•Discontinuous fatigue loading tests of cracked red sandstone were performed.•There is a linear law between energy storage and dissipation.•A discontinuous fatigue damage model was established based ...on dissipated energy.•The failure mode and damage mechanism of the sample were revealed.
This paper explores the energy storage and dissipation, damage evolution and failure mode characteristics of red sandstone with prefabricated cracks under discontinuous multilevel fatigue (DMLF) loading. It was found that the three energy density parameters increase quadratic polynomial with the increase of stress level. The insertion of creep loading increased energy dissipation during fatigue deformation, but had no significant effect on elastic energy. There was a linear energy storage and dissipation law in the sample loading process, and the strength variation laws of samples with different angles were explained in terms of energy. Fatigue damage accumulated slowly and then increased rapidly with loading, and a damage evolution model was established based on dissipated energy. Moreover, the 0° and 30° samples showed shear failure, while the 45°, 60°, and 90° samples showed tensile-shear composite failure. Combined with macro-micro analysis, the damage mechanism of the samples under DMLF loading was further revealed.
Abstract
Systems that evolve towards a state from which they cannot depart are common in nature. But the fluctuation-dissipation theorem (FDT), a fundamental result in statistical mechanics, is ...mainly restricted to systems near-stationarity. In processes with absorbing states, the total probability decays with time, eventually reaching zero and rendering the predictions from the standard response theory invalid. In this article, we investigate how such processes respond to external perturbations and develop a new theory that extends the framework of the FDT. We apply our theory to two paradigmatic examples that span vastly different fields—a birth–death process in forest ecosystems and a targeted search on DNA by proteins. These systems can be affected by perturbations which increase their rate of extinction/absorption, even though the average or the variance of population sizes are left unmodified. These effects, which are not captured by the standard response theory, are exactly predicted by our framework. Our theoretical approach is general and applicable to any system with absorbing states. It can unveil important features of the path to extinction masked by standard approaches.
AbstractThis paper introduces and provides the experimental results related to a new self-centering steel tension-only brace with energy dissipation characteristics. The introduced brace has ...nonpinching behavior with no stiffness and strength degradation because of the performance of the resilient slip friction joint (RSFJ) at the brace end(s). These features can ascertain structural damage-avoidance criteria for building performance to minimize the need for retrofitting, demolishing, and rebuilding of structures after earthquakes. Quasi-static and dynamic tests were carried out on a full-scale, two-dimensional steel frame with a height of 3.1 m and a length of 5.05 m. The designed joints and frame were capable of accommodating a 5% lateral drift as the limit for a maximum earthquake (MCE), which was twice the 2.5% limit considered for the ultimate limit state (ULS). The test observations indicate that the frame response was fully repeatable, self-centering, and energy-dissipative. Dynamic effects were observed and resulted in an increase in the slipping force of the system and not in the maximum force. Die-springs were used to minimize these effects. Additionally, a numerical study was performed on a prototype building that was designed elastically using the equivalent static method (ESM) and a ductility of 1.25 to illustrate the ability of the introduced system to reduce the seismic base shear of a conventional tension-only braced structure that can be considered for retrofitting purposes. For this, the RSFJ tension-only braces were designed for 1.5% and 2% drift limits of the structure and considering a ductility of 3 for both cases. Pushover and nonlinear time history analyses were conducted to design the RSFJs. The results indicated that, in both cases, the RSFJ tension-only braces could significantly reduce the base shear by providing ductility greater than 3. From the experimental and numerical studies in this research, the conclusion can be reached that this new bracing system can provide dependable low-damage structural solutions for new and existing structures through the provided damping and self-centering.
The performance of a heat exchanger is directly related to the energy conversion efficiency of the thermal storage system, and its optimal design is an important method to improve the performance of ...the heat exchanger. This paper uses the distributed parameter method to analyze the effect of the structural parameters and operating parameters of a heat exchanger on the entransy dissipation rate, the entransy dissipation number, the entransy dissipation heat resistance, entropy production rate, and entropy production number in a molten salt–supercritical CO2 concentric tube heat exchanger. The results show that the entransy dissipation rate and entropy production rate have the same trend, with the structural parameters and operating parameters, as well as the changes in the entransy dissipation number and entransy dissipation thermal resistance, jointly affected by the entransy dissipation rate and the heat exchange. Based on the above indicators, single-objective and multi-objective optimization calculations were carried out. The results show that taking the minimum entropy dissipation number, entransy dissipation heat resistance, and improved entropy production number as the objective functions, and using the heat transfer effectiveness as the evaluation index, the optimization effect is better. The ε value is increased by 41.2%, 39.5%, and 40.3% compared with the reference individual. In the multi-objective optimization, taking the minimum number of entransy dissipation and entropy production as the objective function, and using the efficiency of heat transfer and the pressure drop of the working fluid as the evaluation indicators, the optimization effect is better. Compared with the reference individual, the ε value increased by 23.5%, and ΔPh and ΔPc decreased by 51.9% and 32.5%, respectively. This study provides a reference for the optimization of supercritical CO2 heat exchangers by utilizing parameters such as entransy and entropy, which reflect the irreversible loss of the heat transfer process.
