Staff

Abstract:
NiTi exhibits an excellent wear resistance, which can be further enhanced by ion implantation. However, there are some limitations to the implantation effects: only a thin layer of about 100 nm can be created. In this paper, the effect of nitrogen ion implantation on the NiTi wear response is investigated. The different loads and durations of tests are taken into account to show that the implanted layer has the most beneficial effect only in a certain range of contact pressure. It was found that the wear volume changes in a nonlinear manner with respect to the load and sliding length, for both non- and implanted samples. For the latter, two distinct stages can be distinguished in the wear process: an initial stage characterized by a low wear-rate and a low coefficient of friction, and a second stage in which the wear-rate drastically increases. The duration of the first stage is longer for lower loads. This specific behavior is explained by differences in the hardness distribution, energy dissipation due to the normal load, and differences in the microstructure of the wear tracks. Our results show that the lifetime of NiTi can be improved by ion implantation, thus boding well for applications in harsh environments.

Keywords:
ion implantation, NiTi shape memory alloy, sliding wear, pseudoelasticity, abrasion, dry friction, surface treatments

Abstract:
The paper presents the modeling of transmission of the ultrasonic plane wave through an uniform liquid layer. The considered sources of the ultrasonic wave were normal (straight) beam longitudinal wave probes and angle beam sheer waves probes commonly used in non-destructive testing. Coupling losses (CL) introduced by the presence of the coupling layer are discussed and determined applying the numerical procedure. The modeling applies to both monochromatic waves and short ultrasonic pulses with a specified frequency bandwidth. Model implementation and validation was performed using a specialized software. The predictions of the model were confirmed by coupling losses measurements for a normal beam longitudinal wave probe with a delay line made of polymethyl methacrylate (PMMA). The developed model can be useful in designing ultrasonic probes for high-speed rail track inspections, especially for establishing the optimal thickness of the water coupling layer and estimation of coupling losses, due to inevitable changes of the water gap during mobile rail inspection.

Keywords:
non-destructive testing, ultrasonic examination, plane wave propagation

Abstract:
The paper presents an investigation of the railhead steel R260. Brinell hardness was measured, spherical microindentation tests were performed and the microstructure was analysed using optical microscopy and SEM images. Fatigue damage in various near-surface regions of the railhead was estimated on the basis of local deterioration of elastic modulus and the development of plastic work in cyclic microindentation tests. There is a clear difference in hardness and fatigue damage between the used and the virgin regions of the railhead. The Brinell hardness and ultrasonic measurements show a change in material properties due to rail manufacturing process.

Keywords:
Fatigue damage, Microindentation, Brinell hardness, Ultrasonic measurements, Railhead

Abstract:
D6A and C75S spring steels were implanted by nitrogen ions at an energy of 65 keV and a fluence of 6 × 10^17 ions/cm2. The hardness and tribological properties of implanted samples were studied. In a reciprocating ball-on-plate wear test, a relatively low contact pressure was applied (9 mm diameter ball under load up to 0.5 N). In the implanted samples two stages of wear process can be distinguished. In the first stage a low friction coefficient and low wear rate are observed while in the second stage their values considerably increase. The duration of the first stage depends on the applied load (contact pressure). At low contact pressure, the wear life can be extended considerably, and at high contact pressure the first stage practically disappears and the effect of ion implantation is negligible. The evolution of wear process in the second stage is similar to that of non-implanted sample. Analysis of morphology and chemical composition of wear tracks with SEM reveals different mechanisms at different stages of wear in implanted samples wear. At the first stage, adhesive and abrasive wear predominate, while in the second stage, oxidative and fatigue mechanisms occur that are similar to those observed for a non-implanted material.

Keywords:
ion implantation, indentation, sliding wear, scanning profilometry, surface topography, electron microscopy

Abstract:
The difference in wear behaviour of superelastic NiTi and conventional Ti6Al4V alloys with similar elastic limits was investigated. Reciprocating dry-sliding wear tests were performed with a ball-on-plate tribometer machine at different normal loads and sliding times. Mechanical properties were also examined using different microindentation tests, and wear track morphologies and chemical compositions in the scar area were characterised by scanning electron microscopy and energy dispersive spectrometry. Superelasticity of the NiTi alloy was significantly beneficial to its tribological properties, and wear resistance of the NiTi alloy was up to forty times higher than that of the Ti6Al4V alloy. Such a significant difference cannot be attributed only to the relatively small difference in hardness (H) or H/E ratios (E - elastic modulus) of the materials but, rather, a difference in strain energy, which dissipates in the material and results from normal load of the sliding ball. To estimate this energy, spherical microindentation tests in wear scar were carried out. Understanding the difference between tribological behaviour of the NiTi shape memory alloy and the conventional Ti6Al4V material provides insight into wear progression, and an investigation of its mechanisms can effectively prevent the destruction of components, prolonging their safe service life.

Keywords:
wear mechanisms, unlubricated friction, non-ferrous alloys, titanium

Abstract:
The aim of the work is to show both the similarities and differences in the formation of deformation-induced roughness in contact compression in the presence of oil and the problem of free surface roughing during uniaxial stretching in a plastic area. The relationships between changes in the roughness are caused by the deformation of the sample and the viscosity of oil at the contact area. It has been shown that normal contact loading with the presence of oil initially leads to an increase in surface roughness, then to its smoothening. The results of the experimental research have been compared with numerical simulation made using FSI (Fluid Structure Interaction) and ABAQUS systems. Using finite element calculations, it was possible to explain the phenomenon of roughness formation on the surface of a smooth steel sample. The changes in the structure of the smooth surface resulting from compression in the presence of oil are caused by the rotation and deformation of surface grains. The roughness of this structure is dependent on the viscosity of oil: the more viscous the liquid is, the rougher texture is formed.

