Archives of Civil and Mechanical Engineering最新文献

The research concerns the oxidation process of SPS-sintered zirconium-based materials. The sintered materials are characterized by residual porosity, which can affect the intensification of the oxidation process, compared to melted materials. On the basis of TGA tests carried out at temperatures of 500, 600, 700, and 800 °C, activation energies in air of sintered Zr, Zr–2.5Cu, Zr–2.5Nb, and Zr–2.5Mn, respectively, were calculated. Based on the studies of the microstructure and phase composition of oxidized materials, degradation mechanisms were analyzed for materials after annealing at 700 °C. In sintered Zr, Zr–2.5Cu, and Zr–2.5Nb oxidized at 700 °C, Si₃N₄ was detected. Si₃N₄ is formed at the zirconium oxide–zirconium boundary, in oxygen-deficient conditions. Zr–2.5Cu demonstrated the lowest tendency to oxidize despite the low values of activation energy for the oxidation process in air of 61 kJ/mol. This material showed the highest resistance to oxidation in air.

It is known that the most critical factor affecting fire resistance requirements in buildings is the building envelope, and therefore, improving the thermal properties of facade materials has become an important research area. In this context, studies examining the high temperature resistance properties of various facade materials have indicated that concrete has higher heat resistance compared to many facade materials. Lightweight concrete facade elements stand out for their structural durability and fire resistance at high temperatures. However, the different thermal expansion coefficients of aggregates and cement paste in concrete mixtures can lead to adverse outcomes under high temperatures, such as cracking or structural degradation. To mitigate these adverse effects, it has been suggested that adding fibers to lightweight concrete mixtures could enhance durability and improve fire resistance. This study investigated the effects of different fiber types, lengths, and usage rates on the high temperature resistance of lightweight concrete mixtures. In the experimental study, three different types of fibers—polypropylene, polyamide, and glass—were used in varying proportions of 0%, 0.25%, 0.5%, and 0.75% of the total volume. Polypropylene fibers were included at lengths of 3, 6, and 12 mm; polyamide fibers at 6 and 12 mm; and glass fibers at 13 and 25 mm. When observing the behavior of the mixtures under high temperatures, it was noted that mixtures with glass fibers performed best at 300 °C, while those with polypropylene fibers showed superior performance at 600 °C. This demonstrates the advantages of glass and polypropylene fibers in providing resilience at different temperature ranges. Furthermore, the optimal fiber usage rate for high temperature resistance was determined to be 0.25%. These findings highlight the importance of considering factors such as fiber type, length, and usage rate in the development of fire-resistant facade materials.

Water and concrete are the materials humans consume the most on earth. By 2040, several countries are expected to face extreme water stress and the need for significant growth in their infrastructure simultaneously. Water is a fundamental ingredient for concrete production, and the need for infrastructure growth can further increase the water demand for concrete production and thus affect these regions facing water scarcity. Including supplementary cementitious materials (SCMs), non-metallic fibres, and coated/polymer reinforcements can increase the feasibility of producing concrete with seawater (SW). There is a lack of information on the long-term strength and durability properties of SW-mixed concretes (SWC) produced with SCMs. This paper optimises binder compositions with CEM I, fly ash, ground granulated blast furnace slag (slag), and metakaolin suitable for adapting SWC based on performance indicators. Binary and ternary blended concretes of similar binder content (360 kg/m³) and w/b (0.45) were designed and cast with the SCMs mentioned above. Compressive strength, surface resistivity, and accelerated carbonation tests were conducted on the concrete produced with freshwater (FW) and seawater (SW). SWC produced with 30% slag and 15% metakaolin had higher electrical resistivity and an improvement in compressive strength (up to 30%) than other combinations used for producing SWC. Life cycle assessment identified that the concretes produced with fly ash, and ternary combination of fly ash and metakaolin had the least water depletion potential (WDP) compared to other SW-mixed concretes. Also, the replacement of FW by SW reduces the WDP up to 50%.

This study examined the strength features of 3D printed fly ash-based geopolymer mortar with silica fume using the Taguchi and ANOVA methods. This study used the Taguchi L₁₈(2¹ × 3²) orthogonal array. Silica fume was used in the mixtures at 0% and 10% of the binder weight. 40 × 40 × 160 mm specimens were manufactured by a 3D printer. After the specimens were produced, they were cured at 80 °C for 24, 48, and 72 h. Then, these specimens were kept at 20 ± 2 °C for 3, 7, and 28 days. Lastly, the microstructural features, compressive strength, and flexural strength of the samples were determined. The ANOVA results found that the most affecting parameter for the strength properties of geopolymer mortars was found to be silica fume. Also, the Taguchi method found that optimum values of silica fume, curing time, and curing day for strength properties of geopolymer mortar were 10%, 48 h, and 28 days, respectively.

