Investigation of Microcrack Propagation and Energy Evolution in Brittle Rocks Based on the Voronoi Model

The cracking of rock mass under compression is the main factor causing structural failure. Therefore, it is very crucial to establish a rock damage evolution model to investigate the crack development process and reveal the failure and instability mechanism of rock under load. In this study, four different strength types of rock samples from hard to weak were selected, and the Voronoi method was used to perform and analyze uniaxial compression tests and the fracture process. The change characteristics of the number, angle, and length of cracks in the process of rock failure and instability were obtained. Three laws of crack development, damage evolution, and energy evolution were analyzed. The main conclusions are as follows. (1) The rock’s initial damage is mainly caused by tensile cracks, and the rapid growth of shear cracks after exceeding the damage threshold indicates that the rock is about to be a failure. The development of micro-cracks is mainly concentrated on the diagonal of the rock sample and gradually expands to the middle along the two ends of the diagonal. (2) The identification point of failure precursor information in Acoustic Emission (AE) can effectively provide a safety warning for the development of rock fracture. (3) The uniaxial compression damage constitutive equation of the rock sample with the crack length as the parameter is established, which can better reflect the damage evolution characteristics of the rock sample. (4) Tensile crack requires low energy consumption and energy dispersion is not concentrated. The damage is not apparent. Shear cracks are concentrated and consume a large amount of energy, resulting in strong damage and making it easy to form macro-cracks.     

Damage Fracture Characterization of Asphalt Mixtures Considering Freeze–Thaw Cycling and Aging Effects Based on Acoustic Emission Monitoring

Freeze–thaw (F–T) cycling and aging effects are the main factors contributing to the deterioration of asphalt mixtures. The acoustic emission (AE) technique enables real-time detection regarding the evolution of internal damage in asphalt mixtures during the loading process. This study set out to investigate the effects of F–T cycling and aging on the damage characteristics of asphalt mixture under splitting loads. Firstly, the Marshall specimens were prepared and then exposed to various numbers of F–T cycles (one, three, five, and seven) and different durations of aging (short-term aging and long-term aging for 24, 72, 120 and 168 h), after which the specimens were loaded by means of indirect tensile (IDT) testing, and corresponding parameters were synchronously collected by the AE acquisition system during the fracture process. Finally, the energy, cumulative energy and peak frequency were selected to investigate the damage mechanisms of asphalt mixtures. The findings demonstrate that the AE parameters provided effective identification of the deterioration for all specimens in real-time, and that the F–T cycling and aging effects altered the damage characteristics of asphalt mixtures, causing early damage, exacerbating the formation of micro-cracks in the early stage, accelerating the expansion of macro-cracks and advancing the debonding between the asphalt and aggregates. The findings of this study provide further insight into the mechanism of F–T cycling and aging effects on the deterioration of asphalt mixture.     

Feasibility Study on the Bamboo Grid Instead of Geogrid for Soil–Rock Mixture Subgrade Reinforcing

To promote the application of the bamboo grid in the soil–rock mixture subgrade in mountain areas, the mechanical properties of bamboo reinforcement were investigated in this study, and the reinforcement effect and interface characteristics of uniaxial/biaxial bamboo grid on the soil–rock mixture under different vertical loads was comparatively analyzed. The results show that the tensile force (2% elongation) of the bamboo reinforcement is 50.21 kN/m, and its average tensile strength is 236.01 MPa. Moreover, bamboo reinforcement has excellent shear and flexural properties. In general, the reinforcement effect of the biaxial bamboo grid on the soil–rock mixed subgrade is better than that of the uniaxial bamboo grid. In the case of using a uniaxial bamboo grid, its pull-out curve is generally a strain-softening type. As for the biaxial bamboo grid, due to the existence of bite force, its pull-out curve usually presents a strain-hardening type. Compared with the uniaxial bamboo grid, the friction coefficient of the reinforcement–soil interface using the biaxial bamboo grid is higher, and the interfacial shear stress is increased by 72.2–91.2%.     

