Search Results (155)

To study the fracture performance of the concrete-epoxy mortar interface (CEMI) pre-coated with epoxy solutions with different concentrations, a total of nine specimens were fabricated to be subjected to four-point bending tests. DIC technology was used to monitor the deformation of the pure bending region of specimens. A triple-fold stiffness model was developed based on the test results of applied load–displacement curves. A generalized method for determining the parameters of the bilinear softening model was proposed and validated by the test results. Additionally, the fracture performance and crack extension of CEMI specimens were deeply analyzed using the double-K fracture criterion. The fracture initiation toughness K_ICⁱⁿⁱ was calculated by introducing the cohesive fracture toughness, and the crack extension resistance K_R curves of the CEMI specimens were calculated by combining the linear-elastic fracture mechanics and the proposed bilinear softening model. It was indicated that the initiation locations and extension paths of interfacial cracks could be effectively identified by the DIC technique, with an error of less than 8% between test results and predictions. The bridging effect was strengthened by pre-coating with an epoxy solution of the CEMI specimens by filling the microscopic defects on the concrete surface, thereby improving K_ICⁱⁿⁱ, delaying unstable crack extension, and enhancing interfacial fracture resistance. Full article

This study investigates the thermo-mechanical performance of fused filament fabrication (FFF)-printed polyphthalamide reinforced with 15 wt.% short carbon fibers (PPA CF15) for engineering applications under elevated temperature and cyclic loading conditions. The material was characterized by quasi-static tensile testing, fatigue testing, dynamic mechanical analysis (DMA), scanning electron microscopy (SEM), and finite element analysis (FEA). Tensile tests performed from 20 to 180 °C revealed a strong temperature-dependent reduction in mechanical properties: the elastic modulus decreased from 2.437 to 0.401 GPa, while the ultimate tensile strength decreased from 64.537 to 9.190 MPa. In contrast, elongation at break generally increased with temperature, indicating a transition toward more ductile deformation governed by thermal softening of the polymer matrix. Fatigue tests showed reduced fatigue resistance at higher temperatures and stress levels; however, stable cyclic performance was achieved when the applied stress remained below approximately 60–70% of the ultimate tensile strength, with several specimens reaching 10⁶ cycles. DMA confirmed the viscoelastic nature of PPA CF15 and enabled the construction of frequency–temperature superposition master curves for numerical modelling. SEM observations revealed increased matrix deformation and fiber pull-out at elevated temperatures. FEA of an automotive intake manifold (IM) case study demonstrated that experimentally derived material data can be used to predict deformation, stress redistribution, and viscoelastic stabilization under combined thermal and mechanical loading. The results indicate that FFF-printed PPA CF15 is a promising lightweight composite for thermally and mechanically demanding automotive applications. Full article

This study presents the fabrication of a Cr₃C₂-25NiCr/NiCr coating on Co₃W₃ steel utilizing high-velocity oxygen fuel (HVOF) spraying. The effects of the vacuum heat treatment process on the microstructures, mechanical properties, and wear mechanisms of the coating were systematically analyzed. The results indicated that the microstructure became denser following heat treatment. During the spraying procedure, decarburization resulted in transformation of the metastable phase structure into a stable one. In comparison to the sprayed coating, there was a 93.8% reduction in porosity. The precipitation of nano-secondary carbides shifted the mechanism of solid-solution strengthening to precipitation strengthening, resulting in a 29.1% increase in microhardness. Meanwhile, the thermal softening effect led to a 114.3% increase in fracture toughness. Wear experiments demonstrated that the friction-induced amorphous structure effectively mitigated stress concentration and inhibited crack initiation. The polycrystalline interface transition region between the nano-secondary carbides and the matrix facilitated the shedding of nano-secondary carbides, forming abrasive particles that generated a rolling effect, which significantly reduced the coefficient of friction. The semi-coherent interface between secondary carbides and NiCr decreased the interfacial energy and enhanced the bonding strength, effectively preventing the shedding of carbides during the wear process. Consequently, a dense microstructure, the type of interface, and high hardness and toughness were critical factors in enhancing its wear resistance. Full article

