Tribological properties of PTFE‐filled thermoplastic polyimide at high load,velocity, and temperature

The effect of 20 wt% polytetrafluoroethylene (PTFE) fillers on the friction and wear properties of thermoplastic polyimides (TP) are investigated, under dry sliding in line contact against steel under 50 to 200 N, 0.3 to 1.2 m/s, and 60 to 260°C. Besides the lubricating mechanisms of PTFE based on mechanical shear, the thermal and tribophysical interactions in the sliding interface are considered in this research by using thermoanalytical measurements, Raman spectroscopy, and calculating the maximum polymer sliding temperature T*. The effect of hydrolysis of the TP bulk material, causing high friction at 100 to 140°C, is covered by PTFE. A transition at pv‐values 2.2 MPa m/s (T* = 120°C) is due to thermally controlled sliding of PTFE, while a transition at pv‐values 3.2 MPa m/s (T* = 180°C) remains controlled by degradation of the TP bulk material into monomer fractions. The reduced coefficient of friction in the presence of PTFE leads to smaller degradation and orientation of the molecular back‐bone and side‐chains within the TP structure. The formation of a homogeneously mixed transfer film is only observed at 180 to 260°C. The PTFE forms a fibrillar structure during wear at high sliding velocities, while they wear as separate particles at high normal loads. POLYM. COMPOS., 2009. © 2009 Society of Plastics Engineers     

Survey of excitation signals for FFT based signal analyzers

The properties of ten different excitation signals are studied to analyze their suitability as excitation signals for fast Fourier transform (FFT)-based signal and network analyzers. Their influence on the measurement time, accuracy, and sensitivity to nonlinear distortions is described. The flexibility to create a customized amplitude spectrum is investigated. With this information it becomes possible to select the best excitation signal for many applications     

A weighted total least squares estimator for multivariable systemswith nearly maximum likelihood properties

The aim of the present paper is to develop a parametric estimator for linear time-invariant multivariable systems with nearly maximum likelihood properties. The estimator is based on the total least squares (TLS) method. It can be seen as an “optimally” weighted iterative generalized TLS (GTLS) estimator, combining the nice asymptotic properties of the maximum likelihood (ML) method with the global minimization property of the GTLS estimator     

Frequency domain system identification using arbitrary signals

It is the common conviction that frequency domain system identification suffers from the drawback that it cannot handle arbitrary signals without introducing systematic errors. This paper shows that it is possible to deal with nonperiodic signals without any approximation and under the same assumptions as in the time domain, by estimating simultaneously some initial conditions and the system model parameters     

Wear transitions and stability of polyoxymethylene homopolymer in highly loaded applications compared to small-scale testing

Wear and failure mechanisms of polyoxymethylene homopolymer (POM-H) loaded above its yield strength are studied on test samples with a 22500 mm² sliding area at 8–150 MPa contact pressures. Test results are compared to small-scale cylinder-on-plate tests. Plastification of the sliding surfaces at high loads is favourable for low friction, while different wear mechanisms compared to small-scale testing are induced. Small-scale tests show a transition from mild adhesive/abrasive wear to severe wear due to softening, which is characterised by the formation of shear lips. Softening of large-scale sliding surfaces does not cause overload but it contributes to stable wear rates. Overload of large-scale samples is characterised by the transition from softening to melting and degradation. The dimensional stability of polymer elements is influenced by creep and it is verified that deformation of small-scale samples and large-scale samples loaded at 8 MPa is recovered after sliding, while it remains as permanent deformation for large-scale tests at 16–150 MPa. The wear transitions are further analysed by optical microscopy and available temperature models. The flash temperature concept can be applied for small-scale tests and large-scale tests up to 8 MPa. Calculated flash temperatures indicate softening and are in agreement with visual observations of the polymer surfaces. Flash temperatures for large-scale tests at 16–150 MPa indicate melting and degradation that was not visually observed on the polymer surfaces. The bulk temperature model prevails during large-scale sliding and only indicates melting at 150 MPa. Thermal analysis of the worn polymer surfaces confirms that crystallisation happened during small-scale sliding and large-scale sliding up to 55 MPa, occurring between 120 and 150 °C. Thermo-oxidative degradation above 200 °C is evidenced at 150 MPa.     

Variance analysis of frequency response function measurements using periodic excitations

The influence of disturbing noise and nonlinear distortions on frequency response function measurements using periodic excitations has been studied in detail in the literature. A variance analysis method has been developed that allows one to detect and quantify the nonlinear distortions and the disturbing noise. In this paper, the variance analysis is generalized to detect and quantify the following nonstationary disturbances: 1) nonsynchronous periodic signals, for example, the 50 Hz mains and its harmonics, and 2) nonstationary behavior of the device under test, for example, phase or frequency modulation.     

Broadband versus stepped sine FRF measurements

In this paper, the required measurement time to measure a transfer function with a specified accuracy is analyzed. The waiting time that is needed to balance the transient induced errors against the noise errors is calculated. This allows one to decide under what conditions broad-band measurements are faster than stepped sine measurements     

Computing an initial estimate of a Wiener-Hammerstein system with a random phase multisine excitation

Wiener-Hammerstein systems consist of two linear dynamic systems placed around a static nonlinearity. These models are difficult to identify due to the presence of two dynamic systems. Usually, a nonlinear estimation procedure is necessary to estimate the parameters of the different parts. This nonlinear estimation procedure needs good starting values to converge quickly and/or reliably to a global minimum. This paper proposes a method to compute a first estimate based on one measurement record only.     

On the use of signals with a constant signal-to-noise ratio in thefrequency domain

Most parametric frequency-domain estimation methods, applied to linear time-invariant systems, do not take into account the disturbing input noise. It is shown that these estimators can still be used to obtain unbiased estimates from noisy input-output measurements if an input signal with a constant signal-to-noise ratio is used. By means of an experiment in the field of modal analysis, it is demonstrated that a multisine excitation can be used to create an input signal with a constant signal-to-noise ratio, even when the disturbing noise at the input is strongly colored. It is shown that multisines allow a very efficient (low crest factor and time factor) and fast synthesis of the wanted input signal