Archives of Civil and Mechanical Engineering最新文献

Despite its relevance to engineering design, fatigue life assessment of additively manufactured materials subjected to variable-amplitude loading remains insufficiently understood. This paper investigates the fatigue response of L-PBF-processed maraging steel subjected to strain-controlled variable-amplitude loading. The loading sequences consisted of three increasing levels followed by three decreasing levels, repeated up to failure. Strain amplitudes varying from 0.175 to 1.0%, covering a representative spectrum of cyclic deformation levels, were investigated. The cyclic stress-strain response under variable-amplitude loading was modelled with an elasto-plastic constitutive model. A numerical framework combining well-known fatigue damage parameters along with classical and modified cumulative damage laws was developed to estimate the fatigue lifetime. In addition, EBSD and SEM observations were carried to investigate the effect of strain level on failure mechanisms and grain misorientation. Regarding the proposed methodology for fatigue life assessment, Miner’s law combined with the SWT model effectively predicted the fatigue lifetime, contributing to the safe and optimised design of additively manufactured components and structures. In addition, local deformation patterns and dislocation density tend to intensify with increasing strain amplitudes, while the fracture mode changes from ductile to mixed-mode characteristics as the strain level decreases.

The effect of pulse welding current (DC, 1 kHz) on the progression of projection welding process of nuts was analysed. The study involved a comparative analysis using identical average value for pulsed DC current with continuous direct current (DCc), which is commonly used in conventional mode. The analysis was conducted for nuts featuring four trapezoidal projections, made of grade 10B21 steel (AISI1017-SORPAS library), with a projection height of 1.2 mm and an initial contact area between the projection and the sheet metal of 7.8 mm². The second welded component was a 3.0 mm thick DC04 sheet metal. Numerical analyses in both cases were conducted using the SORPAS^™ software. The characteristic parameters evaluated were weld strength, volume of melted material (in both the sheet and the projection), area of molten material in the contact region, welding energy, and average welding current. The results of the numerical calculations were verified experimentally. The study demonstrated the beneficial effect of pulsed welding current (DC, 1 kHz) on weld quality, particularly in terms of improved strength. It is possible to both reduce welding time and increase weld strength for pulsed DC welding. With comparable weld strength, welding time can be reduced by over 30%. In turn, for the same welding time (and thus the same energy), the weld strength can be increased by 35%.

Hybrid wire and arc additive manufacturing (Hybrid WAAM) offers new potentials for local customization of steel structures by adding material precisely where it is required. While the serial production of steel profiles has improved the process efficiency in construction, it is often associated with underutilization of materials, as standard beams are over dimensioned to meet the peak load demands. Hybrid-WAAM I-section beams offer a novel processing solution by combining the benefits of serial production with individualized WAAM strengthening, aiming for efficient material usage. This study evaluates the material efficiency and assesses the carbon footprint of Hybrid-WAAM I-section beams through a cradle-to-gate Life Cycle Assessment (LCA). Results indicate substantial material savings of up to 28% and reductions of CO₂ equivalent up to 27% in the most favorable cases. However, the environmental benefits of Hybrid-WAAM I-section beams depend on structural system, loading conditions and design strategy. In some scenarios, local strengthening may even result in a higher cradle-to-gate CO₂ footprint than conventional profiles. Moreover, the LCA outcomes are highly sensitive to assumptions regarding steel production and regional differences of energy sources, leading to significant variations in CO₂ equivalent estimates. As such, this study highlights that Hybrid-WAAM I-section beams can lead to eco-efficient solutions in steel construction, but the design needs to be supported by a true-to-life LCA.

