Search Results (356)

This article investigates an axisymmetric contact problem involving the interaction between a rigid cylindrical stamp and a poroelastic cylinder of finite dimensions, based on the Cowin–Nunziato theory of media with voids. The stamp is assumed to have a flat base and to be in frictionless contact with the cylinder. The cylinder, in turn, rests on a rigid base without friction, with no normal displacements or tangential stresses on its lateral surface. Under an applied vertical force, the stamp undergoes displacement, compressing the poroelastic cylinder. The mathematical formulation of this problem involves expressing the unknown displacements within the cylinder and the variation in pore volume fraction as a series of Bessel functions. This representation reduces the problem to an integral equation of the first kind, describing the distribution of contact stresses beneath the stamp. The kernel of the integral equation is explicitly provided in its transformed form. The collocation method is employed to solve the integral equation, enabling the determination of contact stresses and the relationship between the indenter’s displacement and the applied force. A comparative model parameter analysis is performed to examine the effects of different material porosity parameters and model geometrical characteristics on the results. Full article

In this study, the authors carried out a multiparametric assessment of the influence of swirl patterns during aerosol flow on the shape of the interfacial area that forms the cone based on data obtained from experimental measurements using the PIV and LLS methods. The results were correlated with the disinfection process occurring in the near and far fields of the aerosol (direct surface disinfection and volume fogging). In this study, parameters such as turbulent kinetic energy (TKE), swirl strength (SS), pressure fields, and Sauter mean diameter (d₃₂) are used to investigate the relationship between aerosol spray morphology and flow dynamics under different operating conditions. Three different geometrical settings of the aerosol-generating system and two different pressures corresponding to the air supply to the spray nozzle have been adopted. By evaluating the results obtained, the influence of each parameter on the formation of the aerosol displacement trajectory, the stabilization of the spray cone, and its degradation was identified. The shape of the boundary between the dynamically moving aerosol and the surrounding air was also evaluated. The conditions for swirling and straight-line flows within the aerosol cone, and, thus, the conditions for the volumetric development of swirling phenomena, were further clarified. Full article

A common approach to optimising hydrodynamic journal bearings for power loss is to reduce the lubricant supply and direct the oil to specific bearing areas where it is needed to guarantee safe operation. This requires information on the processes in the gap and the surrounding pocket areas for both pre-design and simulation. In this paper, a system consisting of a total of eight cameras is used to determine the void fraction in deep grooves outside the lubricant film. The void fraction in the lubrication gap is determined using a novel method for the evaluation of two proximity measurements. While the variation of the deep groove void fraction is realised by a special oil supply and radially adjustable deep groove elements, the gap void fraction is adjusted by the oil supply in the lube oil pockets at the pad leading edges. On the one hand, the experimental investigations show that the void fraction of the deep groove areas has hardly any influence on the general operating behaviour. On the other hand, the void fraction in the lubrication gap can be measured quantitatively for the first time, and the operating point-dependent gas fractions can be visualised. It is also shown that gaseous cavitation is the main mechanism in partially filled regions of the lubrication gap. Full article

One technique for sintering green compacts and imparting the required qualities to meet the specific application requirements is spark plasma sintering (SPS). This study examines the effects of SPS parameters (sintering temperature and pressure, holding time, and heating rate) and plantain peel ash (PPA) reinforcement concentrations (0, 5 wt%, 10 wt%, 15 wt%, and 20 wt%) on the microstructure, compressive strength, and wear characteristics of the fabricated Al–Mg–PPA composites. As a result of the ball milling machine’s high efficiency, the PPA reinforcement was evenly dispersed throughout the aluminum matrix after 90 min of milling. At lower sintering temperatures and pressures, microstructural flaws such as weak grain boundaries, micro-pores, and micro-cracks were more noticeable than at higher ones. The PPA reinforcement and magnesium powder (wetting agent) increased the composites’ compressive strength by improving the wettability between the PPA reinforcement and the Al matrix. At a weight fraction of 5 wt% PPA, the maximum compressive strength of 432 MPa was attained for the sintered composites, which is a 222% improvement over the sintered aluminum matrix. Additionally, the PPA reinforcement enhanced the wear properties of the sintered Al–Mg–PPA composites by reducing the wear loss. Increasing the wear load resulted in a higher wear rate. The COF for the sintered composites ranges from 0.049 to 0.727. The most consistent correlation between the wear rate and the COF is that as the wear rate decreases, the COF decreases, and vice versa. Abrasive wear was the dominant wear mechanism observed. Tear ridges, shear steps, micro-voids, and cleavages were seen on the composites’ fracture surfaces, an indication of a ductile-brittle fracture. Full article

