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Modeling and testing of elastomer impact deformation under high strain rates
Elastomers are commonly used in applications which experience repeated impacts by hard abrasive particles at high or moderate strain rates. The goal of the current work is to develop a simple and cost effective methodology involving scratch testing and modeling for capturing the failure and wear behavior of elastomeric materials. The high failure strains as well as the extremely good wear resistance of elastomers make this task a formidable challenge. Modeling of a scratch test, however, seems to be promising in this regard.
This paper highlights the future perspectives of sustainable tribology by examining the economic, environmental and social impact of three tribological case studies. One case study examines the sustainability and durability of micro-CHP systems looking the tribological phenomena generated within a scroll expander system. The scroll is the main part of a specific micro-CHP system and experiences wear and cavitation damage. The tribological optimization of the scroll expander improves the sustainability of the micro-CHP unit while it has a serious economic and environmental impact to the consumers and to the society in general. Another case study is focused on friction and wear performance of lifeboat launch slipways. The causes of high friction and wear during the RNLI's lifeboat launches along an inclined slipway are investigated with a view to reducing the environmental impact due to slipway panel wear and lubricant release into the marine environment. The project encompasses the sustainable design of slipway panels using design modifications based on tribological investigations to double their lifespan, while environmental and economic impact was significantly reduced by the use of biodegradable greases and water as lubricants. The final case study involves an investigation of recycled plastic materials to replace polyurethane used on skateboard wheels, scooters and similar applications. Polyurethane (PU) is difficult to recycle. With the dwindling resources and environmental problems facing the world today, recycling for both waste reduction and resource preservation has become an increasingly important aspect of sustainability. The tribological results showed that recycled polycarbonate plastic can effectively act as a substitute to polyurethane wheels. Moreover, sustainability considerations showing the environmental benefits of the use of recycled plastics over PU include reducing the CO2 footprint by 50% and the energy consumed by 60%, among other benefits. These case studies emphasise the importance of sustainable tribology in our epoch showing that increased sustainability performance can be achieved through tribology to a significant extent in many cases, providing stability to our world and more viable long term growth to our societies.
Material deformations and the influence of coating thickness and elastic modulus were analysed by three-dimensional finite element method (FEM) modelling on microlevel, by stress, strain, and displacement computer simulations and by experimental studies with a scratch tester. The studied tribological contact was a diamond ball sliding with increasing load on a thin titanium nitride (TiN) coating on a flat steel substrate. The ball was modelled as rigid, the coating was linearly elastic, and the steel substrate was elastic – plastic, taking into account strain hardening effects. It was shown that a thin TiN ceramic coating on a steel substrate has only a very slight effect on friction and on the plastic deformations (i.e., the groove formation) in the surface, but changes considerably the stress pattern at the surface. The stress simulations showed how a thicker hard coating on a soft substrate has a better load-carrying capacity that a thinner one. Higher tensile stresses at the coating/substrate interface increase the risk for interface cracks and delamination of the thicker coating. A stiffer hard coating on a soft substrate has a better load-carrying capacity than a more elastic one. The stiffer coating will accommodate higher tensile stresses with the same indentation depth compared to a more elastic one. The results show that much more attention should be given to optimizing the elastic properties of the coating than previously has been done. In many cases, it can be much more effective to improve the wear resistance of the coated surface by focusing on the elastic modulus of the coating than changing the coating thickness. D 2005 Elsevier B.V. All rights reserved.
The objective of process control in the mineral industry is to optimise the recovery of the valuable minerals, while maintaining the quality of the concentrates delivered to the metal extraction plants. The paper presents a survey of the control approaches for ore size reduction and mineral separation processes. The present limitations of the measurement instrumentation are discussed, as well as the methods to upgrade the information delivered by the sensors. In practice, the overall economic optimisation goal must be hierarchically decomposed into simpler control problems. Model-based and AI methods are reviewed, mainly for grinding and flotation processes, and classified as mature, active or emerging.
