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Traditionally, stress fluctuations in flowing and deformed materials are overlooked, with an obvious focus on average stresses in a continuum mechanical approximation. However, these fluctuations, often dismissed as "noise," hold the potential to provide direct insights into the material structure and its structure-stress coupling, uncovering detailed aspects of fluid transport and relaxation behaviors. Despite advancements in experimental techniques allowing for the visualization of these fluctuations, their significance remains largely untapped as modeling efforts continue to target Newtonian fluids within the confines of Gaussian noise assumptions. In the present work, a comparative analysis of stress fluctuations in two distinct microstructural models is carried out: the temporary network model and the hydrodynamic dumbbell model. Despite both models conforming to the upper convected Maxwell model at a macroscopic level, the temporary network model predicts non-Gaussian fluctuations. We find that stress fluctuations within the temporary network model exhibit more pronounced abruptness at the local scale, with only an enlargement of the control volume leading to a gradual Gaussian-like noise, diminishing the differences between the two models. These findings underscore the heightened sensitivity of fluctuating rheology to microstructural details and the microstructure–flow coupling, beyond what is captured by macroscopically averaged stresses. [ABSTRACT FROM AUTHOR]

One of the challenge that face the effectiveness of Polyvinyl Chloride geomembranes (PVC GMs) as a hydraulic barrier is the capacity to withstand unexpected mechanical actions, particularly tensile forces, during installation and throughout their lifespan. These forces pose risks of premature failure and impermeability degradation. In this study, the characterization of the short and long-term mechanical response of PVC GMs to uniaxial tensile forces has been investigated. Uniaxial tensile test have been performed for tensile rates spanning several orders of magnitude. Analysis of the true stress-strain curves reveals a significant decrease in tensile modulus, strength, and strain at failure at low strain rates, which are relatively close to those applied in situ. Long-term investigations have been conducted as well, through relaxation tests. Our key results unveil two distinct characteristic times in stress relaxation, with the fast relaxation occurring over the first 4 h. During this phase, the pre-relaxation loading rate affects the relaxation behavior. Beyond this phase, the relaxation behavior becomes independent from the pre-relaxation loading rate. Burger's rheological model is proposed to measure the stress relaxation at different rates. The model's results validate the existence of two characteristic times. • Higher tensile loading rates significantly increase the rigidity modulus of PVC geomembranes, while lower rates lead to reduced strength and strain at failure. • Laboratory tests at standard speeds may not accurately predict field performance, highlighting the need for testing at lower rates to avoid the risk of premature geomembrane failure. • Stress relaxation tests demonstrate two distinct relaxation stages, with initial rate-dependent behavior transitioning to a long-term rate-independent response after approximately 4 h. [ABSTRACT FROM AUTHOR]

The complex interplay between high-temperature and rarefaction effects poses ongoing challenges to model hypersonic nonequilibrium flows, affecting the accurate prediction of aerodynamic and aerothermal loads for hypersonic vehicles. To address this issue, a multispecies Nonlinear Coupled Constitutive Relations (NCCR) model is first derived from the modified Boltzmann equations with vibrational mode. The proposed NCCR model incorporates a thermochemical nonequilibrium suite comprising chemical kinetic packages, Park's two-temperature model, and a modified Landau–Teller energy transfer formula, thereby accounting for the high-temperature effect. Furthermore, an undecomposed algorithm with popular numerical methods is also applied to solve the NCCR model within the finite volume framework. The capability of the NCCR solution to simulate thermochemical nonequilibrium flows was validated through three numerical tests, spanning the continuum to rarefied regimes. The comparative benchmark data were derived from wind tunnel experiments, in-flight tests, and direct simulation Monte Carlo method. It is noteworthy that the results of the NCCR solution show a superior alignment with the benchmark data than those obtained by the Navier–Stokes equations. Moreover, it is found that the rarefaction effect captured by the NCCR solution has a pronounced impact on the energy relaxations and chemical reaction processes in hypersonic thermochemical nonequilibrium flows. These findings underscore the potential of NCCR theory as a valuable tool for simulating thermochemical nonequilibrium flows including rarefaction effects, ultimately shedding light on the prospective development of hypersonic cross-domain vehicles. [ABSTRACT FROM AUTHOR]

The article "Bursting bubble in an elastoviscoplastic medium" delves into the dynamics of bubble bursting in a complex fluid medium, examining the effects of elasticity and yield stress. Through numerical simulations, the study identifies four distinct regimes based on Deborah and plastocapillary numbers, shedding light on how elastic stress relaxation influences jet development and droplet formation. The research underscores the intricate interactions between elasticity, yield stress, and interfacial phenomena in elastoviscoplastic fluids, offering valuable insights into bubble bursting dynamics. [Extracted from the article]

The results of metallophysical studies of the specific features of steel and alloy structure after a thermal-deformation exposure to laser radiation are presented. It is shown that the locality of laser treatment in combination with high heating and cooling rates, lack of soaking at the heating temperature, and high temperature gradients along the depth of the irradiated layer lead to the appearance of thermostrictive stresses of up to ~ 320 MPa in value, which play an important role in strengthening the surface of steels and alloys. It has been discovered and confirmed that under the influence of stresses, a localized plastic deformation develops in the microvolumes of the irradiated material, accompanied by the movement of dislocations along the slip planes. Schemes are proposed for the formation of slip lines or bands, kink bands, relief grain boundaries, etc. during high-speed laser treatment. It has been shown that as a result of laser treatment, the defect density of the crystalline structure of the studied single-phase steels and alloys increases to (2–4)×1011 cm-2. This contributes to the improvement of mechanical properties (including hardness and shear stress) of the surface layers of irradiated materials. As a result of metallophysical studies, structural relaxation effects of thermostrictive stresses in the laser-treated zones of steels and alloys have been confirmed. By using practically single-phase ("model") alloys, such as copper and nickel alloys, corrosion resistant steels, and technical iron, it was shown that partial stress relaxation occurs due to dynamic polygonization and recrystallization processes that occur within the surface layers of the alloys, resulting in the formation of small fragmented grains measuring 0.5–2.0 μm in size. This reduces the risk of crack formation due to extreme thermal effects of laser radiation. It has been established that the level of hardening and the ultimate structure of steel are determined by the superposition of stresses, generated in the laser treatment zones, and by the processes of relaxation of the laser radiation energy as a result of local plastic deformation, dynamic polygonization, and recrystallization. [ABSTRACT FROM AUTHOR]

Ethylene propylene diene monomer (EPDM) rubber is used in critical sectors such as national defense, military, and aerospace. However, it has performance limitations under high temperatures, affecting its dynamic mechanical properties, creep, and stress relaxation. To overcome these limitations, rare earth cerium oxide (CeO2) was incorporated as a filler into EPDM in this study, which enhances its properties and makes it suitable for the demanding applications. It could be seen that the CeO2‐enriched EPDM has significantly improved mechanical characteristics at elevated temperatures. The Young's modulus of CeO2‐enriched EPDM increased by about 100 %, while the energy storage modulus reaches a maximum increase of around 140 % at 165 °C. The addition of CeO2 improves its damping properties, provides better creep and stress relaxation resistance, and results in greater residual strain recovery. At room temperature, the impact on the stress relaxation rate at 50 % strain is minimal for the material EPDM, and it is almost negligible for the material EPDM/CeO2.These findings demonstrate that CeO2‐enriched EPDM has enhanced performance under high‐temperature conditions. Under elevated temperature conditions, EPDM enriched with CeO2 exhibits enhanced mechanical properties, superior creep resistance, and stable stress relaxation behavior, rendering it more adept at withstanding the rigorous conditions encountered in aerospace applications. [ABSTRACT FROM AUTHOR]

