Time‐Dependent Mechanical Response of APbX3 (A = Cs,CH3NH3; X = I,Br) Single Crystals

The ease of processing hybrid organic–inorganic perovskite (HOIPs) films, belonging to a material class with composition ABX₃, from solution and at mild temperatures promises their use in deformable technologies, including flexible photovoltaic devices, sensors, and displays. To successfully apply these materials in deformable devices, knowledge of their mechanical response to dynamic strain is necessary. The authors elucidate the time‐ and rate‐dependent mechanical properties of HOIPs and an inorganic perovskite (IP) single crystal by measuring nanoindentation creep and stress relaxation. The observation of pop‐in events and slip bands on the surface of the indented crystals demonstrate dislocation‐mediated plastic deformation. The magnitudes of creep and relaxation of both HOIPs and IPs are similar, negating prior hypothesis that the presence of organic A‐site cations alters the mechanical response of these materials. Moreover, these samples exhibit a pronounced increase in creep, and stress relaxation as a function of indentation rate whose magnitudes reflect differences in the rates of nucleation and propagation of dislocations within the crystal structures of HOIPs and IP. This contribution provides understanding that is critical for designing perovskite devices capable of withstanding mechanical deformations.     

A single electric relaxation time in Ba(1-x)Sr(x)TiO(3) nanoparticles at low temperatures

It is shown that the dielectric response of Ba(0.77)Sr(0.23)TiO(3) nanoparticles at temperatures below 200?K has a frequency and temperature dependence in agreement with the Debye theory with a single relaxation time, which exhibits the Arrhenius law. By contrast, at temperatures above 210?K the dielectric response exhibits a broad range of relaxation times characteristic of relaxor-ferroelectrics. We suggest that the single relaxation time at low temperature originates from a frustration effect, in analogy with frustrated antiferromagnetism.     

Discrete element method investigation on thermally-induced shakedown of granular materials

Temperature change, as a common kind of internal perturbation performed on granular materials, has a significant effect on the bulk properties of granular materials. However, few studies on thermally-induced shakedown under a long-term thermal cycling were reported. In this work, the discrete element method was used to give insight into the thermally-induced shakedown on the fabric and stress states within non-cohesive, frictional granular assemblies. Assemblies were submitted to thermal cycling at a stationary boundary condition after experiencing a one-dimensional compression. Evolution of coordination number, entropy and anisotropy was investigated as well as boundary forces and contact forces. At the same time, effects of the heating rate, the initial vertical load and the magnitude of temperature change were examined. It demonstrates that thermal cycling induces a significant force relaxation within granular materials, while the corresponding granular fabric has a small change. In addition, the entropy and anisotropy decreases with thermal cycling. Moreover, the initial vertical load can constrain the development of thermally-induced fabric change, thereby limiting force relaxation to some degree. Both high heating rate and larger magnitudes of temperature change contribute to more significant force relaxation. However, they cause smaller fabric changes even though they provide larger perturbations.     

The effect of loading time on flexible pavement dynamic response: a finite element analysis

Dynamic response of asphalt concrete (AC) pavements under moving load is a key component for accurate prediction of flexible pavement performance. The time and temperature dependency of AC materials calls for utilizing advanced material characterization and mechanistic theories, such as viscoelasticity and stress/strain analysis. In layered elastic analysis, as implemented in the new Mechanistic-Empirical Pavement Design Guide (MEPDG), the time dependency is accounted for by calculating the loading times at different AC layer depths. In this study, the time effect on pavement response was evaluated by means of the concept of “pseudo temperature.” With the pavement temperature measured from instrumented thermocouples, the time and temperature dependency of AC materials was integrated into one single factor, termed “effective temperature.” Via this effective temperature, pavement responses under a transient load were predicted through finite element analysis. In the finite element model, viscoelastic behavior of AC materials was characterized through relaxation moduli, while the layers with unbound granular material were assumed to be in an elastic mode. The analysis was conducted for two different AC mixtures in a simplified flexible pavement structure at two different seasons. Finite element analysis results reveal that the loading time has a more pronounced impact on pavement response in the summer for both asphalt types. The results indicate that for reasonable prediction of dynamic response in flexible pavements, the effect of the depth-dependent loading time on pavement temperature should be considered.     