To investigate the shear performance of steel fiber-reinforced high-strength concrete (SFRHSC) corbels subjected to concentrated loading, an experimental program was executed on six specimens featuring welded anchorage for the upper longitudinal reinforcement. The control variables included shear span-to-depth ratios of 0.2 to 0.5 and steel fiber volume fractions of 0%, 0.75%, and 1.50%. During the testing phase, strain evolution within the steel reinforcement and concrete matrix was monitored to analyze the structural deformation sequence and ultimate failure modes. Anchored in the Mohr–Coulomb failure criterion and the foundational strut-and-tie model (STM) framework, a softened strut-and-tie calculation approach for corbel shear capacity was formulated; this method explicitly accounts for the softening effect of the steel fiber-reinforced concrete (SFRC) and incorporates a size effect correction. The established shear capacity calculation model, alongside STM-based provisions from ACI 318-19, EN 1992-1-1, and CSA A23.3-19, was deployed to forecast the shear capacities of the six fabricated specimens and 18 additional units sourced from existing literature. Ultimately, a rigorous comparative analysis was conducted between the theoretical predictions generated by each method and the empirical test data. The results indicate that the failure process of the SFRHSC corbels primarily involves three distinct stages: initial cracking, through cracking, and ultimate failure. The addition of steel fibers can alleviate stress concentration at cracks and limit crack growth, thereby improving the tensile performance of the cracked concrete. Test results indicate that the strain in the longitudinal tensile reinforcement increased with the shear span-to-depth ratio but decreased as the steel fiber volume fraction increased. At the point of specimen failure, all longitudinal tensile reinforcement had reached the yield strength, while the horizontal stirrups only partially yielded. The concrete strain distribution across the normal section of the corbel did not follow the plane section assumption. Furthermore, incorporating steel fibers increased both the cracking load and the ultimate load of the corbel normal sections. The mean value of the experimental-to-predicted ratios obtained from the STM provisions of various international codes was 1.453, with a variance of 0.029, indicating conservative calculation results. In contrast, the mean value of the experimental-to-predicted ratios for the calculation model developed in this study was 1.048, with a variance of 0.004, demonstrating closer agreement with the experimental results and less dispersion. Simultaneously, by explicitly considering the softening effect in SFRHSC and the size effect, it provides a better prediction for the shear capacity of corbels. Full article

Sustainable production in dental and orthodontic 3D printing has gained increasing attention due to environmental concerns and the need for cost-effective and resource-saving solutions. This study presents a proof of concept for using recycled polymers and fused granular fabrication (FGF) in a closed-loop 3D printing approach, omitting intermediate filament manufacturing. A desktop 3D printer served as the kinematic platform and was modified with a pellet-based extruder to directly process recycled polyethylene terephthalate glycol (PETG) flakes, obtained by shredding previously printed PETG parts, into dental models. Dimensional accuracy was evaluated using optical 3D scanning analysis. The results indicate that models produced from recycled PETG are, in principle, suitable for dental and orthodontic applications within the investigated scope. This technical note provides initial evidence supporting the integration of recycled thermoplastics into dental and orthodontic model fabrication as part of sustainable additive manufacturing workflows. Potential pathways for workflow integration in clinical and laboratory environments, as well as directions for future research, are outlined, including the optimization of printing parameters and process stability. The main technical challenges were unreliable feedstock flow, causing bridging and jamming, while thermal creep from insufficient inlet cooling promoted premature softening of the flakes, causing torque spikes and unstable feeding. Full article

