Malvern Instruments to exhibit at easyFairs Solids
The focus on the Malvern Instruments stand at easyFairs Solids 2006 will be the Insitec T on-line, and the Insitec Aliss at-line, particle size analysers.
The focus on the Malvern Instruments stand at easyFairs Solids 2006 (scheduled for 1-2 November 2006 at the Ricoh Arena in Coventry) will be the Insitec T on-line, and the Insitec Aliss at-line, particle size analysers. These laser diffraction-based systems are from the extensive range of Malvern on-line, in-line, at-line and off-line solutions for particle characterisation. Designed to produce continuous particle sizing information for real time process control, Insitec systems are both rugged and robust, reflecting the exacting demands placed on analytical instrumentation in a process environment.
The instrument, the process interface, analysis software, automation and data reporting can all be customised to the unique needs of individual clients.
The recently introduced Insitec Aliss at-line analyzer has applications in a wide range of processing industries, including powder coatings, minerals and cement.
It uses the same proven Malvern Insitec technology and platform normally employed for particle sizing on-line.
Users therefore gain access to this robust laser diffraction technology to make measurements in an automated laboratory, or manually close to the process line.
Alloy Steels – AISI Designations
Alloy steels differ from carbon steels in that they have compositions that extend beyond the limits set for carbon steels. Usually this refers to constituents such as boron, carbon, chromium, manganese, molybdenum, silicon and vanadium. They also have chromium contents less than 4%. Steels with chromium contents of greater than 4% become classified as stainless or tool steels. As a general guide, an alloy steel will have:
·        Manganese content >1.65%
·        Silicon content >0.5%
·        Copper content >0.6%
The American Iron and Steel Institute (AISI) naming system is one of the most widely accepted systems.
Designations usually consist of a four digit number, but sometimes this extends to five. The first two digits indicate what the major alloying element is, while the last 2 or three indicate the carbon content in hundredths of a percent.
Example: AISI 1340 is a manganese containing alloy steel with a 0.40% average carbon content.
Table 1. Summarises AISI designations for alloy steels.
|
 |
 |
Main Alloying Elements |
|
13 |
xx: 1.75Mn |
Manganese |
|
23 |
xx: 3.50Ni |
Nickel |
|
31 |
xx: 1.25Ni, 0.65-0.80Cr |
Nickel-Chromium |
|
40 |
xx: 0.20-0.25Mo |
Molybdenum |
|
41 |
xx: 0.50-0.95Cr, 0.12-0.30Mo |
Chromium-Molybdenum |
|
43 |
xx: 1.82Ni, 0.50-0.80Cr, 0.25Mo |
Nickel-Chromium-Molybdenum |
|
44 |
xx: 0.40-0.52Mo |
Molybdenum |
|
46 |
xx: 0.85-1.82Ni, 0.20-0.25Mo |
Nickel-Molybdenum |
|
47 |
xx: 1.05Ni, 0.45Cr, 0.20-0.35Mo |
Nickel-Chromium-Molybdenum |
|
48 |
xx: 3.5Ni, 0.25Mo |
Nickel-Molybdenum |
|
50 |
xx: 0.27-0.65Cr |
Chromium |
|
50 |
xx: 0.50Cr, 1.00C |
Chromium |
|
51 |
xx: 0.80-1.05Cr |
Chromium |
|
51 |
xx: 1.02Cr, 1.00C |
Chromium |
|
52 |
xx: 1.45Cr, 1.00C |
Chromium |
|
61 |
xx: 0.60-0.95Cr, 0.10-0.15V |
Chromium-Vanadium |
|
81 |
xx: 0.30Ni, 0.40Cr, 0.12Mo |
Nickel-Chromium-Molybdenum |
|
86 |
xx: 0.55Ni, 0.50Cr, 0.25Mo |
Nickel-Chromium-Molybdenum |
|
87 |
xx: 0.55Ni, 0.50Cr, 0.25Mo |
Nickel-Chromium-Molybdenum |
|
88 |
xx: 0.55Ni, 0.50Cr, 0.20-0.35Mo |
Nickel-Chromium-Molybdenum |
|
92 |
xx: 1.45-2.0Si, 0.65-0.85Mn,<0.65Cr |
Silicon-Manganese |
|
93 |
xx: 3.25Ni, 1.20Cr, 0.12Mo |
Nickel-Chromium-Molybdenum |
|
94 |
xx: 0.45Ni, 0.40Cr, 0.12Mo |
Nickel-Chromium-Molybdenum |
Ageing of Composites
Fibre reinforced plastics (FRPs) offer many advantages over conventional structural materials. They have high strength and modulus-to-weight ratios, are fatigue and corrosion resistant, tailorable and require low maintenance. However, because of their unknown long term properties when exposed to a combination of in-service loads and environments, designers are still reluctant to use FRPs in primary load bearing structures. The effect of exposure to heat, moisture, hydrocarbons, fatigue and static loads etc, and more importantly a combination of these parameters may degrade the material’s stiffness and strength. The lack of long term data for FRPs and of an accelerated ageing methodology that will predict the effect such a degradation might have on the residual properties and future life of the structure are two of the major issues hindering their wider use.
