Archives of Civil and Mechanical Engineering最新文献

To precisely elucidate the deformation characteristics of Ti65 sheets under uniaxial and biaxial stress states during the superplastic forming (SPF) process, a series of uniaxial hot tensile and biaxial bulging tests were conducted to explore the superplastic deformation behavior within a temperature range of 900–960 °C and a strain rate range of 0.001–0.03 s⁻1. The deformation behavior and uniform strain under uniaxial stress states were characterized through the DIC real-time strain measurement system. In addition, the key microstructures evolutions during different stress states were characterized and analyzed to determine the deformation mechanisms. Based on the test results, the constitutive model for both uniaxial and biaxial behavior of Ti65 sheets was developed and calibrated. The results of this research indicated that Ti65 exhibited superplastic deformation at 940 °C—0.0014 s⁻1 and 960 °C—0.0075 s⁻1, which led to an enlargement of its forming limit. Simultaneously, the evolution mechanism of the microstructure under biaxial stress was revealed. As the temperature increased, the proportion of high angle grain boundaries rose, the grain size decreased, and the forming limit increased accordingly. The established constitutive model, which takes into account the evolution of the microstructure, successfully captured the forming limit points. The accuracy of the predicted uniaxial stress–strain and the bulging grain size reached 91.2% and 96.24%, respectively. This research provides theoretical guidance for the selection of the process window of titanium alloys.

Sapphire, known for its high hardness and excellent optical properties, is widely used in fields such as aerospace, optoelectronics and defense. Micro-milling technology for sapphire demonstrates significant potential in the field of sapphire surface processing due to its efficiency, high quality, low loss and flexibility. In this study, a double-edged helical polycrystalline diamond (PCD) micro-end mill with a diameter of 1 mm is designed and fabricated by electrical discharge machining (EDM). Then, the micro-slots on sapphire material are prepared with EDM-fabricated micro-end mills, and the surface quality, surface morphology, micro-milling forces and tool wear involved in micro-milling process are investigated. Experimental results indicate that three types of damages are observed on sapphire micro-slot surface including wavy cracks, individual small cracks and layered tear structures. The minimum surface roughness S_a for sapphire micro-slot obtained with PCD micro-end mill can reach to 0.73 µm. In addition, the major wear forms of PCD micro-end mill when machining sapphire include mechanical wear, thermal chemical wear, adhesive wear, and micro-chipping. The research of adopting PCD micro-end mills for sapphire holds significant application value, which can advance technological progress in machining efficiency and surface quality of hard brittle material.

H13 steel is a key material in the field of hot work dies. Despite its excellent strength and toughness, surface coatings are often needed to improve its hardness and wear resistance in high-temperature environments. In this research, laser cladding of Inconel 718 (IN718)/hexagonal boron nitride (h-BN) composite coatings on H13 steel is investigated to enhance its performance. The effect of process parameters on the macroscopic morphology, represented by the width-to-height ratio and dilution rate, is investigated to obtain coatings with enhanced interfacial bonding strength and improved metallurgical compatibility. The powder composition is determined by characterizing the microstructure, surface hardness, wear resistance, and other properties of composite coatings with varying h-BN content. Additionally, the roles of solution treatment and age hardening as post-treatment methods in further improving coating performance are evaluated. The results indicate that the composite coatings prepared under the applied laser cladding parameters exhibit no significant defects. The 35 wt.% h-BN composite coating (the optimal composition) demonstrates superior ambient/elevated-temperature hardness, 35.4% lower friction coefficient, and 76.8% reduced wear loss compared to the H13 steel substrate. As h-BN content increases, the volume of nitrides and borides in the coatings also rises. Precipitates such as alumina, Metal Carbide (MC), Metal Nitride (MN), and Laves phases are observed both inside and outside the grains, with grain sizes ranging from 5 to 100 µm. After solution treatment, the dissolution and diffusion of intergranular precipitates are evident. Following age hardening, hard phases enriched with B and N fully diffuse and precipitate at the grain boundaries. Post-treatment effectively releases residual stress in the coating, resulting in enhanced material properties. This research provides a novel strategy for surface strengthening of H13 steel in high-temperature applications.

