Mechanics of Time-Dependent Materials最新文献

With the rapid development and application of ultra-short pulses in the micro-machining of viscoelastic structures, the transient thermodynamic response at the micro/nano scale have become of great importance. Increasingly crucial at the microscale are the scale and memory effects in elastic deformation and heat transfer. A multitude of experimental and theoretical studies indicate that, for practical analyses, the thermal conductivity of materials ought not to be treated as a fixed value. This work aims to formulate a thermoviscoelastic model by incorporating a fractional-order three-phase-lag heat conduction model. The proposed framework utilizes the Atangana-Baleanu definition of fractional derivative, which features a non-singular kernel, along with an extended Caputo definition to describe the time-dependent characteristics of heat conduction. Additionally, the nonlocal elasticity model is taken into account in the stress-strain relationships. Then, the modified model is applied to investigate the nonlinear electro-magneto-thermo-viscoelastic response of a polymer spherical nanoshell with variable thermal conductivity subjected to ramp-type heating load under the effect of a magnetic field. Taking into account the variable thermal conductivity, the nonlinear governing equations are derived. Employed to derive and solve the governing equations are the Laplace and Kirchhoff transformations. In-depth discussions are carried out regarding the influences of diverse factors such as the fractional-order parameter, the nonlocal parameter and the variable thermal conductivity on physical quantities.

A high-throughput characterization scheme to determine yielding and permanent strain accumulation in polymers is presented. Measurement of permanent strains in polymers is challenging because of the extended duration associated with viscoelastic recovery, especially when tests need to be repeated under different loading histories. The proposed testing method can dramatically reduce the experimental efforts for long-term testing of time-dependent materials by producing a multitude of data from a single experiment. This is achieved by imposing a known stress gradient and employing a full-field strain measurement technique to obtain a series of creep curves under varying stress levels simultaneously. The method is demonstrated on low density polyethylene thin films used in superpressure balloons. Uniaxial creep-recovery tests are performed on polyethylene in two different material directions. Residual strains are measured after viscoelastic recovery to determine yielding and the subsequent evolution of permanent strain.

The extensive use of materials in the construction industry imposes risks of depleting natural aggregates and increased carbon emissions, thereby motivating the search for sustainable alternatives. This work investigates the influence of supplementary cementitious materials (SCMs), including fly ash (FA), silica fume (SF), and ground-granulated blast-furnace slag (GBFS) on the durability and mechanical behavior of recycled heterogeneous aggregate concrete (RAC) subjected to sulfate erosion and dry–wet cycles (SEDWCs). Durability performance was evaluated through observation of surface appearance and mass variation, and digital image correlation (DIC) was used to monitor crack evolution and failure modes under uniaxial compression. Microscale mechanisms were examined using X-ray diffraction (XRD), scanning electron microscopy (SEM), and mercury intrusion porosimetry (MIP). Incorporating SCMs mitigated surface deterioration, stabilized mass change, and enhanced compressive strength. DIC observations revealed delayed crack initiation and less severe damage in SCM-modified RAC, with failure modes shifting from predominant shear to combined shear–cohesion failure. Microstructural analyses confirmed that SCMs densified the cement matrix and refined the interfacial transition zone (ITZ). Among the SCMs, silica fume exhibited the strongest resistance to SEDWCs, while fly ash and GBFS also improved durability compared with RAC without SCMs. These results demonstrate that the combined use of recycled aggregates and supplementary cementitious materials can significantly enhance the long-term performance of concrete in sulfate-rich cyclic environments, advancing the development of sustainable construction materials.