The produced water from the No. 1 Oil Production Plant of Xinjiang Oilfield is rich in divalent ions, including Ca²⁺, Mg²⁺, and SO₄²⁻, leading to extremely high scaling tendency that fails to meet the reinjection standard. Therefore, highly efficient water softening technology is urgently required for such wastewater treatment. In this study, a novel negatively charged nanofiltration (NF) membrane was fabricated via interfacial polymerization using 2-carboxypiperazine and trimesoyl chloride as monomers. The membrane was systematically characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared spectroscopy (FTIR), and its rejection performance was investigated under various conditions. Results show that the maximum rejection rates of the NF membrane reached 99% for SO₄²⁻, 81% for Ca²⁺, and 94% for Mg²⁺, respectively. With increasing ion concentration, the removal efficiencies of Ca²⁺ and Mg²⁺ decreased, while that of SO₄²⁻ increased slightly. Higher operating pressure significantly enhanced both ion removal and membrane flux, which was mainly attributed to the synergistic effects of Donnan electrostatic exclusion, membrane surface adsorption, and mass transfer resistance. When applied to treat real produced water from the No. 1 Oil Production Plant, the membrane achieved 100% removal of SO₄²⁻, and 91% and 95% removal of Ca²⁺ and Mg²⁺, respectively. The scaling tendency of the treated effluent was completely eliminated. This work provides theoretical and technical support for the engineering application of nanofiltration technology in oilfield wastewater treatment. Full article

Sandwich structures are increasingly employed in high-performance applications due to their excellent strength-to-weight ratio. However, their mechanical reliability often depends on the structural core, which remains susceptible to failure under shear and flexural loads. Additive manufacturing (AM) enables the design and fabrication of complex, bio-inspired core architectures, such as those derived from Voronoi tessellations, which can potentially enhance energy absorption and mechanical performance. This study investigates the mechanical behavior of PLA-based cellular cores, produced via Fused Filament Fabrication (FFF), under quasi-static and intermediate strain rates (up to 33 s⁻¹). Two infill geometries were compared: a standard cubic pattern and an open Voronoi-based structure inspired by biological morphologies. The results demonstrate strain-rate sensitivity in both configurations, characterized by increased stiffness and peak stress at higher loading rates. While the Voronoi structure exhibited lower maximum strength compared to the cubic pattern, it demonstrated a more gradual post-peak softening, indicating potentially superior energy dissipation capabilities. These findings support the potential of bio-inspired, additively manufactured structures in energy-absorbing applications. Full article

This paper proposes the use of laser-assisted cutting technology to control the brittle–plastic transition of single-crystal CaF₂ through local thermal softening, thereby suppressing its processing anisotropy. Nano-scratch experiments show that heating significantly increases the critical plastic cutting depth of each crystal plane and reduces the inter-plane differences. Based on this, laser-assisted ultra-precision turning was used to fabricate CaF₂ optical microcavities with a surface roughness below 10 nm, achieving a maximum quality factor of ~7.79 × 10⁷, and significantly reducing the performance differences among different crystal orientations. The research indicates that this method can effectively promote uniform plastic flow on each crystal plane, providing an effective approach for the high-performance and consistent fabrication of anisotropic brittle optical components. Full article

Granular materials exhibit complex mechanical behaviors during unloading, yet the underlying micro- and meso-scale mechanisms remain unclear. This study employs a discrete element method to simulate a series of triaxial tests on sand and pebble specimens with varying initial densities under different unloading stress paths. While dense specimens demonstrate strain softening and dilatancy, loose samples exhibit shear contraction. To quantify the underlying fabric evolution, persistent homology (PH) theory is adopted to analyze the particle contact force networks. The results reveal that the average strength of this network correlates strongly with the macroscopic stress–strain response. For dense samples, network strength rapidly increases to a peak coinciding with the deviatoric stress maximum, then gradually decreases with further shear. Crucially, this evolution is path-dependent: the average contact force network strength increases approximately 20% more during unloading in the minor principal stress direction compared to unloading in the major principal stress direction. This quantitative analysis of force chain degradation provides a mechanistic explanation for the observed strain softening, highlighting the dominant role of the unloading stress path. In contrast, loose specimens, which initially lack an obvious force chain network, show negligible microstructural evolution during unloading. Full article

