Materials

28 pages, 17257 KiB

Open AccessArticle

A Crystal Plasticity Phase-Field Study on the Effects of Grain Boundary Degradation on the Fatigue Behavior of a Nickel-Based Superalloy

by Pengfei Liu, Zhanghua Chen, Xiao Zhao, Jianxin Dong and He Jiang

Materials 2025, 18(14), 3309; https://doi.org/10.3390/ma18143309 (registering DOI) - 14 Jul 2025

Abstract

Grain boundary weakening in high-temperature environments significantly influences the fatigue crack growth mechanisms of nickel-based superalloys, introducing challenges in accurately predicting fatigue life. In this study, a dislocation-density-based crystal plasticity phase-field (CP–PF) model is developed to simulate the fatigue crack growth behavior of [...] Read more.

Grain boundary weakening in high-temperature environments significantly influences the fatigue crack growth mechanisms of nickel-based superalloys, introducing challenges in accurately predicting fatigue life. In this study, a dislocation-density-based crystal plasticity phase-field (CP–PF) model is developed to simulate the fatigue crack growth behavior of the GH4169 alloy under both room and elevated temperatures. Grain boundaries are explicitly modeled, enabling the competition between transgranular and intergranular cracking to be accurately captured. The grain boundary separation energy and surface energy, calculated via molecular dynamics simulations, are employed as failure criteria for grain boundary and intragranular material points, respectively. The simulation results reveal that under oxygen-free conditions, fatigue crack propagation at both room and high temperatures is governed by sustained shear slip, with crack advancement hindered by grains exhibiting low Schmid factors. When grain boundary oxidation is introduced, increasing oxidation levels progressively degrade grain boundary strength and reduce overall fatigue resistance. Specifically, at room temperature, oxidation shortens the duration of crack arrest near grain boundaries. At elevated service temperatures, intensified grain boundary degradation facilitates a transition in crack growth mode from transgranular to intergranular, thereby accelerating crack propagation and exacerbating fatigue damage. Full article

(This article belongs to the Section Metals and Alloys)

► Show Figures

Figure 1

15 pages, 5452 KiB

Open AccessArticle

Roughness and Gloss of 3D-Printed Crowns Following Polishing or Varnish Application

by Silvia Rojas-Rueda, Tariq Aziz Alsahafi, Mohammed Hammamy, Neeraj Surathu, Nitish Surathu, Nathaniel C. Lawson and Taiseer A. Sulaiman

Materials 2025, 18(14), 3308; https://doi.org/10.3390/ma18143308 (registering DOI) - 14 Jul 2025

Abstract

The aim of this study was to evaluate and compare the surface roughness and gloss—both initially and after simulated toothbrushing—of three 3D-printed crown materials subjected to different surface treatments: varnishing, polishing with diamond-impregnated rubber polishers, and polishing with a bristle brush and paste. [...] Read more.

The aim of this study was to evaluate and compare the surface roughness and gloss—both initially and after simulated toothbrushing—of three 3D-printed crown materials subjected to different surface treatments: varnishing, polishing with diamond-impregnated rubber polishers, and polishing with a bristle brush and paste. Disc-shaped specimens (n = 90) were 3D-printed using three commercially available crown resins (Rodin Sculpture, VarseoSmile TriniQ, and OnX Tough 2) and post-processed per manufacturers’ instructions. Specimens were divided into three surface treatment groups: application of a light-cured varnish, polishing with a two-step diamond-impregnated rubber polisher, or polishing with a bristle brush and abrasive paste. Surface roughness and gloss were measured after treatment and again following 20,000 cycles of simulated toothbrushing. Additional specimens were prepared for Vickers microhardness testing and determination of filler weight percentage (wt%). Statistical comparisons were performed using two-way ANOVA with significance set at p < 0.05. Results: The varnish provided the statistically lowest roughness of all surface treatments for all materials. The bristle brush and abrasive paste polishing protocol produced the greatest gloss for the softest material (VarseoSmile TriniQ) and lowest gloss for the hardest material (Rodin Sculpture), whereas the two-step diamond-impregnated rubber polisher produced an equivalent gloss on all materials. Following toothbrushing, roughness was minimally affected; however, gloss was considerably reduced. Conclusions: All tested polishing and varnishing methods achieved clinically acceptable surface roughness (Ra < 0.2 µm) that persisted after simulated toothbrushing. Notably, the two-step diamond-impregnated rubber polisher produced consistent gloss across all materials, while the bristle brush and abrasive paste polishing protocol performed better on softer materials, and varnish application resulted in equal or superior gloss and roughness retention compared to polishing. Full article

(This article belongs to the Special Issue Innovations in Digital Dentistry: Novel Materials and Technologies)

► Show Figures

Figure 1

18 pages, 4996 KiB

Open AccessArticle

Mechanical Properties and Microstructures of Solid Waste Composite-Modified Lateritic Clay via NaOH/Na₂CO₃ Activation: A Sustainable Recycling Solution of Steel Slag, Fly Ash, and Granulated Blast Furnace Slag

by Wei Qiao, Bing Yue, Zhihua Luo, Shengli Zhu, Lei Li, Heng Yang and Biao Luo

Materials 2025, 18(14), 3307; https://doi.org/10.3390/ma18143307 (registering DOI) - 14 Jul 2025

Abstract

The utilization of steel slag (SS), fly ash (FA), and ground granulated blast furnace slag (GGBFS) as soil additives in construction represents a critical approach to achieving resource recycling of these industrial by-products. This study aims to activate the SS-FA-GGBFS composite with a [...] Read more.

The utilization of steel slag (SS), fly ash (FA), and ground granulated blast furnace slag (GGBFS) as soil additives in construction represents a critical approach to achieving resource recycling of these industrial by-products. This study aims to activate the SS-FA-GGBFS composite with a NaOH solution and Na₂CO₃ and employ the activated solid waste blend as an admixture for lateritic clay modification. By varying the concentration of the NaOH solution and the dosage of Na₂CO₃ relative to the SS-FA-GGBFS composite, the effects of these parameters on the activation efficiency of the composite as a lateritic clay additive were investigated. Results indicate that the NaOH solution activates the SS-FA-GGBFS composite more effectively than Na₂CO₃. The NaOH solution significantly promotes the depolymerization of aluminosilicates in the solid waste materials and the generation of Calcium-Silicate-Hydrate and Calcium-Aluminate-Hydrate gels. In contrast, Na₂CO₃ relies on its carbonate ions to react with calcium ions in the materials, forming calcium carbonate precipitates. As a rigid cementing phase, calcium carbonate exhibits a weaker cementing effect on soil compared to Calcium-Silicate-Hydrate and Calcium-Aluminate-Hydrate gels. However, excessive NaOH leads to inefficient dissolution of the solid waste and induces a transformation of hydration products in the modified lateritic clay from Calcium-Silicate-Hydrate and Calcium-Aluminate-Hydrate to Sodium-Silicate-Hydrate and Sodium-Aluminate-Hydrate, which negatively impacts the strength and microstructural compactness of the alkali-activated solid waste composite-modified lateritic clay. Full article

(This article belongs to the Section Construction and Building Materials)

► Show Figures

Figure 1

44 pages, 14734 KiB

Open AccessArticle

Influence of Zn Content on the Corrosion and Mechanical Properties of Cast and Friction Stir-Welded Al-Si-Mg-Fe-Zn Alloys

by Xiaomi Chen, Kun Liu, Quan Liu, Jing Kong, Valentino A. M. Cristino, Kin-Ho Lo, Zhengchao Xie, Zhi Wang, Dongfu Song and Chi-Tat Kwok

Materials 2025, 18(14), 3306; https://doi.org/10.3390/ma18143306 (registering DOI) - 14 Jul 2025

Abstract

With the ongoing development of lightweight automobiles, research on new aluminum alloys and welding technology has gained significant attention. Friction stir welding (FSW) is a solid-state joining technique for welding aluminum alloys without melting. In this study, novel squeeze-cast Al-Si-Mg-Fe-Zn alloys with different [...] Read more.

