Scratch hardness as a quasi-intrinsic parameter to measure the scratch resistance of polymers

In this work four different polymers (acrylonitrile-butadiene-styrene, high-impact polystyrene, rubber-toughened polybutylene terephthalate, linear low-density polyethylene) were characterized in terms of their bulk (modulus and yield stress) and surface (scratch hardness) mechanical properties. The intrinsic time-dependence of the materials was addressed by performing DMA and compression tests at varying testing speed/frequency, exploiting time-temperature superposition and Eyring’s model to obtain data at strain rates compatible with scratch experiments. The latter were performed by applying different loading histories (constant depth or load) and indenters. Scratch hardness was determined using Pelletier’s model; it was demonstrated that such a parameter provides a reliable and almost intrinsic (i.e. loading history independent) evaluation of scratch resistance, seen as the resistance the material opposes to indenter penetration. The material compressive yield stress (evaluated at the strain rate relevant to scratch phenomena) was found to be the key controlling factor in determining scratch hardness. It can therefore be taken as a measure of the mechanical scratch resistance when evaluating the possible effects of variables such as material composition, crystallinity, physical ageing… Its relation with other aspects of the scratch phenomenon (in particular deformation recovery) was also explored, accounting for the specific deformation regime imposed by the indenter (transitioning from elastic to predominantly plastic)

Image-based finite element method applied to in-situ X-ray tomography compression tests of open cell polymeric foams

Polymeric foams are widely used in different industries such as automotive, aviation and military due to their wide density range, high specific strength, strong ability to absorb impact loads, and good thermal insulation properties [1-2]. In this study, taking advantage of 3D imaging technology based on X-ray computed tomography (CT), in-situ compression tests were carried out to explore the global and local deformation behaviour of open cell polyurethane foams. Different strain levels were recorded and relevant images reconstructed from x-ray CT during the in-situ tests. To better understand the deformation behaviour of the interior structure of foams under uniaxial loading, an advanced CT image-based finite element model was used, providing a non-destructive and non-invasive way to study the real internal structure of the foam for different loading stages [3]. The finite element models of the foam microstructure for each strain level were generated from images obtained by X-ray CT by applying level set method (LSM) and Delaunay triangulation (DT). Moreover, in order to improve the mesh quality of the FE models and calculation accuracy, Taubin smoothing algorithm was utilized. Then, numerical simulations on the smoothed mesh were implemented to compare with the experimental tests and understand deformation process occurring during compression. Good agreement was observed between the deformations obtained by simulations and in-situ compression experiments at different strain levels. Morphological features of deformed models at different strain level were identified. Three main features were analysed in this study, namely: strut length, strut thickness and strut orientation. The comparison of these features for different deformation stages was carried out. The results suggest that large amounts of cells collapse during the compression process. Under uniaxial stress, the open-cell foam deforms by struts bending followed, at sufficiently large loads, by nonlinear deformation within the struts. The deformation behaviour of the interior structure strongly depends on the initial orientation and thickness of struts

Applicazione della meccanica della frattura viscoelastica alla previsione della vita di tubi in polibutene

Il Polibutene-1 isotattico (i-PB1) è un materiale polimerico usato per la produzione di tubi per il trasporto di fluidi in pressione. In questo lavoro si sono studiati due tipi di i-PB1 prodotti da Basellche differiscono per grado di isotatticità.Si sono condotte prove di frattura a diverse temperature e velocità di spostamento imposte. Si è utilizzata una configurazione di flessione su provini con singolo intaglio (SENB) unitamente a quella di doppia trave a sbalzo (DCB), quest’ultima limitatamente allo studio della fase di propagazione. Al fine di individuare con precisione l’innesco della frattura e la velocità di propagazione della stessa si è fatto ricorso a metodi ottici. Dal punto di vista fenomenologico durante la propagazione si assiste alla formazione di zone in cui il materiale risulta fortemente stirato. La frattura in esse avanza con una lacerazione continua che si alterna a salti repentini in occasione del brusco cedimento di queste zone, associato a conseguenti cadute del carico. Questaparziale instabilità è stata osservata sui due materiali per entrambe le configurazioni di prova.I risultati ottenuti sono stati interpretati seguendo l’approccio della meccanica della frattura e applicando uno schema di riduzione di tipo tempo-temperatura che ha permesso di descrivere il comportamento viscoelastico del materiale su un intervallo temporale di diverse decadi.I risultati hanno permesso di applicare un modello analitico per la previsione della vita utile di tubi in pressione.Il modello si è mostrato in buon accordo con i dati sperimentali disponibili da prove condotte su tubi dello stesso materiale