The characterization of lignocellulosic biomass present in archaeological wood is crucial for understanding the degradation processes affecting wooden artifacts. The lignocellulosic fractions in both the external and internal parts of Moroccan archaeological cedar wood (9th, 12th, and 21st centuries) were characterized using infrared spectroscopy (FTIR-ATR deconvolution mode), X-ray diffraction (XRD), and SEM analysis. The XRD demonstrates a significant reduction in the crystallinity index of cellulose from recent to aging samples. This finding is corroborated by the FTIR analysis, which shows a significant reduction in the area profiles of the C-H crystalline cellulosic bands (1374, 1315, and 1265 cm⁻¹) and C-O-C (1150–1000 cm⁻¹). The alterations in the lignin fraction of aging samples (from the 9th and 12th centuries) were demonstrated by a reduction in the intensity of the bands at 1271 and 1232 cm⁻¹ (C_ar-O) and the formation of new compounds, such as quinones and/or diaryl carbonyl structures, within the 1700–1550 cm⁻¹ range. The SEM images of cedar wood samples from the 9th and 12th centuries reveal voids, indicating that the entire cell wall component has been removed, a characteristic feature of simultaneous white rot fungi. In addition, horizontal “scratches” were noted, indicating possible bacterial activity. Full article

A theoretical model for porous viscoelastoplastic (VEP) materials in the dry state is investigated in this research study. The model is based on the principles of conservation of mass and energy using the rheological dynamic theory (RDT). The model provides expressions for the creep coefficient, Poisson’s ratio, modulus of elasticity, damage variable, and strength as a function of porosity and/or void volume fraction (VVF). The reliability of the proposed model was analyzed by comparing numerical results with experimental ones on hardened concrete. A numerical model was created and analyzed in the commercial software Abaqus and validated by comparison with experimental data obtained by geodetic measurements on a composite wood–lightweight concrete girder. The deflections and stresses of the beam resulting from the influence of concrete creep and porosity were analyzed at the initial moment of time and after 6 years. The results showed that the RDT provided a reliable model for estimating parameters after exposure to long-term loads. Full article

Power-to-heat (P2H) systems offer an efficient solution for decarbonization by facilitating the integration of renewable energy into the industrial, heating, and transport sectors. Its key requirements include high thermal efficiency and an appropriate electrical resistivity to meet application-specific electrical needs. When designing P2H systems, materials and electrical boundary conditions are often limited by application-specific requirements, whereas geometric structures offer high degrees of freedom. While thermal design calculations are often straightforward due to a variety of available Nusselt and pressure loss correlations, simplified design pathways, particularly for porous structures, are lacking in electrical design. Given the wide range of geometric degrees of freedom for porous structures and the fact that detailed modeling involves substantial computational effort, this work employed electrical tortuosity to capture and correlate the geometry-dependent impacts on the effective electrical resistance in a compact way. Honeycomb and triply periodic minimal surface (TPMS)-based structures were selected for this purpose, as they are characterized by high specific surfaces, allowing for high total heat transfer coefficients. The results show that the effective electrical resistance of both TPMS and honeycomb structures can be adjusted by the geometric structure. It was found that the electrical tortuosities of the investigated TPMS structures are nearly identical, while honeycomb structures show slightly higher values. Furthermore, the electrical tortuosity is mainly a function of the void fraction and does not change with the specific surface when the void fraction is kept constant. Finally, correlations for electrical tortuosity depending on geometric parameters with a mean error below 5% are derived for the first time, thereby providing a basis for simplified and computationally efficient electrical design calculations for P2H systems. Full article