With the ongoing development of lightweight automobiles, research on new aluminum alloys and welding technology has gained significant attention. Friction stir welding (FSW) is a solid-state joining technique for welding aluminum alloys without melting. In this study, novel squeeze-cast Al-Si-Mg-Fe-Zn alloys with different Zn contents (0, 3.4, 6.5, and 8.3 wt%) were friction stir welded (FSWed) at a translational speed of 200 mm/min and a rotational speed of 800 rpm. These parameters were chosen based on the observations of visually sound welds, defect-free and fine-grained microstructures, homogeneous secondary phase distribution, and low roughness. Zn can affect the microstructure of Al-Si-Mg-Fe-Zn alloys, including the grain size and the content of secondary phases, leading to different mechanical and corrosion behavior. Adding different Zn contents with Mg forms the various amount of MgZn₂, which has a significant strengthening effect on the alloys. Softening observed in the weld zones of the alloys with 0, 3.4, and 6.5 wt% Zn is primarily attributed to the reduction in Kernel Average Misorientation (KAM) and a decrease in the Si phase and MgZn₂. Consequently, the mechanical strengths of the FSWed joints are lower as compared to the base material. Conversely, the FSWed alloy with 8.3 wt% Zn exhibited enhanced mechanical properties, with hardness of 116.3 HV_0.2, yield strength (YS) of 184.4 MPa, ultimate tensile strength (UTS) of 226.9 MP, percent elongation (EL%) of 1.78%, and a strength coefficient exceeding 100%, indicating that the joint retains the strength of the as-cast one, due to refined grains and more uniformly dispersed secondary phases. The highest corrosion resistance of the FSWed alloy with 6.5%Zn is due to the smallest grain size and KAM, without MgZn₂ and the highest percentage of {111} texture (24.8%). Full article

(This article belongs to the Special Issue Study on Electrochemical Behavior and Corrosion of Materials)

► Show Figures

Graphical abstract

11 pages, 2014 KiB

Open AccessArticle

Attachment of Human Epithelial Cells to an Anodized Titanium Surface

by Yoshihiko Akashi, Hayato Hashiguchi, Yoshitaka Yamaoka, Kei Nakajima, Katsutoshi Kokubun, Yoshiaki Shimoo and Kenichi Matsuzaka

Materials 2025, 18(14), 3305; https://doi.org/10.3390/ma18143305 (registering DOI) - 14 Jul 2025

Abstract

The attachment of the oral epithelium to the abutment surface is crucial for the long-term success of dental implants. This study aimed to evaluate the attachment of human epithelial cells to anodized titanium surfaces. Anodized titanium discs were used as the experimental group, [...] Read more.

The attachment of the oral epithelium to the abutment surface is crucial for the long-term success of dental implants. This study aimed to evaluate the attachment of human epithelial cells to anodized titanium surfaces. Anodized titanium discs were used as the experimental group, while machined titanium discs served as the control. Surface roughness and wettability were first measured for each group. Next, human epithelial cells were seeded onto each disc at a density of 4.0 × 10⁴ cells/cm² and evaluated 3, 6, and 24 h later for cell proliferation, as well as mRNA expression and protein levels of laminin and integrin β₄. Surface roughness was comparable between the two groups; however, wettability was significantly higher in the experimental group. Cell proliferation increased over time in both groups and showed no significant difference. Notably, the expression levels of both laminin and integrin β₄ were significantly higher in the experimental group at 24 h. Furthermore, protein localization of laminin and integrin β₄ was observed along the cell margins on the anodized surface. These findings suggest that anodization enhances epithelial cell attachment by promoting the expression and peripheral organization of key adhesion molecules. Full article

(This article belongs to the Special Issue Advances in Dental Implants and Prosthetics Materials)

► Show Figures

Figure 1

20 pages, 917 KiB

Open AccessArticle

Numerical Investigation of Buckling Behavior of MWCNT-Reinforced Composite Plates

by Jitendra Singh, Ajay Kumar, Barbara Sadowska-Buraczewska, Wojciech Andrzejuk and Danuta Barnat-Hunek

Materials 2025, 18(14), 3304; https://doi.org/10.3390/ma18143304 (registering DOI) - 14 Jul 2025

Abstract

The current study demonstrates the buckling properties of composite laminates reinforced with MWCNT fillers using a novel higher-order shear and normal deformation theory (HSNDT), which considers the effect of thickness in its mathematical formulation. The hybrid HSNDT combines polynomial and hyperbolic functions that [...] Read more.

The current study demonstrates the buckling properties of composite laminates reinforced with MWCNT fillers using a novel higher-order shear and normal deformation theory (HSNDT), which considers the effect of thickness in its mathematical formulation. The hybrid HSNDT combines polynomial and hyperbolic functions that ensure the parabolic shear stress profile and zero shear stress boundary condition at the upper and lower surface of the plate, hence removing the need for a shear correction factor. The plate is made up of carbon fiber bounded together with polymer resin matrix reinforced with MWCNT fibers. The mechanical properties are homogenized by a Halpin–Tsai scheme. The MATLAB R2019a code was developed in-house for a finite element model using C⁰ continuity nine-node Lagrangian isoparametric shape functions. The geometric nonlinear and linear stiffness matrices are derived using the principle of virtual work. The solution of the eigenvalue problem enables estimation of the critical buckling loads. A convergence study was carried out and model efficiency was corroborated with the existing literature. The model contains only seven degrees of freedom, which significantly reduces computation time, facilitating the comprehensive parametric studies for the buckling stability of the plate. Full article

(This article belongs to the Special Issue Mechanical Behavior of Advanced Composite Materials and Structures)

► Show Figures

Figure 1

13 pages, 3050 KiB

Open AccessArticle

Unveiling the Microstructure Evolution and Mechanical Strengthening Mechanisms in Mg–2Y–xZn Alloys

by Luyan Xu, Huanjian Xie, Kuan Chen, Ruizhi Feng, Donghui Zheng and Haoge Shou

Materials 2025, 18(14), 3303; https://doi.org/10.3390/ma18143303 (registering DOI) - 14 Jul 2025

Abstract

This work systematically investigates the Zn-content-dependent phase evolution (1–12 at.%) and its correlation with mechanical properties in as-cast Mg–2Y–xZn alloys. A sequential phase transformation is observed with the Zn content increasing: the microstructure evolves from X-phase dominance (1–2 at.% Zn) through [...] Read more.

This work systematically investigates the Zn-content-dependent phase evolution (1–12 at.%) and its correlation with mechanical properties in as-cast Mg–2Y–xZn alloys. A sequential phase transformation is observed with the Zn content increasing: the microstructure evolves from X-phase dominance (1–2 at.% Zn) through W-phase formation (3–6 at.% Zn) to I-phase emergence (12 at.% Zn). Optimal mechanical performance is attained in the 2 at.% Zn-containing alloy, with measured tensile properties reaching 239 MPa UTS and 130 MPa YS, while maintaining an elongation of 12.62% prior to its gradual decline at higher Zn concentrations. Crystallographic analysis shows that the most significant strengthening effect of the X-phase originates from its coherent orientation relationship with the α-Mg matrix and the development of deformation-induced kink bands. Meanwhile, fine W-phase particles embedded within the X-phase further enhance alloy performance by suppressing X-phase deformation, revealing pronounced synergistic strengthening between the two phases. Notably, although both the I-phase and W-phase act as crack initiation sites during deformation, their coexistence triggers a competitive fracture mechanism: the I-phase preferentially fractures to preserve the structural integrity of the W-phase, effectively mitigating crack propagation. These dynamic interactions of second phases during plastic deformation—synergistic strengthening and competitive fracture—provide a novel strategy and insights for designing high-performance Mg–RE–Zn alloys. Full article

(This article belongs to the Section Metals and Alloys)

► Show Figures

Figure 1

21 pages, 13164 KiB

Open AccessArticle

Surface Modification by Plasma Electrolytic Oxidation of Friction Surfacing 4043 Aluminum-Based Alloys Deposited onto Structural S235 Steel Substrate

by Roxana Muntean and Ion-Dragoș Uțu

Materials 2025, 18(14), 3302; https://doi.org/10.3390/ma18143302 (registering DOI) - 13 Jul 2025

Abstract

The friction surfacing (FS) process has emerged over the past few years as a method for joining both similar and dissimilar materials, for volume damage repair of defective components, and for corrosion protection. The possibility to produce a metallic coating by FS, without [...] Read more.

