Advanced Testing of Soft Polymer Materials

Manufacturers of soft polymer products, as well as suppliers and processors of polymers, raw materials, and compounds or blends are compelled to use predictive and advanced laboratory testing in their search for high-performance soft polymer materials for future applications. The collection of 12 publications contained in this edition therefore presents different methods used to solve problems in the characterization of various phenomena in soft polymer materials, asks relevant questions and offers appropriate solutions

Dynamic stiffness and damping prediction on rubber material parts, FEA and experimental correlation

The final objective of the present work is the accurate prediction of the dynamic stiffness behaviour of complex rubber parts using finite element simulation tools. For this purpose, it becomes necessary to perform a complex rubber compound material characterisation and modelling work; this needs two important previous steps. These steps are detailed in the present document together with a theoretical review of viscoelastic visco-elasto-plastic models for elastomers. Firstly, a new characterisation method is proposed to determine the degree of cure of rubber parts. It is known that the degree of cure of rubbers bears heavily on their mechanical properties. This method consists of the correlation of swelling results to rheometer data achieving a good agreement. Secondly, the influence of the strain rate used in static characterisation tests is studied. In this step, a new characterisation method is proposed. The latter characterisation method will be used to fit extended hyperelastic models in Finite Element Analysis (FEA) software like ANSYS. The proposed method improves the correlation of experimental data to simulation results obtained by the use of standard methods. Finally, the overlay method proposed by Austrell concerning frequency dependence of the dynamic modulus and loss angle that is known to increase more with frequency for small amplitudes than for large amplitudes is developed. The original version of the overlay method yields no difference in frequency dependence with respect to different load amplitudes. However, if the element in the viscoelastic layer of the finite element model are given different stiffness and loss properties depending on the loading amplitude level, frequency dependence is shown to be more accurate compared to experiments. The commercial finite element program Ansys is used to model an industrial metal rubber part using two layers of elements. One layer is a hyper viscoelastic layer and the other layer uses an elasto-plastic model with a multi-linear kinematic hardening rule. The model, being intended for stationary cyclic loading, shows good agreement with measurements on the harmonically loaded industrial rubber part

Fourier transform rheology of complex, filled rubber materials

The presence of reinforcing fillers has a tremendous impact on the structure and the rheological properties of rubber materials. In this work, the nonlinear viscoelastic properties of two types of complex rubber materials, thermoplastic elastomers synthesized by anionic polymerization and carbon black filled SBR, were investigated by means of Fourier transform rheology. The results indicate a large influence of the interface on the third higher harmonic contribution

Achievements and Prospects of Functional Pavement

In order to further promote the development of functional pavement technology, a Special Issue entitled “Achievements and Prospects of Functional Pavement” has been proposed by a group of guest editors. To achieve this objective, the articles included in this Special Issue are related to different aspects of functional pavements, including green roads to decrease carbon emissions, noise, and pollution, safety pavements to increase skid resistance through water drainage and snow removal, intelligent roads for monitoring, power generation, temperature control and management, and durable roads to increase service life with new theories, new design methods, and prediction models, as highlighted in this editorial

New innovations in pavement materials and engineering: A review on pavement engineering research 2021

Sustainable and resilient pavement infrastructure is critical for current economic and environmental challenges. In the past 10 years, the pavement infrastructure strongly supports the rapid development of the global social economy. New theories, new methods, new technologies and new materials related to pavement engineering are emerging. Deterioration of pavement infrastructure is a typical multi-physics problem. Because of actual coupled behaviors of traffic and environmental conditions, predictions of pavement service life become more and more complicated and require a deep knowledge of pavement material analysis. In order to summarize the current and determine the future research of pavement engineering, Journal of Traffic and Transportation Engineering (English Edition) has launched a review paper on the topic of “New innovations in pavement materials and engineering: A review on pavement engineering research 2021”. Based on the joint-effort of 43 scholars from 24 well-known universities in highway engineering, this review paper systematically analyzes the research status and future development direction of 5 major fields of pavement engineering in the world. The content includes asphalt binder performance and modeling, mixture performance and modeling of pavement materials, multi-scale mechanics, green and sustainable pavement, and intelligent pavement. Overall, this review paper is able to provide references and insights for researchers and engineers in the field of pavement engineering