In this study, a multiscale model is developed through secondary development (UMAT and UEXTERNALDB) in Abaqus with the objective of simulating the thermal deformation process with dynamic recrystallization behavior. The model couples the finite element method (FEM) with the multiphase field model (MPFM), thereby establishing bidirectional coupling between macroscopic mechanical behavior and microstructural evolution. A comparison between the single-element hot compression simulation and experimental results demonstrates that the model accurately simulates both the macroscopic mechanical behavior and microstructural evolution during the thermal deformation process, thereby exhibiting high precision. Simulations of the reduction pretreatment (RP) process under different reduction amounts and billet surface temperatures demonstrate that increasing the reduction amount and billet surface temperature significantly enhances both plastic deformation and the volume fraction of dynamic recrystallization in the billet core. This results in the closure of core voids and the refinement of the core microstructure, thereby providing valuable guidance for the development of optimal reduction pretreatment (RP) processes. Full article

As a critical component of a train, the railway wagon bogie adapter has higher quality requirements. During the forging process, external loads can induce voids, cracks, and other defects in the forging, thereby reducing its service life. Hence, studying the damage behavior of the forging material, specifically AISI 1035 steel, becomes crucial. This study involved obtaining stress–strain curves for AISI 1035 steel through uniaxial tensile tests at temperatures of 900 °C, 1000 °C, and 1100 °C, with strain rates of 0.1 s⁻¹, 1 s⁻¹, and 10 s⁻¹. Subsequently, SEM was used to observe samples at various deformation stages. The damage parameters, $q_1$, $q_2$ and $q_3$ in the GTN model “a computational model used to analyze and simulate material damage which can effectively capture the damage behavior of materials under different loading conditions” were then calibrated using the Ramberg–Osgood model and stress–strain curve fitting. Image Pro Plus software v11.1 quantified the sample porosity as $f_0$, $f_n$, $f_c$ and $f_F$. A finite element model was established to simulate the tensile behavior of the AISI 1035 steel samples. By comparing the damage parameters of $f_0$, $f_n$, $f_c$ and $f_F$ obtained by the finite element method and experimental method, the validity of the damage parameters obtained by the finite element inverse method could be verified. Full article

This study investigates the impact of polyvinyl alcohol (PVA) fibers on the mechanical properties and durability of high-performance shotcrete (HPS). Results demonstrate that PVA fibers have a dual impact on the performance of HPS. Positively, PVA fibers enhance the tensile strength and toughness of shotcrete due to their intrinsic high tensile strength and fiber-bridging effect, which significantly improves the material’s splitting tensile strength, deformation resistance, and toughness, and the splitting tensile strength and peak strain have been found to be increased by up to 30.77% and 31.51%, respectively. On the other hand, the random distribution and potential agglomeration of PVA fibers within the HPS matrix can lead to increased air-void formations. This phenomenon raises the volume content of large bubbles and increases the average bubble area and diameter, thereby elevating the pore volume fraction within the 500–1200 μm and >1200 μm ranges. Therefore, these microstructural changes reduce the compactness of the HPS matrix, resulting in a decrease in compressive strength and elastic modulus. The compressive strength exhibited a reduction ranging from 10.44% to 15.11%, while the elastic modulus showed a decrease of between 8.09% and 12.67%. Overall, the PVA-HPS mixtures with different mix proportions demonstrated excellent frost resistance, chloride ion penetration resistance, and carbonation resistance. The electrical charge passed ranged from 133 to 370 C, and the carbonation depth varied between 2.04 and 6.12 mm. Although the incorporation of PVA fibers reduced the permeability and carbonation resistance of shotcrete, it significantly mitigated the loss of tensile strength during freeze–thaw cycles. The findings offer insights into optimizing the use of PVA fibers in HPS applications, balancing enhancements in tensile properties with potential impacts on compressive performance. Full article

A new approach to an automatable fiber impregnation and consolidation process for the manufacturing of fiber-reinforced composite parts is presented in this article. Therefore, a vacuum chamber sealing machine classically used in food packaging is modified for this approach—Vacuum Chamber Infusion (VCI). Dry fiber placement (DFP) preforms, made from 30 k carbon fiber tape, with different layer amounts and fiber orientations, are infused with the VCI and with the state-of-the-art process—Vacuum Assisted Process (VAP)—as the reference. VCI uses a closed system that is evacuated once, while VAP uses a permanently evacuated open system. Since process management greatly influences material properties, the mechanical properties, void content, and fiber volume fraction (FVF) are analyzed. In addition, the study aims to identify how the complexity of a resin infusion process can be reduced, the automation potential can be increased, and the number of consumables can be reduced. Comparable material characteristics and a reduction in consumables, setup complexity, and manufacturing time by a factor of four could be approved for VCI. A void content of less than 2% is measured for both processes and an FVF of 39% for VCI and 45% for VAP is achieved. Full article