A viscoplastic model for representing plastic silts and clays in geotechnical static slope stability applications is presented. The PM4SiltR model builds on the stress ratio-controlled, critical state-based, bounding surface plasticity model PM4Silt and is coded as a dynamic link library for use in the finite difference program FLAC 8.1. PM4SiltR incorporates strain rate-dependent shear strength, stress relaxation, and creep using a consistency approach combined with an internal strain rate and auto-decay process. The model does not include a cap, and as such cannot simulate strain rate-dependent consolidation under increasing overburden stress. Six parameters control the viscous response for PM4SiltR, while the parameters controlling the nonviscous components of the response are the same as for PM4Silt. Single element simulations are presented to illustrate the influence of viscoplasticity on the constitutive response in direct simple shear loading and undrained creep. Single element responses are shown to be consistent with observed experimental results. Simulations of a hypothetical tailings dam constructed using the upstream method are performed to illustrate use of PM4SiltR at field scale. Results of field-scale simulations show PM4SiltR can model undrained creep and progressive failure leading to delayed slope instability after relatively minor changes in loading conditions at field scale. [ABSTRACT FROM AUTHOR]

[Display omitted] In this paper, microgels with uniform particle size were prepared by physically cross-linking the hydrophobically modified chitosan (h -CS) with sodium phytate (SP). The effects of cross-linking density on the interfacial adsorption kinetics, viscoelasticity, stress relaxation, and micorheological properties of the hydrophobically modified chitosan microgels (h -CSMs) at the oil–water interface were extensively investigated by the dilatational rheology, compressional rheology, and particle tracing microrheology. The results were correlated with the particle size, morphology, and elasticity of the microgels characterized by dynamic light scattering and atomic force microscopy. It was found that with the increase of cross-linking density, the h -CSMs changed from a polymer-like state to ultra-soft fussy spheres with higher elastic modulus. The compression isotherms demonstrated multi-stage increase caused by the interaction between the shells and that between the cores of the microgels successively. As the increase of cross-linking density, the h -CSMs diffused slower to the oil–water interface, but demonstrating faster permeation adsorption and rearrangement at the oil–water interface, finally forming interfacial layers of higher viscoelastic modulus due to the core-core interaction. Both the initial tension relaxation and the microgel rearrangement after interface expansion became faster as the microgel elasticity increased. The interfacial microrheology demonstrated dynamic caging effect caused by neighboring microgels. This article provides a more comprehensive understanding of the behaviors of polysaccharide microgels at the oil-water interface. [ABSTRACT FROM AUTHOR]

Expanded polypropylene (EPP) bead foam mainly consists of entrapped gas within closed polymer cells. This numerical study presents a method to account for the influence of this entrapped gas on the compression behavior of EPP foam. The method developed combines a finite element (FE) model of the foam structure with a smoothed particle hydrodynamics model to simulate the effect of the cell gas. The foam structure is modeled using the open‐source software neper, the FE simulations are conducted using the explicit FE solver of LS‐Dyna. Numerically obtained stress–strain curves for the investigated foam materials, both with and without considering the cell gas, are compared with experimental data from tests using a specially designed vacuum test chamber. The comparison shows a good agreement between numerical and experimental results, indicating that entrapped cell gas increases the structural stiffness under compression. However, in load‐hold‐unload tests, the numerical model fails to accurately capture the stress relaxation behavior observed during the hold phase of the experiment. This study highlights the significant impact of cell gas on the compression behavior of EPP foam and the need for further refinement in simulation strategy to capture effects like the stress relaxation and multiaxial loading. [ABSTRACT FROM AUTHOR]

Alumina (Al 2 O 3) based nanocomposites mark a paradigm shift in materials science, offering unprecedented opportunities for multifunctional applications. This study meticulously examines the mechanical properties of polymer nanocomposites (PNCs), with a specific focus on the impact of varying sizes and concentrations of Al 2 O 3 nanoparticles. By investigating these parameters, the research enables the tailoring of mechanical properties to meet specific requirements. The study reveals remarkable enhancements in the reduced modulus (E r), reaching up to 350%, 175%, and 100% under load and displacement control modes for 25 nm, 80 nm, and 200 nm average particle sizes, respectively. Beyond mechanical strength, the research delves into the nanocomposites resistance to long-term deformation (creep) and their ability to maintain consistent performance under load (stress relaxation). Additionally, the tribological performance lays the groundwork for the development of high-performance materials. The insights gained from this study represent more than an incremental improvement; they signify a transformative leap forward in addressing the multifaceted challenges of advanced material technology, with the potential to revolutionize various industries. [Display omitted] • Al 2 O 3 nanoparticles in PDMS modify interfaces, improving material performance. • The reinforcement boosts PDMS elastic modulus, hardness and mechanical strength. • Coactive effects of crystallinity and interface crosslinking optimize performance. • Nanoindentation shows structural changes during creep and stress relaxation. • Enhanced mechanical properties render nanocomposites ideal for diverse applications. [ABSTRACT FROM AUTHOR]

The use of high contents of recycled materials in asphalt mixtures has an adverse effect on the low-temperature cracking performance of these mixtures if not accounted for properly due to the high stiffness and poor relaxation properties of the reclaimed asphalt pavement (RAP) binder. This concern has driven some to suggest using recycling agents (RAs) in the asphalt mixture to restore the rheological properties of the RAP binder without compromising the short- and long-term performance of the mixture. However, the relationships between RA type and dosage, virgin and RAP binder characteristics, and low-temperature properties, i.e., creep stiffness and m -value, need to be investigated further to assess the acceptance of RAs. In this study, 26 recycled binder blends from a combination of three virgin binders, four RAP sources, and eight RAs were used to study these relationships. First, the use of a method based on the dynamic shear rheometer to predict low-temperature properties of recycled binder blends successfully is demonstrated. Second, a linear relationship is found to exist between the low-temperature properties and RA dosage of these blends. Third, a parameter based on low-temperature properties is proposed to select an RA that yields a balance in the stiffness and relaxation properties of the recycled binder blends. Fourth, a conceptual framework is proposed to assess the acceptance of RAs based on low-temperature performance-related parameters. [ABSTRACT FROM AUTHOR]

The non-exponential character of the structural relaxation is considered one of the hallmarks of the glassy dynamics, and in this context, the relatively narrow shape observed by dielectric techniques for polar glass formers has attracted the attention of the community for long time. This work addresses the phenomenology and role of specific non-covalent interactions in the structural relaxation of glass-forming liquids by the study of polar tributyl phosphate. We show that dipole interactions can couple to shear stress and modify the flow behavior, preventing the occurrence of the simple liquid behavior. We discuss our findings in the general framework of glassy dynamics and the role of intermolecular interactions. [ABSTRACT FROM AUTHOR]

Nanostructures with high surface area to volume ratio, such as oxidized and coated Cu nanowires (NWs), exhibit unique mechanical properties due to their size and surface effects. Understanding the complex oxidation process of Cu NWs at nanoscale and quantifying its resulting effects on mechanical behavior and properties are significantly essential for effective usage of Cu NW devices in a wide range of applications in nanoelectronics. Here, we perform molecular dynamics simulations using ReaxFF (reactive force field) to investigate the oxidation process and mechanisms of [001]-oriented cylindrical Cu NWs and its contribution on the mechanical deformation behavior and material properties as a function of NW sizes. The relatively thin oxide CuxOy layer is formed on the surface of Cu NWs in an O2 environment, creating a core/shell (Cu/CuxOy) NW structure that played a key role in governing the overall tensile mechanical deformation behavior and properties of Cu NW. The formation of oxide layer effects, including the resulting interface and defects, leads to a reduction in the initial dislocation nucleation barrier, which facilitates the onset of plasticity and stress relaxation, ultimately resulting in a negative impact on the tensile strength, Young's modulus, yield stress and strain, and flow stress when compared to pristine counterparts. It is worth noting that the tensile mechanical response and properties of the Cu NWs are highly dependent on the pre-existing oxide shell layer associated with the size of NW, determining the overall mechanical performance and properties of Cu NWs. [ABSTRACT FROM AUTHOR]

