The Effect of COVID-19 Pandemic on the Global Supply Chain Operations: A System Dynamics Approach

Epidemic outbreaks are one of the important sources of the risk in the global supply chains. Before the COVID-19 pandemic, global industries that were unprepared for disruptions experienced a decline due to the pandemic. A global supply chain is a complex system set of dynamics that could be analyzed by the system dynamics approach. In this article, the impact of the recent pandemic on the global supply chain is simulated in different scenarios. A system dynamic model is developed to carry out the simulations. In order to consider the impact of the pandemic on the exogenous and endogenous variables, a force majeure factor is defined in the model. Global features considered in this article are the export and import operations, the exchange rate and the rate of tariff. In this article, a scenario analysis is performed to analyze two important factors of the global supply chain: force majeure factor and delivery delay. Results showed that improving the flexibility of production capacity is one of the important strategies that global supply chain managers should pursue. JEL Codes: F23, P45, C15, C63, E37, F17

Concurrent Validity of Power From Three On-Water Rowing Instrumentation Systems and a Concept2 Ergometer

Purpose: Instrumentation systems are increasingly used in rowing to measure training intensity and performance but have not been validated for measures of power. In this study, the concurrent validity of Peach PowerLine (six units), Nielsen-Kellerman EmPower (five units), Weba OarPowerMeter (three units), Concept2 model D ergometer (one unit), and a custom-built reference instrumentation system (Reference System; one unit) were investigated.Methods: Eight female and seven male rowers [age, 21 ± 2.5 years; rowing experience, 7.1 ± 2.6 years, mean ± standard deviation (SD)] performed a 30-s maximal test and a 7 × 4-min incremental test once per week for 5 weeks. Power per stroke was extracted concurrently from the Reference System (via chain force and velocity), the Concept2 itself, Weba (oar shaft-based), and either Peach or EmPower (oarlock-based). Differences from the Reference System in the mean (representing potential error) and the stroke-to-stroke variability (represented by its SD) of power per stroke for each stage and device, and between-unit differences, were estimated using general linear mixed modeling and interpreted using rejection of non-substantial and substantial hypotheses.Results: Potential error in mean power was decisively substantial for all devices (Concept2, –11 to –15%; Peach, −7.9 to −17%; EmPower, −32 to −48%; and Weba, −7.9 to −16%). Between-unit differences (as SD) in mean power lacked statistical precision but were substantial and consistent across stages (Peach, ∼5%; EmPower, ∼7%; and Weba, ∼2%). Most differences from the Reference System in stroke-to-stroke variability of power were possibly or likely trivial or small for Peach (−3.0 to −16%), and likely or decisively substantial for EmPower (9.7–57%), and mostly decisively substantial for Weba (61–139%) and the Concept2 (−28 to 177%).Conclusion: Potential negative error in mean power was evident for all devices and units, particularly EmPower. Stroke-to-stroke variation in power showed a lack of measurement sensitivity (apparent smoothing) that was minor for Peach but larger for the Concept2, whereas EmPower and Weba added random error. Peach is therefore recommended for measurement of mean and stroke power.

Generation I and Generation II Elastomeric Chains Characteristics Comparison in Artificial Saliva Immersion

Objective: To determine the force decay and discoloration in Generation I and Generation II elastomeric chain on artificial saliva immersion. Material and methods: Generation I and Generation II elastomeric chains stretched on an acrylic board and immersed in artificial saliva for 1, 7, 14, 21, and 28 days according to the group of days. On each specified day, the force of each sample measured by an orthodontic force gauge and the magnitude of the force obtained is entered to the formula to find out the force decay in percentage, whereas discoloration of each sample analyzed by the color reader with CIE Lab analysis. Results: The force decay between Generation I and Generation II elastomeric chains showed a statistically significant difference in every group of days, in which Generation II is more effective in maintaining stretch force. Similar to force decay, the discoloration in Generation I elastomeric chains on the 14th day showed significant value and Generation II elastomeric chains were more stable in maintaining color compared to Generation I. Conclusions: over the entire research period time, Generation II elastomeric chains are more stable in maintaining stretch forces and color compared to Generation I elastomeric chains. KEYWORDS Discoloration; Elastomeric chain; Force decay; Generation I; Generation II.

