Mechanics of Time-Dependent Materials最新文献

Relaxation behavior of fiber-reinforced polymer (FRP) bars is one of the main issues considered in prestressing applications. Recently, basalt FRP (BFRP) bars have been developed as an alternative to other types of FRP bars. However, the available studies on their relaxation behavior are still limited, especially under severe environmental conditions. In this study, the long-term relaxation behavior of BFRP bars was studied, the initial stress level under different environmental conditions are the variables considered in this investigation. A total of twenty-four specimens of BFRP bars of nominal diameter 6 mm were used in this study. The relaxation behavior and the residual tensile strength of BFRP after relaxation were experimentally assessed. The expected relaxation loss after a million hours was estimated based on the extrapolated statistical analysis of the experimental results. The test results showed that the relaxation behavior of BFRP bars under different initial stress levels have linear relationship with logarithm of time, similar to other types of FRP bars, and are significantly affected by the initial stress level. Furthermore, the test results demonstrated that the relaxation behavior of BFRP bars is slightly affected by seawater (pH 8.1) and can be recommended for use in marine structures. On the other hand, the test results showed that the acidic (5% HCl) and alkaline (10% NaOH) environmental conditions greatly impacted the relaxation behavior of BFRP bars, such that although BFRP bars stressed to 50% of their ultimate strength survived 1000 h under normal conditions, exposure to different environmental conditions resulted in their rupture under the same initial stress level. Finally, the expected million-hour relaxation losses of BFRP bars range between 7.76% and 13.75% under normal conditions based on the initial tensile stress level, while introducing seawater can increase this ratio to an average of 16.13%.

Stress relaxation is a time-dependent mechanical behavior of textile materials, which can affect the performance of the fabrics in various applications, especially technical applications such as tensile structures, geotextiles, and medical textiles. The present study aims to consider the effect of fabric structure (Tricot, Locknit, Satin, Queen’s cord, Reverse Locknit, and Sharkskin) on the tensile stress relaxation of biaxial warp-knitted fabrics. According to the results, the fabric structure remarkably influences its stress relaxation. The stress relaxation of the fabric in the course direction increases by increasing the length of underlaps in the front and back guide bars. Using elastic yarns as weft yarns in the fabric structure reduces the stress relaxation of the fabric in the course direction without affecting the fabric’s stress relaxation in the wale direction. Finally, to improve the functionality of pressure garments under stress relaxation, using elastic weft yarns in the fabric structure and bounding the warp and weft yarns in the biaxial warp-knitted fabrics by the Tricot structure is proposed.

This study examines the MHD channelized flow of Maxwell fluid with suction/injection at the boundaries to control fluid velocity and boundary layer development. The governing equations, accounting for momentum, energy, heat generation, and chemical reactions, are analytically solved using the Laplace transform. Explicit solutions for velocity, temperature, and concentration fields are derived in exponential form and further expressed using summation notation for efficient inversion. Graphical analysis reveals that suction reduces velocity, while injection enhances it, significantly improving the heat transfer process. The findings highlight the critical role of suction/injection in optimizing heat and mass transfer, offering valuable insights for engineering applications.

In developing considered occurrence phenomena, the proffered research study is conducted on account of blood motion along with chemically reactive Casson fluid exposed to a circular cylinder, including a rough surface. Moreover, Lorentz force is invoked across the hybrid nanoliquid. The innovation behind this influential approach is based on the assumption of heat production and consumption. Given cooling procedures and the thermal energy mechanism, copper, silver, and titanium oxide within the blood occurrence are used in the proposed study. For the development of the current flow problem, we have considered the Cartesian coordinate system. Due to the complexity of the proffered formulated model, the governing dimensionless set of equations is handled using a traditional numerical approach, the finite element method (FEM). Further, the efficient role of the pertinent constraints arises across the flow phenomena demonstrated graphically and presented in tabular form. Comparative analysis demonstrates that the movement of copper, silver, and titanium oxide in the blood is more intense than the movement of copper and silver in the blood. Meanwhile, thermal energy produced by using copper, silver, and titanium oxide in the blood is much higher in comparison to thermal energy for copper and silver with blood. Moreover, the Nusselt number also depicts an accelerated demeanor for copper, silver, and titanium oxide in the blood in contrast to the production of silver and copper with blood. We have emphasized the proffered study relevance with biomedical applications, specifically its incorporation for understanding blood occurrence within complex geometries and the effects of nanoliquid dispersion in the flow dynamics.

