A relationship between non-exponential stress relaxation and delayed elasticity in the viscoelastic process in amorphous solids: Illustration on a chalcogenide glass

Inorganic glasses are viscoelastic materials since they exhibit, below as well as above their glass transition temperature, a viscoelastic deformation under stress, which can be decomposed into a sum of an elastic part, an inelastic (or viscous) part and a delayed elastic part. The delayed elastic part is responsible for the non-linear primary creep stage observed during creep tests. During a stress relaxation test, the strain, imposed, is initially fully elastic, but is transformed, as the stress relaxes, into an inelastic and a delayed elastic strains. For linear viscoelastic materials, if the stress relaxation function can be fitted by a stretched exponential function, the evolution of each part of the strain can be predicted using the Boltzmann superposition principle. We develop here the equations of these evolutions, and we illustrate their accuracy by comparing them with experimental evolutions measured on GeSe₉ glass fibers. We illustrate also, by simple equations, the relationship between any kind of relaxation function based on additive contribution of different relaxation processes and the delayed elastic contribution to stress relaxation: the delayed elasticity is directly correlated to the dispersion of relaxations times of the processes involved during relaxation.     

Influence of the pelletizing process parameters on the breakage behaviour of the received alumina oxide pellets

Pelletizing is a common optimization process of the mechanical properties of various powder materials. The improvement of flow behaviour, dosage and the increase of bulk density are only a few the goals that this size enlargement concedes. Anyway, the properties of the produced pellets depend on many factors, whose influence is not completely studied. In order to investigate the effect of process parameters on the mechanical strength of final products, industrially produced alumina oxide (\({\gamma }\)-\(\mathrm{Al}_{2}\mathrm{O}_{3})\) granules as primary model particles were pelletized in a laboratory rotating pan-pelletizer (spheronizer). The processing time for all pelletizing batches was 20 min. Solution of viscoelastic polymer—hydroxypropyl methylcellulose was used as binder. Its concentration was increased until finding of the most suitable binder content for the model pellets. The rotational velocity of the pan-pelletizer was varied in order to find the optimal speed, which provides pellets with improved properties. The influence of the process parameters on the received pellet product properties like density and porosity, size distribution, breakage behaviour and breakage probability was also analysed. The solid bonds between the single primary particles were investigated using light and scanning electron microscope. The conclusions from the experimental work help us to understand the basics of pelletizing processes and tend to develop and facilitate the operating principle with particle populations in science and industry. The result should provide base for carrying out of Discrete Element Method simulations, which should enlighten the process of irregular pellet breakage.     

Effect of binder properties on the strength,porosity and leaching behaviour of single nickel laterite pellet

Nickel laterite pellets with controlled mass, porosity and binder were made by using a pellet press. Both water and sulphuric acid solution were used as binders. The wet pellets were then dried at different conditions and their mechanical strength was measured. Leaching tests were also conducted on single pellet with irrigation of sulphuric acid solution from the top of the pellet. The leached out solutions were collected and nickel recoveries were analysed. The time taken for the pellets to disintegrate during leaching test was also recorded. It was found that the mechanical strength of the pellets was directly related to their dryness, with completely dried pellets having much higher strength. The dry pellet strength was found to increase with increasing binder content and decreasing pellet porosity. The time taken for the pellets to disintegrate during leaching test increased with increasing pellet strength. In comparison to sulphuric acid solution-bound pellets at the same condition, water-bound pellets exhibited higher mechanical strength. Although the nickel leaching rate for water-bound pellets was low at the beginning of the leaching test, the pellets lasted for more than 200 h without disintegration, with 70% of nickel recovered.     