Long Term and Accelerated Ageing
Ideally, composite materials and their structures that are intended for long term use should be tested in real time and with realistic in-service environments. Often this is not viable because the time involved would significantly delay product development. However, these long term data are invaluable when generated as in a programme currently running at Westland Helicopters. Test coupons and main rotor blade sections were naturally aged at a hot/wet site in Australia, in stressed and unstressed condition for periods up to ten years. Further specimens were exposed in environmental chambers at 45°C and 85%RH. Others were exposed for a five year period at a hot/dry site in Australia. The fatigue strength of the structures and coupons was unaffected by the 10 year wet natural exposure. Coupon static degradation was greatest in matrix dominated properties, although the fibre dominated properties of the glass fibre materials degraded more than that of the carbon. The coupons in the hot/dry conditions were unaffected after 5 years.
Such a long term approach is generally not viable, and accelerated ageing techniques are required. In polymers (thermosets, thermoplastics or elastomers) free space exists between molecular chains. This free space allows the polymer to absorb fluids to which they are exposed, especially those with similar solubility parameters. Such absorption physically weakens the polymer and may also chemically attack the polymer. The kinetics of these processes are governed by diffusion and chemical kinetics, both of which are governed by Arrhenius relationships with regard to the influence of temperature. Accelerated testing can therefore be performed at elevated temperatures, with the results being extrapolated back to service temperature for life prediction purposes. Diffusion characteristics can be measured by liquid mass uptake or gas permeation experiments. Chemical kinetics, classically involving concentrations of reactants and products, can employ the fact that for crosslinked polymers, the concentration of crosslinks is approximately proportionate to modulus or stiffness. Hence, measurements of changes in modulus from ageing can be plotted logarithmically against linear time (for 1st order reactions) at each temperature. From a series of such ageing plots at different temperatures, times to attain the same degree of modulus change can be used to develop the Arrhenius plot.
Thermal Ageing in Aerobic and Anaerobic Environments
For high mach number aircraft structures, aircraft engines, space satellites and other environments, composite materials are expected to be durable at high temperatures. Carbon bismaleimides (IM7/5260), polyimides (IM7/K3B) and amorphous thermoplastics (IM7/8320) may be considered for the next generation High Speed Civil Transport fuselage and wing structure. These materials may be exposed to temperatures in the region of 125°C and 175°C, representing Mach 2.0 and Mach 2.4 flight respectively. Weight loss, glass transition temperature (Tg), and tensile strength data were shown for ±45 and notched quasi-isotropic laminates exposed at these temperatures for up to 5000 hours. Significant reduction in properties, including Tg and strength, occurred for the bismaleimide after only 2000 hours at 175°C. There are differences in properties when test coupons are aged, as opposed to panels being aged and test coupons cut after the ageing, figure 1. This difference arises because oxidation occurs at the edges of the aged specimens, where damage also initiates in the post exposure test. The notched bismaleimide laminates showed extensive matrix cracking at the surfaces but the ultimate tensile strength was not significantly reduced because the 0° fibres in the load direction contribute to the majority of the laminate strength.
