TRIBOLOGY

Tribology is the branch of science and technology concerned with interacting surfaces in relative motion, friction, wear, and lubrication. Peter Jost (1966) chairman of a working group of lubrication engineers introduced and defined the term tribology in the "Department of Education and Science Report” published in England.   This report suggested that British industry and government by improving the lubrication practices could save considerable amount of money. The field of tribology is extremely diverse with challenges in many different areas of study including: materials science, fluid dynamics, contact mechanics, dynamics, heat transfer, chemistry, physics, controls, surface characterization, and many more, (Davis & Hu 2009).

The development of internal combustion engine forced the experts to develop the system platform of engine tribology design where the problem of engine tribology could be analyzed and the function module of software system and frame work of whole system could be designed. The collaboration of different software and experts helped to improve the analysis ability and the knowledge on engine tribology (WANG et al. 2009).

The inherent elastic properties results in detachment waves also termed as Schallamach waves development in sliding polymer-polymer surface contacts and these waves helps in initiation and propagation of wear along the sliding contacts. Ashraf et al. (2007) used finite element quasi steady-state analysis of such sliding surfaces. The nodal solution for pressure obtained for small sliding steps of the developed contact model and analysis of orthogonal pressure components at the contact nodes reveals the formation of Schallamach wave phenomenon. Further, appropriate wear law is used for calculation of wear at nodal level.

ANSYS, a general purposes finite element modelling and analysis package is used to solve variety of mechanical problems as static/dynamic, structural analysis (both linear and nonlinear), heat transfer, and fluid problems, as well as acoustic and electromagnetic (Stolarski, Nakasone & Yoshimoto 2011).

ANSYS used in this research can also be utilized to handled Material and structural non-linearities, i.e., plasticity, viscoelasticity, and friction. As far as the contact problems are concerned the stated software present Lagrange multiplier, penalty function and direct constraint approaches to model contact conditions with different types of contact element. When meshing a model, the nodes on potential contacting surfaces comprise the layer of contact elements whose four Gauss integral points are used as contacting checkpoints, (Ashraf et al. 2009).

As the wear occurs when the surfaces are in contact with each other, it is utmost important to know how much material is lost in the given operation time.  The pressure distribution depends upon the surface shape in contact which may vary due to their function, manufacturing tolerances etc and are subject to change due to wear and plastic deformation. It’s the versatility of finite element method that it solves the stress and strain problems regardless of the geometry of the body.  In this paper, ANSYS 5.0A has been used for the Finite Element analysis of contact pressure determination and wear simulation. (Põdra & Andersson 1999).

POLYMER BEHAVIOUR UNDER ADHESIVE WEAR LOADING CONDITIONS

Krasnov et al. 2010 studies different processes having polymers and polymer based materials running in contact. The studies related to systems and regularities of friction and wear of such materials covers under polymer tribology. However  Mitrus, Wojtowicz&Moscicki, (2009) explained that a single layer molecule developed by joining the chain of  small molecules in a  repeated manner is named as polymer. Moreover the main purpose behind the development of polymers is to have a product having unique physical and chemical properties. Chemists manipulate large, complex molecules and capitalize on the connections between their molecular structure and properties that make them light weight, hard strong and flexible with distinct thermal, electrical and optical properties, thus making them useful in major industries as packaging, transportation, fiber and communication industries. Harsha &Tewari, (2003) emphasize that the polymer composites are developed to enhance the wear resistance capability and also to minimize the coefficient of friction. It concludes that polymer composites thus developed are for tribological applications.  Furthermore they also studies the five types of wear observed in the practical scenarios. These wear are abrasive, adhesive, fretting, erosion and fatigue wear.   Chiu et al. (2012) explained that the thermoplastics as polyaryletherketone (PEEK) and polyimides when blend with solid lubricants results in reducing the coefficient of friction. Unal, Sen & Mimaroglu, (2005) defined wear as loss of material when two material are placed in contact with each other and there exist a relative motion between the contact surfaces, thus wear is a damage to a solid surface.