Enhancing the melting rate of Phase Change Materials (PCMs) is critical for improving the efficiency of Thermal Energy Storage (TES) systems. Although optimized fin geometries have been widely studied, the combined role of nanoparticle-enhanced molten-salt PCMs with fins has received limited attention. A numerical investigation is conducted on the melting behavior of molten salt PCMs incorporated with Cu and TiO₂ nanoparticles in a cylindrical TES unit featuring three fin geometries: longitudinal, Y-shaped, and tuning fork-shaped. Simulations are performed using ANSYS Fluent software and validated against published experimental data. The primary performance indicators, including the temperature distribution, velocity fields and liquid fraction, are analyzed. At a comparable melt fraction (∼97%), the addition of nanoparticles significantly reduced the melting time across all fin configurations, from 345 s to 192 s for longitudinal fins, from 244 s to 137 s for Y-shaped fins and from 202 s to 114 s for fork-shaped fins. These results demonstrate that nanoparticle integration accelerates the melting process by approximately 44-50%, with performance strongly dependent on the fin design. These findings indicate the synergistic benefits of fin geometry and nanoparticle additives, which provide practical guidelines for the design of high-efficiency TES devices.

Effective heat transfer is crucial for engineering applications such as heat exchangers, electronics cooling, and HVAC systems. Moreover, this paper provides an extensive examination of the physical properties of viscoelastic liquids that are emerging in relation to important profiles, considering the copper oxide (left ( CuO right )), silver (left ( AgO right )), and zirconium dioxide (left ( Zr O_{2} right )), immersed in the base fluid ethylene glycol (EG). Viscous dissipation, porosity, and joule heating effects are introduced in the formulation of the problem. The differential equations are dimensionless after the mathematical formulations via a non-similarity conversion. The local non-similarity method transforms non-similar PDEs into ODEs, which are then solved numerically using MATLAB’s bvp4c. Important physical parameters are given in tabular form, such as skin friction and Nusselt values. The temperature distribution has been seen to increase when the viscoelastic parameter (second-grade fluid) increases. It is discovered that the findings converge more quickly and are validated for limited circumstances. Here are some key applications of the present work: industrial cooling systems, polymer processing, chemical engineering, biomedical engineering, heat exchangers, energy systems, cooling of microelectronic devices.

Negative stiffness honeycombs are energy absorbers that absorb energy by transitioning from one buckling mode to another, and their snap-through behaviour in cells. They have a fundamental property as an energy absorber; in this type of absorber, negative stiffness causes reversibility after impact. In this study, the effect of negative stiffness honeycomb material in increasing energy absorption is investigated experimentally and numerically. Quasi-static tests are performed on negative stiffness honeycombs made of polyamide 11 (PA11) and polyamide 12 (PA12). Then, the finite element (FE) model of the structure is simulated under quasi-static compression. The FE model requires the nonlinear elastic and viscoelastic properties of the materials used to make the honeycombs. For this purpose, tensile and stress relaxation tests are performed. The Prony series coefficients for polyamide materials 11 and 12 are extracted and used in the FE model. Next, the energy absorption performance of negative stiffness honeycombs of PA11 and PA12 is compared experimentally and numerically. The comparison shows that the energy absorption per unit mass (SEA) for a negative stiffness honeycomb made of PA11 is 1.7 times higher in the experimental data and 1.62 times higher in the FE result than negative stiffness honeycomb made of PA12. Other fundamental parameters of energy absorption also confirm the higher efficiency of negative stiffness honeycombs made of PA11. This study provides a unique contribution by integrating experimentally determined nonlinear viscoelastic properties into the FE model and offering a comparison of energy absorption performance between PA11 and PA12 in negative stiffness honeycombs.

Fine recycled concrete aggregate (RCA) exhibits inferior bonding properties with cement due to its high water absorption and porosity. Incorporating fine RCAs in cementitious materials leads to the significant reduction in dynamic mechanical performance. To address this issue, a compound inspired by tea stains, tannic acid (TA), is used to treat the fine RCAs. The results suggest that the width of the interface transition zone (ITZ) between fine RCA and the new matrix is significantly reduced after this treatment, indicating the improvement in bonding properties. Dynamic compression tests are conducted to examine the impact resistance performance of cement mortar containing fine RCAs through employing the Split Hopkinson Pressure Bar (SHPB) apparatus. When subjected to impact loads, mortars with treated fine RCAs indicate higher dynamic compressive strength and enhanced completeness. The dynamic compressive strength and total energy absorption exhibit the exponential positive correlation with the strain rate. The strain rate effect is more pronounced when the strain rate is below 100 s⁻¹. Importantly, mortars with treated fine RCAs manifest superior total energy absorption, enabling them to absorb and dissipate more impact energy. The mortar containing fine RCAs treated with 0.5%TA reveals excellent impact resistance performance, surpassing even that of the cement mortar with natural fine aggregates. Moreover, 0.5%TA group shows better economic benefits than the plain mortar, indicating that the TA treatment method has good application prospects.