In aircraft engines, composites may be used in structures ranging from outer nacelle to core bearing housing structures. The range of different applications causes a wide variation in operating environments. For civil engines, such as the Rolls Royce RB211, component life requirements may be in the order of 25 years. To evaluate woven carbon bismaleimide (T300/52502) thrust reversers, test specimens were aged in air circulating ovens at 230°C and 250°C for up to 2000 hours. Weight loss and the effects of ageing on the flexural and impact properties were measured, and thermal analysis and microscopy were used to investigate chemical and structural changes. Components fabricated from carbon/polyimide (PMR-15) for higher temperature parts were cycled between extremes of -50°C and +350°C. The mechanical stresses resulting from anisotropic thermal expansion of crossply laminates lead to microcracking degrading the PMR-15. Much of the damage occurred from the peak temperature rather than the cycling.
In the space environment, temperature extremes are typically -150°C to +120°C with up to 30,000 cycles in a geostationary orbit, and -90°C to +90°C in low earth orbit. The principal concern is the dimensional stability of components such as communications antenna dishes with effectively zero overall coefficient of thermal expansion. The resultant microcracking from thermal cycling can be reduced by using lower cure temperatures and new toughened epoxy and cyanate ester resins.
Ageing in Liquids
For carbon/toughened epoxy (T800/924C) laminates, maximum moisture content attained in unidirectional specimens was about 1.4% (by weight), reached in 36 days in boiling water. The moisture uptake mode was Fickian. For multidirectional laminates, maximum moisture content was reached in 70 days. This difference in diffusivity is a function of laminate stacking sequence. Similarly, in carbon/PEEK (APC2) and carbon bismaleimides (5245C), the diffusivity rates are different for different lay-ups and thicknesses when exposed to liquids such as water, jet fuels and other aviation fluids. In addition, the diffusivity rate changes when the laminate is loaded mechanically, changing the internal stresses. Thermal spikes on different configurations of the 5245C material in the temperature range of 100-200°C caused enhanced moisture absorption by an increase in the free volume of the matrix at the spiking temperatures.
Liquid diffusion may also alter the strength and failure mode of composites. The compressive strength of the saturated unidirectional T800/924C laminates was reduced by 50% over the dry laminates. The failure mode also changed from in-plane fibre microbuckling, figure 2, to out-of-plane microbuckling for the saturated case.
Physical Testing
Physical tests such as dynamic mechanical thermal analysis (DMA) and differential scanning calorimetry (DSC) were sensitive to changes in plasticisation of two woven carbon toughened epoxies (T300/924C-833 and T300/914C-833). These materials were exposed to various combinations of temperatures up to 70°C for up to 90 days in a variety of aerospace fluids including engine oil, hydraulic, anti-icing and cleaning fluids. The DMA results exhibited good correlation with high temperature interlaminar shear strengths. In a damage tolerance study of carbon and glass epoxy (F913C and F913G), laminates were impacted after immersion in water. The impact damage area was more wide spread and the subsequent compression after impact strength was lower than in non-exposed laminates.
Raman spectroscopy may be used to monitor swelling in matrix materials and determine diffusion coefficients. By monitoring the peak position of strain-sensitive Raman bands, the axial deformation of the fibre may be determined and used to define the states of localised stress and strain in the composite matrix. The dielectric technique may be used as a non-destructive examination method. The frequency domain can provide data on the extent of water ingress into a structure and the conversion of oxide to hydroxide at the interface. Time domain measurements can be used to identify regions of ingress and disbonding within the structure.
Modification of the resin system may enhance the composite’s hydrophobic performance. The synthesis of halogen-substituted tetraglycidyl methylenedianiline (TGDDM) resins can reduce the water uptake of TGDDM resins by as much as 40%, while showing relatively minor changes in Tg.
Advanced Materials take on Tennis
Advanced Materials take on Tennis
Overview
Materials technology has played a vital, lately controversial role in the history of the game, especially during the modern era with the advent of powerful composite rackets. However, while the likes of Andre Agassi could beat most of us using a tea tray, for the average player modern rackets offer a range of benefits, such as oversized sweet spots and efficient vibration damping, that make the game hugely more attractive. In fact, new technology could lead to the effective elimination of vibration.
Materials Evolution
Wooden and Aluminium Rackets
The very first rackets were made from solid sections of woods such as ash, maple and okume. However, the anisotropic nature of these materials necessitated a change in racket construction to a laminated structure, which allowed the stiffness and strength of the racket to be increased in directions both parallel and perpendicular to its main axis. Although adopting laminates significantly increased racket performance, the problem of water absorption, resulting in pronounced warping in the structure and therefore variable performance, persisted.