Segregation pattern of binary-size mixtures in a double-walled rotating drum

Size-induced granular segregation was performed systematically and experimentally in an almost fully filled double-walled rotating drum at 10 different rotation speeds and two different side wall types. The motion of the granular materials was recorded using a high-speed camera for image analysis of particle segregation development in the drum. With continual tracking of the particle movements, the velocity, fluctuations, and granular temperatures were measured. The experimental results indicate that both rotation speeds and friction coefficient of side walls significantly affect segregation phenomena in binary-size mixture granular flows. The results demonstrate similar situations to the Brazil-nut effect and its reverse in the radial direction at either high or a low rotational speed (where the Froude number (Fr) is far from 1). At these instances, the maximum granular temperature occurs near the side walls. Specifically, a double segregation effect (DSE) is found at Froude number (Fr) close to 1. These results can be used in many industrial processes, for example, size grading of materials, screening of impurities, and different structures of functionally graded materials. Moreover, the maximum granular temperature occurs in the middle of the ring space. It causes small particles to move toward both side walls as it pushes bigger particles to accumulate in the middle of the ring space of rotating drum.     

FEM simulation of viscous properties for granular materials considering the loading rate effect

The deformation and strength characteristics of sandy soils as a kind of granular materials are very complex. The experimental results show that when the strain rate suddenly changes in monotonic loading (ML) case, the stress–strain curve of sandy soils changes sharply and then gradually converges into the original inferred one that would be obtained by continuous ML at constant strain rate after having exhibited clear yielding. Similar behaviors are also observed when ML is restarted at a constant strain rate following a creep loading or stress relaxation stage. An elasto-viscoplastic constitutive model for granular materials is developed, which consists of three components. One of the most important features of the model is that it can take into account the effects of loading rate due to viscous properties on the stress–strain behavior. The stress ratio-axial strain–time relations from four drained plain strain compression (PSC) tests on the saturated Toyoura sand are successfully simulated by the finite element method (FEM) code incorporating the proposed constitutive model. It is shown that the FEM code can simulate the viscous behaviors of sand accurately under arbitrary loading history.     

Comprehensive study of the effects of rolling resistance on the stress–strain and strain localization behavior of granular materials

This paper presents the results of a comprehensive study of the effects of rolling resistance on the stress–strain and strain localization behavior of granular materials using the discrete element method. The study used the Particle Flow Code (PFC) to simulate biaxial compression tests in granular materials. To study the effects of rolling resistance, a user-defined rolling resistance model was implemented in PFC. A series of parametric studies was performed to investigate the effects of different levels of rolling resistance on the stress–strain response and the emergence and development of shear bands in granular materials. The PFC models were also tested under a range of macro-mechanical parameters and boundary conditions. It is shown that rolling resistance affects the elastic, shear strength and dilation response of granular materials, and new relationships between rolling resistance and macroscopic elasticity, shear strength and dilation parameters are presented. It is also concluded that the rolling resistance has significant effects on the orientation, thickness and the timing of the occurrence of shear bands. The results reinforce prior conclusions by Oda et al. (Mech Mater 1:269–283, 1982) on the importance of rolling resistance in promoting shear band formation in granular materials. It is shown that increased rolling resistance results in the development of columns of particles in granular materials during strain hardening process. The buckling of these columns of particles in narrow zones then leads to the development of shear bands. High gradients of particle rotation and large voids are produced within the shear band as a result of the buckling of the columns.     

