B.J. Briscoe

Scratching maps for polymers

Abstract The scratching technique has gained interest in recent times due to its varied applications to a number of engineering materials, especially for the evaluation of surface scratch resistance of plastics. Scratching provides a convenient and reliable means to investigate the mechanical properties of organic polymers under various contact conditions. The scratch hardness method is widely adopted to provide a first-order evaluation of the relative scratch resistance of materials for comparison purposes. The method also allows the identification and the assessment of the surface deformation processes and maps defining the scratch deformation modes as a function of contact conditions may be generated. These scratching maps may present experimental results in terms of the deformation mechanism, the scratch hardness and the friction coefficient. This paper primarily provides a review of the application of scratching maps for polymers. Results for the scratch hardness and the deformation mechanisms for a poly(methylmethacrylate) (PMMA), a poly(tetrafluoroethylene) (PTFE) and an ultra-high molecular weight poly(ethylene) (UHMWPE) are presented. The PTFE system is also described following the effects of γ-irradiation; radiation produces a marked reduction in toughness. The scratches were produced on the polymer surfaces by cones and spheres of various size under a number of contact conditions (e.g. applied normal load, strain, scratch velocity, etc.). SEM imaging and laser profilometry are used for the study of the deformation mechanisms and the measurements of the scratch profiles. It is shown that polymers exhibit a wide range of scratch deformation characteristics and that the deformation mechanism is determined by the most efficient energy dissipation process for the particular external constraints.

Wear of polymers: an essay on fundamental aspects

Abstract This paper reviews fundamental aspects of the wear of organic polymers and represents a personal view of the state of our current understanding of this subject

The ballistic impact characteristics of aramid fabrics: The influence of interface friction

Abstract This study reports the effect of the surface friction characteristics of aramid fabrics with respect to their static deformation behaviour and their ballistic capture performance. Various fabric weaves were studied and differing levels of interfibre-interyarn friction within the fabrics were achieved by the removal (Soxhlet extraction) or the addition (polydimethyl-siloxane treatment) of surface lubricants. Simple interyarn friction measurements have been performed, as well as fabric penetration studies, both quasi-statistically and under high speed ballistic conditions. A simple thin elastic plate model was found to described adequately the quasi-static response of the fabrics to a normal indentation deformation. A simulation of the ballistic deformation process in fabrics was devised on the basis of this first-order model of the quasi static indentation process. The predictions of the relative fabric ballistic captive performance, derived from this model, were found to be in reasonable agreement with the results obtained directly from the ballistic studies. The investigation has also shown that the interface frictional work dissipated at the filament-filament and yarn-yarn junctions is a critical factor in determining the static tensile yarn and (transverse) fabric stiffness. The changes in these static parameters is considered to be the origin of the subtle changes observed in the ballistic performance of the corresponding fabrics.

The effects of hydrogen bonding upon the viscosity of aqueous poly(vinyl alcohol) solutions

Abstract This paper describes the rheological properties of a number of aqueous solutions of a poly(vinyl alcohol) (PVA) as a function of the degree of polymer hydrolysis, ambient temperature, pressure and variations induced upon the addition of electrolytes to these solutions. Experimental results reported show that the rheological behaviour of these aqueous PVA solutions mainly depends upon the relative strength of the hydrogen bonding existing between the polymer chains and water molecules, compared with that of the inter and intra chain hydrogen bonding. Changes in temperature, pressure, degree of hydrolysis of the PVA, and the addition of electrolytes produce a corresponding effect in varying these two types of hydrogen bonding and thus the resulting rheological properties of the PVA solutions.

The effect of indenter geometry on the elastic response to indentation

The paper describes various analytical procedures that account for the influence of the geometry of a rigid indenter upon the measured contact compliance of a smooth perfectly elastic half space. The analytical solutions provide a means of interrelating the reaction force, P, and the displacement, h, characteristics in terms of the contact geometry and the reduced elastic modulus E*. The general form is P=gE*hn, where g and n are functions only of the indenter geometry. This relationship is incorporated into a curve-fitting procedure and used to evaluate the influence of the indenter geometry upon the computed modulus of a poly(isobutylene) rubber. The method described provides a viable means of incorporating the imperfections, associated with the geometry of an indenter, into the interpretation of contact compliance data.