•We consider the Newmark and ρ∞-Bathe time integrations for dynamic solutions.•New insights into the Newmark and ρ∞-Bathe methods are presented.•We show that the Newmark method with α=0.25(δ+0.5)2 ...and δ⩾0.5 is a special case of the ρ∞-Bathe method.•Some example solutions of structural dynamics and wave propagations are presented to illustrate the theoretical findings.
We consider the unconditionally stable Newmark and ρ∞-Bathe methods for the direct time integration of the finite element equations in structural dynamics and wave propagations. In our evaluation of the Newmark method we consider the parameters δ and α, and in the ρ∞-Bathe method we consider the parameters γ and ρ∞, with 0<γ<∞,γ≠1 and ρ∞∈-1,+1. We show that the Newmark method as usually used with its δ and α parameters, α=0.25(δ+0.5)2 and δ⩾0.5, is a special case of the ρ∞-Bathe method. We also show that the β1/β2-Bathe method is a special case of the ρ∞-Bathe scheme. The study of the curves of numerical dissipation and dispersion shows that the ρ∞-Bathe method provides effective dissipation and dispersion whereas the Newmark method lacks in that regard. To illustrate our theoretical findings we give the results of some example solutions of structural dynamics and wave propagations. Our study also shows that further research is needed to identify the optimal use of the ρ∞-Bathe scheme and other implicit methods in wave propagation analyses.
Context.
Coronal rain consists of cool and dense plasma condensations formed in coronal loops as a result of thermal instability.
Aims.
Previous numerical simulations of thermal instability and ...coronal rain formation have relied on the practice of artificially adding a coronal heating term to the energy equation. To reproduce large-scale characteristics of the corona, the use of more realistic coronal heating prescription is necessary.
Methods.
We analysed coronal rain formation and evolution in a three-dimensional radiative magnetohydrodynamic simulation spanning from convection zone to corona which is self-consistently heated by magnetic field braiding as a result of convective motions.
Results.
We investigate the spatial and temporal evolution of energy dissipation along coronal loops which become thermally unstable. Ohmic dissipation in the model leads to the heating events capable of inducing sufficient chromospheric evaporation into the loop to trigger thermal instability and condensation formation. The cooling of the thermally unstable plasma occurs on timescales that are comparable to the duration of the individual impulsive heating events. The impulsive heating has sufficient duration to trigger thermal instability in the loop but does not last long enough to lead to coronal rain limit cycles. We show that condensations can either survive and fall into the chromosphere or be destroyed by strong bursts of Joule heating associated with a magnetic reconnection events. In addition, we find that condensations can also form along open magnetic field lines.
Conclusions.
We modelled, for the first time, coronal rain formation in a self-consistent 3D radiative magnetohydrodynamic simulation, in which the heating occurs mainly through the braiding and subsequent Ohmic dissipation of the magnetic field. The heating is stratified enough and lasts for long enough along specific field lines to produce the necessary chromospheric evaporation that triggers thermal instability in the corona.
This review presents an exhaustive overview on the mechanisms of Fe3+ cathodic reduction within the context of the electro-Fenton (EF) process. Different strategies developed to improve the reduction ...rate are discussed, dividing them into two categories that regard the mechanistic feature that is promoted: electron transfer control and mass transport control. Boosting the Fe3+ conversion to Fe2+ via electron transfer control includes: (i) the formation of a series of active sites in both carbon- and metal-based materials and (ii) the use of other emerging strategies such as single-atom catalysis or confinement effects. Concerning the enhancement of Fe2+ regeneration by mass transport control, the main routes involve the application of magnetic fields, pulse electrolysis, interfacial Joule heating effects, and photoirradiation. Finally, challenges are singled out, and future prospects are described. This review aims to clarify the Fe3+/Fe2+ cycling process in the EF process, eventually providing essential ideas for smart design of highly effective systems for wastewater treatment and valorization at an industrial scale.
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