In the 1970s aluminium frames offering increased stiffness and reduced mass enjoyed a brief period of success. However, towards the end of the decade new continuous fibre composites were introduced that rapidly superseded aluminium as a frame material. The first of these composite materials consisted of glass fibres held within a polyester resin matrix, and later rackets went on to encompass various grades of carbon fibres within epoxy resin matrices.
Fibre Reinforced Composites
The short-lived success of aluminium has been attributed to a number of factors. Both glass and carbon fibre composites have a higher specific stiffness (modulus/density) than aluminium, so rackets made from composites can be much lighter, particularly in the case of carbon fibre. Continuous fibres can be woven into a variety of weave styles, giving increased control of the racket’s characteristics. For example, unidirectional fibres are incorporated along the main racket axis for high bending stiffness, and 0/90° weaves are stacked at ±45° for high shear strength and stiffness, (figures 1). A variety of fibre grades are used, each with different levels of strength and stiffness. These fibres are coupled with epoxy resin matrices that often contain one or more property modifiers, such as rubber particles and thermoplastics that increase the toughness of the resin.
On top of these advantages the fatigue performance of the composite rackets was superior to aluminium constructions. Tests on aluminium rackets have shown that a marked decrease in stiffness occurs at around 6000 impacts, compared with a change in stiffness for carbon fibre rackets of around 4% after 50,000 impacts. Another important factor in aluminium’s decline was the comparative damping properties of the frame materials, aluminium has a lower damping capacity than composite materials, and this has implications for the health of players.
Design Aspects
Damping Properties
The damping properties of a tennis racket’s frame are extremely important. When a ball hits a racket resonant modes are excited within the frame and strings, and these modes are felt by the player as vibration through the handle. The level of vibration perceived by the player depends on several factors. For instance, the modal response of a racket depends on whereabouts on the racket face the impact occurs. There is an area on the strings known as the `sweet spot’ in which the modal density is low. A ball striking this area excites few modes and the player perceives little vibration. However, if a ball hits outside this region the resulting frame vibration is significantly increased, and the degree to which these vibrations are transmitted to the handle of the racket is then determined by the damping capacÂity of the frame materials.
Tennis Elbow
If too much vibration is transmitted from the handle to the hand and arm of the player the painful condition known as lateral epicondylitis, or `tennis elbow’ often results. Tennis elbow affects around 45% of people who play regularly and is a particular problem for beginners, who often find striking the ball with the sweet spot more difficult. Hitting with areas other than the sweet spot, where the modal density is higher, has the effect of increasing the amount of vibration transmitted to the hand, which tightens its grip on the racket’s handle to compensate and exacerbates the problem.
Due to the low damping capacity of aluminium, players using aluminium rackets in the 1970s experienced a high transmission of vibration to their hands and arms, and the number of reportÂed cases of tennis elbow increased. Both glass and carbon fibre composites exhibit higher damping capacities than aluminium and in the case of epoxy resin matrices the rubber toughÂness modifiers further increase the damping.
“Feel†V Damping
While too much vibration causes tennis elbow, it is important that a player can feel a ball’s impact. Complete elimination of vibration would result in a loss of impact information and a corresponding reduction in the player’s perception of the impact’s characteristics. Damping vibrations may produce a safer racket but may also create a useless one, devoid of any information and `feel: Research in racket design is now attempting to balance the health benefits of damping with the performance of the racket.
Various attempts have been made to control the transmitted vibration, ranging from the attachment of string dampers to cushioned grip tape. While studies have shown that the effect of string dampers is negligible, and that using cushioned grip tape can reduce the transmitted vibration by 50%, none of the measurements have taken account of the effect of the hand and arm on the vibrational response and transmission in the racket.
The damping capacity of the frame is another key factor. The rebound velocity of the ball can be increased, while the mechanical energy transmitted to the racket and the maximum force transferred to the player can be reduced, all by increasing the damping. However, once again, studies have not accounted for the hand arm system.
The impact of a ball on a racket face initiates a sequence of events. The nature of the impact and the material choice of the frame determines the modal response of the racket and the degree of transmission of vibration to the handle. The level of vibration perceived by the player determines the grip force they apply, which in turn determines the level of vibration transmitted to the hand and forearm and the incidence of tennis elbow.
Piezoelectrics - The Future?