Modelling the mechanical response of elastomers: the roles of the network, the filler and the deformation history

The mechanical response of elastomers is influenced by a large range of factors including the elastomeric network, the deformation history (or Mullins effect) and the type and content of filler. The aim of this work is to develop modelling frameworks and models to capture the influences of these factors in selected elastomer systems. Several experimental studies were conducted to aid in the development and validation of the models. The impact of the elastomer network on the mechanical response is explored using a set of soft custom-made cast ultraviolet light curable silicone-acrylate elastomers eventually intended for elastomer 3D printing. Five different compounds are explored, varying the silicone content from 30 to 70 wt%. Uniaxial tensile tests were performed on all of the compositions. The results from these tests suggest that varying the silicone content results in a systematic variation of the stress-strain response. To study the variation of the underlying network structure with the silicone content, an EdwardsVilgis (EV) strain energy function was fitted to the stress-strain data. By examining the evolution of EV parameters with respect to the proportion of silicone in the system, it was shown that: 1) the network density increases with increasing silicone content, 2) the limiting extensibility decreases with increasing silicone content, i.e., smaller deformations are possible, and 3) the slip-link mobility increases, i.e., there is more freedom for the motion of topological constraints. Simple functions were fitted to quantify the evolution of EV parameters with composition. Based on the evolution of these parameters, a model was proposed to tune the mechanical properties of this class of materials. As these materials were developed for 3D printing, a printability assessment was carried out. It was also shown that further optimisation of the printing process is necessary to achieve mechanical response identical to that of the cast material. A phenomenological constitutive model was developed and validated to describe the impact of deformation history (or Mullins effect) on the mechanical response of an EPDM rubber compound. This model was inspired by recent experimental observations made on the uniaxial tensile cyclic stressstrain response. The third unloading-reloading loop after preconditioning was decomposed into elastic and viscous contributions. The viscous contribution was shown to form a master curve that was independent of the deformation history and only dependent on the effective network stretch. To the author’s knowledge, a constitutive model that incorporates a strain dependent viscosity has not been developed before. This model was shown to reasonably predict the stress in the third loop response, and provided an excellent prediction of the energy dissipated in this loop, making the model particularly useful for applications, such as vibration isolation where prediction of energy dissipation is vital. The initial loading case was underestimated by the model, and this is attributed to the time-dependent nature of the Mullins effect. The applicability of this model formulation to the deformation of CR and NBR rubber compounds was also explored. It was shown that the model was able to capture the response of NBR successfully but less so for CR. The failure was attributed to the inability of the strain energy function in capturing the upturn at small strains. Lastly, it was shown that the material requirement and the number of experiments required to parameterise the model can be reduced by utilising pseudo-cyclic tests. An investigation on the impact of pre-deformation on the small strain dynamic response was also conducted. In this part of the study, the influence of filler type and filler content were explored in SBR compounds filled with three different filler types and amounts. A rheometer was used for both the quasi-static and the dynamic deformations. Uniaxial tensile tests were conducted for the pre-deformations, and oscillatory torsion was applied in the dynamic tests on the same specimens. Two test protocols were employed, and the Kraus model was used to study the small strain dynamic response (or Payne or Fletcher-Gent effect). The first protocol was concerned with the impact of pre-deformation on the small strain dynamic response. Prior to pre-deformation increasing the amount of filler and the surface area of the filler particles results in an increase in the storage modulus plateau at small strains G′ 0 . As increasing pre-deformation is applied, G′ 0 begins to decrease. Other parameters in the Kraus model, the strain γc at which half the van der Waals type interaction between aggregates are broken and the rate m at which it occurs, are approximately independent of pre-deformation, but are impacted by the filler type and content. The results indicate that it may be possible to control the stiffness, and to some extent the onset of the Payne effect (or Fletcher-Gent effect), by a combination of filler selection and pre-conditioning. To the author’s knowledge models describing the Payne effect have yet to incorporate the pre-deformation. To address this issue, an exponential function for G′ 0 is proposed to link it to the pre-deformation. In the second test protocol, alongside pre-deformation, a static tensile strain was also imposed. In the shear strain range explored, for any given static strain, the storage modulus G′ was independent of the shear strain. With increasing static strain, G′ 0 was seen to increase. For a given filler type, G′ 0 increased with increasing filler amount. However, unlike the first test protocol G′ 0 , the behaviour between different filler types is more complex. Several models are presented in this work to describe the roles of the network, of the filler and of the deformation history on the mechanical response of elastomers. Although the models were developed for the materials used in this work, much of the modelling framework is applicable to a range of different materials. These models and the modelling framework supplement existing models in the prediction of the mechanical response of elastomers and are of value to engineers in the design process of elastomeric components. Owing to their physical nature, these models could also be leveraged by scientists to better understand the behaviour of elastomer network