Keywords:
deformation-induced roughening, viscosity, finite element method

Abstract:
Contact between a deformable, random rough surface and a rigid flat counterpart was investigated experimentally and theoretically. In the experimental setup, two modes of rough surface flattening were performed, namely normal compression and sliding (tangential load) in presence of normal compression. The load-approach relationship and friction coefficient were measured. After unloading the surface, the topography was measured using scanning profilometry. The deformation of roughness zone, i.e. evolution of the real contact area (RCA), and roughness parameters were analysed. A model based on statistical analysis and a finite element solution was applied to predict the load–approach relationship. The numerical and experimental results were compared. The effect of friction coefficient was discussed.

Keywords:
contact mechanics, roughness, normal load, tangential load

Abstract:
The manuscript presents experimental investigations of an unusual behaviour of shape memory alloys (NiTi) having different characteristic temperatures in wear test. The studies are focused on the different wear mechanism at varying loads and sliding times and include a description of the phenomena accompanying the wear process. The ball-on-plate reciprocating sliding wear tests were conducted on NiTi shape memory alloys against a sapphire ball. We show that the wear resistance of NiTi is affected by its specific stress-strain characteristic. The understanding of the tribological behaviour of NiTi, a knowledge of the course of wear and an examination of its mechanisms can enable an effective prevention of the destruction of devices components and prolong their safe working life.

Keywords:
wear behaviour of NiTi SMA, friction coefficient, superelasticity, shape memory effect

Abstract:
In the paper a problem of contact of rough surface with rigid flat plane is investigated experimentally and numerically. Samples made of three different steels with roughness constituted in a sand-blasting process were compressed in a special experimental setup. 3D surface topographies were measured in initial and deformed state using scanning profilometry. An experimental procedure has been designed that enables specifying load-approach and load-real contact area relations corresponding to plastic deformation of roughness zone. These relations were also simulated using a simple model based on statistical approach with special procedure proposed for a proper specification of sampling interval. The experimental and numerical results have been compared.

Keywords:
Contact mechanics, Roughness, Real contact area, Asperity definition, Sampling interval

Abstract:
We used the fractal theory based on a single variable Weierstrass–Mandelbrot function to obtain the normal contact stiffness if rough and smooth isotropic surfaces are pressed against each other. Because in the original fractal theory the distribution of contact area is assumed geometrically, we propose the method in which the actual deformation of asperities and a correction due to asperity coupling (interaction) will be taken into account. This correction is equivalent to an increase of the effective separation by a quantity proportional to the nominal pressure and it has a significant effect on contact stiffness at larger normal loads (low separations). The numerical results demonstrate a nonlinear evolution of the contact stiffness with the normal load in particular in the first stage of loading at low squeezing pressures. We have compared the results of the theoretical contact stiffness using the fractal method with the experimental ultrasonic measurements. Experimental results made on real surfaces agree remarkably well with the theoretical predictions.

Keywords:
fractal model, contact stiffness, ultrasonic measurements

Abstract:
We have used an ultrasonic method to determine the normal and shear stiffness for three different surfaces. The degree of hysteresis for the loading/unloading and stiffness ratio is a function of roughness. Nonlinear contact stiffness characteristics are obtained. The ratio of tangential to normal stiffness KT/KN slowly increases in proportion to normal loading. The novelty of our setup is that at the same time we can measure the reflection coefficient, obtain results for three transducers simultaneously, and measure the approach as a function of load. The presented experimental results of normal contact stiffness measurements have been used for the verification of our theoretical model based on a fractal description of rough surfaces (Buczkowski et al., “Fractal Normal Contact Stiffness of Rough Surfaces,” Arch. Mech. (submitted for publication).

Keywords:
Surface roughness, Stress, Reflectance, Stiffness, Ultrasonic measurement, Waves

Keywords:
badania ultradźwiękowe, pomiary współczynnika odbicia, ocena powierzchni kontaktu, pomiary chropowatości

Abstract:
This paper presents a simulative method which takes into consideration the influence of a surface layer on the stresses observed during low-cycle fatigue regions of tension and torsion and after loadings. The cyclic loading model is described for an axial-symmetric cylinder with a surface layer. The layer is divided into several zones (shells), which enable the introduction of material property changes and residual stresses as a function of the distance from the surface. This work also introduces a comparison of the evolution of residual stresses counted and measured experimentally with shot peened samples after cyclic tension and torsion. The experimental results agree well with the simulation.

Keywords:
Fatigue modeling, Residual stresses, Tensile loading, Torsional loading, Surface treatments

Abstract:
In the paper an experimental study of compression of rough surface in the presence of lubricant and in dry state is presented. The evolution of roughness parameters was analyzed. In the lubrication conditions an existence of asymptotic state was observed where roughness parameters and real contact area stabilize due to hydrostatic response of lubricant. The simplified model was proposed allowing to predict the roughness parameters and real contact area of the deformed surface in the asymptotic state. The model is based on analysis of topography of the undeformed surface and compression of a singular lubricant pit. The analysis was performed both numerically and experimentally.

Keywords:
Hydrostatic lubrication, Normal contact, Surface topography