The next step in the evolution of the tennis racket may be the inclusion of piezoelectric materials that are capable of controlling the frame vibration. Advances in the technology used in skiing have already led to piezoelectric materials being attached to the surface of skis. At present these materials act in a passive way. The piezoelectric plates have a damping effect by converting the mechanical vibrations into electrical energy that is dissipated through a shunt circuit. A future possibility may be the conversion of the passive configuration to an active form in which the vibrations in the frame are sensed and then cancelled by inverting the electrical signal applied to piezoelectric actuators sited in the handle. It could even be this technology that finally leads to the eradication of tennis elbow.
Advanced Materials bring Fencing into the 20th Century
The sport of fencing is one of only four to have featured at every modern Olympic games. Until recently much of the equipment used had changed relatively little since its inception, but lately technological advances, and challenges to its Olympic status, have led to the adoption of several innovative materials, with varying degrees of success. Swords, masks and clothing have all seen changes, the most recent development being the transparent mask, intended both to improve visibility for the competitor, and make the sport more enjoyable for the spectator.
The Blade
There are three disciplines in modern fencing foil, epee and sabre, each involving a different target area, employing a different weapon, and demanding a different technique. A feature common to all three disciplines is that the blades regularly break, turning a safe piece of sports equipment into a potentially lethal weapon. Analysis of the fracture surfaces reveals the characteristic features of fatigue crack growth, which is understandable given that a successful attacking lunge can cause a blade to bend with a radius of curvature in the region of 200mm. Repeated bending during a blade’s lifetime, coupled with surface damage from impacts with the opponent’s blade, result in the accumulation of damage in the form of microcracks, which initiate fatigue failure. This is a particular problem with cheaper blades made from medium carbon steel.
Blade Materials and Manufacture
These blades are manufactured by quenching and tempering at 300 to 500°C, resulting in a tempered martensitic structure with a yield stress of between 1500 and 1700MNm-2. A more expensive foil material is maraging steel, which has a yield stress in the region of 2000MNm-2 and an increased lifetime. The critical defect size for fast fracture of the maraging steel blades is over four times larger than in standard carbon steel types, which explains the extended lifetime. However, although the lifetime of the maraging blades is longer, they still fail by brittle fracture, resulting in a sharp edge that can penetrate the opponents clothing and cause serious injury.
A variety of other materials have been investigated for the foil, from glass and carbon fibre composites, to dual phase steels that contain fibres of martensite with an interpenetrating austenite phase. This dual phase steel material has demonstrated high strength and exceptional toughness, with Charpy impact tests showing that blade samples are able to absorb 360J impact energy without breaking into two pieces, compared to 10J for conventional medium carbon steel blades. The toughness originates from the inclusion of the austenite phase, which is ductile under the impact conditions and deflects the crack along the length of the blade, requiring re-initiation in an adjacent fibre of martensite if the growth is to continue.
Surprisingly this grade of steel has never become a standard production grade for fencing blades. Two reasons have been cited, one of which is the higher cost, the other the different feel of the blade, which makes it unpopular with fencers. The same explanations apply to glass and carbon fibre composite blades and, despite these investigations into new materials for blades, conventional materials have not yet been replaced.
Face Masks
This is not the case for the protective face masks used in the sport. There has long been a desire within fencing to enable clear vision of the opponent, something that is compromised by the current designs based on a close metal mesh. Although numerous attempts have been made to replace the metal designs over the years, no solution has offered any degree of consistent safety or clarity of vision. The pace of development has quickened in recent years due to threats from the International Olympic Committee to exclude fencing from the games unless it modernised itself. A key point in this demand was the need to make competitors’ faces visible to spectators and television audiences during competition. The IOC’s demand catalysed a development drive that had until that point been very gradual.
Masks for the Future
Many fencing equipment manufacturers have responded to the challenge to produce a clear mask, and one company that has been successful is the UK-based manufacturer Leon Paul. The company has developed a clear mask made from the Lexan grade of polycarbonate (figure 1), a polymer chosen for its combination of excellent optical clarity and high impact resistance. The material is incorporated in two forms, a single 3mm section covered with another layer 0.5mm thick. The thinner, disposable outer layer protects the base layer from environmental stress cracking that could occur on exposure to organic solvent. It also protects the base layer from the effects of stress concentration due to blade damage on the surface of the mask.