Numerical Simulations of the Soil–Rock Mixture Mechanical Properties Considering the Influence of Rock Block Proportions by PFC2D

Soil-rock mixtures (S-RMs), as a kind of special engineering geological material, need to be studied because of the special structure and complex movement mechanism of their rock blocks, their physical and mechanical properties, and the factors underlying rock block movement in the process of their deformation and failure. In this paper, a series of discrete-element numerical models are constructed in particle flow code software (PFC^2D). First, the random structure numerical models of S-RMs with different rock block proportions are established. Then, the parameters of the soil meso-structure are inversed by the biaxial simulation test, and a series of biaxial compressive tests are performed. The characteristics of stress and strain, deformation and failure, and rock block rotation and energy evolution are systematically investigated. The results show the following. (1) As the rock block proportion (confining pressure 0.5 MPa) increases, the peak strength of increases, the fluctuations of the post-peak become more obvious, and the dilatancy of the sample increases. (2) As the rock block proportion increases, the width of the shear band increases, the distribution of cracks becomes more complex and dispersed, and the range of the shear zone increases. (3) The number of rock blocks with rotation also increases significantly as rock block proportion increases, and the rotation angles are mostly between −5° and 5°. (4) The strain energy of S-RMs with different rock block proportions follows the same change rule as axial strain, showing a trend of first increasing and then decreasing, like the stress–strain curve.     

Experimental Study on Timber−Lightweight Concrete Composite Beams with Ductile Bolt Connectors

A timber–lightweight−concrete (TLC) composite beam connected with a ductile connector in which the ductile connector is made of a stainless−steel bolt anchored with nuts at both ends was proposed. The push−out results and bending performance of the TLC composite specimens were investigated by experimental testing. The push−out results of the shear specimens show that shear–slip curves exhibit good ductility and that their failure can be attributed to bolt buckling accompanied by lightweight concrete cracking. Through the bending tests of ten TLC composite beams and two contrast (pure timber) beams, the effects of different bolt diameters on the strengthening effect of the TLC composite beams were studied. The results show that the TLC composite beams and contrast timber beams break on the timber fiber at the lowest edge of the TLC composite beam, and the failure mode is attributed to bending failure, whereas the bolt connectors and lightweight concrete have no obvious breakage; moreover, the ductile bolt connectors show a good connection performance until the TLC composite beams fail. The ultimate bearing capacities of the TLC composite beams increase 2.03–3.5 times compared to those of the contrast beams, while the mid-span maximum deformation decrease nearly doubled.     

Numerical Simulation Study of Expanding Fracture of 45 Steel Cylindrical Shell under Different Detonation Pressure

Detonation and fragmentation of ductile cylindrical metal shells is a complicated physical phenomenon of material and structural fracture under a high strain rate and high-speed impact. In this article, the smoothed particle hydrodynamics (SPH) numerical model is adopted to study this problem. The model’s reliability is initially tested by comparing the simulation findings with experimental data, and it shows that different fracture modes of cylindrical shells can be obtained by using the same model with a unified constitutive model and failure parameters. By using this model to analyze the explosive fracture process of the cylindrical shells at various detonation pressures, it shows that when the detonation pressure decreases, the cylindrical metal shell fracture changes from a pure shear to tensile–shear mixed fracture. When the detonation pressure is above 31 GPA, a pure shear fracture appears in the shell during the loading stage of shell expansion, and the crack has an angle of 45° or 135° from the radial direction. When the pressure is reduced to 23 GPA, the fracture mode changes to tension–shear mixing, and the proportion of tensile cracks is about one-sixth of the shell fracture. With the explosion pressure reduced to 13 GPA, the proportion of tensile cracks is increased to about one-half of the shell fracture. Finally, the failure mechanism of the different fracture modes was analyzed under different detonation pressures by studying the stress and strain curves in the shells.     