In this research, the microstructure, tensile properties, hardness, and wear behavior of the Al/ZK60 Mg laminated composite produced by the accumulative roll bonding (ARB) and vacuum plasma cleaning methods were investigated. ARB process continued until the sixth pass at ambient temperature and at each stage, the surfaces cleaning was done by the vacuum plasma method. The results showed that with the rise in ARB passes, the reinforcement distribution in the matrix has become more homogeneous, which caused a 60% increase in the hardness of the sixth pass compared to the first pass. The results of the tensile tests showed that with the rise in ARB passes up to the fourth pass, the strength and elongation increased simultaneously by 40% and 225% compared to the first pass, and with the rise in ARB passes up to the sixth pass, with the increase of strength, the elongation decreased slightly. The wear tests were performed at loads of 10, 20, 40, and 80 N for different passes at room temperature. The findings indicated that as the number of ARB passes increased, the wear rate decreased up to 65%. Also, the examination of the worn surfaces showed that the abrasive wear mechanism prevailed in the lower passes and the delamination mechanism prevailed in the higher passes.

This study examines a new 3D printing molding process for magnesium phosphate cement (MPC)-based materials. The process involves inputting dry powder and outputting wet material. The study proposes an evaluation method for the integrated 3D printer with functions of mixing, stirring, and extrusion for rapid-setting concrete is used for the research. The study proposes an evaluation method for the printability of materials based on the consistency of MPC paste measured by a Vicat apparatus. The study systematically examines consistency, setting time, extrudability, buildability, and mechanical properties of MPC concrete extruded by the 3D print head under different water-to-material ratios (mass ratio of water to dry mix). The results show that MPC concrete sets rapidly with setting times consistently within 4 min. The consistency of the printing paste decreases as the water-to-material ratio increases. When the extrusion consistency is between 25 and 34 mm, the 3D printed MPC concrete exhibits good extrudability, and when the extrusion consistency is between 23 and 34 mm, the 3D printed MPC concrete shows excellent buildability. A correlation has been established between the water-to-material ratio for rapid 3D printing of MPC concrete and the water-to-binder ratio used in traditional casting processes. The compressive and flexural strengths of the 3D printed MPC concrete generally decrease as the concrete consistency decreases. Under equal consistency and age, the compressive and flexural strengths of the printed specimens are approximately 1/3 and 1/2 of the casting strength, respectively. However, the printed specimens have better crack resistance than those cast. Therefore, the Vicat consistency could be used as an evaluation method for the printability of rapid-setting materials in the rapid-setting and hardening MPC-based material 3D printing process. When the extrusion consistency of MPC paste is in the range of 25–34 mm, the integrated mixing–stirring–extrusion 3D printing molding process for MPC-based materials can be realized. This method simplifies the operation process and procedures of the 3D printing equipment, ensuring accurate shaping and stability of performance of the 3D printed cement-based materials.

In response to the instability and failure of concrete linings in deep tunnels due to obliquely incident stress waves from mining disturbances, this study investigates the failure characteristics of concrete tunnel linings under varying wave incidence angles (0°, 15°, 30°, and 45°) using numerical modeling with PFC2D software. The model applicability is verified by stress wave propagation theory and spalling tests. Results demonstrate that under blast loads, failures predominantly occur in the concrete lining rather than the rock mass. Concrete linings effectively reduce blast damage to the surrounding rock and maintain stability. Vertical stress wave incidence causes the most severe damage to the concrete lining, whereas the least damage occurs at a 45° angle. The concrete tunnel lining failure is primarily caused by P-wave. When P-wave is large enough, the S-wave generated by oblique incidence make the tensile crack expand, and lead to second damage to concrete tunnel lining. As incident wave energy increases, concrete lining damage worsens, followed by rock spalling. The strain energy in concrete tunnel lining accumulates with increasing burial depth, resulting in more cracks in concrete tunnel lining under the action of the same stress wave. Thus, even small blast loads can severely damage deep-buried concrete tunnel linings.