Tests at the Italian Fencing Federation laboratory comparing metal and polymer masks have shown that the polymer sections outperformed the conventional metal mesh sections. In drop tests from heights of 55cm with a mass of 2.4kg fastened to a steel spike (section 3 x 3mm square, with a pyramidal point angle of 60°) the metal mesh was penetrated, whereas the Lexan visor was only marked with an impression of the pyramidal point. Tests on complete masks also showed that the Lexan based masks did not deform, while the metal mesh versions did and to an extent deemed sufficient to injure the fencer. Although the Olympic future of fencing is still uncertain the new mask technology will certainly play a major role in preserving the current status of the sport.
Protective Clothing
A third area of fencing to evolve in recent years is the protective clothing. Traditionally made from heavy gauge cotton fabric, the advent of Kevlar led to protective panels being woven into the garments to afford a high degree of localised protection. However, Kevlar suffers from a major disadvantage. It degrades when washed in biological washing powder or exposed to UV light. With due care and attention there is no degradation in protection, but the legal implications of selling such a garment encouraged manufacturers to look for alternatives. Today fencing clothing is made from a blend of cotton and high strength polymer fibres such as ballistic nylon, a combination that affords resistance to penetration of over 800N, and protection that is not localised.
Advanced Materials for Gas Turbine Engines – Fan Blades
Blade Materials
Fan blades for high by pass aero-engines were, for many years, manufactured from solid titanium alloy forgings and were designed with mid span snubbers to control vibration. However, snubbers impeded airflow and reduced aerodynamic efficiency, penalising fuel consumption. Modern designs have deleted the snubber to provide a more aerodynamically efficient aerofoil, and increased the blade chord for mechanical stability, reducing the number of blades by approximately one third. This has been achieved at reduced weight with a hollow construction and an internal core.
For both snubbered and wide chord blades, a conventional fine grain titanium alloy - 6% aluminium and 4% vanadium (Ti6Al4V) is used. It is simple in terms of chemistry, with the aluminium offering strengthening and low density, and the vanadium making hot working of the material easier. It is used for discs and compressor blades up to about 350°C, but excellent superplastic forming and solid state diffusion bonding capabilities make it particularly suitable for the wide chord blade.
The low density core for the hollow design is an integral part of the structure. The two external skins are separated by either honeycomb filler or a superplastically formed corrugation which carries a share of the centrifugal load. Both panel-to-panel and core-to-panel joints must achieve parent material properties to withstand the effects of foreign body impact and fatigue.
For the first generation design the joints are made by a transient liquid phase diffusion bonding process, whereas the second generation employs solid state diffusion bonding in association with superplastic forming of the assembly. The cavity of the bonded construction is inflated at elevated temperatures between contoured metal dies using an inert gas to expand the core and simultaneously develop the blade’s external aerodynamic profile.
The reliability of these wide chord blades has been second to none. The step in technology produced a major competitive advantage and ten years passed before an equivalent design appeared from a manufacturer other than Rolls Royce. This service record was the result of thorough development testing. Fatigue testing in both low and high cycle modes was essential. Groups of blades were repeatedly accelerated to maximum speed in vacuum to establish low cycle endurance, and high cycle fatigue was investigated on a static vibration rig up to the maximum stress levels likely to be encountered in service.
With a large forward facing area, resistance to bird ingestion is required. Ingestion of a number of medium size birds has to be demonstrated by running an engine at take-off power and requiring it to ingest four birds within the space of one second. The engine continued to deliver power, accelerating and decelerating for a total period of thirty minutes to simulate the likely operating procedure following a severe ingestion incident.
In the very unlikely event of a blade mechanical failure, the engine has to be shown to be structurally sound and to contain all the debris, even if the failure occurs at maximum power. Containment in modern engines is achieved with aluminium or titanium casings through which the blade fragments can penetrate, to be caught in external windings of Kevlar.
As an indication of the benefits of materials development and design enhancements, engines incorporating the wide chord blade have fan modules that are approximately 24% lighter and an engine which is 7% lighter (typically the Trent 800 engine as used in the Boeing 777).
