Archives of Civil and Mechanical Engineering最新文献

Nickel-based superalloy GH4169 has gained significant prominence in aerospace and power-generation sectors due to its exceptional resistance to fatigue, creep and corrosion at high temperatures. However, wear on the contact surface remains a concern during its operational lifespan, as it can lead to crack initiation and reduce the service life of the alloy. Therefore, this study aims to comprehensively investigate the dry-sliding wear behaviors of the GH4169 superalloy under various conditions, including temperatures, applied loads and sliding speeds via a ball-on-disc tribometer. The results obtained from the study indicate that the wear rate of the GH4169 superalloy is initially high at room temperature. However, as the temperature increases to 525 °C, the wear rate significantly decreases. Subsequently, with further temperature increase to 650 °C, there is a slight rise in the wear rate. However, the findings regarding the friction coefficient present a conflicting trend compared to the wear rate. Under the same testing conditions, the wear rate of the GH4169 superalloy at 525 °C and 650 °C demonstrates a significant reduction of 21% (0.0774 × 10^–4 mm³/N.m) and 19% (0.0785 × 10^–4 mm³/N.m), respectively; when compared to room temperature (0.0935 × 10^–4 mm³/N.m). Besides, at room temperature, the wear track depths measure approximately 34 ± 2 µm and 55 ± 3 µm for applied loads of 20 N and 60 N, respectively. These values indicate a significant wear depth under ambient temperature conditions. On the other hand, at 525 °C, the wear track depths are slightly lower compared to those at RT. The measured depths at 525 °C amount to approximately 25 ± 2 µm and 38 ± 2 µm for applied loads of 20 N and 60 N, respectively. At high temperatures and low applied loads, the wear mechanism of the GH4169 superalloy is mainly abrasion. However, at low temperatures and high applied loads, the wear mechanism becomes more complex, involving abrasion as well as oxidation, delamination, softening and melting. This study offers valuable insights into how different wear testing parameters impact the tribological characteristics of the GH4169 superalloy.

Developing novel methods for manufacturing multilayered composites has been the central effort of researchers for years. This study presents the fabrication of Aluminum/steel/Aluminum (Al/St12/Al) multilayered composites using the accumulative roll bonding (ARB) technique, with an examination of the impact of normal load and strain accumulation on microstructure, mechanical properties, and tribological properties. As the applied accumulative strain increased, more instabilities were detected in the St12 layers, with no defects present. The mechanical properties exhibited enhancements as the ARB passes increased, with ultimate and yield strength values as well as microhardness rising, while elongation decreased due to the uneven distribution of hard layers within the Al matrix. The composite achieved a maximum ultimate tensile strength (UTS) of 260 MPa and a breakpoint elongation of 7% after six passes. Scanning electron microscopy (SEM) images revealed both ductile and cleavage fracture modes on the surfaces. Also, based on the results of wear tests, the application of higher accumulative strain improves wear resistance while normal load deteriorates it. Moreover, wear tests indicated various wear mechanisms, including adhesion, abrasion, and delamination, with a reduction in weight loss observed with an increase in the number of rolling passes attributed to the increased hardness of the strain-hardened layers.

In this study, the impact of asymmetric cross-rolling on the microstructure, crystallographic texture, and tensile anisotropy of a low-carbon steel sheet was evaluated. In the microstructures of the cross-rolled sheets, elongated α grains appeared in TD–ND and RD–ND sections, while equiaxed and irregular grains were seen in the RD–TD plane of the sheets. The results indicated that the asymmetric cross-rolling weakens the texture and γ-fiber due to the change in the path of strain during the deformation. The high increasing rate of hardness from 50 to 75% thickness reduction was related to the elimination of ⟨100⟩∥ND (with a low elastic modulus (129 GPa)) in the 75% cross-rolled sheet. Compared with the as-received sheet, the average yield and tensile strengths after 75% cross-rolling were increased to 620.9 and 705.6 MPa, respectively, due to strain hardening. As the cross-rolling strain increased, the rate of increase in strength decreased. For all samples (except 75% cross-rolled sheet), the maximum yield and tensile strengths were obtained along transverse direction owing to the presence of strong 〈111〉∥TD and 〈110〉∥TD texture. The anisotropy results indicated that large-strain asymmetric cross-rolling decreased the mechanical anisotropy degree of low-carbon steel. With an increase in imposed strain to 50% and 75%, the fracture gradually changed from fully ductile to a combination of fully ductile and shear ductile types. The presence of uniform dimples in the 0°-loaded and 45°-loaded sheets for 25% and 50% cross-rolled samples demonstrated the comparable rate of nucleation and growth of microvoids, which led to similar behavior of stress–strain curves after the necking for these sheets.