The friction surfacing (FS) process has emerged over the past few years as a method for joining both similar and dissimilar materials, for volume damage repair of defective components, and for corrosion protection. The possibility to produce a metallic coating by FS, without melting the material, classifies this technique as distinct from other standard methods. This unconventional deposition method is based on the severe plastic deformation that appears on a rotating metallic rod (consumable material) pressed against the substrate under an axial load. The present study aims to investigate the tribological properties and corrosion resistance provided by the aluminum-based FS coatings deposited onto a structural S235 steel substrate and further modified by plasma electrolytic oxidation (PEO). During the PEO treatment, the formation of a ceramic film is enabled, while the hardness, chemical stability, corrosion, and wear resistance of the modified surfaces are considerably increased. The morpho-structural characteristics and chemical composition of the PEO-modified FS coatings are further investigated using scanning electron microscopy combined with energy dispersive spectroscopy analysis and X-ray diffraction. Dry sliding wear testing of the PEO-modified aluminum-based coatings was carried out using a ball-on-disc configuration, while the corrosion resistance was electrochemically evaluated in a 3.5 wt.% NaCl solution. The corrosion rates of the aluminum-based coatings decreased significantly when the PEO treatment was applied, while the wear rate was substantially reduced compared to the untreated aluminum-based coating and steel substrate, respectively. Full article

(This article belongs to the Section Metals and Alloys)

► Show Figures

Figure 1

19 pages, 4331 KiB

Open AccessArticle

Optimization of Grain Boundary Structure and Dielectric Properties in SrTiO₃ Ceramics via Hot Isostatic Pressing

by Yilong Feng, Zhenya Lu, Ming Lv, Dan Qie and Zaiyun Long

Materials 2025, 18(14), 3301; https://doi.org/10.3390/ma18143301 (registering DOI) - 13 Jul 2025

Abstract

This study fabricated SrTiO₃ grain boundary layer ceramics using hot isostatic pressing (HIP), achieving a remarkably high dielectric constant of 60,350 and a superior breakdown strength of 1722 kV/m. Microstructural characterization via scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed [...] Read more.

This study fabricated SrTiO₃ grain boundary layer ceramics using hot isostatic pressing (HIP), achieving a remarkably high dielectric constant of 60,350 and a superior breakdown strength of 1722 kV/m. Microstructural characterization via scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that HIP treatment significantly refined grain size uniformity and homogenized bismuth distribution at grain boundaries, thus enhancing the interfacial barrier effect. Probe-based impedance spectroscopy elucidated the dielectric behavior and conduction mechanisms of individual grain boundaries. HIP promotes the formation of interfacial barrier layers (IBLs), significantly improving electrical performance. Compared to untreated samples (average breakdown strength: 555 kV/m), HIP-processed ceramics exhibited a threefold enhancement in breakdown strength (1722 kV/m). The treated ceramic exhibited excellent temperature stability, with TCC ≤8% over −55 to 125 °C. The optimized dielectric properties stem from HIP-induced structural modifications, including reduced oxygen vacancy concentrations and homogenized electronic distribution at grain boundaries. These findings establish a quantitative correlation between HIP parameters, grain boundary restructuring, and macroscopic performance, providing critical insights for designing high-energy-density dielectric materials. Full article

(This article belongs to the Section Advanced and Functional Ceramics and Glasses)

► Show Figures

Figure 1

25 pages, 7489 KiB

Open AccessArticle

Influence of Recycled Tire Steel Fiber Content on the Mechanical Properties and Fracture Characteristics of Ultra-High-Performance Concrete

by Junyan Yu, Qifan Wu, Dongyan Zhao and Yubo Jiao

Materials 2025, 18(14), 3300; https://doi.org/10.3390/ma18143300 (registering DOI) - 13 Jul 2025

Abstract

Ultra-high-performance concrete (UHPC) reinforced with recycled tire steel fibers (RTSFs) was studied to evaluate its mechanical properties and cracking behavior. Using acoustic emission (AE) monitoring, researchers tested various RTSF replacement rates in compression and flexural tests. Results revealed a clear trend: mechanical properties [...] Read more.

Ultra-high-performance concrete (UHPC) reinforced with recycled tire steel fibers (RTSFs) was studied to evaluate its mechanical properties and cracking behavior. Using acoustic emission (AE) monitoring, researchers tested various RTSF replacement rates in compression and flexural tests. Results revealed a clear trend: mechanical properties initially improved then declined with increasing RTSF content, peaking at 25% replacement. AE analysis showed distinct patterns in energy release and crack propagation. Signal timing for energy and ringing count followed a delayed-to-advanced sequence, while b-value and information entropy changes indicated optimal flexural performance at specific replacement rates. RA-AF classification demonstrated that shear failure reached its minimum (25% replacement), with shear cracks increasing at higher ratios. These findings demonstrate RTSFs’ dual benefits: enhancing UHPC performance while promoting sustainability. The 25% replacement ratio emerged as the optimal balance, improving strength while delaying crack formation. This study provides insights into the mechanism by which waste tire steel fibers enhance the performance of UHPC. This research provides valuable insights for developing eco-friendly UHPC formulations using recycled materials, offering both environmental and economic advantages for construction applications. Full article

(This article belongs to the Section Construction and Building Materials)

► Show Figures

Figure 1

29 pages, 7799 KiB

Open AccessArticle

Substrate Flexibility and Metal Deposition Method Effects on Piezoelectric-Enhanced SERS in Metal–ZnO Nanorod Nanocomposites

by Nguyen Thi Quynh Nhu, Le Tran Thanh Thi, Le Vu Tuan Hung and Vincent K. S. Hsiao

Materials 2025, 18(14), 3299; https://doi.org/10.3390/ma18143299 (registering DOI) - 13 Jul 2025

Abstract

This study investigates the effects of substrate flexibility and metal deposition methods on piezoelectric-enhanced Surface-Enhanced Raman Scattering (SERS) in metal-deposited ZnO nanorod (NR) nanocomposites (NCPs). ZnO NRs were grown on both rigid (ITO–glass) and flexible (ITO-PET) substrates, followed by gold (Au) deposition by [...] Read more.

This study investigates the effects of substrate flexibility and metal deposition methods on piezoelectric-enhanced Surface-Enhanced Raman Scattering (SERS) in metal-deposited ZnO nanorod (NR) nanocomposites (NCPs). ZnO NRs were grown on both rigid (ITO–glass) and flexible (ITO-PET) substrates, followed by gold (Au) deposition by pulsed-laser-induced photolysis (PLIP) or silver (Ag) deposition by thermal evaporation. Structural analysis revealed that ZnO NRs on flexible substrates exhibited smaller diameters (60–80 nm vs. 80–100 nm on glass), a higher density, and diverse orientations that enhanced piezoelectric responsiveness. Optical characterization showed distinct localized surface plasmon resonance (LSPR) peaks at 420 nm for Ag and 525 nm for Au systems. SERS measurements demonstrated that Ag-ZnO NCPs achieved superior detection limits (10⁻⁹ M R6G) with enhancement factors of 10⁸–10⁹, while Au-ZnO NCPs reached 10⁻⁸ M detection limits. Mechanical bending of flexible substrates induced dramatic signal enhancement (50–100-fold for Au-ZnO/PET and 2–3-fold for Ag-ZnO/PET), directly confirming piezoelectric enhancement mechanisms. This work establishes quantitative structure–property relationships in piezoelectric-enhanced SERS and provides design principles for high-performance flexible sensors. Full article

(This article belongs to the Special Issue Advanced Synthesis and Applications of Functional Nanomaterials: From Controlled Fabrication to Enhanced Performance)

► Show Figures

Figure 1

30 pages, 5800 KiB

Open AccessArticle

Mitigating Environmental Impact Through the Use of Rice Husk Ash in Sustainable Concrete: Experimental Study, Numerical Modelling, and Optimisation

by Md Jihad Miah, Mohammad Shamim Miah, Humera Mughal and Noor Md. Sadiqul Hasan

Materials 2025, 18(14), 3298; https://doi.org/10.3390/ma18143298 (registering DOI) - 13 Jul 2025

Viewed by 38

Abstract

Cement production significantly contributes to CO₂ emissions (8% of worldwide CO₂ emissions) and global warming, accelerating climate change and increasing air pollution, which harms ecosystems and human health. To this end, this research investigates the fresh and hardened properties of sustainable [...] Read more.