Myshkin, Petrokovets&Kovalev(2006) explained that a polymer surface which is under elastic, plastic and viscos-elastic deformation depends upon material characteristics. The properties of polymers like wear, friction resistance can be used with high effectiveness. This effectiveness was explained by Sinha& Briscoe (2009) with the explanation that the polymers like films, bulk and composites are being used in shoes, tires, brakes, bearings, MEMS, hair products and artificial human joints.  Wegner (2000) develop the understanding about the importance of polymer surface properties particularly in paints and thin coatings. These coatings are extensively used for corrosion protection as adhesive or anti reflection layers. The best example is the wetting behavior change of the film with respect to water from hydrophilic to hydrophobic. This behavioral change is carried out by the addition of small amount of a surface active component to a polymer material. Shalwan & Yousif(2012)  discussed the significance change in the tribological properties of polymer composites when glass fiber is added.  The common forms of glass fiber produced in the industry are woven or chopped strand mats.  However  Parallel (P-O), Anit-Parallel (AP-O), Normal (N-O) are the three principle orientations of composites with respect to sliding directions. Also there are several studies have been investigated the tribological behaviour of glass/polymer composites considering one orientation (Normal, N-O). Chiu et al(2012) concluded by simulation investigation that the highest  friction corresponding gross interfacial chain rearrangement is on the perpendicular sliding direction and this is independent of the polymer type. Baldan. A,  (2004) suggested that performance of the adhesive bond, the adhered surface (metallic or non metallic) can be improved by using some pre treatment. This pretreatment may be either by using the physical, mechanical, chemical, photochemical, and thermal, or plasma method. However Zhou & Yan (2004) revealed that although the adhesion of the transfer film to the counter face may be enhanced by the fillers which is an attribute of wear reduction. However, the friction reducing and wear resistant mechanisms of various fillers in different polymer matrices are not well understood.  According to Zhou & Yan, (2004), it is feasible to improve the dispersion of fillers by incorporating the compatibilizers in the composites. This will also increase the adhesive ability between the polymers and the fillers.  Interfacial adhesion canbe enhanced and the depressed phase size can be reduced by some copolymers or graft polymers with reactive groups and similar chemical structures. For example, polyethylene-graft-maleic anhydride (PE-g-MAH) has been used as a compatibilizing agent in many polymer–polymer alloys. Yan, Xue& Wang (2002)& Lai, Yue& Li (2007) introduced that preferential load of fillers also help to enhance the wear reducing ability of fillers. This method was proposed by Lancaster and further theoretically by Tanaka and Kawakami. Yan, Xue & Wang (2002) divided the actions of metallic fillers on transfer film in to two aspects. The hard filler exposed on the wear surface of the composite can scrape the polymer transfer film on the counterpart surface; and the transfer film composed of metallic filler can mechanically hold the polymer forming the composite transfer film, because its mechanical strength is higher than that of the polymer matrix. A steady state model of wear of polymer composite material was explained by Blanchet & SUNG (1998). This model also includes the effects of preferential load support by and surface accumulation of wear-resistant filler particles. This theory was further developed by Han and Blanchet both experimentally and theoretically. Yan, Xue& Wang  (2002)  studied the transfer process and preferential load support of hard fillers. The study was conducted to understand the characteristics of metallic fillers under a type of extreme friction condition as high load and reciprocating motion and the influence on the tribological behavior and transfer of polymer composite was recorded. Moreover, Chiu et al (2012) developed the hypothesis that the transfer film  in the polymer/polymer composites during sliding provide the protective and low shear interfaces due to which the wear resistance is increased and friction coefficient is reduced. Yousif & Ku (2012) suggested that natural fiber due to low density, high strength, high flexural modulus and high impact strength are more preferable for developing polymer composites. Furthermore the polymeric composites can be found in housing construction material, industrial and automotive parts. Suresha et al. (2010) explained that by adding the fillers or reinforced fibres,  the mechanical properties of the polymers are improved at higher rate as compared to the effect in the wear rate which do not invariably beneficent. Also, Suresha et al,  (2013) added the extensive applications of high strength and stiffness polymers and its composites like epoxy, vinyl ester, polyamide (PA), polyetheretherketone (PEEK), and polyphenylenesulfide (PPS). These polymers can be manufactured by compression moulding/injection moulding with ease and economics.

Skeist&Miron, (1990) reported adhesive cure is necessary to join together the small resin molecules (mers) so that large molecules known as polymers could be formed. For example, one of some adhesives, such as this wood adhesive, is used only with specific substrates because of the cure time needed to allow for good bonding. Although Adhesion is considered as friction impact and Johnson- Kendal-Roberts model theory used small and big sizes having the properties of polymers (Myshkin et al. 2005).  They also explained the way the special instruments can be used to measure the molecular forces acting between the solids. According to Deryagin et al. (cited in Myshkin et al. 2005, p. 911), molecular attraction forces can be measured by applying the concept of feedback balance. However these molecular forces are now being measured by using atomic force microscopy. Also the mechanical energy dissipates when deformation occurs between the two sliding surfaces. The deformation may be elastic deformation, plastic deformation and visco-elastic deformation. Thus this concludes that the adhesion and deformation is the source of fractional forces (Myshkin et al. 2005, p. 913).  Load, velocity and the temperature has the direct effect on the friction. Firstly, friction force is proportional to the applied load and coefficient of friction staying constant at load between 10-100 Newton. Secondly, sliding velocity has a relationship with friction. By increasing velocity, viscous resistance will increase and at high velocity, friction force will reduce due to short duration of contact. Thirdly, temperature resulted from friction by converting mechanical energy to heat and this heat is generated from friction and the source of this heat is deformation of materials.