Potential use of carbon nanotubes as a nanofiller for natural rubber latex condoms

The recent advancement in the field of nano-technology has raised much interest in the area of natural rubber latex (NRL) processing. This interest stems from the exceptional properties of nano-material and the promising results obtained by several researchers. Studies have shown that very low loadings of inorganic nanomaterials such as carbon nanotube (CNT) in NRL matrix leads to enhanced tensile strength, tensile modulus, tear resistance and aberration resistance. Thus providing a great prospect for reinforcement of thin film NRL articles such as condom. In this research, prevulcanised natural rubber latex (PvNRL) composite blends containing single walled carbon nanotubes (SWCNTs) were prepared via direct mixing. A progressive discolouration of PvNRL was observed with increased loadings of CNTs. Thermal analysis revealed faster drying rates for the composite blends containing SWCNT. Results from equilibrium swelling experiments also suggested a slight increase in crosslink density in the presence of SWCNT. There was a significant influence on flow behaviour of PvNRL as a result of varying loadings of SWCNT suspension. This was reflected as a change in pseudoplasticity and apparent viscosity. For Instance, apparent viscosity at a shear rate of 1 s-1 at 25°C for PvNRL with ~0.08% SWCNT was 2.5 Pa.s, compared to 0.49 Pa.s for the blends with 0.02% SWCNT. Condoms were moulded via the straight dipping technique using custom made glass formers. A series of dilutions was performed to correct the viscosity differences. This also ensured good consistency and promoted uniform deposition of PvNRL on the glass former. The average dimensions of the condoms produced in terms of length and width were ~191.17 ± 5.17 mm and 52.67 ± 5.17 mm respectively. Thickness measurement varied slightly according to the method of determination. The water leakage test suggested the absence of holes in the condoms produced. However, results from electrical leakage test contradicted those from water leak test. The results from infrared spectroscopy (FTIR) did not confirm the presence of chemical interactions between the SWCNT and PvNRL matrix. Glass transition temperature (Tg) was also unaffected across the blends. The stiffness (or modulus) was unaffected in all the condoms, as revealed by results from indentation hardness analysis. The SWCNT showed no significant influence on thermal decomposition temperatures of the condoms. Nonetheless, images from optical microscopy revealed increased surface roughness corresponding to higher loadings of SWCNT. Results from stress relaxation studies revealed improved retention of modulus under constant strain for condom samples containing SWCNT

Modelling the mechanical response of elastomers: the roles of the network, the filler and the deformation history