Viscoplastic–plastic modelling of IN792

At high temperatures metallic materials behave in a viscous manner exemplified by strain rate dependence, stress relaxation and creep deformation. At low temperatures however, these effects are extremely small, and the behaviour is strain rate independent and shows no or very small relaxation effects. Finally there exists an intermediate region, in which the material behaviour is close to strain rate independent for high strain rates but at the same time shows time dependent inelastic effects, such as stress relaxation and creep. For IN792 this occurs at temperatures around 650 °C. The article describes the extension of a power-law viscoplastic model describing the behaviour of IN792 at 850 °C, also to describe the behaviour at 650 °C, by bounding the elastic–viscoplastic stress-space by a plastic yield surface. The model parameters have been estimated using data from creep test and tailored step relaxation tests, and the model fits well to both the step relaxation data aimed at resembling relevant component conditions and long term creep data.     

Mechanical relaxation in simple molecular liquids from the liquid to the supercooled state

Attenuation and velocity of acoustic waves have been revealed at ultrasonic frequencies (2, 5 and 10 MHz) in some glass-forming liquids. The mechanical response has been studied following continuously the materials from the liquid to the supercooled state, using an experimental set-up developed to this purpose. A peak in the attenuation of longitudinal acoustic waves has been observed in a temperature region in which the liquids are supercooled. Correspondingly, the sound velocity shows a dispersion, increasing from liquid-like to solid-like values for decreasing temperatures. Both features develop above the calorimetric glass transition temperature (T_g). In the deeply supercooled liquids, nearly 10 K above their calorimetric T_g, also the propagation of transverse wave sound (which is a characteristic behaviour of solid-like materials) has been experimentally detected. Shear and longitudinal relaxation times are not decoupled in the time–temperature region investigated. Compared to the mechanical one, the dielectric relaxation studied as a function of temperature at the same frequency of the ultrasonic experiments shows a loss peak centred at the same temperature. Depending on the liquid investigated, the mechanical relaxation spectrum can be broader than the dielectric one, specially in the low temperature flank, suggesting that some dissipative processes at lower energies can contribute to the mechanical loss, even though they do not couple to the electric probe field.     

Relaxation of perturbed axial segregation bands

The mixture of grains inside a rotating horizontal cylinder segregate into alternating bands of big and small grains along the rotation axis at appropriate conditions. However, the response of these bands to perturbations is largely unexplored. Here, we report that, when deformed by a weak localized perturbation from one end of the cylinder, the axial bands relax to their original positions after several rotations of the cylinder. Considering that relaxation in other physical systems is related to their structure and dynamics, this robust phenomenon could have interesting implications on our understanding of granular materials.     

一种考虑尺寸效应的颗粒材料流变模型及其验证

经典连续介质理论的粘塑性本构关系缺乏材料尺度的相关性,难以表征颗粒材料流变的尺寸效应,而Cosserat连续体中的内禀特征长度为刻画材料的尺寸效应提供了一种可能途径。该文旨在Cosserat连续体的理论框架下发展Perzyna粘塑性模型,以探讨颗粒材料流变的尺寸效应与影响机制。首先基于Drucker-Prager屈服准则导出了Cosserat连续体粘塑性模型的一致性算法,获得了过应力本构方程积分算法与一致切向模量的封闭形式,并在ABAQUS二次平台上采用用户自定义单元(UEL)予以程序实现。有限元数值算例模拟了软岩试样的三轴压缩蠕变和两种堆石料试样在常规三轴条件下的蠕变和应力松弛,数值预测结果与相应试验结果具有较好的一致性,表明该流变模型的适应性。同时,将颗粒的球型指数、圆度和平均粒径作为表征颗粒材料内禀特征长度的一种度量,以反映颗粒材料的试样尺寸及其颗粒粒径与形状对流变过程中的轴向应变、偏应变和偏应力的影响关系,表明所发展的流变模型可以捕捉颗粒材料流变行为的压力相关性和尺寸效应。     

Dielectric and optical properties of Ba5AFe0.5Ta9.5O30 (A = K, Li) tungsten bronze ceramics