Applicazione della meccanica della frattura viscoelastica alla previsione della vita di tubi in polibutene

Isotactic polybutene-1 (i-PB1) is a polymer used for the manufacturing of pressurized pipes. In this work two grades of i-PB1 with a different degree of isotacticity have been investigated; they have been supplied by Basell Polyolefins. Fracture tests have been performed at various temperatures and testing speeds. Two configurations have been used, single edge notch bending (SENB) and double cantilever beam (DCB), the latter only to study crack propagation. Optical methods have been used to detect crack initiation and measure propagation speed. From the phenomenological point of view, the formation of highly stretched material regions has been observed during crack propagation. A continuous tearing of these regions as the crack advances has often been interrupted by their sudden rupture, with the load decreasing accordingly. This partial instability has been observed on both material grades, with both testing configurations. Results of the tests have been interpreted using the fracture mechanics framework; a time-temperature superposition scheme has been adopted to represent viscoelastic behavior over several decades. An analytical model has been applied to predict the lifetime of pressurized pipes. A good agreement has been reported between model predictions and experimental data obtained from tests on polybutene pipes

Use of recycled fillers in bituminous mixtures for road pavements

This paper reports the results of a laboratory investigation on the use of recycled fillers in bituminous mixtures for road pavements. The fillers were obtained by crushing and sieving (0.00-0.063 mm) Stabilized Bottom Ashes from municipal waste incinerators and Electric Arc Furnace Steel Slags. Moreover, a currently used calcareous filler was included in the research for comparative purposes. Two filler dosages were considered in the experiments. Laboratory tests aimed at investigating compaction properties, volumetric characteristics, and mechanical performance of the bituminous mixtures. The results suggest that both types of recycled filler are suitable to be used in bituminous mixtures, also demonstrating that in certain conditions the investigated fillers increase the performance of the corresponding mixtures in comparison to standard (calcareous) filler

The effects of fibers on the performance of bituminous mastics for road pavements

The experimental investigation herein described is aimed at understanding the effects of cellulose-based fibers on the mechanical properties of bituminous mastics for paving applications. Three bitumen (two of which modified with SBS polymers), a calcareous filler and four different types of fiber with varying content were used to prepare the investigated mastics. The filler to bitumen ratio was maintained constant. The laboratory investigations were focused on empirical tests (Needle Penetration and Ring and Ball – R&B – Temperature) and dynamic-mechanical tests, the latter performed in a wide range of temperatures. Results suggest that fibers improve the behavior of mastics for hot mix asphalts, particularly with respect to the prevention of rutting phenomena at high service temperatures

Modelling mixed-mode fracture in poly(methylmethacrylate) using peridynamics

AbstractPeridynamics (Silling (2000)) is a non-local continuum theory that is particularly suited to handle discontinuities in the displacement field, such as those arising during fracture. Peridynamics prescribes that each material point interacts with all its neighbors contained in a sphere of given radius; this assumption introduces a characteristic length scale in the continuum description. In a nutshell, the interactions between material points depend on their relative distance; in the peridynamics framework this distance is called the “bond length”. The equations of motion, holding at each material point, link the material point acceleration to the integral over the point neighborhood of a force density field, whose strength depend on bond-stretches, i.e. the ratio of the actual bond-length over the initial one. In these equations the displacement gradient does not appear, thus naturally allowing for discontinuities in the displacement field to occur. As to failure, the simplest possible damage description is provided by an interaction law prescribing the force to vanish when a critical bond-stretch threshold is crossed; this parameter can be related to the Mode I critical strain energy release rate. A single parameter is needed to describe failure, in principle under every possible loading condition.In this work the predictive abilities of peridynamics were checked against experimental results in the case of mixed-mode failure of brittle polymers. Pre-cracked poly(methylmethacrylate) (PMMA) samples were tested using different specimens, in order to obtain Mode I, Mixed-Mode and Mode II loading conditions. The material was assumed to behave according to a peridynamics brittle elastic material model; the parameters needed to calibrate the elastic behavior were determined from Mode I tests, as was the critical stretch.The peridynamics simulations of mixed-mode tests were able to catch the correct fracture initiation load and to provide a fair description of the crack path under different conditions. The peridynamics model was also able to qualitatively capture the typical “nail” shape assumed by the crack front during propagation

