Contact mechanics of high-density polyethylene: Effect of pre-stretch on the frictional response and the onset of wear

Nowadays, in many applications metal parts are replaced by light-weight polymer products. As a result of the processing history, these polymer fabricates are, more often than not, anisotropic, leading to a direction dependent mechanical performance. Recently we showed the frictional response of isotactic polypropylene is improved by pre-stretching the crystalline network. In the present work, the scratch response of isotropic high-density polyethylene (HDPE) is compared with that of several pre-stretched samples of the same material, subjected to a single-asperity contact with a rigid diamond indenter. The surface penetration and lateral force are measured in-situ for a range of applied loads and sliding velocities. In the direction perpendicular to the orientation, the observed response is comparable to that of isotactic polypropylene (iPP). Contrary, in the direction parallel to the oriented crystals abrasive wear is observed in HDPE already for relatively low applied loads. As the amount of anisotropy increases, the wear-rate also increases, leading to a decrease in global scratch resistance of these materials. The discrepancies between iPP and HDPE are explained by the intrinsic material behaviour; the lack of strain softening in HDPE prevents strain localization, hence the ever increasing local stress reaches its maximum value and brittle machining is observed

Contact mechanics of isotactic polypropylene: effect of pre-stretch on the frictional response

Polymers are increasingly used in applications where relative moving parts are in contact. The dissipation of energy due to friction, i.e. heat production, reduces a product's lifetime significantly. Since in processing often an extrusion or injection moulding step is used in product formation, the induced anisotropic microstructure leads to a spatial variation of mechanical properties, for example frictional resistance. In this work we compare the scratch response of isotropic isotactic polypropylene (iPP) to the response of various oriented iPP systems. Subjected to single-asperity contact with a rigid diamond, the surface penetration and lateral force are measured. For various combinations of applied normal load and sliding velocity, the surface penetration and lateral force are measured. Optical profilometry measurements are used to explain the (large) differences in residual scratch profile between tests performed in the direction parallel and transverse to the orientation direction, respectively. As the anisotropy increases with the amount of orientation, both the maximum tensile stress and the strain hardening increase substantially. The penetration depth, for oriented systems governed by the transverse viscoelasticity and yield stress, is comparable for all loading angles and decreases with increasing amount of orientation. The direction of lowest frictional resistance is shown to be the direction parallel to the oriented crystals. The combination of decreasing global deformation and friction reduction as a result of pre-stretch decelerates strain localization, therewith delaying crack initiation which eventually leads to abrasive wear. Along with that, the substantial amount of elastic recovery after scratching in the transverse direction is related to the pre-tension of the perpendicular crystal network

Contact mechanics of isotactic polypropylene:Effect of pre-stretch on the frictional response

\u3cp\u3ePolymers are increasingly used in applications where relative moving parts are in contact. The dissipation of energy due to friction, i.e. heat production, reduces a product's lifetime significantly. Since in processing often an extrusion or injection moulding step is used in product formation, the induced anisotropic microstructure leads to a spatial variation of mechanical properties, for example frictional resistance. In this work we compare the scratch response of isotropic isotactic polypropylene (iPP) to the response of various oriented iPP systems. Subjected to single-asperity contact with a rigid diamond, the surface penetration and lateral force are measured. For various combinations of applied normal load and sliding velocity, the surface penetration and lateral force are measured. Optical profilometry measurements are used to explain the (large) differences in residual scratch profile between tests performed in the direction parallel and transverse to the orientation direction, respectively. As the anisotropy increases with the amount of orientation, both the maximum tensile stress and the strain hardening increase substantially. The penetration depth, for oriented systems governed by the transverse viscoelasticity and yield stress, is comparable for all loading angles and decreases with increasing amount of orientation. The direction of lowest frictional resistance is shown to be the direction parallel to the oriented crystals. The combination of decreasing global deformation and friction reduction as a result of pre-stretch decelerates strain localization, therewith delaying crack initiation which eventually leads to abrasive wear. Along with that, the substantial amount of elastic recovery after scratching in the transverse direction is related to the pre-tension of the perpendicular crystal network.\u3c/p\u3

Global and local large-deformation response of sub-micron, soft- and hard-particle filled polycarbonate