Cement production significantly contributes to CO₂ emissions (8% of worldwide CO₂ emissions) and global warming, accelerating climate change and increasing air pollution, which harms ecosystems and human health. To this end, this research investigates the fresh and hardened properties of sustainable concrete fabricated with three different replacement percentages (0%, 5%, and 10% by weight) of ordinary Portland cement (OPC) using rice husk ash (RHA). The hardened properties were evaluated at 14, 28, 60, 90, and 120 days of water curing. In addition, data-based models were developed, validated, and optimised, and the models were compared with experimental results and validated with the literature findings. The outcomes reveal that the slump values increased (17% higher) with the increased content of RHA, which aligns with the lower temperatures (12% lower) of freshly mixed concrete with RHA than the control mix (100% OPC). The slopes of the stress–strain profiles decreased at early ages and improved at longer curing ages (more than 28 days), especially for mixes with 5% RHA. The compressive strength decreased slightly (18% at 28 days) with increased percentages of RHA, which was minimised with increased curing ages (8% at 90 days). The data-based model accurately predicted the stress–strain profiles (coefficient of determination,

R^{2}

≈ 0.9950–0.9993) and compressive strength at each curing age, including crack progression (i.e., highly nonlinear region) and validates its effectiveness. In contrast, the optimisation model shows excellent results, mirroring the experimental data throughout the profile. These outcomes indicate that the 10% RHA could potentially replace OPC due to its lower reduction in strength (8% at 90 days), which in turn lowers CO₂ emissions and promotes sustainability. Full article

(This article belongs to the Special Issue Sustainability and Performance of Cement-Based Materials)

► Show Figures

Figure 1

15 pages, 2369 KiB

Open AccessArticle

Optimization of the Sintering Densification, Microstructure, Mechanical Properties, and Oxidation Resistance of Tib₂–Tic–Sic Composite Ceramics via a Two-Step Method

by Fei Han, Wenzhou Sun, Youjun Lu, Junqing Ma and Shidiao Xu

Materials 2025, 18(14), 3297; https://doi.org/10.3390/ma18143297 (registering DOI) - 13 Jul 2025

Viewed by 112

Abstract

In this investigation, TiB₂–TiC composite powders, synthesized via the boron/carbon thermal reduction process, were employed as precursor materials. SiC, serving as the tertiary constituent, was incorporated to fabricate TiB₂–TiC–SiC composite ceramics utilizing spark plasma sintering technology. The present study [...] Read more.

In this investigation, TiB₂–TiC composite powders, synthesized via the boron/carbon thermal reduction process, were employed as precursor materials. SiC, serving as the tertiary constituent, was incorporated to fabricate TiB₂–TiC–SiC composite ceramics utilizing spark plasma sintering technology. The present study initially elucidates the densification mechanisms and investigates the influence of sintering temperature on the densification behavior, microstructural evolution, and mechanical properties of the resultant ceramics. The experimental findings reveal that the sintering process of TiB₂–TiC–SiC ceramics exhibits characteristics consistent with solid-phase sintering. As the sintering temperature escalates, both the relative density and mechanical properties of the ceramics initially improve, reaching a maximum at an optimal sintering temperature of 1900 °C, before subsequently declining. Microstructural examinations conducted at this optimal temperature indicate a homogeneous distribution of the two primary phases, with no evidence of excessive grain growth. Furthermore, this research explores the effects of SiC addition on the mechanical performance and oxidation resistance of TiB₂–TiC–SiC composite ceramics. The results demonstrate that the incorporation of SiC effectively suppresses grain growth and promotes the formation of rod-like TiB₂ microstructures, thereby enhancing the mechanical attributes of the ceramics. Additionally, the addition of SiC significantly improves the oxidation resistance of the composite ceramics compared to their TiB₂–TiC binary counterparts Full article

► Show Figures

Figure 1

27 pages, 3121 KiB

Open AccessReview

A Critical Review of Membrane Distillation Using Ceramic Membranes: Advances, Opportunities and Challenges

by Francesca Alessandro and Francesca Macedonio

Materials 2025, 18(14), 3296; https://doi.org/10.3390/ma18143296 (registering DOI) - 12 Jul 2025

Viewed by 24

Abstract

Membrane distillation (MD) has attracted increasing attention as a thermally driven separation process for water purification, desalination, and wastewater treatment. Its primary advantages include high rejection of non-volatile solutes, compatibility with low-grade or waste heat sources, and operation at ambient pressure. Despite these [...] Read more.

Membrane distillation (MD) has attracted increasing attention as a thermally driven separation process for water purification, desalination, and wastewater treatment. Its primary advantages include high rejection of non-volatile solutes, compatibility with low-grade or waste heat sources, and operation at ambient pressure. Despite these benefits, large-scale implementation remains limited due to the lack of membrane materials capable of withstanding harsh operating conditions and maintaining their hydrophobic character. Polymeric membranes have traditionally been used in MD applications; however, their limited thermal and chemical stability compromises long-term performance and reliability. In contrast, ceramic membranes are emerging as a promising alternative, offering superior mechanical strength, chemical resistance, and thermal stability. Nevertheless, their broader adoption in MD is hindered by several challenges, including high thermal conductivity, surface wettability, high fabrication costs, and limited scalability. This review provides a critical assessment of current developments, key opportunities, and ongoing challenges associated with the use of ceramic membranes in MD. Particular emphasis is placed on advances in surface modification techniques and the emerging applications in advanced MD configurations. Full article

(This article belongs to the Special Issue Next-Generation Membranes for Enhanced Membrane Distillation Performance)

► Show Figures

Figure 1

30 pages, 5062 KiB

Open AccessReview

State-of-the-Art Review of Studies on the Flexural Behavior and Design of FRP-Reinforced Concrete Beams

by Hau Tran, Trung Nguyen-Thoi and Huu-Ba Dinh

Materials 2025, 18(14), 3295; https://doi.org/10.3390/ma18143295 (registering DOI) - 12 Jul 2025

Viewed by 24

Abstract

Fiber-reinforced polymer (FRP) bars have great potential to replace steel bars in the design of reinforced concrete (RC) beams since they have numerous advantages such as high tensile strength and good corrosion resistance. Therefore, many studies including experiments and numerical simulations have focused [...] Read more.

