Impact of variable loading conditions on fretting wear

Fretting is considered as a specific type of reciprocating sliding. It is defined as a small displacement amplitude oscillatory motion between two solids in contact, usually induced by vibrations. Depending on the loading conditions (displacement amplitudes, normal loading), fretting causes damage by surface fatigue and wear induced by debris formation. To prevent such damage, numerous hard coatings have been developed which improve the wear resistance of contacts. However, one difficulty is to estimate how long it will be before the coating wears through. Studies have been conducted to analyze the effect of displacement amplitude, normal force or ambient atmosphere, but usually under constant loading conditions. Such a situation is far from real operating components, where elements are subjected to variable loadings implying variable displacement amplitudes. To predict the durability of a coating under variable fretting displacements, wear depth is quantified as a function of the maximum accumulated dissipated energy density by derivation from a global energy wear approach. This model is compared to TiC vs. alumina fretting experiments. Very good correlation is observed between the prediction and the wear depth, independently of the applied variable amplitude sequences. An equivalent “Miner-Energy” wear model is introduced which permits the durability of the coating to be estimated

Comparison of shot peening and nitriding surface treatments under complex fretting loadings

Considered as a plague for numerous industrial assemblies, fretting associated with small oscillatory displacements is encountered in all quasi-static contacts submitted to vibrations. According to the sliding conditions, fretting cracks and/or fretting wear can be observed in the contact area. On the other hand an important development has been achieved in the domain of surface engineering during the past three decades and numerous new surface treatments and coatings are now available. Therefore there is a critical challenge to evaluate the usefulness of these new treatments and/or coatings against fretting damage. To achieve this objective, a fast fretting methodology has been developed. It consists in quantifying the palliative friction, cracking and wear responses through a very small number of fretting tests. With use of defined quantitative variables, a normalized polar fretting damage chart approach is introduced. Finally, to evaluate the performance of the assemblies after these protective surface treatments under complex fretting loadings, an original sequence of partial slip and gross slip sliding procedure has been applied. It has been demonstrated that performing of a very short sequence of gross slip fretting cycles can critically decrease the resistance of the treated surfaces against cracking failures activated under subsequent partial slip loadings

Fretting wear of TiN PVD coating under variable relative humidity conditions – development of a “composite” wear law

Fretting is defined as a small oscillatory displacement between two contacting bodies. The interface is damaged by debris generation and its ejection from the contact area. The application of hard coatings is an established solution to protect against fretting wear. For this study the TiN hard coating manufactured by a PVD method has been selected, and tested against a polycrystalline alumina smooth ball. A fretting test programme has been carried out at a frequency of 5 Hz, 100 N normal load, 100 μm displacement amplitude and at five values of relative humidity: 10, 30, 50, 70 and 90% at a temperature of 296 K. The intensity of the wear process is shown to be significantly dependent on the environmental conditions. A dissipated energy approach has been employed in this study to quantify wear rates of the hard coating. The approach predicts wear kinetics under constant medium relative humidity in a stable manner. It has been shown that an increase of relative humidity promotes the formation of hydrate structures at the interface and modifies the third body rheology. This phenomenon has been characterised by the evolution of wear kinetics associated with a significant variation of the corresponding energy wear coefficient. Hence, a ‘composite’ wear law, integrating the energy wear coefficient as a function of relative humidity, is introduced. It permits a prediction of wear under variable relative humidity conditions from 10 to 90% within a single fretting test. The stability of this approach is demonstrated by comparing various variable relative humidity sequences

Development of a Wöhler-like approach to quantify the Ti(CxNy) coatings durability under oscillating sliding conditions

The selection of a proper material for the particular engineering application is a complex problem, as different materials offer unique properties and it is not possible to gather all useful characteristics in a single one. Hence, employment of different surface treatment processes is a widely used alternative solution. In many industrial applications, coating failure may be conducive to catastrophic consequences. Thus, to prevent the component damage it is essential to establish the coating endurance and indicate the safe running time of coated system. To this study PVD TiC, TiN and TiCN hard coatings have been selected and tested against polycrystalline alumina smooth ball. The series of fretting tests with reciprocating sliding at the frequency 5Hz have been carried out under 50-150N normal loads and under wide rage of constant as well as variable displacement amplitudes from 50µm to 200µm at a constant value of relative humidity of 50% at 296K temperature. To quantify the loss of material a dissipated energy approach has been applied where the wear depth evolution is referred to the cumulative density of friction work dissipated during the test. Different dominant damage mechanisms have been indicated for the investigated hard coatings, which is debris formation and ejection in case of TiC coating and progressive wear accelerated by cracking phenomena in case of TiN and TiCN coatings. Energy-Wöhler wear chart has been introduced, in which the critical 1 dissipated energy density corresponds to the moment when the substrate is reached after a given number of fretting cycles. Two different methods to determine the critical dissipated energy density are introduced and compared. The Energy-Wöhler approach has been employed not only to compare the global endurance of the investigated systems but also to compare the intrinsic wear properties of the coatings. It has been shown that the fretting wear process is accelerated by the stress-controlled spalling phenomenon below a critical residual thickness and a severe decohesion mechanism is activated. Finally the applicability of the investigated method to other coated systems subjected to wear under sliding conditions is discussed and analyzed. The perspectives of this new approach are elucidated