Dental resin composites are routinely exposed to chemically aggressive beverages that may compromise long-term functional performance. This study investigated the structure–property–tribology relationships of four restorative composites (Filtek Z250, Filtek Z550, Herculite, and Herculite Ultra) subjected to cyclic immersion in beverages with different pH values. A total of 120 cylindrical specimens (7 mm diameter, 2 mm thickness; n = 5 per material per condition) were fabricated and exposed to mineral water, tea, coffee, Coca-Cola^®, Cola Light^®, and red wine for 28 days under cyclic conditions. Microhardness, surface roughness (Ra), steady-state coefficient of friction (COF), and mass variation were evaluated. All composites exhibited significant microhardness reduction after acidic exposure (p < 0.05), with the greatest decrease observed for Herculite Ultra in red wine (−47.4%) and Coca-Cola^® (−35.3%). Filtek Z250 demonstrated the highest baseline hardness and the lowest degradation susceptibility. Surface roughness changes were formulation-dependent, with Herculite Ultra showing pronounced roughening (ΔRa up to +0.074 µm), whereas Filtek Z550 exhibited erosion-driven smoothing (ΔRa down to −0.068 µm). Tribological behaviour was primarily governed by matrix softening rather than roughness alterations, with softened systems displaying unstable frictional responses (COF range: 0.127–0.697; p < 0.05). The results indicate that polymer matrix stability plays a more critical role in long-term functional performance than surface roughness or mass variation alone. Clinically, frequent exposure to acidic and solvent-containing beverages may accelerate mechanical and tribological degradation of susceptible composite formulations. Full article

The microstructure and mechanical properties of the joint of a novel Al-Mg-Zn-Er-Zr alloy fabricated by multi-pass MIG welding using ER5E61 filler wire were investigated first. The results show that multi-pass MIG welding induces heterogeneous grains in the weld metal: equiaxed grains, columnar grains, and cover-pass feather-like grains. The weld metal exhibits coarse grains (45.81 ± 19.68 μm), a high proportion of high-angle grain boundaries (83.3%), and a low dislocation density compared with the base metal. The joint achieves 316 MPa ultimate tensile strength, 10.5% elongation, and 0.80 joint efficiency with minimum hardness (77.2 HV) in the weld metal. Strengthening mechanism analysis reveals that joint softening mainly stems from the disappearance of deformed structure, reduced dislocation density, and the coarsening and reduction in Al3(Er, Zr) nanophases. Diffuse precipitation of the Al3(Er, Zr) nanophases (19.61 nm, 0.53%) under multi-pass MIG welding compensates for the softening of the welded joint, leading to the retention of high tensile strength despite marked hardness loss, thus demonstrating effective strength preservation. Full article

Heat creep is a critical failure mechanism in fused filament fabrication (FFF) extrusion systems, arising from insufficient thermal isolation between the hot end and cold end. It causes premature polymer softening, extrusion instability, and nozzle clogging, especially when active cooling is reduced or lost. This study experimentally evaluates passive cooling strategies for mitigating heat creep in consumer-class printers by exploiting ambient thermal stratification within the build volume. Vertical air-temperature gradients above heated build plates were measured for enclosed, semi-enclosed, and open-frame architectures, revealing pronounced stratification. Cold-end temperatures were then quantified for a stock extruder under forced and natural convection while printing polylactic acid (PLA) and acrylonitrile butadiene styrene (ABS). Finally, a modified cold-end using a heat pipe to relocate heat rejection to an elevated heat sink was tested under identical conditions, assuming fan failure. Elevated heat-rejection locations experienced lower ambient temperatures and improved natural-convection heat transfer. Relative to the stock configuration, the augmented design reduced cold-end temperatures and improved thermal stability during representative printing cycles without continuous active cooling—the improvement percent is ~8%. The results demonstrate that coupling heat-pipe conduction with environmental thermal gradients can mitigate heat creep and improve extruder reliability with lower energy demand. Full article