Fiber-reinforced polymer (FRP) bars have great potential to replace steel bars in the design of reinforced concrete (RC) beams since they have numerous advantages such as high tensile strength and good corrosion resistance. Therefore, many studies including experiments and numerical simulations have focused on the behavior of FRP RC beams. In this paper, a comprehensive overview of previous studies is conducted to provide a thorough understanding about the behavior, the design, and the limitations of FRP RC beams. Particularly, experimental studies on FRP RC beams are collected and reviewed. In addition, the numerical analysis of FRP beams including the finite element (FE) analysis, the discrete element (DE) analysis, and artificial intelligence/machine learning (AI/ML) is summarized. Moreover, the international standards for the design of FRP RC beams are presented and evaluated. Through the review of previous studies, 93 tested specimens are collected. They can be a great source of reference for other studies. In addition, it has been found that the studies on the continuous beams and deep beams reinforced with FRP bars are still limited. In addition, more studies using DE analysis and AI/ML to analyze the response of FRP RC beams under loading conditions should be conducted. Full article

(This article belongs to the Section Construction and Building Materials)

► Show Figures

Figure 1

25 pages, 7859 KiB

Open AccessArticle

Methodology for the Early Detection of Damage Using CEEMDAN-Hilbert Spectral Analysis of Ultrasonic Wave Attenuation

by Ammar M. Shakir, Giovanni Cascante and Taher H. Ameen

Materials 2025, 18(14), 3294; https://doi.org/10.3390/ma18143294 (registering DOI) - 12 Jul 2025

Viewed by 28

Abstract

Current non-destructive testing (NDT) methods, such as those based on wave velocity measurements, lack the sensitivity necessary to detect early-stage damage in concrete structures. Similarly, common signal processing techniques often assume linearity and stationarity among the signal data. By analyzing wave attenuation measurements [...] Read more.

Current non-destructive testing (NDT) methods, such as those based on wave velocity measurements, lack the sensitivity necessary to detect early-stage damage in concrete structures. Similarly, common signal processing techniques often assume linearity and stationarity among the signal data. By analyzing wave attenuation measurements using advanced signal processing techniques, mainly Hilbert–Huang transform (HHT), this work aims to enhance the early detection of damage in concrete. This study presents a novel energy-based technique that integrates complete ensemble empirical mode decomposition with adaptive noise (CEEMDAN) and Hilbert spectrum analysis (HSA), to accurately capture nonlinear and nonstationary signal behaviors. Ultrasonic non-destructive testing was performed in this study on manufactured concrete specimens subjected to micro-damage characterized by internal microcracks smaller than 0.5 mm, induced through controlled freeze–thaw cycles. The recorded signals were decomposed from the time domain using CEEMDAN into frequency-ordered intrinsic mode functions (IMFs). A multi-criteria selection strategy, including damage index evaluation, was employed to identify the most effective IMFs while distinguishing true damage-induced energy loss from spurious nonlinear artifacts or noise. Localized damage was then analyzed in the frequency domain using HSA, achieving an up to 88% reduction in wave energy via Marginal Hilbert Spectrum analysis, compared to 68% using Fourier-based techniques, demonstrating a 20% improvement in sensitivity. The results indicate that the proposed technique enhances early damage detection through wave attenuation analysis and offers a superior ability to handle nonlinear, nonstationary signals. The Hilbert Spectrum provided a higher time-frequency resolution, enabling clearer identification of damage-related features. These findings highlight the potential of CEEMDAN-HSA as a practical, sensitive tool for early-stage microcrack detection in concrete. Full article

(This article belongs to the Section Construction and Building Materials)

► Show Figures

Figure 1

19 pages, 6209 KiB

Open AccessArticle

Structural and Thermal Effects of Beeswax Incorporation in Electrospun PVA Nanofibers

by Margarita P. Neznakomova, Fabien Salaün, Peter D. Dineff, Tsvetozar D. Tsanev and Dilyana N. Gospodinova

Materials 2025, 18(14), 3293; https://doi.org/10.3390/ma18143293 (registering DOI) - 12 Jul 2025

Viewed by 31

Abstract

This study presents the development and characterization of electrospun nanofibers composed of polyvinyl alcohol (PVA) and natural beeswax (BW). A stable emulsion containing 9 wt% PVA and 5 wt% BW was successfully formulated and electrospun. The effects of beeswax incorporation on solution properties-viscosity, [...] Read more.

This study presents the development and characterization of electrospun nanofibers composed of polyvinyl alcohol (PVA) and natural beeswax (BW). A stable emulsion containing 9 wt% PVA and 5 wt% BW was successfully formulated and electrospun. The effects of beeswax incorporation on solution properties-viscosity, conductivity, and surface tension—were systematically evaluated. Electrospinning was performed at 30 kV and a working distance of 14.5 cm, yielding nanofibers with diameters between 125 and 425 nm. Scanning electron microscopy (SEM) revealed increased surface roughness and diameter variability in PVA/BW fibers compared to the PVA. Fourier transform infrared spectroscopy (FTIR) confirmed physical incorporation of BW without evidence of chemical bonding. Thermogravimetric and differential scanning calorimetry analyses (TGA/DSC) demonstrated altered behavior and an expanded profile of temperature transitions due to the waxy components. The solubility test of the nanofiber mat in saline indicated that BW slows dissolution and improves the structural integrity of the fibers. This study demonstrates, for the first time, the incorporation of beeswax into electrospun PVA nanofibers with improved structural and thermal properties, indicating potential for further exploration in biomedical material design. Full article

► Show Figures

Graphical abstract

19 pages, 1302 KiB

Open AccessArticle

Low-Carbon, Low-Shrinkage Concrete Design Based on Paste–Aggregate Binary Model

by Chunming Lian, Xiong Zhang, Lu Han, Weijun Wen, Wenbiao Lin and Lifang Han

Materials 2025, 18(14), 3292; https://doi.org/10.3390/ma18143292 (registering DOI) - 12 Jul 2025

Viewed by 20

Abstract

This study presents a performance-based concrete mix design methodology rooted in the paste–aggregate binary framework, aiming to reduce binder content while ensuring optimal workability and strength. We found that inter-particle spacing (SPT) and paste rheology jointly govern fresh concrete behavior, with slump increasing [...] Read more.

This study presents a performance-based concrete mix design methodology rooted in the paste–aggregate binary framework, aiming to reduce binder content while ensuring optimal workability and strength. We found that inter-particle spacing (SPT) and paste rheology jointly govern fresh concrete behavior, with slump increasing nonlinearly with SPT and a critical transition zone around 20–35 µm; paste yield stress controls slump, while plastic viscosity governs segregation resistance. A two-level strength model was developed to predict concrete strength from paste properties based on compactness and hydration (R² = 0.90). Fixing SPT at 25 µm was identified as optimal for achieving balanced flowability with minimal paste volume. This approach effectively decouples aggregate packing optimization from paste calibration, offering a physically interpretable and practical framework for designing sustainable, low-carbon, and low-shrinkage concrete. Full article

(This article belongs to the Special Issue Construction Materials: Innovations in Recyclable, Low-Carbon, and Durable Solutions)

► Show Figures

Figure 1

14 pages, 7269 KiB

Open AccessArticle

A Study on Three-Dimensional Flexible Mesh Influence on the Stability of Reserved Tunnels in Cemented Backfill

by Xiaosheng Liu, Weijun Wang and Hao Li

Materials 2025, 18(14), 3291; https://doi.org/10.3390/ma18143291 (registering DOI) - 12 Jul 2025

Viewed by 13

Abstract

Ordinary backfill has characteristics such as low compressive strength, low tensile strength, and easy bending, which cannot meet the stability requirements of reserved tunnels, but three-dimensional flexible mesh can be added to improve it. In this paper, mechanical characteristics and displacement were taken [...] Read more.