Yousif (2008) explained the performance evaluation of polyester in terms of frictional and wear performance using dry sliding contact against the smooth stainless steel using block-on-disk (BOD) machine.  The polyester is reinforced by the multilayered (mainly three or four layers) of coir fiber. The results were the worn surface analyzed on scanning electronic microscope and wear rate and friction coefficient were calculated in different aspects including;

·         Different applied load between 10N to 30N.

·         Different sliding distance between, (0km) to (4.2km).

This experiment concluded that the coir fibre-reinforced polyester (CFRP) composites with three layers are better than four layers with better frictional and wear performance. Moreover, CFRP composites with four layers gave a high friction coefficient (Yousif 2008). Yousif (2008) concludes with some outcomes as follows:

·         Strong interfacial adhesion between the polyester and the coir fibres which overcome the pull out of fibres at the interface.

·         High coefficient of friction was observed.

·         Reinforced polyester with coir fibres has improved adhesive wear characteristic of (NP).

World’s Largest Solar Bridge

The World’s Largest Solar Bridge Launched This January In The City of London
 
The city of London is a proud owner of the world’s largest solar bridge. The massive construction located at the Blackfriars station is equipped with 4,400 photovoltaic panels, providing half of the energy required to operate all services there.

The solar panels, together with the electric trains that run through the station, are not only going to reduce the carbon footprint of the station, but they will also serve as a symbol of what a sustainable city should look like. The project was executed by Network Rail, with partnership of Solarcentury, who was responsible for installing the panels, while trying not to interfere with commuting times during London 2012 Olympics. Thanks to careful planning, and strict organization, however, the specialists managed to complete the task in time despite heavy traffic, safety standards and restrictions.

The panels are expected to contribute to reduction in carbon emissions by 511 tonnes annually. The routes that run through the solar bridge provide a connection for the citizens from southeast England with central London, making the Blackfriars station one of the busiest locations in the city. Of course, having the bridge at such a central spot, allows people to admire it and hopefully it makes them realize how important sustainable living and green energy are. Besides raising awareness and encouraging local people and tourists to embrace a more sustainable lifestyle, the developers hope to inspire other major infrastructure developers to include renewable energy into their projects.

RESIDUAL STRESS

Residual stresses can arise in materials in almost every step of processing. The origins of residual stresses in a component may be classified as:

ü  Mechanical

ü  Thermal

ü  Chemical

Mechanically Generated

These are the residual stresses which occur due to plastic deformation as a result of manufacturing process. They may develop during processing or treatment and the operation that may develop theses stresses, which are ofcource undesirable, are rod or wire drawing, welding and machining processes as turning, milling and grinding. Whereas shot peening, autofrettage of pressure vessels, toughening of glass or cold expansion of holes may introduce compressive residual stresses in the components.

The stresses which are undesirable for the component are due to the reason that they contribute towards fatigue failure, quench cracking and stress-corrosion cracking. Such stresses are tensile stresses developed in the surface of the component. On the other hand compressive residual stresses contribute towards fatigue strength and resistance to stress-corrosion cracking, and increase the bending strength of brittle ceramics and glass.

During grinding process the undesirable tensile residual stresses are generated as a result of a conventional or highly abrasive grinding wheel whereas if a soft gentle grinding wheel is used beneficial compressive residual stresses will be generated as shown in the figure below.

Therefore it can be concluded that if a body is in equilibrium its tensile residual stresses are balanced by its compressive residual stresses. Also if the residual stresses are being operated in the plane of applied load and are on opposite in sense, then these residual stresses are termed to be as beneficial.

Figure 1.1. Characteristic residual stress distributions in hardened steel for three different grinding operations [1]

THERMALLY GENERATED

The residual stresses may be developed in a material during manufacturing and processing which results in differences on coefficient of thermal expansion between different constituents resulting in a thermal mismatch. These are he micro-stresses as they changes occurs at atomic scale.   Macro stresses results during the forming processes or non uniform heating or cooling as in casting, welding, heat treatment etc. [2]. If the stresses generated as a result of thermal imbalance exceed the yield strength, plastic deformation will occur.

