Creep-Fatigue Relationship in Polymer: Molecular Dynamics Simulations Approach

The creep‐tensile fatigue relationship is investigated using MD simulations for amorphous polyethylene, by stepwise increasing the R‐ratio from 0.3 for fatigue to an R‐ratio = 1 for creep. The simulations can produce similar behavior as observed in experiments, for instances strain‐softening behavior and hysteresis loops in the stress‐strain curves. The simulations predict the molecular mechanisms of creep and fatigue are the same. Fatigue and creep cause significant changes of the van der Waals and dihedral potential energies. These changes are caused by movements of the polymer chains, creating more un‐twisted dihedral angles and the unfolding of polymer chain

The effects of the van der Waals potential energy on the Young’s modulus of a polymer: comparison between molecular dynamics simulation and experiment

Molecular dynamics simulation were employed to investigate the effect of changing the potential energies describing primary and secondary bonds on the Young’s modulus of a polymer. The energies were changed by arbitrarily modifying the parameters of the potential energy model function. The parameters influence the structure of the polymer and its global energy, eventually causing changes to the Young’s modulus. The van der Waals energy describing secondary bonds gives the most significant contribution to the changes. Increasing the energy increases the density and Young’s modulus. The trends are in agreement with experimental data

Effects of temperature and strain rate on the deformation of amorphous polyethylene: a comparison between molecular dynamics simulations and experimental results

Molecular dynamics simulations are used to investigate the effects of temperature and strain rate on the deformation of amorphous polyethylene. The simulations predict the effects of temperature and strain rate on the stress-strain responses, Youngs modulus and Poissons ratio similar to those observed in laboratory experiments performed by other researchers. The time-temperature superposition principle is applied to the Youngs modulus and Poissons ratio to form a master curve to address the discrepancies in strain rates between the simulations and the experiments. Differences in the numbers of monomers and chains, the degree of crystallinity and molecular orientation lead to discrepancies in the Youngs modulus and Poissons ratio between simulations and experiments

Molecular dynamics simulations of strain-controlled fatigue behaviour of amorphous polyethylene

Fatigue of amorphous polyethylene under low strain was simulated using molecular dynamics. The united atom approach and the Dreiding force field were chosen to describe the interaction between monomers. Molecular dynamics simulations resembling strain-controlled loading fatigue tests in tension-tension mode were performed to study the effect of the R-ratio and mean strain on the mechanical responses. Laboratory fatigue experiments in strain/displacement control were performed at room temperature, and the results were compared to the simulation results. The simulations are able to produce qualitatively similar behaviour to the experimental results, for instance, mean stress relaxation, hysteresis loops in the stress�strain curve, and change in the cyclic modulus. They also show that stress relaxation is enhanced by cyclic loading. The simulations show that cyclic loading changes the total potential energies of the system, especially the van der Waals potential. The changes in the van der Waals potential energy contribute significantly to the increasing of the stiffness of the system. Some changes in dihedral angles with lower energy configurations are observed; however, bond distances and angles do not change significantly. The chains tend to unfold slightly along the loading axis as the fatigue loading progresses

Temperature dependence of the Young's modulus of polymers calculated using a hybrid molecular mechanics-molecular dynamics method

A hybrid molecular mechanics-molecular dynamics (MM-MD) method is proposed to calculate the Youngs modulus of polymers at various temperature. It overcomes the limitation that MD is restricted to extremely high strain rates. A case study based on poly-methyl-methacrylate demonstrates that, contrary to previous MD studies, the method is able to accurately reproduce the effect of temperature on the Youngs modulus in close agreement with experimental data. The method can also predict a more clear transition between the glassy and rubbery states than previous MD studies

A Coarse Grained Model for Viscoelastic Solids in Discrete Multiphysics Simulations

Viscoelastic bonds intended for Discrete Multiphysics (DMP) models are developed to allow the study of viscoelastic particles with arbitrary shape and mechanical inhomogeneity that are relevant to the pharmaceutical sector and that have not been addressed by the Discrete Element Method (DEM). The model is applied to encapsulate particles with a soft outer shell due, for example, to the partial ingress of moisture. This was validated by the simulation of spherical homogeneous linear elastic and viscoelastic particles. The method is based on forming a particle from an assembly of beads connected by springs or springs and dashpots that allow the sub-surface stress fields to be computed, and hence an accurate description of the gross deformation. It is computationally more expensive than DEM, but could be used to define more effective interaction law

A Policy Strategy Evaluation for Covid-19 Pandemic in the City of Surabaya using Vensim Ventana Dynamic System Simulation

