On the efficiency of internal lubricants for polymers under different sliding conditions

When polymers are used as bearing materials or are processed over tooling under dry sliding conditions, the formation of a transfer layer at the interface controls friction and wear properties. The film formation may be enhanced by the addition of internal lubricants. This article reviews some effects of internal lubricants, such as graphite, polytetrafluoroethylene, and internal silicone oil, on the friction and wear of bulk polymers such as polyimide, polyester, and polyamide. Known lubricating mechanisms are complemented by an interpretation of tribophysical reactions in the sliding interface. Test results are obtained during small and large‐scale laboratory testing under high‐load and high‐temperature conditions and are further related to the effect of contact conditions. The study reveals that some lubricants are not effective under the entire range of sliding temperatures and/or normal loads. For internally lubricated polymers, the efficiency of internal lubricants strongly depends on the contact geometry. J. VINYL ADDIT. TECHNOL., 2008. © 2008 Society of Plastics Engineers     

Signal reconstruction for non-equidistant finite length samplesets: a “KIS” approach

The reconstruction of a signal from a finite number of unequally spaced signal samples is considered. Depending on the assumed class of signals, several more or less optimal solutions are proposed in the literature. Mostly, these methods assume perfectly known sampling instants and perfect signal samples. The aim here is to determine the robustness of these methods to the measurement noise and to draw conclusions for their applicability in sampling scope timebase reconstruction     

A maximum likelihood estimator for linear and nonlinear systems-apractical application of estimation techniques in measurement problems

A method is presented for estimating the parameters of linear systems and nonlinear systems. The linear systems are modeled by their transfer function, while the nonlinear systems are described by a Volterra series. The estimator belongs to the class of maximum-likelihood estimators. During the estimation process, the Cramer-Rao lower bound on the covariance matrix of the estimates is derived     

The interpolated fast Fourier transform: a comparative study

The properties of five interpolating fast Fourier transform (IFFT) methods (IFFT) are studied with respect to their systematic errors and their noise sensitivity, for a monofrequency signal. It is shown that windows with small spectral side lobes do not always result in a better overall performance of the IFFT method and that time-domain estimators can be more efficient than the analyzed IFFT methods     

Fast approximate identification of nonlinear systems

In this paper, a method is presented to extend the classical identification methods for linear systems towards nonlinear modelling of linear systems that suffer from nonlinear distortions. A well chosen, general nonlinear model structure is proposed that is identified in a two-step procedure. First, a best linear approximation is identified using the classical linear identification methods. In the second step, the nonlinear extensions are identified with a linear least-squares method. The proposed model not only includes Wiener and Hammerstein systems, it is also suitable to model nonlinear feedback systems. The stability of the nonlinear model can be easily verified. The method is illustrated on experimental data.     

Uniquely connecting frequency domain representations of given order polynomial Wiener–Hammerstein systems

The notion of frequency response functions has been generalized to nonlinear systems in several ways. However, a relation between different approaches has not yet been established. In this paper, frequency domain representations for nonlinear systems are uniquely connected for a class of nonlinear systems. Specifically, by means of novel analytical results, the generalized frequency response function (GFRF) and the higher order sinusoidal input describing function (HOSIDF) for polynomial Wiener–Hammerstein systems are explicitly related, assuming the linear dynamics are known. Necessary and sufficient conditions for this relation to exist and results on the uniqueness and equivalence of the HOSIDF and GFRF are provided. Finally, this yields an efficient computational procedure for computing the GFRF from the HOSIDF and vice versa.     

Influence of stretching on the resilience of LLDPE monofilaments for application in artificial turf

Polymeric filaments have been used successfully in artificial turf, however lack of resilience and excessive fibrillation are still the main problems encountered on artificial turf fields and especially when used for football. Resilience is the ability to recover rapidly from a deformation, especially from a bending deformation. FIFA and EN standards recognize the 0.8 m‐Lisport for predicting filament behavior, but this method does not provide any information concerning the resilience of individual filaments. Furthermore, it is merely a qualitative method that only assesses the system in its entirety. The research presented in this article is twofold, first to develop a test method to assess the resilience of a single filament and to correlate with the established methodology, dynamic bending by Favimat R. Second to characterize fiber morphology and to correlate the morphology characteristics with the resilience measurements. A good correlation of the static bending with dynamic bending is obtained and both test methods provide valuable information about the influence of the processing parameters on the resilience. Dynamic scanning calorimetry, Raman and WAXS measurements clearly prove the influence of the structure and more specifically of the amorphous phase on the resilience. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Experimental extrapolation model for friction and wear of polymers on different testing scales

The friction and wear behaviour for polyoxymethylene homopolymers (POM-H) and polyethylene terephthalate with teflon additives (PET/PTFE) is compared on small-scale cylinder-on-plate tests (50-200 N normal loads) and large-scale flat-on-flat tests (190-3880 kN normal loads). A common parameter to characterise tribological data is the contact pressure×sliding velocity (pv-value), but its use seems restricted to a single testing scale. Four experimental models are presented to extrapolate tribological data from one to another testing scale, based on (i) one single mechanical parameter (normal load or contact pressure), (ii) two mechanical parameters (normal load and sliding velocity), (iii) the contact pressure-sliding velocity model (pv-temperature limit), (iv) macroscopic geometry model. The latter model is most extensive, considering the influences of thermal effects (frictional heat generation and dissipation), sample geometry (geometry factor G) and visco-elastic contact (critical contact pressure p₀). For unfilled polymers, the introduction of macroscopic scaling factors allows for the extrapolation of coefficients of friction obtained on different testing scales. Specific or volumetric wear rates cannot be extrapolated because they strongly depend on the sample geometry, while linear wear rates are in better agreement when considering the transitions between mild wear, softening and melting. For internally lubricated polymers, extrapolation is more difficult. The differences depending on the testing scale are attributed to contact stress concentrations near the sample borders and limited wear debris mobility within large contact areas, promoting a homogeneous film formation onto the polymer surface.     

Practical choices in the FRF measurement in presence of nonlineardistortions

The frequency response function (FRF) of the best linear approximation to a nonlinear system is usually measured by averaging system responses to a normally distributed, filtered, and clipped random excitation (Gaussian noise). This signal is compared to the multisine signal with random phases. It is shown that a random phase multisine signal defined over an even-odd frequency grid is superior to the Gaussian signal in terms of the variance and the bias of the measured FRF of the approximated nonlinear system