Development of a new combined numerical/experimental approach for the modeling of the nonlinear hyper-viscoelastic behavior of highly carbon black filled rubber compound
Abstracts:In this work, a new approach was developed for the determination of the parameters of a complex constitutive model for a highly filled rubber compound. A SSBR sulfur-vulcanized rubber sheet reinforced by carbon black was prepared. Three test specimens in the form of two rubber strips and dumbbell were cut from it. The Arruda-Boyce equation was selected to describe the hyperelastic behavior of the rubber. A parallel rheological framework (PRF) model and a new equation using a sigmoid function were selected to describe the nonlinear viscoelastic and stress softening effects. The model parameters were determined via a novel methodology based on the solution of the inverse problem comprising of the finite element models of the tests. Two multi-objective optimization loops based on the Nelder–Mead simplex method were designed. The accuracy of the predicted parameters was checked via the comparison of the model predictions with their corresponding data obtained from uniaxial and stress relaxation tests. A thorough analysis was made on the predicted results.
Viscoelastic and mechanical properties of large- and small-particle natural rubber before and after vulcanization
Abstracts:Insights into viscoelastic and mechanical properties of large- and small-particle natural rubber (NR) were gained for developing new rubber products. NR latex was first subjected to centrifuging to separate large rubber particles (LRP) and small rubber particles (SRP). The LRP and SRP solid rubbers were prepared to investigate their molecular weight characteristics and mechanical and viscoelastic properties. Before vulcanization, the LRP rubber was found to be better in both tensile strength and elasticity, as a result of natural branch-point networks formed by aggregation of phospholipid groups at the terminating end (α end) of the LRP rubber chains. After vulcanization, the SRP vulcanizate was, on the other hand, found to perform better in all respects. This could be explained by the accelerating effect from the proteins bound to the initiating end (ω end) of the SRP rubber chains in the crosslinking process during vulcanization.
Tribological anisotropy of selective laser sintered PA12 parts
Abstracts:Additive manufacturing techniques like selective laser sintering (SLS) of polymers are on the verge from a pure prototyping technique to producing small lot sizes, as it enables the production of highly complex components without a mould. The parts are build layerwise and therefore the resulting components exhibit specific characteristics like anisotropic mechanical behavior, porosity and a high surface roughness. As SLS enables the production of whole assembly groups in one building step without the need of assembly, a missing factor is the tribological interaction of these parts. The scope of this paper therefore is to identify the influence of the building orientation on the tribological properties. The resulting anisotropic behavior is presented under different loads and in comparison to specimens produced in a pvT-device to point out the influence of the inner structure and surface of the parts during the different stages of a tribological interaction.
Synergistic effect between hybrid nanoparticles of TiO2 and Nb2O5 in the nanostructured materials based on eva matrix
Abstracts:The effect of TiO2/Nb2O5 hybrid nanoparticles added to a poly (ethylene co acetate) (EVA) matrix with 28% vinyl content was evaluated, aiming to shed light on the mechanisms with which oxides, when added as hybrid nanoparticles, can modify specific properties of a polymer matrix. The nanostructured EVA/TiO2/Nb2O5 films, containing different proportions of TiO2/Nb2O5 in the range between 0.25 1% referred to total EVA mass, were prepared by the solution casting method. The films were characterized by X-ray diffraction and nuclear magnetic resonance relaxometry, since this technique allows assessing the molecular dynamics and intermolecular interactions of the new materials. According to the relaxometry and X-ray diffraction results, the addition of the two oxides had a positive effect on the EVA matrix through their synergetic action on the physical characteristics of the nanostructured material. The addition of Nb2O5 caused a change in the crystallinity and EVA chain mobility. According to the relaxation parameter values and the shape of the domain curves, the materials fabricated in the present work had good polymer-nanoparticle interaction and, consequently, a high level of dispersion and distribution of the oxides was found. The niobium oxide intensified the effect of titanium dioxide, indicating that there was synergy of oxides individual effects for the proportions investigated in this study. This finding is very interesting because the polymer-oxide interface action will influence the characteristics and properties of the new nanostructured materials that can be used in food packaging, because the nanoparticles chosen will give special properties to these materials.
Effects of thermal aging on uniaxial ratcheting behavior of vulcanised natural rubber
Abstracts:Vulcanised natural rubber (NR) tends to be hardened and lose damping capability as it is used at high temperature for a long period of time. Therefore, the thermal aging effects on the cyclic response of vulcanised NR need to be considered. In the present work, vulcanised NR sheets were thermally aged in an oven at a temperature ranging from 60 °C to 100 °C for aging time ranging from 24 h to 96 h. Subsequently, a series of uniaxial ratcheting tests were conducted on thermally aged vulcanised NR at room temperature. The thermal aging effects on the ratcheting behavior were experimentally investigated. The ratcheting strain can be restrained in thermally aged rubbers. The ratcheting strain decreases as the aging time or aging temperature increases. The reduction of ratcheting strain in thermally aged rubbers is attributed to the restrained ratcheting strain in the first cycle. Moreover, higher mean stress and stress amplitude cause faster ratcheting strain accumulation. Furthermore, the prior higher mean stress and stress amplitude can restrain the ratcheting strain in the subsequent cycles with lower ones. However, the loading rate has little influence on the ratcheting behavior of thermally aged rubbers.