The global Covid-19 pandemic has been a considerable concern worldwide. In Surabaya city, the government has taken particular measures to establish appropriate policies to overcome the Covid-19 pandemic. However, there has been no precise measure to verify the effectiveness of the policy in the future. This research aims to evaluate the Surabaya Government policy scenario using a dynamic system-based simulation with Vensim Ventana software. The initial model was designed with adopting the Covid-19 model made by Tom Fiddaman, Ventana System in 2020 adjusted to real conditions in Surabaya. The simulation results obtained from the initial validated model estimate that the Covid-19 pandemic would end on May 5, 2021, with total infected 15,876 people. The model was then developed by simulating several policy scenarios: herd immunity, convalescent plasma therapy, and swab test to predict policy’s impact. The best-case scenario is gained by combining the convalescent plasma therapy policy and increasing the number of swab tests at Labkesda to 4,000 samples per day. The simulation results’ prediction shows the pandemic will end 52 days earlier, with the percentage of the infected population 23.77% smaller than the initial model. It can be concluded that the government’s strategy of collaborating the two policies is effective to overcome the pandemic in Surabaya. Nevertheless, the implementation of policies to overcome this pandemic success with contributions from all elements of society

A Policy Strategy Evaluation for Covid-19 Pandemic in the City of Surabaya Using Vensim Ventana Dynamic System Simulation

The global Covid-19 pandemic has been a considerable concern worldwide. In Surabaya city, the government has taken particular measures to establish appropriate policies to overcome the Covid-19 pandemic. However, there has been no precise measure to verify the effectiveness of the policy in the future. This research aims to evaluate the Surabaya Government policy scenario using a dynamic system-based simulation with Vensim Ventana software. The initial model was designed with adopting the Covid-19 model made by Tom Fiddaman, Ventana System in 2020 adjusted to real conditions in Surabaya. The simulation results obtained from the initial validated model estimate that the Covid-19 pandemic would end on May 5, 2021, with total infected 15,876 people. The model was then developed by simulating several policy scenarios: herd immunity, convalescent plasma therapy, and swab test to predict policys impact. The best-case scenario is gained by combining the convalescent plasma therapy policy and increasing the number of swab tests at Labkesda to 4,000 samples per day. The simulation results prediction shows the pandemic will end 52 days earlier, with the percentage of the infected population 23.77% smaller than the initial model. It can be concluded that the governments strategy of collaborating the two policies is effective to overcome the pandemic in Surabaya. Nevertheless, the implementation of policies to overcome this pandemic success with contributions from all elements of society

A wicking measurement approach to evaluate the protection of non-medical face mask fabrics

Purpose The objective of this research is to systematically compare two methods of wicking test for evaluating the quality of the non-medical-mask fabric, i.e. its absorbency property at various conditions, using a design of experiment approach. This research also evaluates the suitability of several fabrics to be used for non-medical masks. Design/methodology/approach Horizontal and vertical wicking tests were selected to evaluate the absorbency property of five fabrics commonly used for the non-medical mask. The tests were performed at three temperatures and using two types of liquid. The design of experiment approach was employed to determine the relationship between the path length of liquid movement in fabric and type of test method, temperature and type of liquid. Findings Both vertical and horizontal wicking tests show the same order of fabrics according to their absorbency. The order is cotton twill, local cotton, Japanese cotton, Oxford and Scuba, where the first in the order has the lowest absorbency and the last has the highest absorbency. Based on the analysis of variance (ANOVA), the range of temperature and types of liquid employed in this research do not affect the path length of the liquid movement in the fabric. Originality/value This research proposes horizontal and vertical wicking tests as a practical tool to evaluate absorbency property of fabric for the non-medical mask. This research also presents a design of experiment approach to evaluate the effect of the test method, temperature and type of liquid on the path length of the liquid movement in the fabric

Temperature and configurational effects on the Young’s modulus of poly (methyl methacrylate):a molecular dynamics study comparing the DREIDING, AMBER and OPLS force fields

The effects of the configuration and temperature on the Youngs modulus of poly (methyl methacrylate) (PMMA) have been studied using molecular dynamics simulations. For the DREIDING force field under ambient temperatures, increasing the number of monomers significantly increases the modulus of isotactic and syndiotactic PMMA while the isotactic form has a greater modulus. The effects of temperature on the modulus of isotactic PMMA have been simulated using the DREIDING, AMBER, and OPLS force fields. All these force fields predict the effects of temperature on the modulus from 200 to 350 K that are in close agreement with experimental values, while at higher temperatures the moduli are greater than those measured. The glass transition temperature determined by the force fields, based on the variation of the modulus with temperature, is greater than the experimental values, but when obtained from a plot of the volume as a function of the temperature, there is closer agreement. The Youngs moduli calculated in this study are in closer agreement to the experimental data than those reported by previous simulations