Two novel Ba₅AFe_0.5Ta_9.5O₃₀ (A = K, Li) ceramics were fabricated through a conventional solid-state sintering process. Both samples were paraelectric phase with tetragonal tungsten bronze structure at room temperature. Dielectric response of the samples was studied by using AC impedance spectroscopy and universal dielectric relaxation law in detail. Dielectric relaxation at high temperatures is attributed to the oxygen vacancies induced by the evaporation of alkali oxide. The relaxation activation energy of the ceramics is lower than half of the band gap E _g obtained by UV–Vis spectroscopy, which resulted from the emergence of oxygen vacancies at high temperatures. In the visible light region, the samples showed strong absorption with a band gap of about 2.7 eV, which could be applied as a visible light irradiation photocatalyst.     

Plastic flow of polypropylene (PP) and a PP-based blend

The compressive deformation at constant strain-rate of pure polypropylene (PP) and one of its blends with high-density polyethylene (HDPE) and an ethylene propylene rubber (EPR) has been investigated in the low temperature range 77 K <T < 300 K. A careful determination of the stress relaxation behaviour at yield reveals the existence of critical temperatures which relate the deformation mechanisms to glass transition molecular mobilities. A self-consistent thermodynamic analysis is performed in order to characterize the thermally activated plasticity developed in these materials.     

Dielectric properties of a ceramic oxide: Li(Ni<Subscript>7/10</Subscript>Fe<Subscript>3/10</Subscript>)VO<Subscript>4</Subscript>

The Li(Ni_7/10Fe_3/10)VO₄ compound has been synthesized by solution-based chemical route. Its dielectric response is investigated using complex impedance spectroscopy technique. Frequency dependence of dielectric constant (ε_r) at different temperatures shows low-frequency dispersion due to polarized structure of the material and mobile charge carriers. Temperature dependence of ε_r at different frequencies exhibits the dielectric anomalies in ε_r at different temperatures. Dielectric relaxation process in the material is signified by the variation of tangent loss with frequency at different temperatures. The variation of relaxation time with temperature obeys the Vogel–Fulcher law.     

The dynamics of wet granular matter under a vertical vibration bed

This study investigates the dynamic properties of convection rolls in a 2D wet vibrated granular bed. A particle tracking method with the help of image-processing technology was used to measure the velocity fields, convection flow rate, and the granular temperatures in the wet vibrated granular bed. This study examines the dynamic behaviors of wet granular materials subjected to external vertical vibration. Different liquid contents, viscosities, and surface tensions were added to glass beads forming cohesive granular materials in the vibrated granular bed. This study presents a systematic investigation of the effects of the addition of liquid content, viscosity, and surface tension on dynamic properties of wet particulates. Results show that the convection flow rate and granular temperature decrease monotonically as the added liquid content and liquid viscosity increase. However, the effects of surface tension on the convection flow rate are more significant at the smaller liquid content than that at a higher liquid content. The convection flow rate also decreases in a power decay as the modified Bond number increases.     

The Glassy Response of Solid 4He to Torsional Oscillations

We calculated the glassy response of solid ⁴He to torsional oscillations assuming a phenomenological glass model. Making only a few assumptions about the distribution of glassy relaxation times in a small subsystem of otherwise rigid solid ⁴He, we can account for the magnitude of the observed period shift and concomitant dissipation peak in several torsion oscillator experiments. The implications of the glass model for solid ⁴He are threefold: (1) The dynamics of solid ⁴He is governed by glassy relaxation processes. (2) The distribution of relaxation times varies significantly between different torsion oscillator experiments. (3) The mechanical response of a torsion oscillator does not require a supersolid component to account for the observed anomaly at low temperatures, though we cannot rule out its existence.     