A sustainable approach to the recycling of rice straw through pelletization and controlled burning

A technique for controlled burning of rice straw is presented. It relies on well-designed rice straw pellets to be burned in fluidized bed. The developed pellets have high burning rate, no fly ashes emissions and minimum bed fouling. The pellets are manufactured from ground rice straw in a disc pelletizer with the aid of bonding and suitable additive materials. The pellets are tested under controlled conditions in a test rig, which represents a single pellet fluidized bed. It is equipped with a nitrogen gun to eject the pellet and freeze the reaction at any predetermined time during combustion. The ejected pellets are weighed as well as elementary analyzed for both carbon and hydrogen, to calculate the burning rate as well as the combustion efficiency, respectively. The effect of several parameters has been evaluated including straw particle size, pellet size, type and concentration of bonding material as well as anti-sintering additives. Also, the pellets’ mechanical characteristics have been evaluated. It has been found that char combustion phase represents the controlling phase of the pellet combustion. The burning rate is higher as the void fraction of the pellet is higher. Starch showed better combustion and mechanical characteristics out of the five tested bonding materials. Adding kaolin to the pellets results in improving the sintering characteristics of the pellets. The experimental results were compared with two combustion models: the oxygen diffusion controlled and the kinetic-diffusion models. It has been found that oxygen diffusion controlled model more accurately simulates the combustion of the pellet during its char combustion phase. The model has been used to evaluate the effect of some operational parameters on the pellet combustion characteristics such as bed temperature, gas flow and oxygen concentration.     

Devolatilization of oil sludge in a lab-scale bubbling fluidized bed

Devolatilization of oil sludge pellets was investigated in nitrogen and air atmosphere in a lab-scale bubbling fluidized bed (BFB). Devolatilization times were measured by the degree of completion of the evolution of the volatiles for individual oil sludge pellets in the 5-15 mm diameter range. The influences of pellet size, bed temperature and superficial fluidization velocity on devolatilization time were evaluated. The variation of devolatilization time with particle diameter was expressed by the correlation, τ(d) = Ad(p)(N). The devolatilization time to pellet diameter curve shows nearly a linear increase in nitrogen, whereas an exponential increase in air. No noticeable effect of superficial fluidization velocity on devolatilization time in air atmosphere was observed. The behavior of the sludge pellets in the BFB was also focused during combustion experiments, primary fragmentation (a micro-explosive combustion phenomenon) was observed for bigger pellets (>10mm) at high bed temperatures (>700 °C), which occurred towards the end of combustion and remarkably reduce the devolatilization time of the oil sludge pellet. The size analysis of bed materials and fly ash showed that entire ash particle was entrained or elutriated out of the BFB furnace due to the fragile structure of oil sludge ash particles.     

Influence of process parameters on the strength of oil palm kernel shell pellets

In this study, the influence of processing and storage parameters on the strength of oil palm kernel shell pellets was determined. The strength of the pellets increased with compaction pressure but pelletizing pressures above 188 MPa had little effect on the pellet strength. At these high pressures, the pellets achieved no or near-zero porosities, indicating that the maximum strength was achieved. The diametrical tensile strengths of the pellets were much weaker than the compressive strengths when compacted at the same compaction pressure. The pellet strengths were found to increase with longer hold times during the compaction process. This was due to a continuous decrease in porosities until near-zero porosities were achieved, when increasing the hold time no longer led to further strengthening of the pellets. The time-dependent characteristic of the pellet strength was also reflected in the reduction in strength as compaction speed was increased. The strength of the pellets decreased significantly after 1 day (24 h) of storage, with a higher decrease observed when the pellets were stored under a higher humidity condition. These changes during storage could create difficulties during handling and transportation and may affect gasification performance.     

Sustained-release alginate-chitosan pellets prepared by melt pelletization technique

Context: Alginate-chitosan pellets prepared by extrusion-spheronization technique exhibited fast drug dissolution.

Objective: This study aimed to design sustained-release alginate pellets through rapid in situ matrix coacervation by chitosan during dissolution.

Methods: Pellets made of alginate with chitosan and/or calcium acetate were prepared using solvent-free melt pelletization technique which prevented reaction between processing materials during agglomeration and allowed such reaction to occur only in dissolution phase.

