and more developed dislocation structures help suppress crack initiation and extend fatigue life. EBSD results further indicate that fatigue life is influenced by grain orientation distribution, grain size, and the fraction of low-strain grains. A higher fraction of fine, low-strain grains improves resistance to fatigue crack initiation. Fracture observations support these mechanisms, showing rougher fracture surfaces for high-stress specimens and flatter surfaces with slower crack growth for low-stress specimens. These findings clarify the microstructural factors that control the high-temperature high-cycle fatigue behavior of additively manufactured GH3625 superalloy.

This experimental study evaluates the influence of recycled metallic fibers (MF) from steel machining waste and polypropylene grids (PPG) on the flexural behavior of concrete slabs. The metallic fibers are randomly dispersed throughout the concrete, while the PPG grids (with fine and large meshes) are positioned through the slab thickness. Three-point bending tests were conducted on slabs measuring 250 × 500 × 70 mm. Results show that metallic fibers at a fiber content of 0.8% significantly improve both the ultimate strength and ductility. The average ultimate load of slabs with this fiber content of 0.8% (W(_{mathrm{f}} = 0.8)%) reaches 17.21 kN, representing an increase of 24% compared to the control slabs without fibers (Fr = 13.91 kN). These slabs exhibit ductile behavior, in contrast to slabs reinforced with PPG grids, which undergo brittle failure despite an increase in mid-span deflection of approximately 25.6%. A comparison with the literature indicates that using recycled metallic fibers at 0.8% yields higher load capacity and stiffness than a hybrid steel-rubber fiber mixture. This study highlights the potential of recycled metallic fibers to sustainably enhance the mechanical performance of concrete while promoting industrial waste recycling.

Concrete in marine tidal and splash zones is exposed to repeated seawater ingress, drying, salt accumulation, and re-saturation. This study comparatively evaluated the effects of water–binder ratio (W/B = 0.35–0.55), fly ash replacement (0–30%), and a commercial mica-based silane/polydimethylsiloxane-modified internal hydrophobic powder, hereafter denoted as YREC (0–5%) on concrete deterioration under a common accelerated wetting–drying protocol. Specimens were exposed to fivefold artificial seawater by 16 h immersion at 20 °C followed by drying at 60 °C for up to 120 cycles. Compressive strength retention, relative dynamic modulus, mass loss, water absorption, apparent chloride penetration depth, cumulative moisture loss during drying, and water uptake during re-immersion were measured. After 120 cycles, increasing W/B from 0.35 to 0.55 reduced compressive strength retention from 96.18% to 84.78% and increased apparent chloride penetration depth from 11 to 50 mm. Fly ash produced a non-monotonic response: the 20% replacement mixture achieved the best overall performance among the tested dosages, with 91.27% strength retention, 0.814% mass loss, and an apparent chloride penetration depth of 21 mm. The 5% YREC mixture showed 92.79% strength retention, 0.681% mass loss, 2.08% water absorption, and an 18 mm apparent chloride penetration depth; its cumulative moisture loss was 30.3 g, compared with 53.6 g for the unmodified control. Extending drying from 6 to 36 h reduced strength retention from 88.15% to 83.01% and increased mass loss from 1.382% to 1.792%, reflecting the combined effects of drying severity and cumulative thermal exposure. Overall, lower post-exposure moisture exchange was associated with better mechanical retention, less surface scaling, and shallower chloride penetration.

Viscoelastic materials differ fundamentally in their retardation spectrum characteristics. Conventional fractional-order models cannot accommodate such spectral diversity within a unified framework. To address this, a generalized (psi )-Caputo fractional-order Burgers ((psi )-CFB) model is developed in this work, which enables flexible kernel function selection according to material characteristics. A spectral width factor (Lambda (t)) is introduced to establish a mathematically grounded kernel function selection criterion. The power-law kernel is suggested for broad-spectrum materials, the exponential kernel for narrow-spectrum materials, and the logarithmic kernel for materials with intermediate spectral width. The Kachanov damage theory and the Schapery nonlinear viscoelastic theory are incorporated according to the specific characteristics of the target materials. Three representative viscoelastic materials with distinct retardation spectrum features, namely asphalt, polylactic acid (PLA), and polycarbonate (PC), are selected for assessment. The results under unconstrained single-stress fitting show that the (psi )-CFB model provides accurate fitting for asphalt under high-stress conditions (Adj-R² = 0.9995, MAPE = 0.598%), reduces the fitting error for PLA relative to the fractional-order Burgers model, and maintains stable fitting accuracy for PC across both stress levels, demonstrating the effectiveness of the proposed approach compared with the integer-order and conventional fractional-order Burgers models. Furthermore, joint fitting across stress levels is performed for each material using one shared set of core (psi )-CFB parameters with a few stress or data-scale factors, and the results confirm that the proposed model also agrees with the experimental data under this joint-fitting scheme.