Strong reduction of the chain rigidity of hyaluronan by selective binding of Ca2+ ions

ABSTRACTThe biological functions of natural polyelectrolytes are strongly influenced by the presence of ions, which bind to the polymer chains and thereby modify their properties. Although the biological impact of such modifications is well-recognized, a detailed molecular picture of the binding process and of the mechanisms that drive the subsequent structural changes in the polymer is lacking. Here, we study the molecular mechanism of the condensation of calcium, a divalent cation, on hyaluronan, a ubiquitous polymer in human tissues. By combining two-dimensional infrared spectroscopy experiments with molecular dynamics simulations, we find that calcium specifically binds to hyaluronan at millimolar concentrations. Because of its large size and charge, the calcium cation can bind simultaneously to the negatively charged carboxylate group and the amide group of adjacent saccharide units. Molecular dynamics simulations and single-chain force spectroscopy measurements provide evidence that the binding of the calcium ions weakens the intra-molecular hydrogen-bond network of hyaluronan, increasing the flexibility of the polymer chain. We also observe that the binding of calcium to hyaluronan saturates at a maximum binding fraction of ~10-15 mol %. This saturation indicates that the binding of Ca2+ strongly reduces the probability of subsequent binding of Ca2+ at neighboring binding sites, possibly as a result of enhanced conformational fluctuations and/or electrostatic repulsion effects. Our findings provide a detailed molecular picture of ion condensation, and reveal the severe effect of a few, selective and localized electrostatic interactions on the rigidity of a polyelectrolyte chain.TOC

The Chain Distribution Tensor: Linking Nonlinear Rheology and Chain Anisotropy in Transient Polymers

Transient polymer networks are ubiquitous in natural and engineered materials and contain cross-links that can reversibly break and re-form. The dynamic nature of these bonds allows for interesting mechanical behavior, some of which include nonlinear rheological phenomena such as shear thickening and shear thinning. Specifically, physically cross-linked networks with reversible bonds are typically observed to have viscosities that depend nonlinearly on shear rate and can be characterized by three flow regimes. In slow shear, they behave like Newtonian fluids with a constant viscosity. With further increase in shear rate, the viscosity increases nonlinearly to subsequently reach a maximum value at the critical shear rate. At this point, network fracture occurs followed by a reduction in viscosity (shear-thinning) with a further increase in shear rate. The underlying mechanism of shear thickening in this process is still unclear with debates between a conversion of intra-chain to inter-chain cross-linking and nonlinear chain stretch under high tension. In this paper, we provide a new framework to describe the nonlinear rheology of transient polymer networks with the so-called chain distribution tensor using recent advances from the transient network theory. This tensor contains quantitatively and statistical information of the chain alignment and possible anisotropy that affect network behavior and mechanics. We investigate shear thickening as a primary result of non-Gaussian chain behavior and derive a relationship for the nonlinear viscosity in terms of the non-dimensional Weissenberg number. We further address the criterion for network fracture at the critical shear rate by introducing a critical chain force when bond dissociation is suddenly accelerated. Finally, we discuss the role of cross-linker density on viscosity using a “sticky” reptation mechanism in the context of previous studies on metallo-supramolecular networks with reversible cross-linkers.

Sprocket - Silent Chain Force Distribution with the Influence of Friction

The aim of this paper is to define and to analyse the theoretical contact forces that appear in a sprocket - silent chain joint. The contact force between the links and the teeth flanks of the sprocket can be considered as a reaction to the axial force in chain (tensioning force), transmitted throughout the pin - links joints. Friction forces will be considered in the joints and in the link - sprocket joints, which will be defined by the corresponding friction angles. The centrifugal force is also considered. This paper is presenting the steps to obtain the analytical model for the forces from the sprocket - link and the pin - link joints, depending on the tensioning force and the centrifugal force. These forces are computed in order to establish their distribution along links.