Analysis of the influence of crushing on the behavior of granular materials under shear

The behavior of granular materials mainly depends on the mechanical and engineering properties of particles in its structural matrix. Crushing or breakage of granular materials under compression or shear occurs when the energy available is sufficient to overcome the resistance of the material. Relatively little systematic research has been conducted regarding how to evaluate or quantify particle crushing and how it effects the engineering properties of the granular materials. The aim of this study is to investigate the effect of crushing on the bulk behavior of granular materials by using manufactured granular materials (MGM) rather than using a naturally occurring cohesionless granular material. MGM allow changing only one particle parameter, namely the “crushing strength”. Four different categories of MGM (with different crushing strength) are used to study the effect on the bulk shear strength, stiffness modulus, friction and dilatancy angle “engineering properties”. A substantial influence on the stress–strain behavior and engineering properties of granular materials is observed. Higher confining stress causes some non-uniformity (strong variations/jumps) in volumetric strain and a constant volumetric strain is not always observed under large shear deformations due to crushing, i.e. there is no critical state with flow regime (with constant volumetric strain).     

Application of Nonlinear Superposition to Creep and Relaxation of Commercial Die-Casting Aluminum Alloys

Die-cast aluminum alloys are heavily used in small engines, where they are subjected to long-term stresses at elevated temperatures. The resulting time-dependent material responses can result in inefficient engine operation and failure. A method to analytically determine the stress relaxation response directly from creep tests and to accurately interpolate between experimental time-history curves would be of great value. Constant strain, stress relaxation tests and constant load, creep tests were conducted on aluminum die-casting alloys: B-390, eutectic Al–Si and a 17% Si–Al alloy. A nonlinear superposition integral was used to (i) interpolate between empirical primary inelastic creep-strain and stress-relaxation time histories and (ii) to determine the stress relaxation response from corresponding creep data. Using isochronal stress-strain curves, prediction of the creep response at an intermediate stress level from empirical creep curves at higher and lower stresses resulted in a correlation (R) of 0.98. Similarly for relaxation, correlations of 0.98 were obtained for the prediction of an intermediate strain level curve from higher and lower empirical relaxation curves. The theoretical prediction of stress relaxation from empirical creep curves fell within 10% of experimental data.This paper has not been submitted elsewhere in identical or similar form, nor will it be during the first three months after its submission to Mechanics of Time-Dependent Materials     

Special features of the compaction and phase transformations of wurtzitic boron nitride nanopowders under the action of high static pressures and temperatures

The experimental results of special features of the compaction and attendant phase transformations of compacts formed in sintering wurtzitic boron nitride nanopowders at high static pressures (p = 7.7 GPa) and temperatures (T = 1100–1800°C) have been considered. The principal possibility to produce polycrystalline superhard materials with nanocrystalline granular structures based on wurtzitic boron nitride has been established. The compaction of nanodispersed wurtzitic boron nitride at high barothermal conditions has been found to correspond to common regularities of the compaction of dispersed powder systems with regard to the effect of the phase and structural transformations.     

Viscoelastic properties of poly(butylene terephthalate)/poly(ethylene naphthalate) blends

Melt blends of poly(butylene terephalate) (PBT) and poly(ethylene naphthalate) (PEN) with 30 and 60 wt% PEN were prepared using a single screw extruder and an injection moulding machine. Stress relaxation tests for the specimens of PBT/PEN blends and the homopolymers were carried out using an Instron testing machine in an Instron environmental chamber. The Taguchi method of experimental design analysed how different levels of temperature, PEN content and initial stress affected the relaxation behaviour of PBT/PEN blends and homopolymers. From the response tables and analyses of main and interaction effects, it was shown that the most significant factor was temperature, followed by PEN content and then the initial stress. Consequently, high temperature, low PEN content and high initial stress speeded up stress relaxation rate of specimens. Interaction effects between factors were insignificant. To fit the relaxation curves of the PBT/PEN blends and the homopolymers at different temperatures, PEN contents and the initial stresses, four different equations were attempted with Matlab™, which determined the coefficients of these functions using the experimental data of stress change with time. The simulated curves from the most suitable function among them were shown using the calculated coefficients to predict the relaxation behaviour of PBT/PEN blends (50% PEN) at temperatures of 30 and 60°C with an initial stress of 7 MPa.