The Future
Looking to the future, some believe that carbon composite materials can be used to reduce weight. At present these materials limit the speeds for which the blade can be designed, requiring a greater diameter for a given thrust. It may be that this alternative approach will converge with the hollow design because airworthiness requirements have led to the incorporation of titanium sheathing around a large part of the composite blade with, of course, some weight penalty. The composite can be considered as an alternative core to the titanium honeycomb or corrugation. Rolls-Royce is studying the future possibility of titanium based metal matrix composites with selective reinforcement provided by silicon carbide monofilaments to control blade untwist.
Advanced Materials for Gas Turbine Engines
Developments in advanced materials, more than anything else, have contributed to the spectacular progress in thrust-to-weight ratio of the aero gas turbine. This has been achieved in the main through the substitution of titanium and nickel alloys for steel, fig 1. Aluminium has virtually disappeared from the aero engine, and the future projection illustrates the potential for composites of various types. The aero engine designer requires a much wider range of materials than the airframe designer because the temperature range is large, whereas a civil airframe, even that of Concorde, lies entirely within the capability of aluminium. Materials also supply the enabling technology for equally significant improvements in performance and reliability.
The RB211 and Trent families of engines provide good illustrations of the link between material capabilities and engine performance. Civil engine programmes are becoming the drivers for materials development, replacing the military programmes that were the leaders at the beginning of the gas turbine era. The earlier approach of technology transfer from military to civil is tending to switch direction.
The turbine entry temperatures of modern civil engines are now approaching those of the latest military combat engines, and the longer operational lives expected by airlines place greater demands on materials technology.
Design Parameters
The key design parameters are fan airflow, which is related directly to thrust, particularly at take-off, and the pressure ratio and flow size of the core, which determine the fuel consumption and climb thrust for a given engine size. Take-off thrust is determined by the airflow, with a direct relationship to fan diameter. Increasing physical size places considerable importance on design, not only for low weight but also for structural stiffness.
Core engine size is equally important The power output to drive the fan is determined by core mass flow and combustor temperature rise. Component development provides increased temperature capability, but the physical size of the compressor is not easily changed and mass flow through the core can only be increased by supercharging to higher overall pressure ratios.
Three examples of aerospace components - the fan blade, the rear of the high-pressure compressor and the high-pressure turbine illustrate how materials are responding to the required performance and design parameters. They also highlight the potential of advanced materials such as titanium and nickel alloys, plus the possibilities for composite materials. In the production of larger diameter, low weight fan blades, the contribution of advanced materials is vital, not only in terms of density but also through advanced methods of fabrication. New materials must also be able to withstand the demands for increasing compressor delivery temperature and turbine entry temperature. Specific fuel consumption depends on thermal as well as propulsive efficiency. Thermal efficiency depends in turn on the maximum temperature of the cycle, as with any heat engine. Maximising efficiency within the design compromise on each component is clearly important for fuel consumption.
Advanced Materials for Athletes Prostheses
Background
Competition amongst paralympians is no less fierce than that experienced by their able bodied compatriots, with competitors producing athletic performances that are truly inspiring. The current world record for the 100 metre sprint by an amputee athlete is 11.03 seconds, only about a second slower than the fastest Olympic sprinters. What makes this possible?
Major factors are, of course, the strength, technique and determination of the competitors, but an important part of the equation is the materials technology found within the prosthetic limbs they use. The days of the wooden leg are long gone and, as they have for many other sports, advances in materials technology have revolutionised performance levels in disabled sport.
Design of Prosthetic Limbs
The process of designing a prosthetic limb is a complex one. Consider the case of an athlete with a below knee amputation. The remaining stump is often very tender, and is composed of a variety of tissue types, some of which are pressure sensitive and some of which are pressure tolerant. The prosthetic practitioner fitting the athlete with the limb begins by designing a hard socket that supports the limb under the stump’s pressure tolerant areas. These hard sockets are made from polypropylene or woven carbon fibre composite materials. To provide protection for the pressure sensitive tissue a soft silicone rubber liner is worn over the stump, and together the hard and soft socket combination provides comfortable support for the athlete.
To replace the tibia and fibula of the lower leg a hollow circular bar is attached to the hard socket via a metallic nut and bolt assembly. The bar is made from several different carbon fibre materials, with woven and unidirectional fibres being used in combination with filament wound fibres. Attached to the base of the circular bar is a curved foot section, also made from carbon fibre composites. Its purpose is to act as a spring to aid forward motion.