Since polymers play an increasingly important role in both structural and tribological applications, understanding their intrinsic mechanical response is key. Therefore in the last decades much effort has been devoted into the development of constitutive models that capture the polymers' intrinsic mechanical response quantitatively. An example is the Eindhoven Glassy Polymer model. In practice most polymers are filled, e.g. with hard particles or fibers, with colorants, or with soft particles that serve as impact modifiers. To characterize the influence of type and amount of filler particles on the intrinsic mechanical response, we designed model systems of polycarbonate with different volume fractions of small, order 100 nm sized, either hard or soft particles, and tested them in lubricated uniaxial compression experiments. To reveal the local effects on interparticle level, three-dimensional representative volume elements (RVEs) were constructed. The matrix material is modeled with the EGP model and the fillers with their individual mechanical properties. It is first shown that (only) 32 particles are sufficient to capture the statistical variations in these systems. Comparing the simulated response of the RVEs with the experiments demonstrates that in the small strain regime the stress is under-predicted since the polymer matrix is modeled by using only one single relaxation time. The yield- and the large strain response is captured well for the soft-particle filled systems while, for the hard-particles at increased filler loadings, the predictions are less accurate. This is likely caused by polymer-filler interactions that result in accelerated physical aging of the polymer matrix close to the surfaces. Modifying the Sa-parameter, that captures the thermodynamic state of the polymer matrix, allows to correctly predict the macroscopic response after yield. The simulations reveal that all rate-dependencies of the different filled systems originate from that of the polymer matrix. Finally, an onset is presented to predict local and global failure based on critical events on the microlevel, that are likely to cause the over-prediction in the large-strain response of the hard-particle filled systems

From firm to fluid – Structure-texture relations of filled gels probed under Large Amplitude Oscillatory Shear

\u3cp\u3eSoft-solid foods show a progressive transition from a viscoelastic solid state to a flowing fluid state when subjected to a large load. The engineering properties and sensory texture of soft-solid foods depend strongly on the rheological properties that characterize this fluidization. In this paper we use Large Amplitude Oscillatory Shear (LAOS) rheometry to quantify the texture of emulsion-filled food gels in terms of measurable material properties. We provide unambiguous rheological definitions for the firmness, rubberiness, softening and fluidization of soft-solid food gels. We propose a new measure for the load-induced solid-fluid transition, the fluidizing ratio, which quantifies the progression of damage and the degree of plastic flow in the soft-solid gel. We use another dimensionless measure, the thickening ratio, to reveal and characterize the resulting sequence of flow regimes. We use our rheological definitions to quantify the texture of zero-fat, low-fat and full-fat semi-hard cheese respectively. Our data provides evidence that the rate of two physical processes, microcrack nucleation and microcrack propagation, are controlled by the amount of fat emulsion in the gel and govern the rubberiness and brittleness of semi-hard cheese. By translating texture terminology into quantitative material properties measured using Large Amplitude Oscillatory Shear, we augment the capabilities of LAOS as an analytical tool for structure-texture engineering of soft-solid food gels.\u3c/p\u3

Contact mechanics of polyolefins: effect of pre-stretch on the frictional response and the onset of wear

As a result of processing history, polymer products are, more often than not, anisotropic. In this work, single-asperity sliding friction experiments are used to characterize the tribology of semi-crystalline polymer films. We show the improved scratch resistance of oriented isotactic polypropylene and comment on the paradoxical results obtained for high-density polyethylene

The effect of an adhesive interaction on predicting the scratch response of PS/PPO blends

\u3cp\u3eSingle-asperity scratching is used as a simplified contact problem to investigate the deformation due to two materials touching each other. Coupling the intrinsic polymer characteristics to the scratch response for blends of polystyrene (PS) and poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) with varying composition is the main challenge of this study. The intrinsic deformation properties of these blends are strongly influenced by their composition. A combination of experiments and simulations is essential to understand the influence of friction on the interplay between intrinsic deformation properties and contact mechanics. Without an adhesive component in the numerical simulations, no influence of scratch velocity on the penetration depth or lateral force is observed. Furthermore, the lateral force is highly underestimated. Inclusion of an adhesive component between the indenter tip and polymer substrate results in a bow-wave in front of the sliding indenter tip. The experimentally measured lateral forces can only be predicted when the velocity-independent constant friction coefficient varies with blend composition. Therefore, knowing the intrinsic material properties, i.e. deformation kinetics and the intrinsic friction parameter, enables a quantitative prediction of the single-asperity scratch response.\u3c/p\u3

Finite element modeling and experimental validation of single-asperity sliding friction of diamond against reinforced and non-filled polycarbonate

Polymer composites used as protective coatings are important, tribology-critical applications. In this study, hard or soft particle-filled model systems with a polycarbonate matrix are tested in single asperity sliding friction tests against diamond tips. A numerical approach developed to simulate scratching on unfilled polycarbonate was adapted by computing the effective material parameters for the hard and soft particle filled systems using representative volume elements. Combining this proper constitutive framework with a rate-independent friction model correlated quantitatively with the results of the current scratching experiments. Polymer composites used as protective coatings are important, tribology-critical applications. In this study, hard or soft particle-filled model systems with a polycarbonate matrix are tested in single asperity sliding friction tests against diamond tips. A numerical approach developed to simulate scratching on unfilled polycarbonate was adapted by computing the effective material parameters for the hard and soft particle filled systems using representative volume elements. Combining this proper constitutive framework with a rate-independent friction model correlated quantitatively with the results of the current scratching experiments