Research on Mechanical Behavior of the Steel–Concrete–Steel Composite Structures Subjected to High Temperature of Fire

A new type of steel–concrete–steel composite structure was adopted and widely used in the immersed tunnel of the Shenzhen–Zhongshan access. The research on the mechanical behavior of the new composite structure under a high temperature of fire is of great engineering significance to the fire protection design of the structure. Both the model test and a numerical simulation were adopted to study the mechanical behavior and damage characteristics of the new composite structure under fire. The RABT standard temperature rise curve was used to simulate the temperature rising law under fire (it reflects the characteristics of temperature rise in case of fire in an enclosed environment: rapidly raised to 1200 °C within 5 min, maintained at 1200 °C for 120 min, then it is cooled to normal temperature after 110 min). The temperature distribution law inside the structure, the deformation development law of the roof and the crack distribution were analyzed based on the thermal–mechanical coupling analysis method. The results showed that the internal part of the composite structure close to the fire surface was directly affected by the high temperature, and the temperature presented a step distribution law, while the part far from the fire surface was affected by the lag effect of the temperature transfer, and the temperature presented a continuous growth law. The roof deformation presented a three-stage model of “rapid growth-deformation stability-deformation recovery” with time. The overall cracks of the composite structure showed a symmetrical distribution under fire. The composite structure presented a shear failure mode as a whole. The results could provide a reference for the study of fire resistance for the new composite structure and support the structural fire protection design of the immersed tunnel of the Shenzhen–Zhongshan access.     

Influence of Stress Intensity Factor on Rail Fatigue Crack Propagation by Finite Element Method

Wheel rail rolling contact fatigue is a very common form of damage, which can lead to uneven rail treads, railhead nuclear damage, etc. Therefore, ANSYS software was used to establish a three-dimensional wheel–rail contact model and analyze the effects of several main characteristics, such as the rail crack length and crack propagation angle, on the fatigue crack intensity factor during crack propagation. The main findings were as follows: (1) With the rail crack length increasing, the position where the crack propagated by mode I moved from the inner edge of the wheel–rail contact spot to the outer edge. When the crack propagated to 0.3–0.5 mm, it propagated to the rail surface, causing the rail material to peel or fall off and other damage. (2) When the crack propagation angle was less than 30°, the cracks were mainly mode II cracks. When the angle was between 30 and 70°, the cracks were mode I–II cracks. When the angle was more than 70°, the cracks were mainly mode I cracks. When the crack propagation angle was 60°, the equivalent stress intensity factor reached the maximum, and the rail cracks propagated the fastest.     

Design,Fabrication, and Mechanical Properties of T-700TM Multiaxial-Warp-Knitting–Needled–C/SiC Composite and Pin

In this paper, the 12k T-700^TM Multiaxial-Warp-Knitting–Needle (MWK–N) C/SiC composite and pin were designed and fabricated using the isothermal chemical vapor infiltration (ICVI) method. The composite’s microstructure and mechanical properties were examined by subjection to tensile and interlaminar shear tests. Three types of double-shear tests were conducted for C/SiC pins, including shear loading perpendicularly, along, and at 45° off-axial to the lamination. The fracture surface of the tensile and shear failure specimens was observed under scanning electronic microscope (SEM). The relationships between the composite’s microstructure, mechanical properties, and damage mechanisms were established. The composite’s average tensile strength was σ_uts = 68.3 MPa and the average interlaminar shear strength was τ_u = 38.7 MPa. For MWK–N–C/SiC pins, the double-shear strength was τ_u = 76.5 MPa, 99.7 MPa, and 79.6 MPa for test types I, II, and III, respectively. Compared with MWK–C/SiC pins, the double-shear strength of MWK–N–C/SiC pins all decreased, i.e., 26.7%, 50.8%, and 8% for test types I, II, and III, respectively. The MWK–N–C/SiC composite and pins possessed high interlaminar shear strength and double-shear strength, due to the needled fiber in the thickness direction, low porosity (10–15%), and high composite density (2.0 g/cm³).     

Principles of Stress-Strength Modelling of the Highly Thermally Loaded Materials—The Influence of an Effect of Strength Differential on the Material Effort

This paper presents an improvement in the Huber–Mises–Hencky (HMH) material effort hypothesis proposed by Burzyński. Unlike the HMH hypothesis, it differentiates the plastic effort between compression and tensile load states, and links shear with tensile limit. Furthermore, it considers the fact that construction materials do not have infinite resistance in the pure tensile hydrostatic load state, which was proved by the static load experiment performed on St12T heat-resistant steel. The asymmetry between tensile and compressive loads is captured by the elastic region asymmetry coefficient ϰ, which was established by experiment for St12T steel in the temperature range between 20 °C and 800 °C.     