Noncentral limit theorem and the bootstrap for quantiles of dependent data

We will show under minimal conditions on differentiability and dependence that the central limit theorem for quantiles holds and that the block bootstrap is weakly consistent. Under slightly stronger conditions, the bootstrap is strongly consistent. Without the differentiability condition, quantiles might have a non-normal asymptotic distribution and the bootstrap might fail

Washboarding of corrugated cardboard

The aims of the thesis were to study how washboarding (the undulations present on the surface of corrugated cardboard used to manufacture boxes) relates to the mechanical properties of paper and the board manufacturing conditions and to examine the impact of washboarding upon the structural integrity and printability of corrugated cardboard packaging. A digital image profilometry technique was developed to measure the washboarding profiles of corrugated board. This technique was used to measure the washboarding depth and profiles for a range of corrugated boards, some constructed manually and some machine manufactured. This enabled a study into how a change in the mechanical properties of paper and glue affect washboarding depth. The effect that the speed of machine manufacturing had upon the degree of washboarding was also determined. A study of how environmental conditions affect washboarding geometry was undertaken. The effects of the extent of washboarding upon a range of board performance measures were tested empirically and modelled using Finite Element Analysis. These were edgewise compression testing (ECT), three-point bend, and MD-Shear (an Amcor Ltd. proprietary test). A method was developed to measure full-tone print coverage of corrugated board and was used to study how washboarding affects the printing quality of corrugated board

A possible cooling effect in high temperature superconductors

We show that an adiabatic increase of the supercurrent along a superconductor with lines of nodes of the order parameter on the Fermi surface can result in a cooling effect. The maximum cooling occurs if the supercurrent increases up to its critical value. The effect can also be observed in a mixed state of a bulk sample. An estimate of the energy dissipation shows that substantial cooling can be performed during a reasonable time even in the microkelvin regime.Comment: 5 pages, to appear in Phys. Rev.

Mean parameter model for the Pekar-Fr\"{o}hlich polaron in a multilayered heterostructure

The polaron energy and the effective mass are calculated for an electron confined in a finite quantum well constructed of $GaAs/Al_x Ga_{1-x} As$ layers. To simplify the study we suggest a model in which parameters of a medium are averaged over the ground-state wave function. The rectangular and the Rosen-Morse potential are used as examples. To describe the confined electron properties explicitly to the second order of perturbations in powers of the electron-phonon coupling constant we use the exact energy-dependent Green function for the Rosen-Morse confining potential. In the case of the rectangular potential, the sum over all intermediate virtual states is calculated. The comparison is made with the often used leading term approximation when only the ground-state is taken into account as a virtual state. It is shown that the results are quite different, so the incorporation of all virtual states and especially those of the continuous spectrum is essential. Our model reproduces the correct three-dimensional asymptotics at both small and large widths. We obtained a rather monotonous behavior of the polaron energy as a function of the confining potential width and found a peak of the effective mass. The comparison is made with theoretical results by other authors. We found that our model gives practically the same (or very close) results as the explicit calculations for potential widths $L \geq 10 \AA$.Comment: 12 pages, LaTeX, including 5 PS-figures, subm. to Phys. Rev. B, new data are discusse

Predictors of olfactory improvement after endoscopic sinus surgery in chronic rhinosinusitis with nasal polyps

Objective. This study aimed to determine the predictors of olfactory improvement after endoscopic sinus surgery among patients with chronic rhinosinusitis with nasal polyps. Method. This prospective cohort study included patients admitted to a university hospital between 2006 and 2012. Assessment using odour identification testing, a sinonasal symptom questionnaire, the Rhinosinusitis Disability Index and mucus biomarker levels was performed at various time points. Correlation of variables with identification score differences at six postoperative time points and at baseline was performed, followed by multiple linear regression to determine significant predictors at each of the six post-operative time points. Results. Baseline absence of acute sinusitis, elevated serpin F2 and anterior rhinorrhoea predict early olfactory improvement, whereas baseline allergic rhinitis predicts late olfactory improvement. Baseline odour identification score was the strongest predictor across all time points. Conclusion. Patients with chronic rhinosinusitis and nasal polyps with worse disease or baseline olfactory function may benefit more from endoscopic sinus surgery in terms of olfactory improvement