Conjugated polymers consist of complex backbone structures and side‐chain moieties to meet various optoelectronic and processing requirements. Recent work on conjugated polymers has been devoted to studying the mechanical properties and developing new conjugated polymers with low modulus and high‐crack onset strain, while the thin film mechanical stability under long‐term external tensile strain is less investigated. Here we performed direct mechanical stress relaxation tests for both free‐standing and thin film floated on water surface on both high‐Tg and low‐Tg conjugated polymers, as well as a reference nonconjugated sample, polystyrene. We measured thin films with a range of film thickness from 38 to 179 nm to study the temperature and thickness effect on thin film relaxation, where an apparent enthalpy–entropy compensation effect for glassy polymer PS and PM6 thin films was observed. We also compared relaxation times across three different conjugated polymers and showed that both crystalline morphology and higher modulus reduce the relaxation rate besides higher glass transition temperature. Our work provides insights into the mechanical creep behavior of conjugated polymers, which will have an impact on the future design of stable functional organic electronics. [ABSTRACT FROM AUTHOR]

The improvement of computing power provides a support for finite element simulation analysis, which can solve more complex structures and nonlinear problems more efficiently. In this study, based on the finite element simulation analysis technology, the complex mechanical properties of Mn-Cu alloy are simulated. Prony series constitutive model and parallel rheological constitutive model are respectively adopted to simulate the stress relaxation test of Mn-Cu alloy under different deformation conditions. The stress relaxation experimental data are extracted and converted into the corresponding model parameters and the finite element simulation calculation is carried out. The simulation model parameters which can accurately describe the nonlinear mechanical properties of Mn-Cu alloy are obtained. By comparing the simulation and experimental results, it is concluded that the parallel rheological framework can more accurately characterize the stress relaxation behavior of Mn-Cu alloy. Under different initial strain conditions, the relative error of the simulation results and the test results under corresponding working conditions is less than 1%. Compared with Prony series constitutive model, parallel rheological framework is more suitable for the simulation of Mn-Cu alloy under complex working conditions. [ABSTRACT FROM AUTHOR]

The development of tests, standards, and specifications necessitates the establishment of technical validity while concurrently optimizing costs associated with the testing requirements. AASHTO TP 125-16 recommends the utilization of the bending beam rheometer (BBR) as a simple, rapid, relatively cost-effective, and reproducible method for assessing the low-temperature properties of asphalt concrete mixtures. However, the standard does not provide specific guidance regarding the representative sample size necessary for the BBR test. Therefore, this study used statistical techniques to calculate the sample size by scrutinizing data for two variables, i.e., creep moduli (stiffness) and stress relaxation capacity (m -value), which were obtained from testing a variety of asphalt concrete mixtures. In this regard, distinct asphalt mixtures were meticulously prepared and subjected to laboratory testing. A representative sample size of eight thin-beam specimens was determined. This recommendation was validated by sampling five different asphalt mixtures from pavement construction projects. It was found that testing and analyzing trimmed means of only eight specimens from a 2 in. thick asphalt concrete layer was adequate to represent the global low-temperature performance of those five pavement sections. So, an asphalt mixture's low-temperature performance can be completed in 2 h by testing only eight specimens in the BBR without sacrificing accuracy and practicality. Therefore, this study contributes to a possible application of the BBR during pavement construction. [ABSTRACT FROM AUTHOR]

This work reports a new methodology using indentation stress relaxation to characterize the dislocation velocity–stress exponent. Through the indentation stress relaxation process, the dislocation structure builds up at the rate governed by dislocation velocity, which is a function of the externally applied stress. The relationship between the dislocation velocity and stress can thus be derived from the indentation stress relaxation data of the stress as a function of time. In this study, instrumented nanoindentation stress relaxation experiments were performed on pure aluminum samples, following three different initial displacement rates of 100, 400, and 800 nm/s. Based on the scaling properties of dislocation kinetics, the data were interpreted to derive a dislocation velocity–stress exponent of 2.5 ± 0.5 for room-temperature aluminum. Crystal plasticity finite-element simulations were performed to illustrate the sensitivity of the proposed nanoindentation stress relaxation methodology to the dislocation velocity–stress exponent value. [ABSTRACT FROM AUTHOR]

Polyamide 12 (PA12) is vastly utilized in many additive manufacturing methods, such as Selective Laser Sintering (SLS), and a better understanding of its mechanical behaviors promotes available knowledge on the behaviors of 3D-printed parts made from this polymer. In this paper, SLS-produced standard tensile specimens are studied under monotonic and cyclic tension tests, as well as stress relaxation experiments, and the obtained force-displacement responses are shown to be consistent with a hyper-viscoelastic material model. This finding is also observed in typical pantographic structures produced by the same manufacturing parameters. To propose a constitutive model for predicting these behaviors, the convolution integral of a strain-dependent function and a time-dependent function is developed where the material parameters are determined with the use of both short-term and long-term responses of the specimens. Numerical results of the presented model for standard test specimens are shown to be in good agreements with the experimental ones under various loading conditions. To prove the capabilities of the proposed model in studying any SLS-produced part, finite element implementation of the constitutive equations is shown to provide numerical results in agreement with the empirical findings for tensile loading of the 3D-printed pantographic structure. [ABSTRACT FROM AUTHOR]

With the development of medical diagnosis and treatment, knowing the mechanical properties of living tissues becomes critical. The aim of this study was to investigation material properties of the fresh porcine kidney and the parametric characterization of its viscoelastic material behavior. The material investigation included uniaxial tension tests in different strain rates, relaxation tests, as well as hydrostatic compression tests on the samples extracted from the fresh porcine kidney cortex. Tension tests and relaxation tests were performed by a planar dog-bone specimen with a micron loading testing machine. Hydrostatic compression tests were performed on the kidney cylinder sample which was placed in a compression chamber. Furthermore, a nonlinear viscoelastic model recently proposed by us was employed to characterize the tension data at different strain rates and relaxation test data. The the experimental and numerical results show that the stress-strain relations of the porcine kidney cortex at different strain rates in tension are presented for the first time and a higher strain rate results in higher ultimate strength and initial Young modulus but a lower rupture strain. A damage-dependent visco-elastic model is employed to model the tension data at different strain rates and relaxation data and exhibits a good agreement with the experimental data, which also demonstrates that the damage has an obvious influence on the stress-strain relation. Through comparison with the existing reference covering the uniaxial compression data, it seems that the mechanical behavior of the porcine kidney cortex manifests a stress state-dependent mechanical behavior. The ultimate strength and rupture strain are larger in compression than that in tension. [ABSTRACT FROM AUTHOR]

Enhancement of thermally activated dislocation motion by solute hydrogen (H) has been envisaged in Fe–Cr–Ni austenitic steel through accelerated stress relaxation and a prolonged creep duration. Nevertheless, differences in the imposed stress/strain between the compared non- and H-charged samples at the starts of these mechanical transients, as well as involvements of other obstacles (e.g., alloying elements and forest dislocations), mask the essential effects of H. We performed stress relaxation and strain rate jump tests at multiple stress/strain for Type310S austenitic steel with ∼7600 at ppm H at 296 K. The measured strain rate sensitivity (SRS) was evaluated via a methodology so-called Haasen plot. By screening the latent factors above, primary role of H was revisited: they work as short-range obstacles, hindering the dislocation movement. Multiple H atoms potentially participate in each thermal activation event, giving rise to a stress-equivalent activation volume and a proportionality between H concentration and yield strength. [Display omitted] • Revision of the tenet on an enhanced thermally activated dislocation motion by H. • Transient mechanical tests involving stress relaxation and strain rate jump. • Strain rate sensitivity (SRS) vs. flow stress as an evaluation method: Haasen plot. • Increase of SRS by solute H as short-range obstacles for dislocation motion. • A new model for rationalizing the H-induced solid solution-hardening. [ABSTRACT FROM AUTHOR]