The vibration pretreatment–microwave curing process can achieve high-quality molding under low-pressure conditions and is widely used in the curing of resin-based composites. This study investigated the effects of the vibration pretreatment process parameters on the void content and the fiber weight fraction of T700/TRE231; specifically, their influence on the interlaminar shear strength and impact strength of the composite. Initially, an orthogonal experimental design was employed with interlaminar shear strength as the optimization target, where vibration acceleration was determined as the primary factor and dwell time as the secondary factor. Concurrently, thermogravimetric analysis (TGA) was performed based on process parameters that corresponded to the extremum of interlaminar shear strength, revealing a 2.17% difference in fiber weight fraction among specimens with varying parameters, indicating a minimal effect of fiber weight fraction on mechanical properties. Optical digital microscope (ODM) analysis identified interlaminar large-size voids in specimens treated with vibration energy of 5 g and 15 g, while specimens subjected to a vibration energy of 10 g exhibited numerous small-sized voids within layers, suggesting that vibration acceleration influences void escape pathways. Finally, impact testing revealed the effect of the vibration pretreatment process parameters on the impact strength, implying a positive correlation between interlaminar shear strength and impact strength. Full article

The mechanical properties of local materials subjected to various stress triaxialities were investigated via self-designed small punch tests and corresponding simulations, which were tailored to the geometry and notch forms of the samples. The finite element model was developed on the basis of the actual test method. After verifying the accuracy of the simulation, the stress, strain, and void volume fraction distributions of the Ti6Al4V titanium alloy under different stress states were compared and analyzed. The results indicate that the mechanical properties of the local material significantly differ during downward pressing depending on the geometric shape. A three-dimensional tensile stress state was observed in the center area, where the void volume fraction was greater than the fracture void volume fraction. The fracture morphology of the samples further confirmed the presence of different stress states. Specifically, the fracture morphology of the globular head samples (with or without U-shaped notches) predominantly featured dimples. Modifying the specimen’s geometry effectively increased stress triaxiality, facilitating the determination of the material’s constitutive relationship under varying stress states. Full article

This study investigates the effect of void features in additively manufactured polymers on water diffusion, focusing on polyethylene terephthalate glycol (PETG) composites. The additive manufacturing (AM) of polymers, specifically, material extrusion AM (MEAM), results in manufacturing-induced voids, therefore affecting the water resistance of the printed parts. The research analyses the effects of size, shape, orientation and the hydrophilicity of voids on moisture diffusion in PETG composites employing numerical (finite-element) simulations. Two void types were examined: voids of Type I that retard the moisture propagation and voids of Type II that enhance it. Simulations demonstrate that a higher volume fraction of voids and their orientation with regard to the diffusion direction significantly hinder the moisture transport for Type I voids. Conversely, due to their high diffusivity, Type II voids serve as channels for rapid moisture transmission. Consequently, for such materials, the global diffusion rates mainly depend on the volume fraction of voids rather than their shape. These findings indicate the critical role of voids in the design of AM parts for environments exposed to moisture, such as marine and offshore applications. Understanding the void effects is critical for optimising the durability and performance of MEAM components underwater exposure. Full article

This paper describes the self-heating effects resulting from mechanical deformation in the additively manufactured aluminium alloy AlSi7Mg0.6. The material’s self-heating effect results from irreversible changes in the material’s microstructure that are directly coupled with the inelastic deformations. These processes are highly dissipative, which is reflected in the heat generation of the material. To describe such effects, a numerical framework that combines an elasto-viscoplastic Chaboche model with the Gurson Tvergaard Needleman damage approach is analysed and thermomechanically extended. This paper characterises the sample preparation, the experimental set-up, the development of the thermomechanical approach, and the material model. A user material subroutine applies the complete material model for the finite element software Abaqus 2022. To validate the material model and the parameters, a complex tensile test is performed. In order to check the finite element model, the energy transformation ratio is included in the evaluation. The numerical analyses of the mechanical stress evolution and the self-heating behaviour demonstrate good agreement with the experimental test. In addition, the calculation shows the expected behaviour of the void volume fraction that rises from the initial value of $0.0373\%$ to a higher value under a complex mechanical load. Full article