Premature failure of polymeric self-lubricating bearings is a limiting factor in the sustainable and continued operation of hydropower plants. In this work, the tribological performance of two novel polymer composites was studied with respect to commercially available materials. The materials were evaluated under dry and lubricated conditions, using water and an environmentally adaptive lubricant (EAL). The two tested commercial materials yielded a low dry sliding friction and wear. Under water lubricated conditions, the in-house developed polyphenylene sulphide (PPS) based composite provided an exceptionally low friction and a wear rate of a factor of 3 lower than the best performing commercial material. Using the EAL wear was reduced for most materials by up to 85%.

Predictions of the real contact area in metal forming processes are essential for the calculation of friction and, consequently, the optimisation of energy consumption and quality of rolled products. This study develops of a model for estimating the real contact area in hot rolling of a 6061 aluminium alloy based on: thermo-viscoplastic material properties, measured roll topography, and hydrodynamic and hydrostatic roles of lubricant in the contact. Results show the interplay between load and contact area, as regions of solid contact, fluid channels and pockets are formed. Quantification in a rolling pass shows both solid contact and lubricant entrapment play the major roles, which is highly determined by the topography and the flow stress of the workpiece.

Hardness is routinely utilised to link the bulk material properties to surface contact mechanics. Besides possible differences in the material properties between bulk and surface, there is no established relation between hardness and the thermo-viscoplastic flow stress of materials used in the context of metal forming processes. The purpose of this study is to investigate such relationship using 6061 and 6016 aluminium alloys manufactured by hot rolling. Optical microscopy and Vickers hardness tests at different loads and temperatures ranging from 22°C up to 450°C were carried out. A time-dependent Finite Element model of an indentation using thermo-viscoplastic material model based on bulk samples was developed and compared with experiments. Overall, no significant difference between bulk and surface for neither alloy was experimentally identified. The hardness decrease with temperature of the alloys is quantified and ready-to-use constraint factor maps associating the thermo-viscoplastic flow stress with hardness of the materials are presented. The numerical model allowed visualisation of viscoplastic effects, whereas comparisons to the experiments in terms of hardness and topography validated the model and constitutive equations. Constitutive equations derived from compression tests of bulk samples can confidently describe deformation at the surface level and so, be used to develop a contact model in the tribological context of metal forming.

Specific wear rates of tribosystems always rely on the data obtained from wear experiments. Nonetheless, the events taking place during an experiment may often lead to wide variations and low repeatability of the results. In this work, the authors attempt to take a closer look into the dynamic contact conditions of a dry linearly reciprocating block-on-flat wear experiment. The finite element method and Archard’s wear model are used through COMSOL Multiphysics® 5.2a and LiveLink™ for MATLAB® software to model the wear and study the influence of different conditions of the block surface and alignment of the sample. Changes of the geometry of the block and the contact pressure are quantified for several back and forth motions, using an extrapolation scheme in the wear modelling methodology. The tracking of such changes allow a dynamic overview of how the block contact area and the contact pressure distribution change throughout time. The results show how the assumption of a constant contact area and use of a nominal contact pressure in calculating the wear rate in such experiments can be inappropriate, especially in the presence of roughness and misalignments of the block.

The contact between solids in metal-forming operations often involves temperature-dependent viscoplasticity of the workpiece. In order to estimate the real contact area in such contexts, both the topography and the deformation behaviour should be taken into account. In this work, a deterministic approach is used to represent asperities in appropriately shaped quadratic surfaces. Such geometries are implemented in indentation finite element simulations, in which the indented material has thermo-viscoplastic properties. By creating a database of simulation data, investigations in terms of contact load and area for the specifically shaped asperities allow for an analysis on the influence of the material properties on the load–area relation of the contact. The temperature and viscoplasticity greatly define how much load is supported by a substrate due to an indenting asperity, but the description of the deformation behaviour at small values of strain and strain rate is also relevant. The pile-up and sink-in regions are very dependent on the thermo-viscoplastic conditions and material model, which consequently affect the real contact area calculation. The interplay between carried load and contact area of a full surface analysis indicates the role that different sized asperities play in the contact under different thermomechanical conditions.

Constitutive models were built based on the results of isothermal hot compression tests for a 6061 aluminium alloy at temperatures of 400, 450, 500, and 550 degrees C and strain rates of 0.1, 1, and 10 s(-1), which reproduced conditions of the hot rolling forming process for this alloy. The Garofalo-Arrhenius, Johnson-Cook, and Hensel-Spittel material models, modified versions of the latter two, as well as a newly proposed Johnson-Cook model were applied based on the experimental data. The predictive power of the constitutive models was assessed for a wide range of plastic strains, from the start of the plastic region up to a strain value of 1, including strain hardening at the beginning of the flow curve. Comparisons between experiments and models by means of the Pearson correlation coefficient and relative errors, considering different strain ranges, showed that the goodness of the models depends strongly on the considered strain range. Results revealed that the Garofalo-Arrhenius model provided the highest accuracy at any strain range, followed by the Hensel-Spittel models and the newly proposed Johnson-Cook model, which performed more accurately than its commonly employed modified version. (C) 2020 The Authors. Published by Elsevier B.V.

The use of computational methods in tribology can be a valuable approach to deal with engineering problems, ultimately saving time and resources. In this work, amodel problem and methodology is dev ...

Contact mechanics and wear of self-lubricating polymer bearings used in hydropowerapplications