This study investigates the machinability of Continuous Fiber-Reinforced Thermoplastic Composite (CFRTP) produced via Material Extrusion (MEX) additive manufacturing, focusing on drilling as a critical post-processing step in hybrid manufacturing. CFRTP components, fabricated from 3K carbon fibers and a PLA matrix, were subjected to systematic drilling tests under varying cutting speeds (50–110 m/min) and feed rates (0.06–0.24 mm/rev). Thrust force (Fz) and torque (Mz) were recorded using a high-precision dynamometer to evaluate the influence of cutting parameters on mechanical loads and damage mechanisms. Results indicate that increasing the feed rate significantly increases Fz and Mz, promoting fiber pull-out, delamination, and edge deformation, particularly at hole entry and exit regions. Conversely, higher cutting speeds reduce Fz and Mz due to thermal softening of the PLA matrix, enabling more controlled fiber–matrix interaction. Microscopic analyses revealed that damage severity correlates strongly with mechanical load levels. While high feed rates caused pronounced surface irregularities and matrix smearing, low feed rates combined with high cutting speeds yielded smoother hole morphology and preserved fiber–matrix integrity. The study concludes that optimal drilling conditions for CFRTP materials involve low feed rates and high cutting speeds, minimizing mechanical loads and suppressing damage formation. These findings provide a scientific basis for precision finishing strategies in hybrid manufacturing, enhancing dimensional accuracy and structural reliability of CFRTP components for advanced engineering applications. Full article

To investigate the dynamic behavior of smart composite structures with embedded heat sources over a wide temperature range, this study employed thermoplastic polypropylene as the matrix, combined with glass/carbon fiber prepregs and Ni80Cr20 alloy heating wires, and fabricated functional laminated specimens with integrated heating elements via a prepreg molding process. Using a self-developed variable-temperature cantilever beam vibration testing system, the evolution of natural frequencies and damping ratios from room temperature to 140 °C was systematically examined. Results indicate that temperature-induced thermal softening of the polypropylene matrix reduces the effective bending stiffness of the composites, leading to a decline in natural frequencies across all modes. For example, the first-order natural frequency of the sample decreased from approximately 30.8 Hz at room temperature to about 28.3 Hz at 140 °C, representing a reduction of approximately 8.12%. The second-order reduction reached about 8.99%, and the third-order reduction was approximately 9.65%. Carbon fiber-reinforced specimens exhibited relatively smaller frequency reductions due to the high modulus of the fibers. Concurrently, elevated temperatures enhance molecular chain mobility and interfacial viscoelastic dissipation at the fiber–matrix interface, causing a sharp increase in damping ratios at high temperatures (>100 °C). For instance, the damping ratio of the first-order mode increased significantly from approximately 1.02% at room temperature to about 2.9% at 140 °C. By comparatively analyzing carbon fiber and glass fiber systems, the study elucidated the distinct mechanisms underlying the “fiber-dominated” stiffness retention effect and the “resin/interface-dominated” damping dissipation effect under thermal influence. These findings provide critical experimental data and theoretical references for the active thermal regulation of structural performance in thermoplastic composite structures with integrated heat sources, thereby mitigating damage caused by external disturbances. Full article

This study evaluates the performance of a copper-based metal matrix composite (Cu-MMC) brake pad fabricated by powder metallurgy for high-speed railway braking applications. The material was produced via homogeneous powder mixing, compaction at 650 MPa, and sintering at 950 °C for 2 h to promote densification and metallurgical bonding. The fabricated Cu-MMC exhibited densities of 5.71–5.98 g/cm³, porosities of 5.85–10.1%, and hardness values of 62–73 HV, indicating effective microstructural control. Tribological performance was assessed using a brake dynamometer at an equivalent speed of 160 km/h and a contact pressure of 0.95 MPa. The composite demonstrated a low specific wear rate of 0.11–0.14 cm³/MJ, meeting the TJ/CL 307-2014 standard for high-energy braking. Surface analysis revealed stable frictional behavior dominated by oxidative–abrasive, adhesive, and delamination wear mechanisms. Thermal evaluation showed a maximum operating temperature of 225–235 °C, below the softening temperature of copper, confirming adequate thermal stability. Full article