The mechanical response of elastomers is influenced by a large range of factors including the elastomeric network, the deformation history (or Mullins effect) and the type and content of filler. The aim of this work is to develop modelling frameworks and models to capture the influences of these factors in selected elastomer systems. Several experimental studies were conducted to aid in the development and validation of the models. The impact of the elastomer network on the mechanical response is explored using a set of soft custom-made cast ultraviolet light curable silicone-acrylate elastomers eventually intended for elastomer 3D printing. Five different compounds are explored, varying the silicone content from 30 to 70 wt%. Uniaxial tensile tests were performed on all of the compositions. The results from these tests suggest that varying the silicone content results in a systematic variation of the stress-strain response. To study the variation of the underlying network structure with the silicone content, an EdwardsVilgis (EV) strain energy function was fitted to the stress-strain data. By examining the evolution of EV parameters with respect to the proportion of silicone in the system, it was shown that: 1) the network density increases with increasing silicone content, 2) the limiting extensibility decreases with increasing silicone content, i.e., smaller deformations are possible, and 3) the slip-link mobility increases, i.e., there is more freedom for the motion of topological constraints. Simple functions were fitted to quantify the evolution of EV parameters with composition. Based on the evolution of these parameters, a model was proposed to tune the mechanical properties of this class of materials. As these materials were developed for 3D printing, a printability assessment was carried out. It was also shown that further optimisation of the printing process is necessary to achieve mechanical response identical to that of the cast material. A phenomenological constitutive model was developed and validated to describe the impact of deformation history (or Mullins effect) on the mechanical response of an EPDM rubber compound. This model was inspired by recent experimental observations made on the uniaxial tensile cyclic stressstrain response. The third unloading-reloading loop after preconditioning was decomposed into elastic and viscous contributions. The viscous contribution was shown to form a master curve that was independent of the deformation history and only dependent on the effective network stretch. To the author’s knowledge, a constitutive model that incorporates a strain dependent viscosity has not been developed before. This model was shown to reasonably predict the stress in the third loop response, and provided an excellent prediction of the energy dissipated in this loop, making the model particularly useful for applications, such as vibration isolation where prediction of energy dissipation is vital. The initial loading case was underestimated by the model, and this is attributed to the time-dependent nature of the Mullins effect. The applicability of this model formulation to the deformation of CR and NBR rubber compounds was also explored. It was shown that the model was able to capture the response of NBR successfully but less so for CR. The failure was attributed to the inability of the strain energy function in capturing the upturn at small strains. Lastly, it was shown that the material requirement and the number of experiments required to parameterise the model can be reduced by utilising pseudo-cyclic tests. An investigation on the impact of pre-deformation on the small strain dynamic response was also conducted. In this part of the study, the influence of filler type and filler content were explored in SBR compounds filled with three different filler types and amounts. A rheometer was used for both the quasi-static and the dynamic deformations. Uniaxial tensile tests were conducted for the pre-deformations, and oscillatory torsion was applied in the dynamic tests on the same specimens. Two test protocols were employed, and the Kraus model was used to study the small strain dynamic response (or Payne or Fletcher-Gent effect). The first protocol was concerned with the impact of pre-deformation on the small strain dynamic response. Prior to pre-deformation increasing the amount of filler and the surface area of the filler particles results in an increase in the storage modulus plateau at small strains G′ 0 . As increasing pre-deformation is applied, G′ 0 begins to decrease. Other parameters in the Kraus model, the strain γc at which half the van der Waals type interaction between aggregates are broken and the rate m at which it occurs, are approximately independent of pre-deformation, but are impacted by the filler type and content. The results indicate that it may be possible to control the stiffness, and to some extent the onset of the Payne effect (or Fletcher-Gent effect), by a combination of filler selection and pre-conditioning. To the author’s knowledge models describing the Payne effect have yet to incorporate the pre-deformation. To address this issue, an exponential function for G′ 0 is proposed to link it to the pre-deformation. In the second test protocol, alongside pre-deformation, a static tensile strain was also imposed. In the shear strain range explored, for any given static strain, the storage modulus G′ was independent of the shear strain. With increasing static strain, G′ 0 was seen to increase. For a given filler type, G′ 0 increased with increasing filler amount. However, unlike the first test protocol G′ 0 , the behaviour between different filler types is more complex. Several models are presented in this work to describe the roles of the network, of the filler and of the deformation history on the mechanical response of elastomers. Although the models were developed for the materials used in this work, much of the modelling framework is applicable to a range of different materials. These models and the modelling framework supplement existing models in the prediction of the mechanical response of elastomers and are of value to engineers in the design process of elastomeric components. Owing to their physical nature, these models could also be leveraged by scientists to better understand the behaviour of elastomer network