Results: Drug release was retarded in pH 2.2 medium when pellets were formulated with calcium acetate or chitosan till a change in medium pH to 6.8. The sustained-release characteristics of calcium alginate pellets were attributed to pellet dispersion and rapid cross-linking by soluble Ca²⁺ during dissolution. The slow drug release characteristics of alginate-chitosan pellets were attributed to polyelectrolyte complexation and pellet aggregation into swollen structures with reduced erosion. The drug release was, however, not retarded when both calcium acetate and chitosan coexisted in the same matrix as a result of chitosan shielding of Ca²⁺ to initiate alginate cross-linkages and rapid in situ solvation of calcium acetate induced fast pellet dispersion and chitosan losses from matrix.

Conclusion: Similar to calcium alginate pellets, alginate-chitosan pellets demonstrated sustained drug release property though via different mechanisms. Combination of alginate, chitosan and calcium acetate in the same matrix nevertheless failed to retard drug release via complementary drug release pattern.     

Dynamic crack extension along the interface of materials that differ in their thermal properties: with and without thermal relaxation

Summary Moving surface stresses cause crack extension along the interface of perfectly bonded thermoelastic materials at a constant sub-critical speed. The materials differ only in their thermal properties, and are governed by coupled thermoelastic equations that admit as special cases Fourier heat conduction as well as thermal relaxation with one or two relaxation times. A dynamic steady state of plane strain is assumed. The exact transform solution for a propagating displacement and temperature discontinuity is used to find solutions to the interface crack valid away from the crack edge for low extension speeds and solutions valid at the crack edge for high speeds. Results show that Fourier heat conduction dominates the former case, but solution behavior in the latter is dependent upon the particular thermal model. Thermal mismatch is seen to by itself cause a solution behavior similar to that for bonded dissimilar isothermal elastic solids. In particular, the two-relaxation time solution exhibits both oscillatory and non-oscillatory terms, and the interface temperature at the crack edge is finite.     

New facet of honey bees dancing language for mining the induction rules

Artificial Bee Colony (ABC) algorithm is used in many domains of computation, including optimization, clustering and classification tasks. Further, honey bees dancing is one of the most fascinating and intriguing behaviours of animal life. Honey bees’ dancing is termed as “waggle Dance” in literature and they perform it for indicating the food sources in their environment. This work presents a novel honey bees dancing language (HBDL)-based algorithm for mining the induction rules from datasets. The proposed HBDL algorithm is implemented and tested against the performance of ABC, Particle Swarm Optimization and nine more traditional algorithms frequently used by researchers. The experimental results showed that HBDL is a suitable and effective technique for data mining and classification task.     

基于单片机的蜂箱蜜蜂监测系统设计

针对蜜蜂养殖业及科学研究中存在的蜂箱内蜜蜂数量监测困难的问题,提出一种用于监测蜂箱中蜜蜂数量的监测系统。系统采用STC89C52单片机作为控制核心,用光电传感器H42B6感应外界光线的变化,通过触发单片机的外部中断识别蜜蜂的经过,根据触发顺序判断蜜蜂的进出方向,通过单片机进行数据处理,并将处理后的数据送入串口,最终通过串口调试工具显示在上位机界面上。实验结果证明:该系统能精确地统计出蜂箱中蜜蜂的进出数目,给养蜂者的管理提供准确依据。     

Penetration of a glass-faced transparent elastomeric resin by a lead–antimony-cored bullet

The penetration of the lead antimony-cored 7.62 mm × 51 mm bullet into a glass-faced polyurethane elastomeric polymer resin has been studied. The resulting craters in the resin contained elongated bullet core material that had a significant amount of porosity. A simple linear viscoelastic model was applied to AUTODYN-2D to describe the behaviour of the resin and numerical results of the penetration mechanism and depth-of-penetration appeared to match experimental observations well. Analysis of the high speed photography and a numerical model of this bullet penetrating a viscoelastic polymer showed that during the initial stages of penetration, the projectile is essentially turned inside out. Furthermore, the shape of the cavity was defined by the elastic relaxation of the polymer that led to compression of the core material. A weight analysis of the penetrated materials showed that using a thicker tile of glass resulted in better ballistic performance.     