The Mechanism of Fracture and Damage Evolution of Granite in Thermal Environment

In the study of rock mechanics, the variation of rock mechanical characteristics in high-temperature environments is always a major issue. The discrete element method and Voronoi modeling method were used to study the mechanical characteristics and crack evolution of granite specimens subjected to the high temperature and uniaxial compression test in order to study the internal crack evolution process of granite under the influence of high temperatures. Meanwhile, dependable findings were acquired when compared to experimental outcomes. A modified failure criterion was devised, and a Fish function was built to examine the evolution behavior of tensile and shear cracks during uniaxial compression, in order to better understand the evolution process of micro-cracks in granite specimens. Shear contacts occurred first, and the number of shear cracks reached its maximum value earliest, according to the findings. The number of tensile contacts then rapidly grew, whereas the number of shear cracks steadily declined. Furthermore, it was found that when temperature rises, the number of early tensile cracks grows. This study develops a fracture prediction system for rock engineering in high-temperature conditions.     

Investigation of Damage Evolution in Heterogeneous Rock Based on the Grain-Based Finite-Discrete Element Model

Granite exhibits obvious meso-geometric heterogeneity. To study the influence of grain size and preferred grain orientation on the damage evolution and mechanical properties of granite, as well as to reveal the inner link between grain size‚ preferred orientation, uniaxial tensile strength (UTS) and damage evolution, a series of Brazilian splitting tests were carried out based on the combined finite-discrete element method (FDEM), grain-based model (GBM) and inverse Monte Carlo (IMC) algorithm. The main conclusions are as follows: (1) Mineral grain significantly influences the crack propagation paths, and the GBM can capture the location of fracture section more accurately than the conventional model. (2) Shear cracks occur near the loading area, while tensile and tensile-shear mixed cracks occur far from the loading area. The applied stress must overcome the tensile strength of the grain interface contacts. (3) The UTS and the ratio of the number of intergrain tensile cracks to the number of intragrain tensile cracks are negatively related to the grain size. (4) With the increase of the preferred grain orientation, the UTS presents a “V-shaped” characteristic distribution. (5) During the whole process of splitting simulation, shear microcracks play the dominant role in energy release; particularly, they occur in later stage. This novel framework, which can reveal the control mechanism of brittle rock heterogeneity on continuous-discontinuous trans-scale fracture process and microscopic rock behaviour, provides an effective technology and numerical analysis method for characterizing rock meso-structure. Accordingly, the research results can provide a useful reference for the prediction of heterogeneous rock mechanical properties and the stability control of engineering rock masses.     

The Influence of Service Temperature and Thickness on the Tensile Properties of Thin T2 Copper Sheets

Thin T2 copper sheets with nine different thicknesses were employed in uniaxial tensile tests to investigate the influence of service temperature and thickness on their tensile properties. A total of 33 groups of tensile samples were separately tested at 20 °C, 100 °C, 150 °C, 200 °C, and 250 °C to obtain their elongation and their tensile and yield strengths. The change laws of the tensile properties of the investigated T2 copper were analyzed using different fitting functions. The main results show that both sheet thickness and temperature have an important influence on the tensile properties of T2 copper. As the sheet thickness increased, the tensile and yield strengths of the tested materials first increased rapidly, then decreased sharply, and finally stabilized. As the temperature increased, the tensile strength increased linearly while the yield strength decreased linearly. The relationships between the elongation and the sheet thickness and temperature were exponential and polynomial functions, respectively. T–t–Rm, T–t–Rel, and T–t–δ empirical formulas were proposed and established to predict the tensile properties of the investigated T2 copper sheet, and the predictive models exhibited solid accuracy.     