CHEMICALLY GENERATED

The chemically generated stresses can develop due to volume changes associated with chemical reactions, precipitation, or phase transformation. Chemical surface treatments and coatings can lead to the generation of substantial residual stress gradients in the surface layers of the component. Nitriding produces compressive stress in the diffusion region because of expansion of the lattice and precipitation of nitrides, and carburizing causes a similar effect. The magnitude of residual stresses generated in coatings can be very high –compressive stresses of the order of 6-8GPa or higher have been measured at the interface of some thermal barrier coatings (TBCs).

[1]E.B. Evans, “Residual Stress in Processing” Encyclopedia of Materials Science and Engineering Vol. 6, 4183-88 (1986) 

[2] G. Totten, M. Howes & T. Inoue, “Handbook of Residual Stress and Deformation of Steel” 2002

RESIDUAL STRESSES IN PIPELINES

The structural integrity and cost of pipelines are of major concern in oil, chemical, and other industries. Pipelines can be subjected to severe thermal, seismic, pressure, and other mechanical loads, and for this reason an increasing amount of attention has been given to their analysis.

Residual stress has got the attention due to the reason it has great effect on the performance and life of the component. It has got more significant importance when considered for pipelines as it plays an important role in issues as Stress Corrosion Cracking (SCC), Hydrogen Induced Cracking (HIC), Fatigue Cracking, Welding Stresses, Heat Treatment Effectiveness, Surface Enhancements due to Cold Work, Bending due to Seismic Activity and Installation Stresses. For a perfect design beneficial stresses should be induced into the component whereas the effect of harmful stresses should be minimized.

Welding being the most important industrial process mostly being used in ship building, pipeline fabrication etc. Residual stresses arising after welding exert a considerable influence on the service characteristics of welded equipment and their control allows to avoid failure of welded joint. The influence of residual stresses on service characteristics of welded equipment has been analyzed by many scientists.

J.T. Assis, V Monin_, J.R.Teodosio, T. Gurova  presented the experimental results of residual stress measurements by X-ray tensometry method. He concluded that the that stress measurements on the outer surface of the welded joint are not enough to examine residual stress states in weld regions and it confirms the need to study stress distribution along the depth. This can be accomplished by means of stress measurements after surface layer removing. It is clear that in the case of layer removal by machining or grinding it is necessary to undertake electropolishing to remove residual stresses introduced by machining.

Analysis of equilibrium equations for circular weld seam is more complicated but the importance of knowing the stress distribution along the depth is obvious. Stress measurements after surface removal are presented in figure 2. The stress distributions for the samples with a linear circular weld seams show that the compressive residual stresses at the centre of the weld seams reverse to tensile stresses after removing a surface layers approximately equal to 0,5 mm.

Head Loss

INTRODUCTION

Head loss is a measure of the reduction in the total head (sum of elevation head, velocity head and pressure head) of the fluid as it moves through a fluid system. Head loss is unavoidable in real fluids.

There are two categories of head loss in pipe. One of them is due to viscous resistance extending throughout the total length of the circuit. Next is due to localized effects such as valves, sudden changes in area of flow and bends. Many factors affect the head loss in pipes, the viscosity of the fluid being handled, the sizes of the pipes, the roughness of the internal surface of the pipes, the changes in elevation within the system and the length of travel of the fluid.

The resistance through various valves and fittings will also contribute to the overall head loss. A method to model the resistances for valves and fittings will be of minor significance to the overall head loss, many designers choose to ignore the head loss for valves and fittings at least in the initial stages of a design.

Frictional loss is that part of the total head loss that occurs as the fluid flows through straight pipes. The head loss for fluid flow is directly proportional to the length of pipe, the square of the fluid velocity, and a term accounting for fluid friction called the friction factor. The head loss is inversely proportional to the diameter of the pipe.

The friction factor has been determined to depend on the Reynolds number for the flow and the degree of roughness of the pipe’s inner surface.

THEORY

For an incompressible fluid flowing through a pipe the following equations apply:

Notation:   

Q  Volumetric flow rate (m3/s) 

V  Mean Velocity (m/s)

A  Cross sectional area (m3) 

Z  Height above datum (m)

P  Static pressure (N/m2)

hL  Head Loss (m)

ρ  Density (kg/m3) 

g   Acceleration due to gravity (9.81m/s2)

HEAD LOSS

The head loss in a pipe circuit falls into two categories: 

That due to viscous resistance extending throughout the total length of the circuit, and; 

That due to localized effects such as valves, sudden changes in area of flow, and bends. 

The overall head loss is a combination of both these categories. Because of mutual interference between neighboring components in a complex circuit the total head loss may differ from that estimated from the losses due to the individual components considered in isolation.

Head Loss in Straight Pipes 

The head loss along a length, L, of straight pipe of constant diameter, d, is given by the expression: 

Where f is a dimensionless constant, which is a function of the Reynolds number of the flow and the roughness of the internal surface of the pipe.