During low‐temperature tempering of wear‐resistant steel, ε‐carbide precipitation can effectively relax residual stress, and its microscopic mechanism is currently unclear. To reveal the stress relaxation mechanism during ε‐carbide precipitation, density functional theory calculations are combined with experiments to explore the ε‐carbide growth behavior and the stress evolution. The results show that ε‐carbide precipitation includes two stages: ε‐precursor formation and ε‐carbide nucleation, and ε‐carbide is in situ nucleated from ε‐precursor matrix. The precipitation‐induced stress relaxation behavior mainly occurs in the early stage of ε‐precursor formation and ε‐carbide nucleation. The early stage of ε‐precursor formation and ε‐carbide precipitation can relax the residual stress to 35.73% and 27.07%, respectively. [ABSTRACT FROM AUTHOR]

A systematic analytical study of the mathematical properties of the previously constructed nonlinear model for shear flow of thixotropic viscoelastic-plastic media, which takes into account the mutual influence of the deformation process and structure evolution, is continued. A set of two nonlinear differential equations describing the processes of shear at a constant rate and stress relaxation is obtained. Equation set describing creep is derived; a general solution of the Cauchy problem for the set is constructed in an explicit form (the equations of the families of creep, and structuredness curves are derived). For arbitrary six material parameters and (increasing) material function that govern the model, basic properties of the families stress-strain curves at constant strain rates, stress relaxation curves and creep curves generated by the model, and the features of structuredness evolution under these types of loading are analytically studied. The dependences of these curves on time, shear rate, stress level, initial strain, and initial structuredness of the material, as well as on the material parameters and function of the model, are studied. Several indicators of the applicability of the model are found which are convenient to check with experimental data. It was examined what effects typical for viscoelastic-plastic media can be described by the model and what unusual effects (unusual properties) are generated by a change in structuredness in comparison with typical stress-strain curves, relaxation curves, and creep curves of structurally stable materials. In particular, it is proved that creep curves always increase in time and have oblique asymptote, and structuredness under constant stress is always monotonous (unlike other loading modes), but can decrease or increase depending on the relation between the stress level and initial structuredness. The same condition controls creep curves to be convex up or down: at a certain (calculated) critical load creep curves change from convexity up (under smaller loads) to convexity down, and the structuredness becomes ascending instead of descending. The analysis proved the ability of the model to describe behavior of not only liquid-like viscoelastoplastic media, but also solid-like (thickening, hardening, hardened) media: creep, relaxation, recovery, a number of typical properties of experimental relaxation curves, creep and stress-strain curves, strain rate and strain hardening, flow under constant stress and so on. [ABSTRACT FROM AUTHOR]

Previous studies have shown that gel‐spun‐ultra‐high‐molecular‐weight polyethylene (UHMWPE) produces thin fibril products that exhibit high tensile moduli (35–200 GPa). The elaborate gel‐spinning process involves complex drawing stages with solvent incorporation. In this study, a previously proposed two‐stage, environmentally friendly solventless methodology was optimized. The two‐stage process included cross‐rolling (Stage 1) and orientation (Stage 2) to obtain oriented HDPE thin rods with an impressively high modulus using conventional HDPE. The optimization of the process was successfully achieved by thoroughly investigating the voiding mechanism. In addition, rapid relaxation during orientation supports the cavitation mechanism. Owing to this optimization, a modulus of 75 GPa was readily attained. The significant enhancement in the mechanical properties was a direct result of the optimization of our processing methodology to achieve a high degree of orientation. Notably, the fabricated oriented HDPE thin rods showed moduli comparable to those of the gel‐spun UHMWPE fibers but were at least 40 times thicker. Our comprehensive characterization of the voiding process and stress relaxation during our two‐stage process indicated the formation of a highly taut network structure and craze‐like configuration with controlled delamination. Thus, our proposed hierarchical model was refined to elucidate the process‐structure‐property relationships in greater detail. Highlights: An optimized two‐stage environmentally friendly solventless process has been developed to create oriented polyethylene thin rods with impressively high modulus (75 GPa).The optimization was achieved by thoroughly investigating the voiding effect during cross‐rolling and crystalline relaxation during orientation.Comparison of the modulus from our process are similar to various commercial, gel‐spun fibers. Our thin rod products are at least 40 times thicker than commercial gel‐spun fibers.The thin rod product has impressively high modulus‐to‐weight and strength‐to‐weight ratios for future study in composite systems. [ABSTRACT FROM AUTHOR]

Extensive experimental studies have demonstrated the time-dependent mechanical behaviors of frozen soil. Nonetheless, limited studies are focusing on the constitutive modeling of the time-dependent stress–strain behaviors of frozen clay soils at different subzero temperatures. The objective of this study is to numerically investigate the time-dependent behavior of frozen clay soils at a temperature range of 0 °C to − 15 °C. The Drucker–Prager model is adopted along with the Singh–Mitchell creep model to simulate time-dependent uniaxial compression and stress relaxation behaviors of frozen sandy clay soil. The numerical modeling is implemented through the finite element method based on the platform of Abaqus. The constitutive modeling is calibrated by a series of experimental results on laboratory-prepared frozen sandy clay soils, where the strain hardening, the post-peak softening, and stress relaxation behaviors are captured. Our results show that both the rate-dependent model and creep model should be adopted to characterize a comprehensive time-dependent behavior of frozen soils. The rate-dependent stress–strain behaviors heavily rely on the rate- and temperature-dependent hardening functions, where the creep strain provides a very limited contribution. Nevertheless, the creep strain should also be adopted when a long-term analysis or stress relaxation behavior is involved. [ABSTRACT FROM AUTHOR]

The properties of MAX phase ceramics that possess metallic and ceramic nature are anisotropic because of their hexagonal and layered crystal structure. Slip casting under a strong magnetic field followed by spark plasm sintering is one of the methods to control the orientation of the MAX phase ceramics. In this study, first, the textured Ti2AlC-MAX phase was fabricated by slip cast under 12 T, where the direction of magnetic field was fixed to the slip cast direction after clarifying the easily magnetizable axis of Ti2AlC. Second, the crystal orientation dependence of the room temperature deformation behavior was investigated by the Vickers hardness test from various orientations. It was found that the easily magnetizable axis of Ti2AlC was the c-axis and the textured body could be obtained by applying the magnetic field from one direction. In the room temperature deformation, Vickers indents show an anisotropic shape and the hardness changed depending on the loading direction against the c-axis due to different deformation modes such as kink deformation and slip deformation. For 0°, since the slip deformation was unlikely to occur, linear crack growth was observed to form from the indent. For 45°, on the other hand, since the slip deformation occurred relatively easily, the crack growth was suppressed through the stress relaxation due to the slip deformation. For 90°, although the slip deformation is unlikely to occur similar to 0°, the crack propagation was suppressed through pile-up due to kink deformations along the c-axis. It can be concluded that kink deformation prevents crack propagation and increases the fracture toughness of the textured Ti2AlC-MAX phase. [ABSTRACT FROM AUTHOR]