Many scholars have obtained preliminary findings regarding the study of the dynamic size effect of concrete; however, a unified explanation for the development law of internal damage in fly ash concrete caused by this dynamic size effect has not yet been achieved. Compression tests were conducted on cylindrical specimens of fly ash concrete with varying sizes under dynamic loads ranging from 1.0 × 10^–5(s⁻¹) to 1.0 × 10^–2(s⁻¹) seismic strain rate. The experimental findings indicate that the peak stresses were increased by 29.07%, 38.19% and 48.18% for the three sizes of specimens, large, medium and small, respectively, under the condition that the strain rate was increased from 1.0 × 10^–5 (s⁻¹) to 1.0 × 10^–2 (s⁻¹). From the overall trend analysis, the impact of strain rate on fly ash concrete gradually increases as the size decreases. The size effect of fly ash concrete can be attributed to the internal heterogeneity of specimens, which results in varying degrees of damage development. Similarly, the strain rate effect of meso-components is also caused by uneven damage development within fly ash concrete. The damage development law of fly ash concrete is then investigated by analyzing the changes in the 3D-DIC strain cloud map, using advanced technology known as 3D digital image correlation (3D-DIC). At strain rates of 1.0 × 10^–4(s⁻¹), 1.0 × 10^–3(s⁻¹), and 1.0 × 10^–2(s⁻¹), the full-stage damage degree factor (D_f1) in the pre-loading phase is 76.47%, 54.90%, and 25.49% of the static strain rate (1.0 × 10^–5(s⁻¹)), respectively. At strain rates of 1.0 × 10^–5(s⁻¹) and 1.0 × 10^–2(s⁻¹), the slopes of post-peak damage change (D_f2) for specimen sizes L, M, and S are 2.09, 2.27, and 2.5, and 2.25, 7.6, and 10.62, respectively. This suggests that in smaller specimens, damage development is primarily concentrated in the post-peak phase. Finally, the uniform static and dynamic size effect law of compressive strength in fly ash concrete is established based on the influence mechanism of damage development on dynamic strength and size effect. The research findings provide a theoretical foundation for the application and advancement of fly ash concrete engineering.

This paper investigates large amplitude vibration responses of a sandwich nanocomposite doubly curved shell composed of a honeycomb core integrated with graphene nanoplatelets reinforced curved face sheets. The kinematic relations are developed with accounting von Karman nonlinear strain components and shear deformable model. The large amplitude governing motion’s equations are derived using Hamilton’s principle. The effective geometric and material composition-dependent properties are estimated using the Gibson's formula and Halpin–Tsai relations for the core and attached layers, respectively. The Galerkin’s approach is employed to convert governing equations to the time-dependent differential equations and finally the perturbation technique and modified Poincare–Lindstedt method are employed to obtain large amplitude nonlinear responses. The results are presented to investigate the impact of geometric parameters of the honeycomb core and material compositions of nanocomposite layers on the nonlinear vibration characteristics of the doubly curved shell. A verification study is presented to valid employed numerical results.

Since the railway axles generally need to bear heavy dynamic loads of impaction, bending, torsion, and vibration under high speed and heavy transporting, it is of great significance to control the microstructure of the railway axle steel. In this study, the material behaviors including hot deformation and microstructure evolution of LZ50 steel are tested. According to the stress–strain and grain size of basic experiments, a mechanism-based constitutive model considering internal state variables of dislocation density, recrystallization fraction, and average grain size (AGS) is established to theoretically describe the macroscopical deformation and microstructure evolution of LZ50 steel. Subsequently, a finite element (FE) model of flexible skew rolling (FSR) bar is further developed via compiling the mechanism-based constitutive model into FE software by user-defined subroutine and its reliability has been verified by comparing the size of geometry and grain of experimental and FE results. The FE results show that the microstructure evolution mechanism of FSR rolling LZ50 steel is that hot plastic deformation leads to dislocation multiplication resulting in dynamic recrystallization and ultimately grain refinement. As the bar is formed by continuous FSR rolling, its microstructure is relatively uniform with the grain size of the outer layer slightly smaller than that of the inner layer because of the more severe deformation of the outer metal. The influence of FSR parameters on grain size reveals that the microstructure of the rolled bar is generally uniform in various situations and the parameter conditions of larger area reduction, longer sizing length, larger forming angle, and smaller skewing angle can improve the microstructure and properties of the FSR rolled bar by grain refinement.