Critical undrained shear strength of sand-silt mixtures under monotonic loading

<p>This study uses experimental triaxial tests with monotonic loading to develop empirical relationships to estimate undrained critical shear strength. The effect of the fines content on undrained shear strength is analyzed for different density states. The parametric analysis indicates that, based on the soil void ratio and fine content properties, the undrained critical shear strength first increases and then decreases as the proportion of fines increases, which demonstrates the influence of fine content on a soil’s vulnerability to liquefaction. A series of monotonic undrained triaxial tests were performed on reconstituted saturated sand-silt mixtures. Beyond 30% fines content, a fraction of the silt participates in the soil’s skeleton chain force. In this context, the concept of the equivalent intergranular void ratio may be an appropriate parameter to express the critical shear strength of the studied soil. This parameter is able to control the undrained shear strength of non-plastic silt and sand mixtures with different densities.</p><p> </p><p><strong>Resumen</strong></p><p>Este estudio utiliza evaluaciones experimentales triaxiales con cargas repetitivas para desarrollar relaciones empíricas y estimar la tensión crítica de corte bajo condiciones no drenadas. El efecto de contenido de finos en la tensión de corte sin drenar se analizó en diferentes estados de densidad. El análisis paramétrico indica que, basado en la porosidad del suelo y las propiedades del material de finos, la tensión de corte sin drenar primero se incrementa y luego decrece mientras la proporción de finos aumenta, lo que demuestra la influencia de contenido de finos en la vulnerabilidad del suelo a la licuación. Una serie de las evaluaciones se realizó en mezclas rehidratadas y saturadas de arena y cieno. Más allá del 30 % de los contenidos finos, una fracción del cieno hace parte principal de la cadena de fuerza del suelo. En este contexto, el concepto de porosidad equivalente intergranular puede ser un parámetro apropiado para expresar la tensión crítica de corte del suelo estudiado. Este parámetro nos permite controlar la tensión de corte sin drenar de cieno no plástico y mezclas de arena de densidades diferentes.</p>

Micro-Analysis Ballast-Geogrid Pull out Tests Interaction

The discrete element method was used to simulate geogrid-reinforced ballast by pull out tests. Ballast particle was made of irregular clumps where its size and shape were considered by bonded spheres. The response of the ballast reinforced with geogrid under loading agrees with pull out experimental results. The micro-interaction between ballast particle and geogrid analyzed by microscopic parameters, contact force chain, force-displacement of the pull out tests was presented. It was also proved that the shape of granular particles, geogrid size and friction played an important role in ballast-geogrid system.

Static and Dynamic Characteristics of the Chain Drive System of a Heavy Duty Apron Feeder

Mechanical models of a chain drive system are proposed and applied to the theoretical analysis of chain drive system of a certain type of heavy duty apron feeder in mobile crushing station, including a five-bar model discussing the speed fluctuation problems of the chain drive system, an elastic collision model probing the effects of impulsive loads and a model of meshing area revealing the stress of chain links. Theoretical analysis indicates that the teeth number and sprocket pitch have the most influential effect on the vibration in the conveying direction; external impulsive loads are caused a significant increment of chain force, especially when loads caused by large materials; the initial pressure angle affects the roller chain stress conditions immensely. Then, multi-body dynamics models are established for the verification of theoretical results and dynamic simulation. Simulation results are in good agreement with the theoretical results and illustrate that impulsive loads affect chain tension significantly.

The Theoretical Analysis of Chain Drive System for Heavy Duty Apron Feeder

Taking a certain type of heavy duty apron feeder inmobile crushing station as the object of study, this paper proposed a five-barmechanism to research the velocity fluctuation problems of chain drive systemin an apron feeder. The effects of impulsive loads were analyzed, and amechanics model of meshing area was proposed. The research results are asfollows. 1. In the conveying direction, the number of teeth and sprocket pitch hasthe most influential effect on the vibration. 2. External impulsive loads causea significant increment of chain force, in particular the loads caused by largematerials. 3. The teeth and rollers near the tight-side chain play a major rolein transferring the torque, and the initial pressure angle affects their stressconditions immensely.