Correlation between process and silica dispersion/distribution into composite: Impact on mechanical properties and Weibull statistical analysis
Abstracts:The aim of this study is to propose a methodology for controlling and monitoring particle dispersion at each step of composite manufacturing. Composites with well and poor particle dispersion were manufactured and characterized by numerous methods to estimate the impact of different dispersion state on mechanical properties. An original Weibull statistical analysis is suggested and shows the impact of this dispersion state on ultimate stress. For this purpose, a “model” matrix of PMMA was chosen. Well-dispersed silica enhanced Young's modulus by ∼9% and ultimate stress by ∼6%, while poorly-dispersed silica does not affect Young's modulus and ultimate stress. Moreover, a size effect is observed for well-dispersed composites; and not with poor particles dispersion.
Comparative study on different methods for determination of activation energies of nuclear cable materials
Abstracts:During the design stage of a nuclear power plant, the qualification testing is obliged to be conducted on the electrical equipment like cables to ensure the reliability and to predict the service lifetime of them under the operational conditions. Thereinto, thermal and oxidative degradation of the cable insulations and sheaths that are made of polymeric materials is the priority concern, and the renowned Arrhenius model is always referred to, which consequently calls for determination of the imperative kinetic constant - the activation energy. Actually, diverse methods for activation energies have been established by the scientists and applied by the engineers, but consensuses still have not been reached for such a method that is both convenient and conservative, and for such value ranges that are both reasonable and representative. Hence, in this paper, totally five different testing and data processing methods including thermal ageing test, differential scanning calorimetry and thermogravimetry analysis were utilized to determine the activation energies of three domestic polymeric materials that are intended for nuclear cables of the advanced pressurized water reactors in China. The results were then collected and the reasons for the differences among them were discussed, based on which the advantages and disadvantages of these methods were compared, and finally a compromising approach was proposed.
Serviceability analysis of wood–plastic composites impregnated with paraffin-based Pickering emulsions in simulated sea water–acid rain conditions
Abstracts:In order to enhance the serviceability of sorghum straw/polyvinyl chloride composites in simulated sea water–acid rain conditions, paraffin-based Pickering emulsions stabilized by attapulgite particles were prepared as an anti-corrosion coating of the composites, and a worst-case simulated sea water and acid rain corrosion scenario was employed. The results showed that Pickering coating can have superior leaching resistance in the sea water-acid rain conditions. The coated composites had higher water-resistance and hardness than non-coated composites. The presence of Pickering coating resists the degradation effects of simulated corrosion through better water-resistance and hardness, as well as better mechanical and wear properties of the composites. Prior to the corrosion, the thermal stability of the composites was slightly reduced due to the presence of paraffin, while after 12 d corrosion, Pickering emulsions endowed the coated composites with higher thermal stability than non-coated composites.
LF-NMR analysis of the water mobility, state and distribution in sorbitol plasticized polyvinyl alcohol films
Abstracts:In this work, high-alcoholysis polyvinyl alcohol (PVA) films with sorbitol as plasticizer are obtained through the melt processing and characterized with low field nuclear magnetic resonance (LF-NMR), differential scanning calorimetry (DSC) etc. to investigate the water mobility, state and distribution as well as its effect on the performance of the films. The results show that two state of water, e.g. bound water and free water, exist and sorbitol can effectively bind w'ater in PVA films. The sorbitol molecules can effectively promote the formation of bound water. Therefore, with the increase of sorbitol plasticizer, the bound water content increases. The water migration study indicates that sorbitol can form strong hydrogen bonds with water and make the bound water more stable. The tensile strength and the haze of the films increase with increased bound water content.
Ductile failure analysis of blunt V-notched epoxy resin plates subjected to combined tension-shear loading
Abstracts:Several rectangular specimens made of a ductile epoxy material containing rounded-tip V-notches were tested under in-plane tension-shear loading to experimentally obtain their fracture initiation angles and load-carrying capacities (LCCs). The epoxy resin specimens with different notch angles, notch tip radii and notch inclination angles were loaded under remote tension. Then, the reformulated Equivalent Material Concept (EMC) was utilized in conjunction with the rounded-tip V-notched maximum tangential stress (RV-MTS) criterion to predict the experimental results. A set of fracture curves was obtained for the first time based on the effective notch stress intensity factor (NSIF) for predicting the onset of failure in the V-notched ductile polymeric specimens. Good agreement was found to exist between the experimental results and the theoretical predictions of the EMC-RV-MTS combined criterion, which demonstrates the capability of these new fracture curves to predict the experimental results.