Rate sensitive deformation behavior of a soft Sn–0.6%Cu alloy was investigated by the macro-indentation and indentation-relaxation tests at room temperature to evaluate the consistency between the results found by different localized testing methods. Testing by cylindrical, pyramidal, and spherical indenters yielded strain rate sensitivity indices, which were in good agreement with each other. It is concluded that despite different geometries, stress ranges, and testing conditions, very similar results could be achieved by these small-scale tests. [ABSTRACT FROM AUTHOR]

The effect of the network-to-molecular structural transformation with increasing phosphorus content in PxSe100−x (30 ≤ x ≤ 67) supercooled liquids on their shear-mechanical response is investigated using oscillatory shear rheometry. While network liquids with 30 ≤ x ≤ 40 are characterized by shear relaxation via a network bond scission/renewal process, a Maxwell scaling of the storage (G′) and loss (G″) shear moduli, and a frequency-independent viscosity at low frequencies, a new relaxation process emerges in liquids with intermediate compositions (45 ≤ x ≤ 50). This process is attributed to an interconversion between network and molecular structural moieties. Predominantly molecular liquids with x ≥ 63, on the other hand, are characterized by a departure from Maxwell behavior as the storage modulus shows a linear frequency scaling G′(ω) ∼ ω over nearly the entire frequency range below the G′–G″ crossover and a nearly constant ratio of G″/G′ in the terminal region. Moreover, the dynamic viscosity of these rather fragile molecular liquids shows significant enhancement over that of network liquids at frequencies below the dynamical onset and does not reach a frequency-independent regime even at frequencies that are four orders of magnitude lower than that of the onset. Such power-law relaxation behavior of the molecular liquids is ascribed to an extremely broad distribution of relaxation timescales with the coexistence of rapid rotational motion of individual molecules and cooperative dynamics of transient molecular clusters, with the latter being significantly slower than the shear relaxation timescale. [ABSTRACT FROM AUTHOR]

Gels made of telechelic polymers connected by reversible cross-linkers are a versatile design platform for biocompatible viscoelastic materials. Their linear response to a step strain displays a fast, near-exponential relaxation when using low-valence cross-linkers, while larger supramolecular cross-linkers bring about much slower dynamics involving a wide distribution of timescales whose physical origin is still debated. Here, we propose a model where the relaxation of polymer gels in the dilute regime originates from elementary events in which the bonds connecting two neighboring cross-linkers all disconnect. Larger cross-linkers allow for a greater average number of bonds connecting them but also generate more heterogeneity. We characterize the resulting distribution of relaxation timescales analytically and accurately reproduce stress relaxation measurements on metal-coordinated hydrogels with a variety of cross-linker sizes including ions, metal-organic cages, and nanoparticles. Our approach is simple enough to be extended to any cross-linker size and could thus be harnessed for the rational design of complex viscoelastic materials. [ABSTRACT FROM AUTHOR]

This paper presents geometric analyses of welded frames after free relaxing and vibratory stress relief (VSR). The tested frames were components of a prototype packaging machine. Two types of relaxation were carried out to remove stresses introduced as a result of the welding process. One of the frames was subjected to free relaxation, while the other one was subjected to accelerated vibration relaxation. Detection of the frame geometry changes was performed using a photogrammetric system. In addition, an evaluation of the geometry change was conducted for fifteen variants of a steel frame support. A comparative analysis of the geometric deviations of the frames after free and vibratory stress relief confirmed the assumption that the frame post vibration stress relief better reproduces the nominal dimensions. Nevertheless, it should be emphasized that after vibratory stress relief, the frame is not subject to further deformation, which is a desirable effect. In the case of free relaxing, the frame undergoes dimensional changes in a random manner. In summary, carrying out accelerated vibratory stress relief allows for control of spontaneous dimensional changes in the designed frame of a packaging machine resulting from spontaneous relaxation of stresses arising from the welding process. The shortening of the relaxation process of the welded frame is also an unquestionable advantage. [ABSTRACT FROM AUTHOR]

The onset of thermal convection in an incompressible Oldroydian fluid‐saturating porous media is examined to study the combined impact of uniform rotation and temperature‐dependent viscosity. A characteristic equation from the basic hydrodynamic equations governing the Brinkman–Oldroyd model is derived using linear stability theory and modified Boussinesq approximation. For various combinations of stress‐free and slip‐free boundaries, the expressions for the Darcy–Rayleigh numbers for both non‐oscillatory as well as oscillatory convection with linear and exponential temperature‐dependent viscosity are derived, using the "weighted residual method." The effects of rotation, variable viscosity parameter, strain retardation and stress relaxation time parameters and other fluid parameters on non‐oscillatory and oscillatory convection are investigated numerically and the results are presented graphically. From the analysis, it is found that overstability is the preferred mode of onset of convection. The rotation, the coefficient of specific heat variations (due to temperature variation), and the strain retardation time have a stabilizing influence on the stability of the system, whereas the stress relaxation imparts a destabilizing effect. Additionally, it is noticed that the variable viscosity parameter and Brinkman‐Darcy number stabilize the system for each set of boundary conditions. [ABSTRACT FROM AUTHOR]

Couple-stress nanofluids have multiple potential applications in numerous industrial and engineering sectors, such as energy production, medical diagnostics, thermal control systems, and the aerospace industry. Couple-stress nanofluids have the ability to improve the heat exchange properties and elevate the performance of nuclear power plants, solar panels, and other renewable energy sources. Therefore, in the current analysis, a non-homogeneous nanofluid model is considered to examine the non-Newtonian Casson nanofluid flow across a prolonging sheet. The flow has been studied under the significance of generalized Fourier's and Fick's laws, convective boundary conditions, and the heat source/sink. The modeled equations are simplified into a dimensionless lowest-order system of ordinary differential equations by using similarity transformation. The numerical outcomes are achieved by using the "ND-Solve" approach. It has been noticed that the energy field decreases because of the Prandtl number's impacts, whereas it increases with the increase in the heat radiation parameter. The couple-stress nanoliquid's velocity decreases vs increasing values of the magnetic field and mixed convection parameter. The influence of thermal relaxation and couple-stress parameters falls off the energy field. Furthermore, the intensifying effect of Rayleigh number and buoyancy ratio increases the fluid temperature. [ABSTRACT FROM AUTHOR]

Polylactic acid (PLA) is considered a promising biodegradable polymer alternative. Due to its high brittleness, composite materials made by melt blending thermoplastic polyurethane (TPU) with PLA can enhance the toughness of PLA. To understand the forced aging caused by stress relaxation in polymer materials, this study explains the stress relaxation experiments of PLA/TPU blends with different mass ratios under applied strain through mechanical model simulations. The Kelvin representation of the standard linear solid model (SLSM) is used to analyze the stress relaxation data of TPU/PLA blends, successfully explaining that the Young's moduli (E1 and E2) of springs decrease with increasing temperature and TPU content. The viscosity coefficient of the PLA/TPU blends decreases with increasing temperature, and its reciprocal follows the Arrhenius law. For TPU/PLA blends with increased concentration of TPU, the activation energy for stress relaxation shows a linear decrease, confirmed by the glass transition point measured by DMA, indicating that it does not involve chemical reactions. [ABSTRACT FROM AUTHOR]