The rheological mechanical properties of the surroundings rock of salt caverns under triaxial stress state are essential for precisely assessing geological stability of salt caverns storage. Triaxial stepwise creep loading tests were carried out to investigate the creep deformations and long-term strength properties of salt rocks under triaxial stress states. The impact of varying deviatoric stresses ((sigma _{d})) and confining pressures ((sigma _{c})) on the creep behavior of the surroundings rocks of salt cavern was analyzed. The findings demonstrated that the proportions of creep deformations to total deformations for salt rocks were associated with (sigma _{d}), but not with (sigma _{c}). As the (sigma _{d}) reaches 33.4 MPa, the percentage stabilizes between 80.7% and 93.7%. The steady creep rates of salt rocks increased exponentially as the (sigma _{d}) increases, while it decreased exponentially as the (sigma _{c}) increased. The sensitivity of salt caverns surrounding rock creep to (sigma _{d}) was greater than its sensitivity to (sigma _{c}). The long-term strength of salt rock was defined as the inflection point, which initially decreased progressively and approached a constant value over time, determined by the isochronous stress-strains curve. The time-dependent strength of salt rocks under triaxial stress states could be effectively described using an exponential empirical formulation. At low (sigma _{c}), plastic deformations were primarily influenced by crack propagation, whereas at high (sigma _{c}), it was governed by dislocation mechanisms. As the (sigma _{c}) increase, the salt rocks become “softer,” and the failures characteristics transitioned from brittle to ductile. The strength degradation of salt rocks under triaxial stress states could be attributed to a progressive transition in its strength mechanism—from cohesion-dominated behavior to one increasingly governed by frictional resistance associated with the internal friction angle. This process revealed the intrinsic mechanism of the time-dependents evolution of salt rocks strength.

Strongly confined timber connections rely on high contact pressure at the steel–wood interface to provide stiffness and load-transfer capacity. However, because wood is viscoelastic, stress relaxation under sustained confinement may reduce interfacial friction and degrade connection performance over time. In this study, 90 Chinese fir specimens were prepared to investigate the effect of stress relaxation on steel–wood interface friction under strong confinement. Three volume compression percentages (VCPs), 33%, 48%, and 55%, and six relaxation durations, 0, 1, 7, 15, 30, and 80 days, were considered. Push-out tests were conducted to characterize the friction force–slip response, static friction stress, and normal stress evolution at the steel–wood interface. The results show that frictional performance is strongly affected by VCP and wood density. Higher VCP and higher density lead to greater peak push-out force and static friction stress. Stress relaxation occurs mainly during the first 30 days, with rapid reductions in friction stress at early relaxation durations. Increasing VCP reduces the loss of friction stress and accelerates relaxation stabilization. A three-element viscoelastic model was used to describe the stress relaxation behavior of confined Chinese fir, and a relationship between friction stress and relaxation duration was established. The proposed model showed good agreement with the experimental results and can support prediction of the long-term mechanical performance of strongly confined timber connections. This investigation provides useful guidance for the design and application of high-performance timber connection systems.

To investigate the mechanical response and damage evolution of surrounding rock subjected to repeated dynamic disturbances in deep underground engineering, cyclic impact tests on granite were conducted using a split Hopkinson pressure bar (SHPB) system under different incremental gas-pressure gradients, with constant-amplitude repeated impact tests performed for comparison. The results indicate that, under constant-amplitude repeated impact loading, the dynamic strength and dynamic elastic modulus generally decrease with increasing impact number, demonstrating a degradation process dominated by damage accumulation. Under incremental cyclic impact loading, the dynamic strength increases markedly with increasing impact amplitude; however, crack propagation during the later loading stage leads to stiffness degradation and sudden failure, reflecting competition among strain-rate strengthening, cyclic compaction, and damage-induced degradation. Energy analysis shows that the cumulative dissipated energy increases continuously with impact number. Under incremental cyclic impact loading, a larger impact-amplitude increment accelerates damage accumulation and causes the specimen to enter the high-damage regime at an earlier stage. The energy-based damage variable (D) effectively characterizes the damage evolution process and exhibits good agreement with the observed macroscopic failure characteristics. The results demonstrate that the impact-amplitude increment governs the rate and critical process by which granite transitions from compaction-induced strengthening to damage-dominated instability, while the stability of surrounding rock primarily depends on whether internal damage approaches the critical state of crack coalescence. These findings provide a theoretical basis for stability assessment of deep surrounding rock and the control of blasting-induced disturbances.

The deep rock mass has long been subjected to a complex geomechanical environment of “three highs and one disturbance,” rendering the structural characteristics and mechanical behavior of deep rock masses more complex. The −850 m roadway in Qujiang Coal Mine is longitudinally tiled under the 805 working face and coal pillar. Affected by the mining of the upper working face and the bias of the coal pillar, the roadway presents various deformation and damage characteristics. The deformation and damage patterns of the dynamic pressure roadway of the back mining are analysed through field investigation, bolt and anchor cable force monitoring, and peripheral rock deformation monitoring, and the proposed support countermeasures are verified and analysed by using FLAC3D numerical software. The results show that: (1) Using bolt/anchor cable axial force monitoring and surrounding rock deformation monitoring, it is found that the mining-induced dynamic pressure −850 m roadway displays distinct zonal damage characteristics, which can be divided into a rear sustained (high-pressure) deformation zone and a forward disturbance (dynamic-pressure) deformation zone. (2) In the forward disturbance (dynamic-pressure) deformation zone, damage mainly occurs in the upper roof of the roadway, predominantly as sliding shear damage; in the rear sustained (high-pressure) deformation zone, damage mainly occurs at the floor and the arch foot. The floor is prone to plastic extrusion deformation and damage, while the arch foot mainly experiences punching shear and internal folding damage. (3) According to the different damage modes of the −850 m roadway, it is proposed to implement “three-anchor” combined support technology in the rear sustained (high-pressure) deformation zone, and to implement “resistance-yielding combined” support in the forward disturbance (dynamic-pressure) deformation zone. (4) FLAC3D numerical simulation results are consistent with the field monitoring data, confirming the rationality and effectiveness of the proposed zonal support measures. The research results provide a certain reference for surrounding rock deformation control in deep mining dynamic pressure roadways in the future.