Ordinary backfill has characteristics such as low compressive strength, low tensile strength, and easy bending, which cannot meet the stability requirements of reserved tunnels, but three-dimensional flexible mesh can be added to improve it. In this paper, mechanical characteristics and displacement were taken as the evaluation index, an optimal three-dimensional flexible mesh was studied by a laboratory experiment of small samples, then backfill with a reserved roadway was used to carry out a large-sample similarity simulation experiment, and finally, a numerical simulation was carried out. The research shows that the three-dimensional flexible mesh had a strengthening effect on the backfill, especially on the tensile strength and shear strength of the backfill. The strengths increased by 1.57~2.00 times and 2.00~2.56 times, respectively. After backfill is damaged by external forces, three-dimensional flexible mesh can also hinder the detachment of backfill fragments. The effect of the three-dimensional flexible mesh on the backfill under static pressure was calculated by using numerical simulation, and it was found that the three-dimensional flexible mesh played an effective support role for the roadway inside the backfill, effectively reducing the displacement of the roadway roof by 21.43% and the strain energy by 40%. Full article

(This article belongs to the Special Issue Sustainable Materials for Engineering Applications)

► Show Figures

Figure 1

12 pages, 1298 KiB

Open AccessArticle

Effect of Deuteration on the Temperature Dependence of the Quadratic Electro-Optic Effect in KDP Crystals

by Marek Izdebski and Rafał Ledzion

Materials 2025, 18(14), 3290; https://doi.org/10.3390/ma18143290 (registering DOI) - 12 Jul 2025

Viewed by 14

Abstract

The results of precise measurements of the temperature dependencies of quadratic electro-optic coefficients, namely

g_{1111} - g_{1122}

and

n_{o}^{3} g_{1111} - n_{e}^{3} g_{3311}

, in KH₂PO₄ (KDP) and KD₂PO₄ [...] Read more.

The results of precise measurements of the temperature dependencies of quadratic electro-optic coefficients, namely

g_{1111} - g_{1122}

and

n_{o}^{3} g_{1111} - n_{e}^{3} g_{3311}

, in KH₂PO₄ (KDP) and KD₂PO₄ (DKDP) crystals at a wavelength of 632.8 nm are presented. We consider electro-optic coefficients describing changes in the optical impenetrability tensor resulting from an applied electric field, as well as intrinsic electro-optic coefficients defined in terms of induced polarization. The results show significant differences in the values of the analogous coefficients for the KDP and DKDP crystals and their temperature dependencies. Therefore, the quadratic electro-optic effect in KDP-type crystals cannot be easily described based solely on the contribution of PO₄ tetrahedra, as assumed in current models of the linear effect. Moreover, the values of the intrinsic coefficients in the KDP and DKDP crystals differ even more than the corresponding usual electro-optic coefficients, which contradicts the conventional belief in their lower variability. Full article

(This article belongs to the Section Optical and Photonic Materials)

► Show Figures

Figure 1

24 pages, 5337 KiB

Open AccessArticle

A Piezoelectric-Actuated Variable Stiffness Miniature Rotary Joint

by Yifan Lu, Yifei Yang, Xiangyu Ma, Ce Chen, Tong Qin, Honghao Yue and Siqi Ma

Materials 2025, 18(14), 3289; https://doi.org/10.3390/ma18143289 - 11 Jul 2025

Viewed by 37

Abstract

With the acceleration of industrialization, deformable mechanisms that can adapt to complex environments have gained widespread applications. Joints serve as carriers for transmitting forces and motions between components, and their stiffness significantly influences the static and dynamic characteristics of deformable mechanisms. A variable [...] Read more.

With the acceleration of industrialization, deformable mechanisms that can adapt to complex environments have gained widespread applications. Joints serve as carriers for transmitting forces and motions between components, and their stiffness significantly influences the static and dynamic characteristics of deformable mechanisms. A variable stiffness joint is crucial for ensuring the safety and reliability of the system, as well as for enhancing environmental adaptability. However, existing variable stiffness joints fail to meet the requirements for miniaturization, lightweight construction, and fast response. This paper proposes a piezoelectric-actuated variable stiffness miniature rotary joint featuring a compact structure, monitorable loading state, and rapid response. Given that the piezoelectric stack expands and contracts when energized, this paper proposes a transmission principle for stiffness adjustment by varying the pressure and friction between active and passive components. This joint utilizes a flexible hinge mechanism for displacement amplification and incorporates a torque sensor based on strain monitoring. A static model is developed based on piezoelectric equations and displacement amplification characteristics, and simulations confirm the feasibility of the stiffness adjustment scheme. The mechanical characteristics of various flexible hinge structures are analyzed, and the effects of piezoelectric actuation capability and external load on stiffness adjustment are examined. The experimental results demonstrate that the joint can adjust stiffness, and the sensor is calibrated using the least squares algorithm to monitor the stress state of the joint in real time. Full article

(This article belongs to the Special Issue Advanced Design and Synthesis in Piezoelectric Smart Materials)

► Show Figures

Figure 1

24 pages, 6254 KiB

Open AccessArticle

Cleaner Production of Metallurgical-Grade Iron from High-Iron Bauxite Residue via Smelting Reduction: Thermodynamic Control, Industrial Application Potential, and Slag Utilization Strategy

by Kun Wang, Ting-An Zhang, Zhi-He Dou, Yan Liu and Guo-Zhi Lv

Materials 2025, 18(14), 3288; https://doi.org/10.3390/ma18143288 - 11 Jul 2025

Viewed by 33

Abstract

Iron-rich bauxite residue (red mud) is a hazardous alkaline solid waste produced during the production of alumina from high-iron bauxite, which poses severe environmental challenges due to its massive stockpiling and limited utilization. In this study, metallic iron was recovered from high-iron red [...] Read more.

Iron-rich bauxite residue (red mud) is a hazardous alkaline solid waste produced during the production of alumina from high-iron bauxite, which poses severe environmental challenges due to its massive stockpiling and limited utilization. In this study, metallic iron was recovered from high-iron red mud using the smelting reduction process. Thermodynamic analysis results show that an increase in temperature and sodium oxide content, along with an appropriate mass ratio of Al₂O₃ to SiO₂ (A/S) and mass ratio of CaO to SiO₂ (C/S), contribute to the enhancement of the liquid phase mass fraction of the slag. During the smelting reduction process of high-iron red mud, iron recoveries for low-alkali high-iron red mud and high-alkali high-iron red mud under optimal conditions were 98.14% and 98.36%, respectively. The metal obtained through reduction meets the industrial standard for steel-making pig iron, which is also confirmed in the pilot-scale experiment. The smelting reduction process of high-iron red mud can be divided into two stages, where the reaction is predominantly governed by interfacial chemical reaction and diffusion control, respectively. The apparent activation energy of high-alkali high-iron red mud is lower than that observed for low-alkali high-iron red mud. The reduced slag can be used as a roadside stone material or cement clinker. This proposed method represents a sustainable process for the comprehensive utilization of high-iron red mud, which also promotes the minimization of red mud. Full article

(This article belongs to the Special Issue Advances in Efficient Utilization of Metallurgical Solid Waste)

► Show Figures

Figure 1

18 pages, 3388 KiB

Open AccessArticle

Influence of Material Optical Properties in Direct ToF LiDAR Optical Tactile Sensing: Comprehensive Evaluation

by Ilze Aulika, Andrejs Ogurcovs, Meldra Kemere, Arturs Bundulis, Jelena Butikova, Karlis Kundzins, Emmanuel Bacher, Martin Laurenzis, Stephane Schertzer, Julija Stopar, Ales Zore and Roman Kamnik

Materials 2025, 18(14), 3287; https://doi.org/10.3390/ma18143287 - 11 Jul 2025

Viewed by 36

Abstract

Optical tactile sensing is gaining traction as a foundational technology in collaborative and human-interactive robotics, where reliable touch and pressure feedback are critical. Traditional systems based on total internal reflection (TIR) and frustrated TIR (FTIR) often require complex infrared setups and lack adaptability [...] Read more.