Compaction of and drug release from coated pellets of different mechanical properties

The aim of this work was to relate the mechanical properties of film-coated pellets to their damage received during compaction, and to establish the significance of this damage for the release of a model drug from the resulting tablets. The formulations contained paracetamol and various excipient combinations chosen to provide different mechanical properties of the pellets, which were film-coated with Surelease® at various film thicknesses, and then compacted into tablets using three different compaction pressures. The drug release from the tablets was determined and compared to that of the uncompacted pellets. The compressibility and compactability of the various types of pellets was significantly influenced by the nature of the excipient combinations and binder liquids used to prepare the pellet cores, which also affected the drug release from the tablets. This could be attributed to the different responses of the pellets to compressive and shear stress. The film thickness and the mechanical properties of the film coating were found to be less important for tablet formation, but the film thickness played an important role in controlling the drug release rate from the tablets.     

Investigation of the physical–mechanical properties of Eudragit® RS PO/RL PO and their mixtures with common pharmaceutical excipients

Ammonio methacrylate copolymers Eudragit^® RS PO and Eudragit® RL PO have found widespread use as key components in various types of extended release solid dosage forms. The deformation behavior of neat polymers and binary mixes was evaluated using Heckel Analysis, strain rate sensitivity, work of compaction and elastic recovery index. Additionally, the compact forming ability of neat materials and binary mixes were evaluated by analyzing their tabletability, compressibility and compactibility profiles. The Heckel analysis of both polymers exhibited a speed-sensitive deformation behavior typical to plastic materials. The yield values of the binary mixes of the polymers with microcrystalline cellulose revealed a linear relationship with the weight fractions of individual components. The yield values of binary mixes of both the polymers with dibasic calcium phosphate exhibited slight negative deviations from linearity. Both polymers exhibited axial relaxation after ejection typical of viscoelastic materials, as measured by the elastic recovery index values. The work of compaction and the elastic recovery index values of the binary mixtures were found to be linearly related to the weight fractions of the individual components thus, confirming ideal mixing behavior based on the composition. Addition of microcrystalline cellulose to both polymers significantly improved their tabletability and compactibility. The tensile strengths of the compacts prepared with neat materials and binary mixes with microcrystalline cellulose, dibasic calcium phosphate and lactose were the function of their solid fraction and independent of the tableting speeds tested; thus, validating compactibility as a reliable parameter in predicting acceptable tablet properties.     

Determination of the properties of viscoelastic materials using spherical nanoindentation

The article is devoted to determining the properties of linearly viscoelastic isotropic materials from the experiment on the introduction of a spherical indenter at a constant-rate displacement in a viscoelastic sample. The results are based on the Lee–Radok (J. Appl. Mech. 27:438–444, 1960) solution of the viscoelastic contact problem. An exact formula is obtained for calculation of the relaxation function using indentation load–displacement data. To illustrate the application of this formula, it is used to find the relaxation function of polymethyl methacrylate (PMMA). The relaxation function found in the article is compared with data measured in a conventional test to evaluate the suitability of the proposed method.     

A stress relaxation model for the viscoelastic solids based on the steady-state creep equation

Creep and stress relaxation are inherent mechanical behaviors of viscoelastic materials. It is considered that both are different performances of one identical physical phenomenon. The relationship between the decay stress and time during stress relaxation has been derived from the power law equation of the steady-state creep. The model was used to analyse the stress relaxation curves of various different viscoelastic materials (such as pure polycrystalline ice, polymers, foods, bones, metal, animal tissues, etc.). The calculated results using the theoretical model agree with the experimental data very well. Here we show that the new mathematical formula is not only simple but its parameters have the clear physical meanings. It is suitable to materials with a very broad scope and has a strong predictive ability.     