Evaluation Approach of Fracture Behavior for Asphalt Concrete with Different Aggregate Gradations and Testing Temperatures Using Acoustic Emission Monitoring

Different aggregate gradations of asphalt concrete possess dissimilar skeleton structures, leading to diverse macroscopic and mechanical characteristics. Acoustic emission (AE) technology can realize real-time monitoring of the whole damage evolution process of materials. The objective of the present investigation was to demonstrate the fracture characteristics of asphalt concrete with three types of aggregate gradations, including dense-graded asphalt concrete (AC), stone mastic asphalt (SMA), and open-graded friction course (OGFC) under indirect tensile load on account of the acoustic emission (AE) technique. The Marshall compaction method was used to prepare specimens, and the indirect tensile test (IDT) and AE monitoring were conducted simultaneously at different temperatures. The corresponding AE parameters containing energy, cumulative energy, count, and cumulative count were adopted to characterize the fracture process of asphalt concrete with different aggregate gradations. The impact of temperature on the damage characteristics of asphalt concrete was also assessed. Test results indicated that the AE parameters could effectively classify the damage stages of asphalt concrete, and specimens with different aggregate gradations exhibited different AE characteristics during failure processes. The combination of AE parameters and cumulative AE parameters can accurately characterize the damage characteristics of asphalt concrete. SMA specimens possessed the best overall performance among these three types of asphalt concrete in terms of the variations in energy and cumulative energy at different temperatures. The findings obtained in this study can provide a practical AE-based evaluation approach for demonstrating the fracture mechanism of asphalt concrete with different aggregate gradations.     

Effect of Elevated Temperature on Rhyolitic Rocks’ Properties

The effect of high temperatures on rock’s thermophysical and mechanical properties is critical to the design of underground geotechnical applications. The current work investigates the impact of temperature on rhyolitic turf rock’s physical and mechanical properties. Intact cylindrical core rock samples were heated to different temperatures (200, 400, 600, and 800 °C). The uniaxial compressive strength (UCS) and elastic modulus of unheated and heated samples were determined as important mechanical properties. In addition, the effect of temperature on the physical properties of rhyolite rock (density, color, and absorption) was investigated in conjunction with its microstructural properties. The hardening of the rhyolitic rock samples was observed below 600 °C, at which point the UCS and elastic modulus decreased to 78.0% and 75.9%, respectively, at 800 °C. The results also show that heating does not significantly affect the density and volume of permeable pore space, but a color change can be observed at 400 °C and above. A microscopic analysis shows the change in microstructural properties of rhyolite rock after heating to 600 °C. Furthermore, the SEM observations of heated materials show structural particle displacements and microcracking, leading to apparent surface cracks.     

Effect of Vulcanization Process Parameters on the Tensile Strength of Carcass of Textile-Rubber Reinforced Conveyor Belts

Textile-reinforced conveyor belts are most widely used in various industries, including in the mining, construction, and manufacturing industries, to transport materials from one place to another. The conveyor belt’s tensile strength, which primarily relies on the property of the carcass, determines the area of application of the belt. The main aim of the current work was to investigate the influence of vulcanization temperature and duration of the vulcanization process on the tensile properties of the carcass part of the conveyor belt. An extensive experiment was carried out on the tensile properties of woven fabrics that were intended to reinforce conveyor belts by aging the fabrics at the temperature of 140 °C, 160 °C, and 220 °C for six and thirty-five minutes of aging durations. Afterward, the textile-reinforced conveyor belts were produced at vulcanization temperatures of 140 °C, 160 °C, and 220 °C for six and thirty-five minutes of vulcanizing durations. The influence of the vulcanization process parameters on the tensile property of fabrics utilized for the reinforcement of the conveyor belt was analyzed. In addition, the effect of the dipping process of woven fabric in resorcinol–formaldehyde–latex on the tensile property of polyester/polyamide 66 woven fabric (EP fabric) was investigated. The investigation results revealed that the tensile strength of the carcass of the conveyor belt was significantly affected by vulcanization temperature. The conveyor belt vulcanized at 160 °C for 35 min has shown the optimum tensile strength, which is 2.22% and 89.06% higher than the samples vulcanized at 140 °C and 220 °C for 35 min, respectively. Furthermore, the tensile strength and percentage elongation at break of conveyor belts vulcanized at 220 °C were almost destroyed regardless of the vulcanization duration.     