Optical tactile sensing is gaining traction as a foundational technology in collaborative and human-interactive robotics, where reliable touch and pressure feedback are critical. Traditional systems based on total internal reflection (TIR) and frustrated TIR (FTIR) often require complex infrared setups and lack adaptability to curved or flexible surfaces. To overcome these limitations, we developed OptoSkin—a novel tactile platform leveraging direct time-of-flight (ToF) LiDAR principles for robust contact and pressure detection. In this extended study, we systematically evaluate how key optical properties of waveguide materials affect ToF signal behavior and sensing fidelity. We examine a diverse set of materials, characterized by varying light transmission (82–92)%, scattering coefficients (0.02–1.1) cm⁻¹, diffuse reflectance (0.17–7.40)%, and refractive indices 1.398–1.537 at the ToF emitter wavelength of 940 nm. Through systematic evaluation, we demonstrate that controlled light scattering within the material significantly enhances ToF signal quality for both direct touch and near-proximity sensing. These findings underscore the critical role of material selection in designing efficient, low-cost, and geometry-independent optical tactile systems. Full article

(This article belongs to the Section Polymeric Materials)

► Show Figures

Figure 1

22 pages, 260894 KiB

Open AccessArticle

Effects of Aging on Mode I Fatigue Crack Growth Characterization of Double Cantilever Beam Specimens with Thick Adhesive Bondline for Marine Applications

by Rahul Iyer Kumar and Wim De Waele

Materials 2025, 18(14), 3286; https://doi.org/10.3390/ma18143286 - 11 Jul 2025

Viewed by 29

Abstract

The use of adhesive joints in naval applications requires a thorough understanding of their fatigue performance. This paper reports on the fatigue experiments performed on double cantilever beam specimens with thick adhesive bondline manufactured under shipyard conditions. The specimens have an initial crack [...] Read more.

The use of adhesive joints in naval applications requires a thorough understanding of their fatigue performance. This paper reports on the fatigue experiments performed on double cantilever beam specimens with thick adhesive bondline manufactured under shipyard conditions. The specimens have an initial crack at the steel–adhesive interface and are tested in unaged, salt-spray-aged and immersion-aged conditions to determine the interface mode I fatigue properties. The strain energy release rate is calculated using the Kanninen–Penado model, and the fatigue crack growth curve is determined using a power law model. The crack growth rate slope for salt-spray-aged specimens is 16.5% lower than for unaged specimens, while that for immersion-aged specimens is 66.1% lower and is shown to be significantly different. The fracture surfaces are analyzed to identify the failure mechanisms and the influence of the aging process on the interface properties. Since the specimens are manufactured under shipyard conditions, the presence of voids and discontinuities in the adhesive bondline is observed and as a result leads to scatter. Hence, Bayesian linear regression is performed in addition to the ordinary least squares regression to account for the scatter and provide a distribution of plausible values for the power law coefficients. The results highlight the impact of aging on the fatigue property, underscoring the importance of considering environmental effects in the qualification of such joints for marine applications. Full article

(This article belongs to the Special Issue Advances in the Experimentation and Numerical Modeling of Material Joining Processes (Second Edition))

► Show Figures

Figure 1

17 pages, 9466 KiB

Open AccessArticle

Effect of Post-Weld Heat Treatment on Residual Stress and Fatigue Crack Propagation Behavior in Linear Friction Welded Ti-6Al-4V Alloy

by Sungkyoung Lee, Hyunsung Choi, Yunji Cho, Min Jae Baek, Hyeonil Park, Moo-Young Seok, Yong Nam Kwon, Namhyun Kang and Dong Jun Lee

Materials 2025, 18(14), 3285; https://doi.org/10.3390/ma18143285 (registering DOI) - 11 Jul 2025

Viewed by 33

Abstract

In this study, the effects of post-weld heat treatment (PWHT) on residual stress distribution and fatigue crack propagation (FCP) behavior in linear friction welded (LFW) Ti-6Al-4V joints were investigated. Microstructural evolution in the weld center zone (WCZ), thermomechanically affected zone (TMAZ), heat-affected zone [...] Read more.

In this study, the effects of post-weld heat treatment (PWHT) on residual stress distribution and fatigue crack propagation (FCP) behavior in linear friction welded (LFW) Ti-6Al-4V joints were investigated. Microstructural evolution in the weld center zone (WCZ), thermomechanically affected zone (TMAZ), heat-affected zone (HAZ), and base metal (BM) was characterized using scanning electron microscropy (SEM) and electron backscatter diffraction (EBSD). Mechanical properties were evaluated via Vickers hardness testing and digital image correlation (DIC)-based tensile testing. Residual stresses before and after PWHT were measured using the contour method. The LFW process introduced significant residual stresses, with tensile stresses up to 709.2 MPa in the WCZ, resulting in non-uniform fatigue crack growth behavior. PWHT at 650 °C and 750 °C effectively reduced these stresses. After PWHT, fatigue cracks propagated uniformly across the weld region, enabling reliable determination of crack growth rates. The average crack growth rates of the heat-treated specimens were comparable to those of the base metal, confirming that PWHT, particularly at 750 °C, stabilizes the fatigue crack path and relieves internal stress. Full article

(This article belongs to the Section Metals and Alloys)

► Show Figures

Figure 1

22 pages, 11082 KiB

Open AccessArticle

Exploring the Impact of Inter-Layer Structure on Glass Fiber-Poplar Composite Board: Mechanical and Thermal Properties Analysis

by Jiong Zhang, Shurui Liu, Jinpeng Li, Jixuan Wang, Haoyu Bai, Peng Wei and Tian Liu

Materials 2025, 18(14), 3284; https://doi.org/10.3390/ma18143284 - 11 Jul 2025

Viewed by 28

Abstract

This study presents the design and fabrication of a glass fiber–poplar veneer composite plate, investigating how varying interlayer configurations of glass fiber (single- and double-layer) and the arrangement of poplar veneer layers (odd and even) impact the mechanical and thermal insulation characteristics of [...] Read more.

This study presents the design and fabrication of a glass fiber–poplar veneer composite plate, investigating how varying interlayer configurations of glass fiber (single- and double-layer) and the arrangement of poplar veneer layers (odd and even) impact the mechanical and thermal insulation characteristics of these composite plates. Compared to plywood made from natural wood, glass fiber significantly improved the properties of fast-growing poplar plywood. The highest impact strength increased by 3.62 times, while the flexural strength increased by 26.22% and the tensile strength by 29.66%. The thermal diffusion coefficient of the experimental group decreased by 40.74%, indicating better insulation. Interestingly, single-layer glass fiber is superior to a double-layer structure in terms of thermal insulation. An optimal interlayer structure was identified, comprising one veneer layer between two layers of glass fiber cloth, repeated three times. Abaqus 2019 was used for finite element analysis (FEA). The simulation results agree with the experimental data to within 5%. These findings confirm the importance of structural configuration in determining the properties of composite materials, providing a theoretical basis for the structural design of fiber–reinforced composite materials. Full article

(This article belongs to the Special Issue On the Residual Strength and Damage Identification of Damaged Composite Structure)

► Show Figures

Figure 1

20 pages, 8257 KiB

Open AccessArticle

Tribological Performance of Bronze Engineering Materials with Environmentally Friendly Lubricants Under Starved Lubrication Conditions

by Marcin Kowalski, Kasper Górny, Szymon Bernat, Arkadiusz Stachowiak, Jacek Wernik and Wiesław Zwierzycki

Materials 2025, 18(14), 3283; https://doi.org/10.3390/ma18143283 - 11 Jul 2025

Viewed by 21

Abstract

This article demonstrated that environmentally friendly lubricants—glycerol–water-based oil (GWB) and rapeseed oil-based oil (RSB)—would provide comparable conditions (wear of node components, friction resistance) in a friction node as a commercial semi-synthetic gear oil (REF). Wear tests were performed on a block-on-ring model friction [...] Read more.