In this article, the vibration analysis of a nanocomposite deep thick cylindrical shell surrounded by an orthotropic medium subjected to thermal load is studied. It is assumed that the shell is fabricated from a polymeric matrix reinforced with graphene nanoplatelets (GNPs) in which the volume fraction of the GNPs varies along the thickness based on several distribution patterns. The modeling of shell is carried out utilizing a quasi-3D shear theory which includes the thickness stretching. The modeling of the medium is conducted based on the orthotropic Pasternak model. The one-dimensional heat conduction equation is solved analytically to find the temperature profile through the thickness of the shell. Moreover, the dependency of properties of the materials on the temperature is considered. A semi-analytical solution is presented to determine the natural frequencies of the shell and associated mode shapes. The effects of several parameters on the natural frequencies are studied, including the mass fraction and distribution pattern of the GNPs, thermal loading, agglomeration parameters, boundary conditions, and characteristics of the orthotropic medium. Owing to considering the thickness stretching effect, removing shallow shell assumptions, and incorporating the agglomeration of the GNPs, the results of the presented work benefit from high accuracy and can be used in the design and analysis of thin to thick and shallow to deep nanocomposite cylindrical shells.

Alkali-activated materials have gained increasing popularity due to their advantages in reducing carbon emissions and promoting environmental sustainability. To gain a comprehensive understanding of the early-stage properties and mechanisms of alkali-activated materials when mixed with various types of water, a study was conducted to investigate the impact of seawater, deionized water, and freshwater on the setting time, fluidity time, strength, and drying shrinkage rate of a slag and desulfurization gypsum composite alkali-activated material (SD-AAM). Additionally, the hydration products and microstructures of the SD-AAM were examined using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and thermogravimetric–differential scanning calorimetry (TG–DSC) to uncover the mechanisms by which mixing water influences these properties. The results indicate that seawater significantly enhances the strength of the SD-AAM after a 7-day curing age compared to deionized water. However, it also reduces fluidity, setting time, and drying shrinkage rate. This phenomenon can be attributed to the ability of seawater to accelerate the hydration process and facilitate the formation of Friedel’s salt. The strengthening effect of the seawater becomes increasingly pronounced as the curing age extends. Furthermore, Friedel’s salt is gradually enveloped by C–(A)–S–H gel, leading to a decrease in the ({text{Cl/Al}}) ratio when seawater is utilized, as evidenced by the combined results of SEM and EDS. In contrast, freshwater consistently exerts a detrimental effect on early-stage strength across various curing ages. These findings are significant for expanding the applications of alkali-activated materials in specialized engineering contexts.

Cork materials, valued for their sustainability and thermal insulating properties, are gaining use as eco-friendly alternatives to synthetic insulation in building facades, among other architectural, construction, and building applications. However, the ageing effects caused by weathering exposure are yet to be investigated. Understanding how these materials respond to outdoor environmental stressors is essential to ensure their long-term performance in facade systems and other exposed building elements. Therefore, this study examines the effects of accelerated ultraviolet (UV) ageing and freeze–thaw (FT) cycles on natural, agglomerated, and expanded cork’s structure, mechanical, thermal, and chemical properties. Each cork type underwent UV and FT ageing, simulating seasonal environmental conditions, and was subsequently analysed through scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), compressive mechanical testing, and thermal analysis. UV exposure leads to the degradation of the cellular structure in all types of cork, with particularly significant in expanded cork. On the other hand, FT cycles primarily affect agglomerated cork. FTIR analysis corroborates these structural changes from SEM observations, showing spectral changes associated with suberin and lignin degradation in UV-exposed cork. However, only expanded cork exhibits changed or erased bands when exposed to FT cycles. Mechanical testing indicates that UV exposure reduces the compressive strength of natural cork, whereas FT cycles led to a slight increase in the agglomerated one and a significant decline for natural and expanded cork. Thermal tests reveal that UV exposure increases thermal conductivity and specific heat in natural cork but reduces diffusivity, while agglomerated cork experiences an increase in conductivity and diffusivity for the same conditions. FT cycles generally increase the conductivity of all cork types, while thermal diffusivity decreases for expanded cork and decreases for both natural and agglomerated cork.