Accounting for friction in the mechanical testing of athletics tracks

This investigation deals with the problem of identifying the mechanical behaviour of rubbers from compression tests, performed on specimens having unfavorable geometry. A typical situation is that of flat specimens obtained from high-friction sports surfaces. To this purpose, experimental tests were conducted, aimed at measuring friction under various conditions and evaluating its effect on the compressive behavior of different rubber samples. The experimental results have been interpreted in view of an existing analytical model proposed by Gent and coworkers. The method was shown to be valid within a relatively broad range of conditions (in terms of materials, lubrication and aspect ratio). Its application allowed the creation of virtual “frictionless” curves, by rescaling experimental data for the stiffening factor predicted by Gent model. These curves represent more closely the intrinsic material behaviour, removing the large frictional contribution present in the experimental tests, and can be used as a more reliable input for numerical simulations

On the relationship between force reduction, loading rate and energy absorption in athletics tracks

In this work, finite element simulations of typical sports surfaces were performed to evaluate parameters, such as the loading rate and the energy absorbed by the surface, in relation to its characteristics (surface structure and material properties). Hence, possible relations between these quantities and the standard parameters used to characterize the shock absorbing characteristics of the athletics track (in particular, its force reduction) were investigated. The samples selected for this study were two common athletics tracks and a sheet of natural rubber. They were first characterized by quasi-static compression tests; their mechanical properties were extrapolated to the strain rate of interest and their dependence on the level of deformation was modelled with hyperelastic constitutive equations. Numerical simulations were carried out for varying sample thicknesses to understand the influence of track geometry on force reduction, loading rate and stored energy. A very good correlation was found between force reduction and the other relevant parameters, with the exception of the loading rate at the beginning of the impact

applicazione della meccanica della frattura viscoelastica alla previsione della vita di tubi in polibutene

Il Polibutene-1 isotattico (i-PB1) è un materiale polimerico usato per la produzione di tubi per il trasporto di fluidi in pressione. In questo lavoro si sono studiati due tipi di i-PB1 prodotti da Basellche differiscono per grado di isotatticità.Si sono condotte prove di frattura a diverse temperature e velocità di spostamento imposte. Si è utilizzata una configurazione di flessione su provini con singolo intaglio (SENB) unitamente a quella di doppia trave a sbalzo (DCB), quest'ultima limitatamente allo studio della fase di propagazione. Al fine di individuare con precisione l'innesco della frattura e la velocità di propagazione della stessa si è fatto ricorso a metodi ottici. Dal punto di vista fenomenologico durante la propagazione si assiste alla formazione di zone in cui il materiale risulta fortemente stirato. La frattura in esse avanza con una lacerazione continua che si alterna a salti repentini in occasione del brusco cedimento di queste zone, associato a conseguenti cadute del carico. Questaparziale instabilità è stata osservata sui due materiali per entrambe le configurazioni di prova.I risultati ottenuti sono stati interpretati seguendo l'approccio della meccanica della frattura e applicando uno schema di riduzione di tipo tempo-temperatura che ha permesso di descrivere il comportamento viscoelastico del materiale su un intervallo temporale di diverse decadi.I risultati hanno permesso di applicare un modello analitico per la previsione della vita utile di tubi in pressione.Il modello si è mostrato in buon accordo con i dati sperimentali disponibili da prove condotte su tubi dello stesso materiale