The Small Amplitude Oscillatory Strain (SAOS) testing is commonly used to measure linear viscoelastic functions such as elastic moduli, which are denoted respectively (G^{*}(iomega )) and (E^{*}(iomega )) in simple shear and tension-compression. Beyond the linear regime, Large Amplitude Oscillatory Strain (LAOS) is a widely used experimental technique to investigate nonlinear phenomena. From mathematical standpoint, Fourier transform and Fourier series are used to handle SAOS and LAOS data. In general, SAOS-tests do not account a strain jump discontinuity to make easy measurement data processing. Thereby, we focus on the influence of a strain jump discontinuity (or not) on the response of the Cauchy stress. First, we investigate the problem of strain discontinuity (or not) within framework of one-dimensional isotropic linear viscoelasticity by considering both the integral approach and the fractional Maxwell constitutive model. Second, we assume that, the Large Amplitude Oscillations Strain (LAOS) is equivalent to small oscillations around a static pre-deformation. It means that, the jump at the origin is shifted to minus infinity in the time scale. Therefore, we investigate the first and third harmonics within framework of the nonlinear Kelvin-Voigt model. We conclude with some future perspectives for the present work.

For crystalline polymers used as a matrix polymer at fiber-reinforced thermo-plastics, effects of crystallinity of polypropylene on creep characteristics were investigated. Measurements taken using DSC, DMA, SAXS, and WAXS elucidated crystallization behavior and crystallinity along with changes in the annealing temperature and annealing time. Although crystallinity increases with the annealing temperature and annealing time, a relation was found between the annealing temperature and time: crystallinity can be shifted by a shift factor. The time–temperature crystallinity superposition holds for crystalline polymers. Using this relation, creep time change caused by aging can be predicted. The relation between crystallinity and storage modulus (E') was described using a Takayanagi type two-phase model. We proposed a mechanical model that extends the Takayanagi type two-phase model to express the phenomenon by which the creep time increases because of increased crystallinity together with a viscous element of a model implementing the Guth–Gold model. This model can express behaviors of increasing creep time caused by increased crystallinity.

This study aimed to predict the yield stress and plastic viscosity of Superabsorbent Polymer (SAP)-modified cement pastes using the Yield stress mODEL (YODEL) and the Krieger-Dougherty (K-D) equation. Predictions were made for cement pastes with SAP dosages of 0.2–0.5% (by weight of cement) and water-to-cement (w/c) ratios of 0.4–0.6, over 5–35 minutes, and were compared with experimental data. In the YODEL model, the percolation threshold ((phi _{0})) and surface-to-surface separation distance (H) were fitted. The (phi _{0}) values (0.20–0.27) decreased over time, indicating paste stiffening and reduced percolation. The values of H (1.5–3 nm) declined with higher SAP dosages and over time due to water absorption and hydration, leading to increased flocculation and stiffening. In the K-D model, intrinsic viscosity [(eta )] was adjusted; [(eta )] increased with higher SAP dosages, greater w/c ratios, and time, consistent with thickening caused by SAP water uptake and hydration. For w/c = 0.4, predictions agreed with experiments from 5–15 min, with larger deviations occurring later. For w/c = 0.5 and 0.6, predictions aligned from 5–20 min, with slight overestimations and underestimations afterward. The K-D equation generally provided close agreement with experimental viscosities, showing only minor deviations. Overall, YODEL effectively captured early-age yield stress behavior, while the K-D equation successfully predicted viscosity trends, demonstrating the combined potential of these models for describing the rheology of SAP-modified cement paste.