Different researchers have perused the bond behavior of concrete and rebar employing the beam or pull-out tests; nonetheless, results were not precisely comparable owing to different test methods. Hence, a comprehensive research study is necessary in this field. In contrast to steel bars, there is no standard approach for FRP bar surface characterization. Moreover, previous studies reveal that FRP bars with different surfaces exhibit various bonding mechanisms. Therefore, determining the bond characteristics of various FRP bars with different surfaces is essential for their application in structures. A recent study experimentally compared beam and pull-out test results for ribbed GFRP bars, with and without anchors, conforming to RILEM standards. The variables included two different test methods (beam test and pull-out test), steel anchor (presence and absence of anchor), and concrete compressive strength level (25 and 45 MPa). It was found that the maximum transferred stress in the ribbed GFRP bar from the beam test was higher than in the pull-out test for all specimens. On average, they are about 24% higher than the pull-out test values. In the pull-out test, the bar slip threshold stress and the initial slope of the stress–slip graph were higher than in the beam test for all specimens. A comparative analysis was conducted on studies using beam and pull-out test results without anchors. The impact of anchors on reducing development length was evaluated by comparing the existing length with the straight and hook development lengths specified by ACI 440.1R-15. The difference suggests a conservative approach in the code.

The research presented in the article indicates the process of building models based on machine learning algorithms, linear regression, decision trees, ensemble learning, random forest, ensemble averaging, Boosting, stacking, and support vector regression (SVR) algorithms. The basis for building these models are experimental data collected during research conducted at the Łukasiewicz Research Network-Krakow Institute of Technology. An analysis of the initial state and the analysis of the state of correlation in the set were performed, which facilitated the development of models. To increase the amount of data, augmentation was performed using the Bootstrapping. For selected results, castings were made and tested in real conditions. The research results indicate the possibility of identifying the most appropriate input parameters for specific production processes of austempered ductile iron (ADI), the possibility of predicting the expected mechanical parameters based on the indicated parameters of the production process, chemical composition, specific parameters of the heat treatment process, and the thickness of the target product. A set of such models constitutes the basis of the system, enabling the end user to estimate the final parameters of the casting planned to be produced.

A correct understanding of the fracture mechanism of Recycled Aggregate Concrete (RAC) plays an important role in the design of RAC structure and also in gaining a better understanding of the behavior of structures made from it. On the other hand, one of the most important parameters that affects cracking behavior and the fracture parameters of concrete is the water to cement ratio. The main objective of this study is to investigate the effect of different water to cement ratios on the fracture behavior of RAC. To achieve this objective, 125 notched concrete beams were subjected to three-point bending experiments, with W ranging 0.35 to 0.7. Specific fracture energy (G_F) and characteristic length (L_ch) from work-of-fracture method and initial fracture energy (G_f), brittleness number, fracture toughness, effective length of fracture process zone (C_f), and the critical crack-tip opening displacement from size effect method were evaluated. The results illustrate that G_F and G_f increase by 34 and 64% when W reduces from 0.7 to 0.35, respectively. Moreover, L_ch and C_f reduce from 378 to 208 mm and from 32.5 to 17.2 mm by decreasing W from 0.7 to 0.35, respectively. On average, G_F/G_f ratio for various Ws attains 2.48 with the variation coefficient of 10.9%. Eventually, multivariate prediction models were developed for RACs with various Ws. A comparison was made between prediction and experimental values of the present and previous research works.

Long-term beam deflection is a key index during the serviceability limit state (SLS) design of timber beams under gravity loads. To accurately calculate the long-term beam deflection, it is necessary to reasonably capture the long-term rotational deformation of beam–column joints. However, the long-term rotational deformation of timber beam–column joints is still not fully understood, especially for post-tensioned timber (PTT) joints. In this study, long-term experiment was conducted over 930 days to investigate the time-dependent rotational deformation for PTT joints under variable environment and sustained gravity loads. Different gravity loading levels and connection details were considered. Time-dependent variations of key parameters including environmental conditions, moisture content, joint rotations, prestressing forces and strain deformation were captured. Based on observed data, a numerical approach about the long-term rotational deformation for PTT joints was developed and verified. Finally, the verified numerical approach was adopted for predicting the long-term joint rotation over a 50-year service life. This study demonstrated that the long-term rotation for post-tensioned timber joints globally increased over time and increased as the gravity loading level increased. Additionally, the long-term rotational deformation can be effectively reduced by steel reinforcement.