Considerable development has been observed in the area of applying fractional-order rheological models to describe the viscoelastic properties of miscellaneous materials in the last few decades together with the increasingly stronger adoption of fractional calculus. The fractional Maxwell model is the best-known non-integer-order rheological model. A weighted least-square approximation problem of the relaxation modulus by the fractional Maxwell model is considered when only the time measurements of the relaxation modulus corrupted by additive noises are accessible for identification. This study was dedicated to the determination of the model, optimal in the sense of the integral square weighted model quality index, which does not depend on the particular sampling points applied in the stress relaxation experiment. It is proved that even when the real description of the material relaxation modulus is entirely unknown, the optimal fractional Maxwell model parameters can be recovered from the relaxation modulus measurements recorded for sampling time points selected randomly according to respective randomization. The identified model is a strongly consistent estimate of the desired optimal model. The exponential convergence rate is demonstrated both by the stochastic convergence analysis and by the numerical studies. A simple scheme for the optimal model identification is given. Numerical studies are presented for the materials described by the short relaxation times of the unimodal Gauss-like relaxation spectrum and the long relaxation times of the Baumgaertel, Schausberger and Winter spectrum. These studies have shown that the appropriate randomization introduced in the selection of sampling points guarantees that the sequence of the optimal fractional Maxwell model parameters asymptotically converge to parameters independent of these sampling points. The robustness of the identified model to the measurement disturbances was demonstrated by analytical analysis and numerical studies. [ABSTRACT FROM AUTHOR]

This article explores the phenomenon of adiabatic shear bands (ASB) in materials under dynamic loading. It discusses the initiation and growth of ASB, as well as the influence of initial defects and thermoplastic instability. The study uses statistical theory and self-similar solutions to understand the behavior of microshears and the scaling properties of shear bands. The findings contribute to a better understanding of ASB phenomena and their impact on material properties. The article includes experimental analysis and numerical simulations to support its conclusions. [Extracted from the article]

To describe the evolution of residual stresses in epoxy resin during the curing process, a more detailed characterization of its viscoelastic properties is necessary. In this study, we have devised a simplified apparatus for assessing the viscoelastic properties of epoxy resin. This apparatus employs a confining cylinder to restrict the circumferential and radial deformations of the material. Following the application of load by the testing machine, the epoxy resin sample gradually reduces the gap between its surface and the inner wall of the confining cylinder, ultimately achieving full contact and establishing a continuous interface. By recording the circumferential stress–strain on the outer surface of the confining cylinder, we can deduce the variations in material bulk and shear moduli with time. This characterization spans eight temperature points surrounding the glass transition temperature, revealing the bulk and shear relaxation moduli of the epoxy resin. Throughout the experiments, the epoxy resin's viscoelastic response demonstrated a pronounced time-temperature dependency. Below the glass transition temperature, the stress relaxation response progressively accelerated with increasing temperature, while beyond the glass transition temperature, the stress relaxation time underwent a substantial reduction. By applying the time-temperature superposition principle, it is possible to construct the relaxation master curves for the bulk and shear moduli of the epoxy resin. By fitting the data, we can obtain expressions for the constitutive model describing the viscoelastic behavior of the epoxy resin. In order to validate the reliability of the test results, a uniaxial tensile relaxation test was conducted on the epoxy resin casting body. The results show good agreement between the obtained uniaxial relaxation modulus curves and those derived from the bulk and shear relaxation modulus equations, confirming the validity of both the device design and the testing methodology. [ABSTRACT FROM AUTHOR]

Curving woodworks exhibit wide applications in wooden furniture manufacturing due to the conformable visual and tactile experience, are welcome among consumers. However, the corresponding long-term shape stability facing the stress relaxation is an attention-worthy issue with the fundamental insights about the mesoscopic behaviors of vessels have not been provided. Here, we proposed the investigation into the effect of vessels on stress relaxation behavior of white oak (Quercus alba L.). Results demonstrated that stress relaxation modulus of specimen decreased with the elevation in MC and temperature, and vessel-containing specimens displayed more evident stress relaxation. Providing more physical connotation, Zener model is more suitable for the numerically investigation into the stress relaxation than logarithmic model. The time-temperature-moisture superposition principle (TTMSP) allows us to predict the~10 d and ~1 d stress relaxation behavior of white oak facing different temperatures and MCs. Additionally, stress relaxation of white oak with vessels is more evident, which can be attributed to the stress concentration and resultant deformation around the vessel. We believe that this work is beneficial for obtaining a mesoscopic understanding of vessels on stress relaxation behaviors of wood. It thus paves a way to exploit the methodologies to improve the shape-stability of curving woodworks. [ABSTRACT FROM AUTHOR]

Mechanical Control of Relaxation refers to the dependence of myocardial relaxation on the strain rate just prior to relaxation, but the mechanisms of enhanced relaxation are not well characterized. This study aimed to characterize how crossbridge kinetics varied with strain rate and time-to-stretch as the myocardium relaxed in early diastole. Ramp-stretches of varying rates (amplitude = 1% muscle length) were applied to intact rat cardiac trabeculae following a load-clamp at 50% of the maximal developed twitch force, which provides a first-order estimate of ejection and coupling to an afterload. The resultant stress-response was calculated as the difference between the time-dependent stress profile between load-clamped twitches with and without a ramp-stretch. The stress-response exhibited features of the step-stretch response of activated, permeabilized myocardium, such as distortion-dependent peak stress, rapid force decay related to crossbridge detachment, and stress recovery related to crossbridge recruitment. The peak stress was strain rate dependent, but the minimum stress and the time-to-minimum stress values were not. The initial rapid change in the stress-response indicates enhanced crossbridge detachment at higher strain rates during relaxation in intact cardiac trabeculae. Physiologic considerations, such as time-varying calcium, are discussed as potential limitations to fitting these data with traditional distortion-recruitment models of crossbridge activity. [ABSTRACT FROM AUTHOR]

The paper presents an experimental study and numerical simulation of stress relaxation behavior of isotropic and anisotropic magnetorheological elastomers (MREs) made from silicone rubber filled with micro-sized carbonyl iron powder. Effects of applied constant strains and magnetic fields of an electromagnet on the stress relaxation of the MREs were investigated for 10 h using the single relaxation test with double-lap shear samples. The isotropic MRE showed a linearly elastic behavior, while the anisotropic MRE indicated a highly nonlinear elastic response. The shear stress and relaxation modulus of anisotropic MRE are much higher than those of isotropic MRE. The shear stress of the MREs increased with increasing the constant strain, while their relaxation modulus decreased. The shear stress and relaxation modulus of the MREs within the first 0.25 h boosted with raising the magnetic flux density to about 0.5 T. However, the shear stress and relaxation modulus of the MREs under strong magnetic fields declined considerably after 0.25 h testing. This reduction was attributed to the temperature rise in the MRE samples caused by the heating of the electromagnet. The stress relaxation behavior of the MREs was examined using a four-parameter fractional derivative model. The model parameters were obtained by fitting the relaxation modulus to the measured data of the MREs. The shear stress and relaxation modulus with long-term predictions estimated from the studied model were in good agreement with the measured data for the MREs at various applied strains and under low magnetic fields. The model-predicted values did not agree well with the experimental data of the MREs under high magnetic fields because of the sample temperature gain caused by heat generation of the electromagnet. Therefore, the investigated model can be used to predict the long-term relaxation stress of the MREs under high magnetic fields of permanent magnets. [ABSTRACT FROM AUTHOR]

During an extensive test programme at the Bundesanstalt für Materialforschung und prüfung, material property changes of EPDM O-rings were investigated at different ageing times and two ageing temperatures of 125 ∘ C and 150 ∘ C . To exclude possible diffusion-limited oxidation (DLO) effects that can distort the data, IRHD microhardness measurements were taken over the cross section of compressed O-rings. Continuous stress relaxation measurements were taken on samples free of DLO effects. The additional effect of physical processes to irreversible chemical ones during a long-term thermal exposure is quantified by the analysis of compression set measurements under various test conditions. By combining the different experimental methods, characteristic times relative to the degradation processes were determined. On the basis of experimental data, a microphysically motivated model that takes into account reversible and irreversible processes was developed. The parameter identification strategy of the material model is based on our experimental investigations on homogeneously aged elastomer O-rings. The simulated results are in good agreement with the experiments. [ABSTRACT FROM AUTHOR]