The surface fracture mechanisms, that are the origin to wear, were analysed by three-dimensional finite element method (FEM) modelling on micro-level by stress and strain computer simulations and by experimental studies with a scratch tester. The studied tribological contact was a diamond ball sliding with increasing load on a thin titanium nitride (TiN) coating on a steel substrate. The ball was modelled as rigid, the coating linearly elastic and the steel substrate elastic–plastic taking into account strain hardening effects. In a sliding contact the first crack is initiated at the top of the coating from bending and pulling actions and it grows down through the coating. The effect of initial residual stress fields on coating cracking was approached by carrying out stress simulations with a pre-stressed coating. The stress fields near the indenter were found to relax by plastic deformation to such extent that the remaining stresses had only a marginal effect to macroscopic behavior. Fracture mechanical evaluation of crack driving force and fracture toughness were performed by determining stress intensity factor (SIF) solutions using boundary element analysis. SIF solutions were evaluated for crack fields of different density, location, crack angle, type of loading and mode of loading. The results were utilized to evaluate fracture characteristics and compute fracture toughness for the TiN to high speed steel coating substrate system.
The stress and fracture conditions of a coated surface, that are the origin to wear, were analysed by three-dimensional finite element method (FEM) modelling on microlevel, by stress and strain computer simulations and by experimental studies with a scratch tester. The studied tribological contact was a 0.2 mm radius diamond ball sliding with increasing load on a thin, 2 μm thick titanium nitride (TiN) coating on a flat high speed steel substrate. The ball was modelled as rigid, the coating linearly elastic and the steel substrate elastic–plastic taking into account strain hardening effects. The stresses and strains generated in the surface during sliding are the result of four different mechanisms: the pulling and pushing by the friction force; the geometrical indent, groove, and torus shaped deformations of the flat surface; the bulk plasticity concentration and curvature minimum effects; and the residual stresses in the coating. In a sliding contact the first crack is initiated at the top of the coating from bending and pulling actions and it grows down through the coating. In the modelled scratch tester system a complex stress field is formed at the surface including remaining residual stresses in the coating behind the sliding contact. The stress fields are very different in a scratched uncoated steel sample. Some residual tensile stresses are formed in the groove behind the tip but they are very much lower than for the TiN coated case. A displacement controlled FEM model was found to better represent the real situation and correspond to experimental results than a force controlled model.
This study presents calculations on the global fuel energy consumption used to overcome friction in passenger cars in terms of friction in the engine, transmission, tires, and brakes. Friction in tribocontacts was estimated according to prevailing contact mechanisms such as elastohydrodynamic, hydrodynamic, mixed, and boundary lubrication. Coefficients of friction in the tribocontacts were estimated based on available information in the literature on the average passenger car in use today, a car with today’s advanced commercial tribological technology, a car with today’s best advanced technology based upon recent research and development, and a car with the best technology forecasted in the next 10 years. The following conclusions were reached:
This paper shall review Metso Minerals’ current 3D Discrete Element Method (DEM) modeling techniques, developed to allow a detailed study of the interactions of crushing design variables with actual rock mechanics.A standard DEM approach cannot be applied to systems where size reduction is an important element in the flow of particles, such as industrial crushers. In these systems, it is necessary to physically decrease the particle sizes in a realistic way, as they are transported through the device. Metso Minerals has therefore developed a breakage model that incorporates elements of Population Balance Modeling (PBM) techniques to describe breakage as a function of the loads on the individual rocks. The combination of this breakage model with DEM has therefore allowed the development of a multi physics based comminution model that can be applied to crusher development. This model is sensitive to all aspects of crusher design including crusher machine parameters and the ore characteristics.This physics based virtual comminution model will enable engineers to optimize the design development of future crushers by allowing a wider range of values for the crusher variables to be investigated in a rapid, yet more detailed manner, without the need for the traditional expensive and lengthy prototype testing.
Wear of grinding mill liners and lifters plays a major role in the overall efficiency and economics of mineral processing. Change in the shape of lifters as they wear has a significant influence on grinding efficiency, and the annual cost of maintenance and mill down-time depends on the life of both liners and lifters. The Discrete Element Method (DEM) is a computational method for simulating the dynamics of particle processes. This paper presents an analysis of D simulation of a grinding mill carried out using the EDEM software package customised to predict the rate of wear of lifter geometry and to enable progressive updating of worn lifter geometry profiles. A simplified breakage rate model is developed as a tool to correlate liner profile to mill performance.The combination of the prediction of liner profile and grinding rate shows promise to provide a powerful tool for advanced mill liner design – providing a means to balance liner life and mill performance over the life of the liner at the liner design stage.
Development of optimum wear-resistant materials relies on design of the total tribosystem based on the mechanical, chemical and thermal demands of the triboenvironment. Further material refinement is based on knowledge of the wear and degradation mechanisms, with material parameters such as composition and microstructure becoming vitally important. Wear-resistant metallic and elastomeric materials developed or adapted for use in the mining and mineral processing industries are reviewed. Tailoring composition and morphology of white cast irons and elastomers such as natural and synthetic rubbers and polyurethanes is described. Performance of metals and elastomers in corrosive-wear environments is compared. Translation from product specification data sheets (hardness, tensile strength, DIN or ASTM abrasion resistance) to field wear performance is discussed. The effect of tribosystem design on wear rate is emphasized. © 2001 Elsevier Science B.V. All rights reserved.