Microbial-induced concrete corrosion (MICC) has been recognized as one of the main factors causing damage to urban sewer pipelines. However, little is known about the neutralization and degradation of the concrete pipe during the early-stage MICC, which is a key point for the trenchless protection of sewer pipelines. In the present study, the corrosion behaviors of concrete pipe were simulated in a pilot-scale sewer system for 180 days and correlated to the change of microbial communities. The results revealed the post-corrosion characteristics in different spatial locations of the sewer during the early-stage MICC. Apart from the biogenic sulfuric acid attack and gypsum formation at the upper part (UP), the bottom part (BP) of the concrete pipe suffered severe degradation due to volatile fatty acids (VFAs) generated. Under the action of microbial metabolism, the decomposition of hydrates and pore coarsening occurred, resulting in decreased mechanical strength. In terms of microbiology, the dominating functional bacteria were fermentation bacteria (FB), such as Macellibacteroides, Trichococcus, Longilinea, etc. FB played a major role in the production of VFAs, which would create suitable conditions for the subsequent development of microorganisms. The early fermentation processes were key factors contributing to concrete pipe corrosion, especially at BP. The relative abundance of sulfate-reducing bacteria (Desulfovibrio) and methanogenic archaea increased with exposure to age. The findings can provide a theoretical basis for the protection of urban concrete sewer pipelines.

This experimental study investigates the effectiveness of retrofitting fiber-reinforced concrete (FRC) beams using nano-graphene oxide (GO) and carbon fiber-reinforced polymer (CFRP) sheets. The primary goal is to evaluate the combined effect of GO and CFRP sheets on the mechanical performance of FRC beams. The experimental program involved preparing and testing multiple concrete beam specimens, with some retrofitted using GO and CFRP sheets. The principal results indicate that the integration of GO significantly enhances the compressive and tensile strength of concrete. Additionally, the application of CFRP sheets markedly improves the flexural strength and ductility of the beams. The retrofitted specimens exhibited higher load-bearing capacity and greater deformation before failure compared to control specimens. The significant conclusions drawn from this study are that the combined use of GO and CFRP sheets provides a synergistic effect, improving the overall mechanical performance of FRC beams. However, failure modes such as debonding and delamination of CFRP sheets highlight the need for optimization in bonding techniques and materials. The primary research outcomes demonstrate that retrofitting FRC beams with GO and CFRP sheets is a promising approach for enhancing the structural performance. This study underscores the potential of advanced materials in structural retrofitting and provides insights for future research to address observed failure modes and further improve the retrofitting techniques.

Fibre-reinforced polymer (FRP) sheets have been extensively used to strengthen existing structures in harsh environments (e.g., freeze–thaw and chemical attacks) due to their excellent mechanical properties and durability. However, the effects of FRP sheets on strengthening reinforced concrete columns eroded by chloride and sulphate ions in freeze–thaw environments remain ambiguous. Therefore, this paper investigates the salt frost resistance of basalt fibre-reinforced polymer (BFRP)-reinforced columns subjected to complex salt-frozen environment. The effects of the freeze–thaw medium, reinforcement method, and pre-damage degree on the salt frost resistance of the columns were evaluated, followed by revealing the freeze–thaw damage mechanism of the columns eroded by sulphate and chloride. Moreover, the correlation between the durability and the mechanical properties of the columns wrapped with BFRP sheets was analysed. Results indicate higher concentrations of sodium chloride and sodium sulphate solutions decelerate the deterioration of high-performance concrete (HPC). The double-layer BFRP sheet wrapping method increases the ultimate load of the columns by 84.8%. The partially damaged epoxy resin coatings can serve as a protective layer for the concrete. The ultimate-transition stress ratio of columns wrapped with double-layer BFRP sheets can be used as an index to evaluate the durability. The proposed BFRP retrofit method demonstrates significant potential for enhancing the durability of HPC in salt frost regions. This method provides a theoretical basis for the durability design of reinforced HPC columns wrapped with BFRP sheets in salt frost regions.