The chemical structure, polymer mobility and mechanical properties are studied for a cross-linked amorphous poly(ether urethane) (PU) from glass transition to rubber elasticity for juvenile dry samples and for water-saturated states after exposure to humid air (r.h. = 29, 67, 95, 100%) at 60 ∘ C during 1 y of ageing. For saturated samples, network chain cleavage is the chemical ageing mechanism, but it is too weak and slow to affect on the physical properties significantly within 1 y. Water acts primarily in a physical manner. Within 1 d, H 2 O molecules replace part of the weak urethane H-bonds by H 2 O –urethane H-bonds and reduce all other physical interactions between network chains by solvating hydrophilic segments. Thus, the cooperative polymer mobility strongly amplifies: The gain of specific conformational entropy doubles across the caloric glass transition, which shifts by −17 K. A H 2 O concentration of only c H 2 O ≈ (0.4 ... 0.5) c H 2 O , max suffices for the major part of these fast rearrangements. Some part of the water slowly forms (during 3–4 months) a finely dispersed water-rich mixed phase with the PU chains. Except the new phase, these molecular processes of physical ageing strongly affect the mechanical properties at damage-free deformation. For dry PU in the glass transition, the shear modulus, μ relaxed (T), after viscoelastic stress relaxation only depends on the deformation-induced entropy change—like in the rubber elastic state. Within one month, water drastically decreases the viscoelastic response, as expected for plasticisation. However, μ relaxed (T) slightly grows in wet PU. H 2 O molecules cause these opposite trends by boosting the cooperative mobility (i.e. extension of the accessible conformational space and entropy by reduction in energy barriers) and by occupation of free volume compartments. Water quickly reduces the fracture parameters by about 50%. We explain that embrittlement by the H 2 O -induced facilitation of cooperative network chain motions, which let fracture proceed with less energy. In summary, our findings provide a detailed conception of the molecular effects the H 2 O molecules have on the PU network, and they explain the consequences for the mechanical properties. [ABSTRACT FROM AUTHOR]

A systematic analytical study of the mathematical properties of the previously constructed nonlinear model of the shear flow of thixotropic viscoelastic-plastic media, which takes into account the mutual influence of the deformation process and structure evolution, is carried out. A set of two nonlinear differential equations describing shear at a constant rate and stress relaxation was obtained. Assuming six material parameters and an (increasing) material function that control the model are arbitrary, the basic properties of the families of stress-strain curves at constant strain rates, stress relaxation curves (Part 2) and creep curves (Part 3) generated by the model, and the features of the evolution of the structuredness under these types of loading were analytically studied. The dependences of these curves on time, shear rate, stress level, initial strain and initial structuredness of material (for example, degree of physical crosslinking), as well as on material parameters and function governing the model, were studied. Several indicators of the model applicability are found, which are convenient to check with experimental data. It was examined what effects typical for viscoelastic-plastic media can be described by the model and what unusual effects (properties) are generated by structuredness changes in comparison to typical stress-strain, relaxation and creep curves of structurally stable materials. The analysis proved the ability of the model to describe behavior of not only liquid-like viscoelastoplastic media, but also solid-like (thickening, hardening, hardened) media: the effects of creep, relaxation, recovery, a number of typical properties of experimental relaxation curves, creep and stress-strain curves at a constant rate, strain rate and strain hardening, flow under constant stress, etc. The first part of the article is devoted to formulation of the model and preparation of basis for the second part: the proof of the uniqueness and stability of the equilibrium point of the nonlinear equations set, analytical study of the equilibrium point dependence on all material parameters, possible types of phase portraits and the properties of integral and phase curves of the model. [ABSTRACT FROM AUTHOR]

The conventional design approaches for reclaimed asphalt pavement (RAP) mix follow blending rules to select the grade of the virgin binder for hot recycled bituminous mixtures. The interactions of RAP binder with the virgin binder are highly complex in the presence of the rejuvenator. Identifying the optimal proportion of RAP binder, virgin binder, and rejuvenator is often a trial-and-error procedure, involving considerable experimental data collection. The current research study proposes the statistical-based design of experiments (DoE) framework to design ternary RAP binder blend, develop response surface prediction models, and optimize the proportions to meet targeted binder properties of viscosity grade (VG30) binder. A desirability-based multiple response optimization is carried out, and the optimal blend proportions with target binder properties are arrived at with different desirability levels. A total of 13 RAP binder blends are formulated using the I-Optimal algorithm in the current study. Experimental data such as softening point, high-temperature performance grade (PG), and linear viscoelastic parameters in the temperature range of 46°C to 70°C and frequency range of 0.1 to 50 Hz are collected. The binder blends are evaluated using master curve parameters and relaxation spectrum. The response surface models are developed for two measured parameters (high-temperature PG and softening point), three computed master curve parameters, and four relaxation spectrum parameters. These models capture the possible individual component effects and combined component effects on the final RAP binder blend through linear and nonlinear terms. A desirability-based simplex optimization is adopted to identify combinations of constituents at various levels of desirability that comprises only direct measurements, direct measurements and master curve parameters, and direct measurements, master curve parameters, and relaxation spectrum. Using this proposed approach, a wide range of solution zones are identified with various levels of desirability. Reclaimed asphalt pavement (RAP) consisting of aged bitumen coated on mineral aggregates is used for recycling in pavement construction. Many challenges exist related to the optimal use of such material. The aged binder may need to have the required properties suitable for recycling. Generally, rejuvenators are added to the RAP to enhance the properties of aged binder. However, how much rejuvenator should be added is unclear. Aged bitumen from different sources of RAP can have different degrees of aging. In addition, it may not be easy to estimate the extent of blending of the RAP with the virgin materials (aggregates and bitumen), as it is linked to the characteristics of aged binder and the choice of fresh bitumen and rejuvenator. Also, the aged bitumen, when mixed with a rejuvenator and virgin bitumen, may not necessarily result in a final mix with bitumen that meets the requirements. As can be seen, the choices are entirely diverse and varied. This research attempts to solve such issues using a statistics-based tool called design of experiments (DoE). Using such a tool, this study demonstrates how to optimally choose the proportions of virgin bitumen, RAP bitumen, and rejuvenator. This work presents a methodology that can be implemented efficiently using commercial software by any asphalt mix designer. [ABSTRACT FROM AUTHOR]