The mechanical properties of isotropic conductive adhesive (ICA) have received increasing attention due to its widespread application in microelectronic packaging. In this work, the loading and strain rate sensitivity of cured epoxy-based ICA were investigated using nanoindentation. The ICA was prepared and indented under quasi-static and continuous stiffness measurement (CSM) modes under varying loading rates ((dot{P})) and loading strain rates ((dot{P} / P)). The results demonstrate a loading/strain rate hardening effect on the hardness of ICA. Compared with quasi-static test measurement, the CSM mode seems to be a more effective measurement for the hardness results of ICA. During nanoindentation, a competitive interaction between hardening and softening mechanisms was observed: softening dominated at higher loading strain rates, while hardening prevailed at lower rates. Under both loading modes, creep displacement and creep strain rate increased with strain/loading rate. In addition, the creep displacement rose rapidly during the initial holding time before stabilizing, while the corresponding creep strain rate decreased progressively to a steady-state creep stage.

Glycidyl azide polymer (GAP)-based propellants, known for their high energy efficiency, exhibit unique nonlinear variations in viscoelastic behavior during thermal aging, which is distinct from the monotonic trends observed in traditional propellants. This paper investigates the relaxation behavior of GAP-based propellants subjected to thermal aging at 60 °C. Nuclear Magnetic Resonance and high-performance liquid chromatography analyses are conducted to reveal the underlying mechanisms driving the nonlinear relaxation response. The aging process is classified into three distinct stages: an initial phase dominated by post-curing reactions, followed by competing effects from crosslink network scission, and plasticizer degradation. These competing mechanisms affect the relaxation through microscopic changes in free volume, resulting in complex viscoelastic responses. A predictive model is developed for the relaxation modulus to take into account of these aging mechanisms, with capability to capture the nonlinear fluctuations in the aging shift factor. The proposed model provides accurate predictions of relaxation behavior during thermal aging, including the long-term performance of GAP-based propellants.

We address a non-destructive testing of bitumen insulations. A new approach to its in situ monitoring is proposed. It is based on single impact microindentation. To describe the straining process of a bitumen coating, we analyzed Maxwell and Voigt rheological models. It is shown experimentally that Maxwell model suits well for this task. Temporal changes of the rigidity coefficient in the coating depending on the ambient temperature were measured. It has been established that microindentation-based method is effective for the assessment of the insulation aging. Thermal aging experiments and measurements were carried out to confirm the applicability of the proposed approach.

An experimental study and numerical simulation of short- and long-term shear stress relaxation behaviors of nonaligned and aligned magnetorheological elastomers (MREs) were investigated. The aligned MRE was created by aligning micro-size carbonyl iron particles in chains in silicon rubber using an external magnetic field during the curing process, while the nonaligned MRE was fabricated without applying a magnetic field. The effects of permanent magnetic fields on the shear stress relaxation of the nonaligned and aligned MREs were examined using the double-lap shear stress relaxation test with a short-term period of 1200 s and a long-term period of (1.08 times 10^{6}text{ s}). The shear stress and relaxation modulus of the nonaligned and aligned MREs increased considerably with the rise of magnetic flux density to about 500 mT and then enhanced slightly above 500 mT. The shear stress and relaxation modulus of the aligned MRE were considerably higher than those of the nonaligned one. The shear stress relaxation of the nonaligned and aligned MREs was numerically simulated using the fractional derivative viscoelastic Kelvin–Voigt model. The model parameters were identified by fitting the relaxation modulus to the short-term measured data of the MREs. The shear stress estimated from the investigated model with fitted parameters was in excellent agreement with the short-term experimental data of the MREs measured under different magnetic fields. Besides, the short-term model-fitted parameters were used to predict the long-term shear stress relaxation of the nonaligned and aligned MREs. The largest difference between model-predicted and long-term measured results for the nonaligned and aligned MREs was less than 1%. Therefore, the studied model can be used to predict the long-term shear stress relaxation of the nonaligned and aligned MREs.

Elastomeric isolation systems are often used as seismic isolation devices for buildings and bridges. These systems are typically designed based on the nominal properties of the elastomer. However, key properties such as stiffness and damping can vary with environmental temperature, affecting the performance of the elastomeric isolation. The coupled thermo-mechanical dynamic behavior of the elastomer must be considered for accurate response evaluation. Experimental assessment of the coupled thermo-mechanical response in a laboratory setting presents a significant challenge. This paper presents a laboratory testing methodology for evaluating the thermo-mechanical dynamic response of elastomeric isolation systems using real-time hybrid simulation (RTHS). The test system consists of a superstructure resting on an elastomeric isolation system. In RTHS, the elastomeric isolation system itself is tested, while an electromagnetic shaker is used to resemble the behavior of different superstructures. The temperature around each elastomeric isolator is controlled using two L-shaped radiation heaters. The control strategy for the RTHS is validated through virtual simulations for different superstructures. After the numerical validation, experiments are conducted at different temperatures to demonstrate the impact of temperature on the dynamic response of the system. The proposed methodology proves to be effective and can be utilized for studying the coupled thermo-mechanical behavior of elastomeric isolation systems.