To understand how this is possible the human walking pattern, or gait cycle, must be considered. Walking is an activity that we seldom think about, but the process can be separated into four distinct stages - heel strike (HS), foot flat (FF), heel off (HO) and toe off (TO), (see figure 1). The point at which the heel contacts the ground is known as the heel strike. At the midpoint of the stride the foot is flat on the ground, and as the stride progresses the heel leaves the ground followed by the toe, the cycle then repeats. At the point of heel off, the foot section begins to bend under the load of the athlete, and in doing so the section stores elastic strain energy. The load on the foot, and the energy stored, reaches a maximum midway between the heel off and toe off stages, beyond this point the stored energy is returned, providing a forward impulse. This spring-like action helps the athlete achieve a more natural gait, which contributes to the faster times for the 100 metre sprint.
Carbon fibre composites are used to manufacture the foot sections partly because the resulting structure exhibits high strength and stiffness together with relatively low mass. These materials are also used because they enable such a high degree of design flexibility. Varying the degree of fibre orientation in the foot varies the bending stiffness, which is fundamentally important given the variation in mass of the competing athletes. The bending stiffness of a given foot can now be tailored to ensure that the loading is elastic and that energy storage is maximised.
The Future
The future of prosthetic limb technology is exciting, and may lie particularly in the area of osteointegration, in which the prosthetic device is fixed directly to the bone via a titanium implant. This technology is not without difficulties. The fact that the prosthetic limb is connected to the bone means that the skin does not form a continuous surface over the area of the limb and bone fixing, increasing the potential threat of infection and subsequent illness. However, if the problem of infection is solved then we could see Paralympic athletes competing alongside their Olympic counterparts.
Adsorption Site of CO on Pt(111) - Density Functional Theory Study To Lead To Better Catalysts and Sensors – Supplier Data By Accelrys
Background
Researchers at the National Institute of Advanced Industrial Science and Technology (AIST), Japan and Accelrys have used MS Modeling’s DMol3 to study the adsorption of CO on the Pt(111) surface.
The study successfully showed the correct site-preference for the CO adsorption site as suggested by experiment.
This finding will enable the design of better catalysts and sensors.
Metal and Carbon Monoxide Interaction
The interaction of CO with metal surfaces has attracted a great deal of interest because it is an important step in many surface and catalytic reactions, such as CO oxidation and hydrogenation.
In particular, the adsorption of CO on Pt surfaces has attracted much attention because of the many potential applications, such as in car exhaust catalysts where it promotes the oxidation of CO to CO2. The heats of adsorption and local bonding geometries of the interaction have been investigated both experimentally and theoretically.
However, in studies of the CO adsorption on Pt(111), theoretical and experimental results differ. Density functional theory (DFT) predicts adsorption at the fcc-hollow site, whereas experiments reveal adsorption occurs at the atop site.
Adsorption Sites
Researchers at the National Institute of Advanced Industrial Science and Technology (AIST), Japan and Accelrys used MS Modeling’s DFT code DMol3 to solve this puzzle, studying the adsorption sites shown in Figure 1.
Figure 1. Side and top views of typical optimized adsorption structures of CO on Pt(111) for the AER-PBE calculation. Numerical values indicate adsorption energies.
Discoveries
Reporting in Chem. Phys. Lett. the researchers discovered:
·        All electron scalar relativistic (AER) calculations are essential to obtain the correct site-preference, atop followed by bridge and then hollow (fcc and hcp)
·        The AER calculations give a deeper Fermi level in good agreement with work function measurements for a Pt surface with all the functionals
·        The deeper Fermi level enables the interaction of the LUMO of CO with the metal substrate to be decreased
·        This effect suppresses backdonation from the metal substrate to the LUMO of CO, hence it destabilizes fcc-site, that, in turn, stabilizes the atop-site.
Dr Orita, a senior research scientist at AIST, said “Since localized d-orbitals were expected to be important in the model studied, we chose to use DMol3 for this work, as it is based on localized basis sets, which is more appropriate than DFT codes based on plane wave basis sets.”
“As DMol3 is good at performing geometry optimization, the code enabled us to perform geometry optimization taking account of all the electrons in a system, even when running on a personal computer. Such a low computational cost of performing fast calculations is essential for the practical analyses of changing calculation parameters systematically, such as core treatment, functional, number of slab layer, k-points, and size of unit cell.”