Microcracking due to thermal stresses affects the mechanical and flow properties of rocks, which is significant for thermally dynamic environments such as volcanoes and geothermal reservoirs. Compared with other crustal rocks like granite, volcanic rocks have a complex and variable response to temperature; it remains unclear how thermal microcracks form and how they are affected by temperature. We heated and cooled samples of low‐porosity basalts containing different amounts of microcracks and a porous andesite over three cycles, whilst monitoring microstructural changes by acoustic emission (AE) monitoring and measurement of P‐wave velocity (vP; up to 450°C) and thermal expansion coefficient (TEC; up to 700°C). During the second and third cycles, the TEC was positive throughout and the rate of detected AE was low. In contrast to studies on granite, we measured a strong and reversible increase in vP with increasing temperature (by 15%–40% at 450°C), which we interpret as due to microcrack closure. During the first cycle, AE and vP measurements indicated thermal microcracking within the andesite and the basalt with a low initial microcrack density. For these samples, strong inflexions in the TEC indicated stress relaxation during heating, preceding significant thermal microcracking during cooling. The basalt with a high initial microcrack density underwent little microcracking throughout all cycles. Our results and a review of the literature relate the initial microstructure to the occurrence of thermal microcracking and explore the potentially significant influence of temperature on volcanic rock properties. Plain Language Summary: Microcracking due to thermal stresses affects the mechanical and flow properties of rocks, playing an important role within volcanoes and geothermal reservoirs. Volcanic rocks exhibit a complex response to temperature, and it remains unclear how thermal microcracks form and how they are affected by temperature. Here, we repeatedly heated and cooled samples of basalt and andesite with different initial porosities and microcrack content over three heating/cooling cycles, whilst monitoring for laboratory‐scale seismicity and changes in acoustic wave velocity (up to 450°C) and sample length (up to 700°C). During the second and third cycles, we measured a strong, reversible increase in wave velocity with increasing temperature (by up to 40% at 450°C): opposite to measurements on granite, and which we interpret as due to crack closure during heating. During cycle one, we detect thermal microcracking within the andesite and the basalt with a low initial microcrack population. For these samples, anomalous thermal expansion during heating is linked to the significant thermal microcracking during cooling. In contrast, the initially highly‐microcracked basalt underwent little microcracking throughout. We relate the initial microstructure to the occurrence of thermal microcracking, and explore how rock properties may significantly change with temperature. Key Points: One basalt cracked significantly during cooling, following an anomalous thermal expansion, indicative of stress relaxation, when heatedAll samples see an increase in wave velocity with temperature during repeated heating, attributed to microcrack closure during heatingOur data literature study suggest low‐porosity volcanic rocks with high P‐wave velocities are most susceptible to thermal microcracking [ABSTRACT FROM AUTHOR]

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Cardiomyocytes are viscoelastic and contribute significantly to right ventricle (RV) mechanics. Microtubule, a cytoskeletal protein, has been shown to regulate cardiomyocyte viscoelasticity. Additionally, hypertrophied cardiomyocytes from failing myocardium have increased microtubules and cell stiffness. How the microtubules contribute to the tissue-level viscoelastic behavior in RV failure remains unknown. Our aim was to investigate the role of the microtubules in the passive anisotropic viscoelasticity of the RV free wall (RVFW) during pulmonary hypertension (PH) progression. Equibiaxial stress relaxation tests were conducted in the RVFW from healthy and PH rats under early (6%) and end (15%) diastolic strains, and at sub- (1Hz) and physiological (5Hz) stretch-rates. The RVFW viscoelasticity was also measured before and after the depolymerization of microtubules at 5Hz. In intact tissues, PH increased RV viscosity and elasticity at both stretch rates and strain levels, and the increase was stronger in the circumferential than longitudinal direction. At 6% of strain, the removal of microtubules reduced elasticity, viscosity, and the ratio of viscosity to elasticity in both directions and for both healthy and diseased RVs. However, at 15% of strain, the effect of microtubules was different between groups – both viscosity and elasticity were reduced in healthy RVs, but in the diseased RVs only the circumferential viscosity and the ratio of viscosity to elasticity were reduced. These data suggest that, at a large strain with collagen recruitment, microtubules play more significant roles in healthy RV tissue elasticity and diseased RV tissue viscosity. Our findings suggest cardiomyocyte cytoskeletons are critical to RV passive viscoelasticity under pressure overload. This study investigated the impact of microtubules on the passive anisotropic viscoelasticity of the right ventricular (RV) free wall at healthy and pressure-overloaded states. We originally found that the microtubules contribute significantly to healthy and diseased RV viscoelasticity in both (longitudinal and circumferential) directions at early diastolic strains. At end diastolic strains (with the engagement of collagen fibers), microtubules contribute more to the tissue elasticity of healthy RVs and tissue viscosity of diseased RVs. Our findings reveal the critical role of microtubules in the anisotropic viscoelasticity of the RV tissue, and the altered contribution from healthy to diseased state suggests that therapies targeting microtubules may have potentials for RV failure patients. [Display omitted] [ABSTRACT FROM AUTHOR]

A solvent‐free approach was developed to incorporate carbon nanotube (CNT) into castor oil derived poly(urethane urea) (PPU) covalent adaptable network (CAN) based on dynamic piperidine‐urea bonds to fabricate CNT reinforced PPU composites. The approach includes two steps i.e., pre‐polymerization of castor with isophorone diisocyanate in flask and subsequent chain‐extension with 1,3‐bis(4‐piperidinyl)propane (PIP) in the presence of CNT in an internal mixer. The effect on CNT content of the morphology, mechanical property, stress relaxation, and reprocessability of the PPU/CNT composites was investigated in detail. The results demonstrated that CNT dispersed well in the PPU networks due to the applied strong shear force which facilitated the dispersion of CNT in the PPU matrix before cross‐linking. The well‐dispersed CNT reinforced the mechanical properties of PPU significantly and the Young's modulus (E) of the composites were enhanced significantly when the content of CNT was ≥6 wt% due to the formation of CNT network in the PPU matrix. When the content of CNT was 10 wt%, the E of PPU‐10%CNT was 927.59 ± 149.05 MPa, which was improved by ~60% compared to PPU. The reprocessability of the PPU network was remained although the stress relaxation rate was reduced with incorporation and increasing content of CNT. In addition, the PPU/CNT composites could be degraded chemically to recycle CNT through reaction of the dynamic piperidine‐urea bonds with additional PIP. [ABSTRACT FROM AUTHOR]

Dynamic liquid crystal elastomers (LCEs) are a class of polymer networks characterized by the inclusion of both liquid crystalline monomers and dynamic covalent bonds. The unique properties realized through the combination of these moieties has produced a plethora of stimuli‐responsive materials to address a range of emerging technologies. While previous works have studied the incorporation of different dynamic bonds in LCEs, few (if any) have studied the effect of the specific placement of the dynamic bonds within an LCE network. A series of dynamic LCE networks were synthesized using a generalizable approach that employs a tandem thiol‐ene/yne chemistry which allows the location of the dynamic disulfide bond to be varied while maintaining similar network characteristics. When probing these systems in the LC regime, the thermomechanical properties were found to be largely similar. It is not until elevated temperatures (160–180 °C) that differences in the relaxation activation energies of these systems begin to materialize based solely on differences in placement of the dynamic bond throughout the network. This work demonstrates that through intentional dynamic bond placement, stress relaxation times can be tuned without affecting the LCE character. This insight can help optimize future dynamic LCE designs and achieve shorter processing times. [ABSTRACT FROM AUTHOR]

The existence of defects in the enclosure structure is the primary cause of water and sand leakage in foundation pits, as well as being a significant source of danger in pit construction, but current research lacks an in-depth investigation of the generation mechanism and gestation process. In this paper, which comprehensively considers the microscopic particles and macroscopic level, the development mechanism of a water and sand leakage disaster in a foundation pit with a water-rich sand layer was studied using the principle of computational fluid dynamics and discrete element method coupled analysis (CFD–DEM); moreover, based on the anisotropy of the particle force and fluid energy analysis, the deformation of the stratum and ground stress field were analyzed. The results show that the stress field will produce a plugging effect at a certain distance from the defect, and the strata exhibit a dominant displacement tendency in the vertical direction, resulting in the emergence of a gradually concave stress relaxation zone and an elliptical contour in the strata displacement map near the defect. The fluid energy describes the displacement of the sand layer very well, and it is separated into the sand layer's centralized loss region and the major loss area based on the high and low levels of the fluid energy class. The impact of fluid at the defect reaches the maximum kinetic energy, which penetrates the structural weakness and causes the loss of sand particles, and the cross-section of the water influx near the defect gradually expands with the loss of particles, indicating that there is a danger of further expansion of the defect under the impact of water flow. These results have technical implications for the management of water and sand leakage disasters in foundation pit engineering. [ABSTRACT FROM AUTHOR]