The friction and wear of poly(tetrafluoroethylene)-poly (etheretherketone) composites: An initial appraisal of the optimum composition☆

Abstract In this paper an experimental study of the friction and wear of a number of poly(tetrafluoroethylene)-poly(etheretherketone) (PTFE-PEEK) composites over a wide composition range is described. The inclusion of PTFE reduces the friction of PEEK although there is a small loss in wear resistance. However, the friction reduction significantly improves the load-bearing capacity at high sliding velocities. The inclusion of the PTFE also reduces the magnitude of certain mechanical properties such as the hardness. A sufficient concentration of PTFE is approximately 10% although the precise value may depend on factors such as the fabrication and operating conditions.

The hardnesses of poly(methylmethacrylate)

The paper describes an experimental study of the normal and scratch hardnesses of a poly(methylmethacrylate). The deformations have been introduced using hard steel cones of a range of included cone angles. The influence of the state of interfacial lubrication is examined and rationalized. The observed time dependence of the two types of computed hardness data is compared and the nature of the correlations between these data is evaluated. It is observed that when the imposed strains are modest, say less than 0.2, the scratch hardness and normal hardness deformations produce self consistent data using first order and rather indiscriminate analyses for both types of deformations. At higher levels of imposed strain, a more critical appraisal of the nature of the deformation produced in the two cases is necessary in order to provide mutually consistent hardness values and hence unequivocal rheological characteristics for this polymer.

The internal form of compacted ceramic components: a comparison of a finite element modelling with experiment

This paper is concerned with the internal form of compacted ceramic green components. The structural inhomogeneity of alumina specimens, compressed in a top pressed cylindrical die, is studied experimentally, by using lead balls as tracers to detect the densification, as well as by numerical computation. The flow behaviour of a ceramic powder, an agglomerated alumina, is described by the modified Drucker-Prager/cap elasto-plasticity material model developed for powder applications. The modified Drucker-Prager/cap constitutive model, implemented by using the finite element code ABAQUS, is discussed. The procedure for selecting the necessary material parameters by the inclusion of material response data and assumptions made in the implementation of the numerical model are described. The accuracy of the presented numerical method is evaluated by comparing the simulation results with experimental data obtained from density measurements. The evolution of the density distributions during the entire compaction process is predicted. The determining step for the formation of the subtle density variations on the central axis of the green ceramic compacts was found, from the modified Drucker-Prager/cap finite element analysis, to be the unloading step of the compaction process. In this respect, a comparison of the finite element and experimental results showed good mutual agreement.

Wear of polymers

Abstract This paper reviews some of the recent progress which has been made in the area of the sliding wear of polymers. Wear mechanisms are classified under three broad approaches which reflect primarily the way this subject has been historically studied. It is demonstrated here that the wear of polymers is influenced by the contact conditions, the bulk mechanical properties of the polymer and the properties of the ‘third body’, which generally appears in the form of transfer film or degraded polymer particles between two sliding surfaces. Further, this paper establishes a link between the different contact and material parameters and shows how they are important in elucidating the generic wear mechanisms for polymers. The effects of environment and lubrication upon polymer wear are briefly explained in terms of the chemical interactions between the liquid phase and the polymer. The capabilities and limitations of current predictive wear models for polymeric contacts are also highlighted.

FTIR Study of the Thermal Degradation of Poly(vinyl Alcohol)

The degradation of poly(vinyl alcohol) was investigated using TG analysis and Fourier transform infrared spectroscopy to determine the effect of atmosphere on the process of degradation. In the spectra, four vibrational modes were identified that characterised the major steps of the degradation process. These were the O-H, C-H, C=O and C=C stretching modes. The mechanism observed for degradation in an inert atmosphere was in accordance with the accepted mechanism of elimination followed by pyrolisation. Evidence of conjugated polyenes, however, was not observed. For the air atmosphere, oxidation in both steps of the degradation process was observed.