Commercially, the alteration of a rubber formulation is usually made in such a way as to keep the hardness of the rubber product constant. This is because a specific hardness of the rubber product sets the limit to its practical applications. Therefore, in this paper, natural rubber (NR) vulcanizates containing various fillers were prepared to have the same hardness level, and their mechanical properties were compared and related to the degree of filler dispersion. The results show that higher amounts of carbon black (CB) and silica are needed for CB- and silica-filled natural rubber vulcanizates to achieve the same hardness value as a NR vulcanizate containing 6 phr of montmorillonite clay. At equal loading of fillers, clay-filled vulcanizate exhibits higher modulus, hardness, tensile strength and compression set, but lower heat build-up resistance and crack growth resistance than those of the vulcanizates containing conventional fillers. For the vulcanizate having the same hardness value, CB-filled vulcanizate gives the better overall mechanical properties followed by the clay-filled and silica-filled vulcanizates, respectively. The explanation is given as the better dispersion of carbon black, as can be seen in the SEM micrograph.
Addition of reclaimed rubber affected mechanical properties, processing and rheological behavior of rubber compounds. Sulfur
left in reclaimed rubber affected curing process of NR/reclaimed rubber blends. The most suitable curing was achieved with
ratio 1:1 of natural rubber and reclaimed rubber which was used in this study. Presence of fillers in reclaimed rubber, non-homogeneity
of phases, non-uniform filler dispersion and lower molecular weight of reclaimed rubber due to chain scission in reclaiming
process caused diminishing of mechanical properties of NR/reclaimed blends, particularly dynamic-based properties. Reclaimed
rubber also affected rheological behavior of natural rubber. In a strain sweep viscoelastic test, it could be observed that
NR/reclaimed blend show more non-linear viscoelastic and viscous behavior than predicted one which was due to non-homogenized
phases. This non-homogenized morphology was detected using scanning electron microscopy (SEM) and the blend with ratio 1:1
had the most homogenized morphology.
Most wear models and equations in the literature were analyzed as to origin, content and applicability. No single predictive equation or group of limited equations could be found for general and practical use. The reasons include the perpetuation of erroneous and subjective expressions for the mechanisms of wear, the slow pace of translation of microscopic observations into macroscopic models of the wearing processes and the paucity of good experiments to verify proposed models.
Surface coatings are increasingly used to improve the tribological performance of advanced products. The novel coating deposition techniques offer numerous possibilities for tailoring surfaces with different materials and structures. The tribological contact of loaded surfaces is, however, a complicated system itself, and further complexity is introduced when functionally graded coating structures are considered or improvement of specific micro-and nanostructural features is pursued. Furthermore, the mechanisms of damage in such a system are from a modeling standpoint highly complex and to great extent remain an active and open field of study. The focus of the current work is in the numerical modeling of graded thin hard coatings on a plastically deforming metallic substrate when loaded by contact that is typically exhibited during a scratch test. A finite element approach is implemented wherein a coating crack initiation and propagation are modeled using cohesive zone formalism. The cracks are considered to initiate and propagate within the coating and also within the coating to substrate interface. The results demonstrate how an optimization of the coating structure can enhance and exceed the performance of simplistic traditional coated systems. The material parameters of the problem and their significance in terms of fracture and failure behavior are discussed. The results are compared to fracture mechanical analyses and experimental information regarding the problem under study.
Tumbling mill scale-up has had largely empirical roots. Energy size and energy size distribution relationships have been derived from relatively large data bases combined with a sprinkling of fundamental science. High fidelity simulation (HFS), which is based on multi-physics models, allows one to examine the interactions that occur in a mill amongst media, particles, wear parts and slurry at the microscale level. Thus, fundamental questions concerning particle capture, energy utilization and fragment creation as they depend on bequipment sizeQ can be addressed. In this paper, the process is taken from the single particle breakage level, through laboratory grinding experiments up to full-scale commercial mill operation. The basis for certain empirical relationships is explained in fundamental terms. A look at the future of scale-up based on the use of such tools is presented. C. 2004 Published by Elsevier B.V.
Green tribology, its history, challenges, and perspectives
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