A new M-integral parameter for mixed-mode crack growth in orthotropic viscoelastic material

This paper deals with a new independent path integral which provides the mixed-mode during a creep crack growth process in viscoelastic orthotropic media. The developments are based on an energetic approach using conservative laws. The mixed-mode fracture separation is introduced according to the generalization of the virtual work principle. The fracture algorithm is implemented in a finite element software and coupled with an incremental viscoelastic formulation and an automatic crack growth simulation. This M-integral provides the computation of stress intensity factors and energy release rate for each fracture mode. A numerical validation, in terms of energy release rate and stress intensity factors, is carried out on a CTS specimen under mixed-mode loading for different crack growth speeds.     

Influence of the standing time of the extrudate and speed of rotation of the spheroniser plate on the properties of pellets produced by extrusion and spheronization

The aim of the investigation was to study the influence of the standing time of the extrudate prior to spheronization and the speed of rotation expressed as linear peripheral velocity of the spheroniser plate on the properties of pellets using a 5² factorial experiment. Pellets composed of diclofenac sodium (5%), lactose monohydrate (20%) and microcrystalline cellulose (75%), prepared with water as the liquid binder (total solids to liquid ratio 1:0.675) using a screen extruder were produced after various standing times of the extrudate (ranging from immediate spheronization to 2 h) and at different rotational speeds ranging from 770 to 2900 rpm, which translates into a linear peripheral velocity of the friction plate from 4.84 to 18.22 m/s. The relative yield in the practically used pellet size fraction of 0.71–1.44 mm depended significantly on the standing time of the extrudate. Pellets produced at the lowest linear peripheral velocity were not round, and this was not affected by the standing time of the extrudate. Both the surface tensile strength and the density of the pellets were related to the extrudate standing time and the linear peripheral velocity, whereby the two factors were found to interact. However, neither of the process parameters nor the pellet properties themselves appeared to have an influence on the dissolution of the drug.     

Study of the interconversion between viscoelastic behaviour functions of PMMA

The use of polymers and polymer-based composites in mechanical, civil and electronic engineering has been growing owing to advances in the technology of materials. The different applications and working conditions of these materials require knowledge about their viscoelastic material functions: relaxation modulus, compliance, complex modulus, etc. Interconversion between these functions may be required for different reasons such as the impossibility of direct experimentation under certain excitation conditions. In this work, a DMA is used to calculate the experimental viscoelastic functions of a linear viscoelastic material (PMMA). The same functions are estimated by interconversion methods and compared with experimental ones. The results show that the interconversion functions fit properly the experimental functions.     

Dynamic steady-state crack propagation in a transversely isotropic viscoelastic body

The problem considered herein is the dynamic, subsonic, steady-state propagation of a semi-infinite, generalized plane strain crack in an infinite, transversely isotropic, linear viscoelastic body. The corresponding boundary value problem is considered initially for a general anisotropic, linear viscoelastic body and reduced via transform methods to a matrix Riemann–Hilbert problem. The general problem does not readily yield explicit closed form solutions, so attention is addressed to the special case of a transversely isotropic viscoelastic body whose principal axis of material symmetry is parallel to the crack edge. For this special case, the out-of-plane shear (Mode III), in-plane shear (Mode II) and in-plane opening (Mode I) modes uncouple. Explicit expressions are then constructed for all three Stress Intensity Factors (SIF). The analysis is valid for quite general forms for the relevant viscoelastic relaxation functions subject only to the thermodynamic restriction that work done in closed cycles be non-negative. As a special case, an analytical solution of the Mode I problem for a general isotropic linear viscoelastic material is obtained without the usual assumption of a constant Poissons ratio or exponential decay of the bulk and shear relaxation functions. The Mode I SIF is then calculated for a generalized standard linear solid with unequal mean relaxation times in bulk and shear leading to a non-constant Poissons ratio. Numerical simulations are performed for both point loading on the crack faces and for a uniform traction applied to a compact portion of the crack faces. In both cases, it is observed that the SIF can vanish for crack speeds well below the glassy Rayleigh wave speed. This phenomenon is not seen for Mode I cracks in elastic material or for Mode III cracks in viscoelastic material.