This research describes the formulation and experimental assessment of an environmentally sustainable ultra-high-performance concrete (UHPC) embedding both agricultural and industrial by-products. Although individual use of agricultural or ceramic waste in concrete has been explored, the combined effect of palm-based ashes and ceramic waste as fine aggregates (CWFA) on UHPC’s properties remains insufficiently investigated. Palm oil ash (POA) and palm leaf ash (PLA) were incorporated as partial substitutes for Portland cement (PC) in proportions of 10%, 20%, 30%, 40%, and 50%; concurrently, CWFA substituted quartz sand at replacement levels of 25%, 50%, 75%, and 100%. Nineteen distinct concrete mixtures were thus prepared and evaluated. Performance metrics comprised slump flow, compressive strength, permeability, scanning electron microscopy (SEM) analysis, and compressive strength retention under elevated-temperature conditions. Data revealed that the introduction of POA and PLA to an extent of 30% generally elevated the compressive strength in comparison to the reference matrix. The most effective mixture, POA30-C100, attained a peak compressive strength of 205.5 MPa at 90 days, exceeding the reference strength of 172.4 MPa. These results underscore the viability of repurposing agricultural and industrial residues for the sustainable fabrication of high-performance UHPC, thereby enhancing both material performance and ecological stewardship.

The present study investigates the dynamic tensile behaviour of a high-manganese austenitic TWIP steel (X45MnAl20-4) using a flywheel-based testing system, with emphasis on strain rate effects. Dynamic tensile tests with strain rates ranging from approximately 300 to 950 s⁻¹. The force vs. time signals were recorded during the course of tests. This allowed the true stress–true strain diagrams to be determined. It has been established that the propagation of elastic stress waves within the test stand construction caused oscillations in the σ_true-ε_true curves. The smoothing of these oscillations has been approached through the utilisation of a power function as an approximation. The influence of strain rate on the tested steel characteristics such as flow stress, plastic deformation work, and absorbed energy (SEA and VEA) was quantitatively evaluated on the basis of the obtained σ_true-ε_true curves. Microscopic observations confirmed that mechanical twinning is the predominant deformation mechanism in the steel studied, with increased twinning density and multisystem twinning under dynamic conditions. A comparison of the X45MnAl20-4 steel with other TWIP steels and DP steel has shown that it exhibits a competitive specific SEA value. This confirms its suitability for use in structural components operating under dynamic loading conditions. The findings of the study underscore the significance of accurate measurement data interpretation and selection of suitable correction methods when analysing the dynamic mechanical responses of TWIP steels by means of a flywheel machine.

The stacking fault energy (SFE)-governed synergy between twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP) mechanisms delivers superior mechanical properties compared to either effect alone. However, precise knowledge of an optimal TWIP and TRIP balance remains elusive and holistic understanding on the contributions from both TWIP and TRIP effects to the mechanical properties is still lacking. In this study, we show that by carefully tailoring the SFE to approximately 10 mJ·m⁻² through adjustment of grain size and deformation temperatures, an optimal synergy between strength and ductility can be achieved in Fe–Cr–Ni austenitic steels with a variety of compositions. This synergy arises from the intricate manipulation of the sustained TWIP and TRIP effects. The optimal combination characterized by approximately 18% deformation twins and 50% strain-induced martensite is revealed by an SFE-dependent physical model which models the austenite → twin → α′-martensite transformation sequence. These findings offer valuable insights for the fast and cost-effective design of austenitic steels.

This study experimentally investigated the correlation between crystallographic, microstructural, and mechanical properties of Wire arc additive manufacturing (WAAM) fabricated ER70S-6 steel, especially focusing on the effects of layer height on the fabricated samples. To examine these properties, the samples were tested at different heights, i.e., bottom segment (BS), middle segment (MS) and top segment (TS). Then, the microstructure, grain scale boundaries, microhardness, and phase transformations were analyzed with scanning electron microscope (SEM), Energy-Dispersive X-ray Spectroscopy (EBSD), microhardness tester, and X-ray Diffractometer. Similarly, the mechanical behavior of ER70S-6 manufactured parts was measured along a vertical direction, i.e., building direction with a universal testing machine (UTM). The research shows that the mechanical properties of the same parts at different heights were not similar. The average microhardness and ultimate tensile strength of the constructed sample were reduced by 13.21% and 6.18%, respectively, from the bottom to the top of the sample. Based on this, it was found that changes in the cooling rate at different levels resulted in considerable variation in the microstructure. It is also concluded that a coarse-grained zone exists in samples along the building axis, with its growth toward the topmost part causing various changes in mechanical attributes such as ductility and strength.