This study was performed to minimize the mechanical anisotropy of dual-phase (DP) steels via a new method. According to the obtained microstructures, the gradual rise in the fraction of martensite was observed as the intercritical annealing time increased from 0.150 to 0.556 in 830–5 and 830–15 samples. The austenite nuclei could effectively restrict the growth of ferrite grains at elevated temperatures, reducing the average ferrite grain size from 7.8 to 6.7 µm in the 830–5 and 830–15 samples, respectively. The martensite distribution in both RD-ND and RD-TD planes gradually enhanced as a result of the formation of interconnected martensite islands with increasing soaking time in the dual-phase region. All DP samples revealed a weak texture, and the γ-fiber vanished completely after only 10 min of annealing. The Vickers hardness measurements demonstrated an increase from 226.4 ± 9.6 HV to 238.0 ± 6.7 HV and 282.2 ± 7.5 HV with increasing the intercritical annealing time from 5 to 10 min and 15 min as a result of higher martensite fraction. All DP samples revealed a continuous yielding behavior after the formation of mobile dislocations during quenching to ambient temperature. The higher martensite fraction did not enhance the strength significantly, owing to the reduction in martensite carbon content at higher annealing times. However, both 830–10 and 830–15 samples exhibited a mechanical isotropic behavior because of the weak overall texture and γ-fiber elimination. The strain-hardening curves of the DP samples exhibited a high initial strain-hardening rate and a three-stage behavior resulting from the plastic deformation of ferrite and martensite. Moreover, the increase in the martensite fraction and the reduction in ferrite grain size resulted in the higher initial work-hardening exponent of the 830–15 samples. The fractured surfaces of DP samples revealed a mixture of coarse and fine dimples, indicating a ductile fracture.

Single-crystal silicon carbide (SiC) is recognized as a promising semiconductor material that exhibits multiple polytypes. Among these polytypes, 4 H-SiC is extensively employed in fabricating electronic devices because of its superior properties. The inherent anisotropy, high hardness, and brittleness of 4 H-SiC present significant challenges in machining. Compared to conventional grinding, ion implantation-assisted grinding enhances machining efficiency and quality; however, the individual and synergistic effects of ion implantation parameters on 4 H-SiC machining remain underexplored. In this investigation, the single-grit diamond grinding process of 4 H-SiC was examined, both with and without ion implantation, using molecular dynamics simulations. The impacts of varying ion implantation energies and doses on material modification behavior and grinding performance were assessed. Particularly, two factors determining the ion implantation dose, namely the number of implanted ions and the size of the implantation area, were considered independently. The results indicate that both the grinding force and the grinding temperature decrease with increasing implantation energy and dose. Furthermore, higher implantation energy or a greater number of implanted ions enhance material modification efficiency and removal rate. Ion implantation modification reduces machining stress and improves machining quality. When adjusting the ion implantation dose, priority should be given to changing the number of implanted ions. This study not only deepens our understanding of the mechanisms behind material removal and damage evolution but also offers a theoretical foundation for optimizing parameters in ion implantation-assisted machining.

The influence of heat curing effect on the mechanical properties of fly ash based geopolymer concrete (FAG) is significant. The purpose of this study is to investigate the effects of curing temperature and Alkali to Binder ratio (AL/B) on the mechanical characteristics of FAG, as well as the connection between macroscopic nonlinear constitutive behavior and mesoscopic damage processes. By performing uniaxial compression tests at different curing temperatures (ambient curing at 20 °C, heat curing at 60 °C, 80 °C, 100 °C, and 120 °C) and AL/B (0.35 and 0.45), combined with nuclear magnetic resonance, scanning electron microscopy, and x-ray diffraction tests, the relationships between mechanical properties, stress-strain full curves, and microscopic features of FAG were investigated. The experimental findings showed that the peak stress and elastic modulus of FAG rose with increasing curing temperature, but the peak strain decreased. The development of N-A-S-H gel and the densification of the microstructure were crucial for improving FAG strength. The mesoscopic damage constitutive relationship of FAG was examined, correlating the deterioration trend of macroscopic mechanical properties with microstructural evolution to systematically clarify the development of FAG strength and its intrinsic deterioration mechanisms. This research established a theoretical foundation for the widespread adoption of FAG in engineering applications.

An experimental study was conducted to examine the uniaxial tensile and biaxial bulging behavior of 304 austenitic stainless steel (ASS) to elucidate its strain-hardening characteristics at cryogenic temperature (CT). Scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) analyses were employed to quantitatively correlate the macroscopic deformation properties with microstructural evolution. Furthermore, ASS samples with pre-strains of 5%, 15%, and 25% were prepared at both room temperature (RT) and CT, which was then followed by tensile testing at CT. The results revealed that the yield strength (YS) of ASS increased with increasing pre-strain at CT, exhibiting a reduction in ductility. Notably, the YS of cryogenically pre-strained samples increased by 15.4%, 26.5%, and 33.8%, respectively, compared with those pre-strained at RT. Under biaxial loading, the bulging load of ASS also increased with decreasing temperature. The highest hardness values were consistently observed at the bulge apex and increased with increasing bulge height. Consequently, the enhanced YS primarily increased from martensitic transformation and dislocation strengthening during cryogenic pre-strainings. The findings of this study offered valuable insights for optimizing the lightweight design and improving the operational safety of cryogenic storage tanks.