A High-Strain-Rate Superplasticity of the Al-Mg-Si-Zr-Sc Alloy with Ni Addition

The current study analyzed the effect of Ni content on the microstructure and superplastic deformation behavior of the Al-Mg-Si-Cu-based alloy doped with small additions of Sc and Zr. The superplasticity was observed in the studied alloys due to a bimodal particle size distribution. The coarse particles of eutectic origin Al₃Ni and Mg₂Si phases with a total volume fraction of 4.0–8.0% and a mean size of 1.4–1.6 µm provided the particles with a stimulated nucleation effect. The L1₂– structured nanoscale dispersoids of Sc- and Zr-bearing phase inhibited recrystallization and grain growth due to a strong Zener pinning effect. The positive effect of Ni on the superplasticity was revealed and confirmed by a high-temperature tensile test in a wide strain rate and temperature limits. In the alloy with 4 wt.% Ni, the elongation-to-failure of 350–520% was observed at 460 °C, in a strain rate range of 2 × 10⁻³–5 × 10⁻² s⁻¹.     

Temperature-Dependent Photoluminescence of ZnO Thin Films Grown on Off-Axis SiC Substrates by APMOCVD

The growth of high-quality ZnO layers with optical properties congruent to those of bulk ZnO is still a great challenge. Here, for the first time, we systematically study the morphology and optical properties of ZnO layers grown on SiC substrates with off-cut angles ranging from 0° to 8° by using the atmospheric pressure meta–organic chemical vapor deposition (APMOCVD) technique. Morphology analysis revealed that the formation of the ZnO films on vicinal surfaces with small off-axis angles (1.4°–3.5°) follows the mixed growth mode: from one side, ZnO nucleation still occurs on wide (0001) terraces, but from another side, step-flow growth becomes more apparent with the off-cut angle increasing. We show for the first time that the off-cut angle of 8° provides conditions for step-flow growth of ZnO, resulting in highly improved growth morphology, respectively structural quality. Temperature-dependent photoluminescence (PL) measurements showed a strong dependence of the excitonic emission on the off-cut angle. The dependences of peak parameters for bound exciton and free exciton emissions on temperature were analyzed. The present results provide a correlation between the structural and optical properties of ZnO on vicinal surfaces and can be utilized for controllable ZnO heteroepitaxy on SiC toward device-quality ZnO epitaxial layers with potential applications in nano-optoelectronics.     

Effect of Solution Temperature on Microstructure and Properties of Thixotropic Back-Extruded Tin–Bronze Shaft Sleeve

To study the heat-treatment process of a semi-solid copper alloy, a thixotropic back-extruded tin–bronze shaft sleeve was heat-treated at 630 °C, 660 °C, 690 °C and 720 °C for 1 h, respectively. Microstructure changes and mechanical properties under different solution temperatures of shaft sleeve were characterized using a metallographic microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM), hardness tester, and tensile tester. The results show that the tensile strength first increases and then decreases; the elongation decreases; and the Brinell hardness increases gradually with increasing solution temperature. When the solution treatment is at 690 °C, the tin–bronze shaft sleeve’s microstructure and comprehensive mechanical properties are the best. The shape factor is 0.75, the average grain size is 82.52 μm, the Brinell hardness is 122 HBW, the tensile strength is 437 MPa, and the elongation is 17.4%, which is 3.4 times higher than that before solution treatment.     

Microstructure and Mechanical Properties of an Al–Mg–Si–Zr Alloy Processed by L-PBF and Subsequent Heat Treatments

The aim of this study was to develop a new Al–Mg–Si–Zr alloy with a high magnesium content to achieve a wide range of mechanical properties using heat treatment and at a lower cost. Additive manufacturing was conducted using a powder bed fusion process with various scan speeds to change the volumetric energy density and establish optimal process conditions. In addition, mechanical properties were evaluated using heat treatment under various conditions. The characterization of the microstructure was conducted by scanning electron microscopy with electron backscatter diffraction and transmission electron microscopy. The mechanical properties were determined by tensile tests. The as-built specimen showed a yield strength of 447.9 ± 3.6 MPa, a tensile strength of 493.4 ± 6.7 MPa, and an elongation of 9.6 ± 1.1%. Moreover, the mechanical properties could be adjusted according to various heat treatment conditions. Specifically, under the HT1 (low-temperature artificial aging) condition, the ultimate tensile strength increased to 503.2 ± 1.1 MPa, and under the HT2 (high-temperature artificial aging) condition, the yield strength increased to 467 ± 1.3 MPa. It was confirmed that the maximum elongation (14.3 ± 0.8%) was exhibited with the HT3 (soft annealing) heat treatment.