This article demonstrated that environmentally friendly lubricants—glycerol–water-based oil (GWB) and rapeseed oil-based oil (RSB)—would provide comparable conditions (wear of node components, friction resistance) in a friction node as a commercial semi-synthetic gear oil (REF). Wear tests were performed on a block-on-ring model friction node stand using GBZ12 (CuSn12), BA1032 (CuAl10Fe3Mn2), and BA1054 (CuAl10Ni5Fe4) bronze samples. Glycerol–water-based oil (GWB) significantly reduced the wear of the samples by several times, compared to semi-synthetic oil (REF) and rapeseed oil-based oil (RSB). The (GWB) oil also provided a stable friction coefficient value at the lowest level of 0.05–0.06. The main disadvantage of the (RSB) oil was the temporary fluctuation of the friction coefficient value (increase above 0.1), which indicated the lack of stability of the boundary layer. The results highlight the potential of (GWB) oil in reducing wear and stabilizing friction under extreme conditions, supporting the shift toward sustainable lubricants in industrial applications. Full article

(This article belongs to the Special Issue Engineering Materials: Friction, Wear and Damage)

► Show Figures

Figure 1

17 pages, 3331 KiB

Open AccessArticle

Strength Enhancement of Clay Through Lime–Sand Stabilization at Various Remolding Water Contents

by Shuai Qi, Jinhui Liu, Wei Ma and Jing Wang

Materials 2025, 18(14), 3282; https://doi.org/10.3390/ma18143282 - 11 Jul 2025

Viewed by 28

Abstract

During the construction of subgrade, the remolding water content w of lime–sand-stabilized clay usually varies in a wide range, leading to inconsistent effectiveness in strength enhancement. Until now, this aspect has not been investigated. In this study, an unconfined compression test and microscopic [...] Read more.

During the construction of subgrade, the remolding water content w of lime–sand-stabilized clay usually varies in a wide range, leading to inconsistent effectiveness in strength enhancement. Until now, this aspect has not been investigated. In this study, an unconfined compression test and microscopic observation were carried out on clay and stabilized clay (adding 4% lime by mass and 50% sand by volume). The results show the following: (1) remolding water content w had a strong effect on the soil fabrics of pure clay and lime-stabilized clay. An increase in the w from the dry to wet side of optimum reduced matric suction, which diminished the aggregation effect among fine-grained particles in both clay and lime-stabilized clay. Correspondingly, fine-grained aggregate progressively disintegrated, and dispersed fine-grained particles increased. As a result, the w increment at w ≤ w_cha made the dispersed fine-grained particles successively fill the large pores between aggregates, densifying the soil fabric. In contrast, at w > w_cha, the ongoing disintegration of aggregate resulted in progressive structural weakening. Herein, w_cha was defined as the characteristic water content at which the soil fabric transitioned from structural densification to weakening. (2) The UCS of both pure clay and lime–sand-stabilized clay followed a bell-shaped pattern as the w increased, with w_cha acting as the turning point. For pure clay soils, the UCS increased with increasing w up to w_cha because of structural densification, but decreased beyond w_cha due to structural weakening. In lime–sand-stabilized clay, where a sand grain skeleton developed, the compression of lime-stabilized clay induced by the movement of sand grains during shearing activated its contribution to the overall strength. The compressive capacity of the lime-stabilized clay varied in a bell-shaped manner with w, and this trend was mirrored in the UCS of lime–sand-stabilized clay. (3) At a low w, the fact that the clay aggregate exhibited sand-like mechanical behavior reduced the effectiveness of incorporating sand and lime for enhancing the UCS. As the w increased at w ≤ w_cha, the breakdown of aggregates enlarged the distinction between pure clay and sand, resulting in a more pronounced improvement in the UCS with the addition of sand and lime. At w > w_cha, the lubrication effect occurring at the contact between sand grains diminished the interlocking between the sand grains. Consequently, the effectiveness of the UCS enhancement decreased. Full article

(This article belongs to the Section Construction and Building Materials)

► Show Figures

Figure 1

21 pages, 3088 KiB

Open AccessArticle

Mechanical Properties of Recycled Concrete with Carbide Slag Slurry Pre-Immersed and Carbonated Recycled Aggregate

by Xiangfei Wang, Guoliang Guo, Jinglei Liu, Chun Lv and Mingyan Bi

Materials 2025, 18(14), 3281; https://doi.org/10.3390/ma18143281 - 11 Jul 2025

Viewed by 27

Abstract

This research focuses on improving the characteristics of recycled concrete and utilizing solid waste resources through the combination of industrial waste pre-impregnation and the carbonation process. A novel pre-impregnation–carbonation aggregate method is proposed to increase the content of carbonatable components in the surface-bonded [...] Read more.

This research focuses on improving the characteristics of recycled concrete and utilizing solid waste resources through the combination of industrial waste pre-impregnation and the carbonation process. A novel pre-impregnation–carbonation aggregate method is proposed to increase the content of carbonatable components in the surface-bonded mortar of recycled coarse aggregate by pre-impregnating it with carbide slag slurry (CSS). This approach enhances the subsequent carbonation effect and thus the properties of recycled aggregates. The experimental results showed that the method significantly improved the water absorption, crushing value, and apparent density of the recycled aggregate. Additionally, it enhanced the compressive strength, split tensile strength, and flexural strength of the recycled concrete produced using the aggregate improved by this method. Microanalysis revealed that CO₂ reacts with calcium hydroxide and hydrated calcium silicate (C-S-H) to produce calcite-type calcium carbonate and amorphous silica gel. These reaction products fill microcracks and pores on the aggregate and densify the aggregate–paste interfacial transition zone (ITZ), thereby improving the properties of recycled concrete. This study presents a practical approach for the high-value utilization of construction waste and the production of low-carbon building materials by enhancing the quality of recycled concrete. Additionally, carbon sequestration demonstrates broad promise for engineering applications. Full article

(This article belongs to the Section Construction and Building Materials)

► Show Figures

Graphical abstract

21 pages, 5109 KiB

Open AccessArticle

Influence Mechanism of Waterborne Polyurethane on the Properties of Emulsified Asphalt

by Jian Tan, Shuguang Hou, Rui Jin, Xiao Zhong and Xiaoxi Zou

Materials 2025, 18(14), 3280; https://doi.org/10.3390/ma18143280 - 11 Jul 2025

Viewed by 28

Abstract

To elucidate the modification mechanism of waterborne polyurethane (WPU) on emulsified asphalt, anionic and cationic WPUs are utilized as modifiers. As well, their effects on physical properties, microstructure, and compatibility are characterized using basic performance tests, Fourier transform infrared spectroscopy (FTIR), and atomic [...] Read more.

To elucidate the modification mechanism of waterborne polyurethane (WPU) on emulsified asphalt, anionic and cationic WPUs are utilized as modifiers. As well, their effects on physical properties, microstructure, and compatibility are characterized using basic performance tests, Fourier transform infrared spectroscopy (FTIR), and atomic force microscopy (AFM). The results show that WPU-modified emulsified asphalt exhibited a higher softening point, reduced penetration, and decreased ductility, suggesting enhanced high-temperature stability but diminished low-temperature flexibility. Among all samples, the combination of cationic WPU with cationic emulsified asphalt shows the highest softening point (54.1 °C), whereas cationic emulsified asphalt alone exhibits the lowest one (52.9 °C). Anionic emulsified asphalt demonstrates the highest penetration (79 mm), while non-ionic WPU combined with cationic emulsified asphalt shows the lowest one (59.3 mm). The ductility decreases from 90.3 cm to 28.7 cm. The storage stability varies with WPU ion type. Cationic WPU-modified samples showed the poorest storage stability (0.7% residue), while anionic-modified samples exhibit the best one (0.4% residue). FTIR analysis confirms the presence of characteristic WPU absorption peaks, indicating that physical blending occurs, and chemical interaction is limited. AFM observations reveal that anionic WPUs provide superior compatibility, forming fine, uniformly distributed particles with the lowest surface roughness (5.655 nm). In contrast, cationic WPUs form chain-like structures that cure effectively but exhibit poor dispersion. This study provides a basis for the development of high-performance WPU-modified emulsified asphalt. Full article

(This article belongs to the Special Issue Sustainable Pavement Materials: Design, Application and Performance Evaluation)

► Show Figures

Figure 1

Journal Menu

Journal Browser

Materials, Volume 18, Issue 14 (July-2 2025) – 101 articles

Further Information

Guidelines

MDPI Initiatives

Follow MDPI