In this work, the asymmetric rolling (ASR) is employed to improve the formability of 2195 Al-Li alloy, which has been widely used in the aerospace and automobile fields. The microstructures and mechanical properties of 2195 Al-Li alloy in ASR under different velocity ratios and strain paths are explored through experimental characterization. Compared with conventional rolling, ASR can effectively refine the grain. As the velocity ratio increases, the average grain size gradually decreases, and the surface grain contacted with the faster roll is finer, which can be attributed to more severe shear deformation. The complex strain path will increase the shear component. Under the same velocity ratio, the grain refinement effect of double-direction asymmetrical rolling is more significant. Moreover, the shear component generated by ASR causes some textures to deviate or split from the ideal positions. Most of the recrystallization textures are eliminated. During ASR, the rolling texture of the surface in contact with the faster roll tends to evolve into the shear texture. In terms of improving the uniformity of microstructure, adjusting the velocity ratio is more effective than changing the strain path. The grain size and texture of the top and inner layers of the ASR sheet with a velocity ratio of 1.8 are closer. These findings provide theoretical guidance for achieving high-performance manufacturing of Al-Li alloy components.

This article presented a comprehensive and accurate nonlinear mathematical model to analyze the mechanical response of multi-directional functionally graded material porous (MDFGMP) plates, for the first time. The model includes a differential quadrature method (DQM) with a quasi-3D theory to investigate bending and vibration responses MDFGMP plates. A nonlinear quasi-3D plate theory is exploited to present the kinematic fields including the effect of normal strain, thickness stretching, and satisfy the zero-shear strain/stress at the top and bottom surfaces without shear correction factor. The power 3D function distribution is used to portray gradation of material constituents through thickness and in-plane directions. Two types of porosity are selected to describe the distribution of voids and cavities through the thickness of the plate. Hamilton’s principle is employed to derive the nonlinear governing differential equations of motions in terms of stress resultants. The differential integral quadrature method (DIQM) is manipulated to discretize the structure spatial domains. The accuracy and reliability of the proposed method have been validated by comparing its numerical results to those of available works. Parametric studies are provided to exhibit the significant impacts of kinematic normal and shear relations, gradation indices, porosity type, and boundary conditions on MDFGMP plates. It is found that frequency mode shapes are symmetric for homogeneous plates but exhibit non-symmetric profiles if the material properties change in the in-plane directions. In contrast, changing material properties in the thick direction, although changing the frequencies, it preserves symmetric mode shape patterns. The present model and results can be implemented as benchmarks for future nonlinear mechanical response of MFGMP plates structures. The proposed model can be implemented in selection and design of the nuclear reactors, marine and aerospace structures manufacture from MDFGMP plates.

Carbon fiber reinforced concrete (CFRC) often suffers from weak bonding between its cement matrix and carbon fibers (CF) because the fibers have an inert surface.To overcome this limitation, we introduce phenylpropyl emulsion (SAE) as an interfacial modifier and examine its strengthening mechanism through macroscopic and microscopic experiments and multiscale simulations using discrete element and molecular dynamics methods. Macroscopic experiments show that both CF and SAE significantly improve the mechanical properties of CFRC, although the effectiveness of SAE depends on its dosage. Specifically, a 4% SAE dosage increases the compressive and shear strengths of CFRC (with 1.5% CF) by 16.5% and 12.1%, respectively, while a 2% dosage boosts early flexural strength by 24.4%. At the microscopic level, SAE enhances the bond at the interface by providing physical anchorage through benzene rings and by forming chemical interactions via COO⁻-Ca²⁺complexes. Discrete element simulations confirm that the modified material has higher ultimate strength and toughness, and molecular dynamics simulations show that SAE lowers the interfacial energy and strengthens the bond between CF and the matrix through ionic and hydrogen bonding. This work offers a solid theoretical basis for improving CFRC interfaces and informs the design of innovative polymer modifiers.

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