bluuebunny/crossref_metadata_2025 huggingface dataset:
Active vibration suppression of a flexible structure using smart material and a modular control patch
Authors: Song G , Schmidt S P , Agrawal B N
Published: October 2002

This paper presents experimental results of vibration suppression of a flexible structure using smart materials and a miniaturized digital controller, called the modular control patch (MCP). The MCP employs a TI-C30 digital signal processor and was developed by TRW for the United States Air Force for future space vibration control. In this research, the MCP is used to implement different control algorithms for vibration suppression of a cantilevered aluminium beam. The beam is equipped with smart sensors and actuators, and both are made of piezoceramics. Positive position feedback (PPF) control, strain rate feedback (SRF) control and their combinations were implemented. Experiments found that PPF control is most effective for single-mode vibration suppression, and two PPF filters in parallel are most effective for multimode vibration suppression. Experiments also demonstrated the capacity of smart material being used as sensors and actuators for vibration suppression. The MCP was shown to be capable of implementing various real-time control laws.
A three-dimensional finite element investigation of the bolt stresses in an aero-engine Curvic coupling under a blade release condition
Authors: Richardson I J , Hyde T M , Becker A A , Taylor J W
Published: October 2002

Curvic couplings transmit torque between parallel in-line shafts or turbine/compressor discs. They are used extensively within the aero-engine industry but can also be found in industrial gas turbines. The manufacture of Curvic couplings involves the precision cutting and grinding of mating teeth on the adjacent ends of the components; the assembly is then clamped together with bolts. The use of Curvic couplings in engineering transmission components is relatively widespread, although the publication of work in the open literature is currently limited.

An attempt has been made to investigate the factors influencing the load imparted to a fan Curvic coupling in a large aerospace gas turbine. Owing to the very complicated nature of the current problem, some geometric and especially loading simplifications have been necessary. The results presented provide an initial solution to the problem of the three-dimensional numerical modelling of a fan Curvic coupling during a blade release event. This work is intended to provide data to help make decisions at the design stage and, together with aero-engine fan blade-off tests, will contribute to an improved design of Curvic couplings. The current work has been carried out using the three-dimensional finite element elastic-plastic contact method in an attempt to achieve realistic displacements of the fan Curvic teeth and bolts.
A systematic approach to aircraft crashworthiness and impact surface material models
Authors: Hashemi S M R , Walton A C
Published: October 2002

The underlying objective of this paper is to describe an approach adopted for investigating the crashworthiness of an airliner fuselage. It is based on detailed Finite Element (FE) modelling and supported by a series of static and dynamic tests on a subfloor section. The detailed FE analysis was undertaken to predict the dynamic collapse mechanism of a fuselage section. A parametric analytical study was performed for one of the dynamic tests modelled here to verify the results by taking into consideration the influence of modelling techniques. The structural response of an aircraft during an impact with the ground is highly dependent upon the terrain encountered. Significant structural response difference are likely to be produced when the impact surface is of a soft soil type as against a rigid surface such as concrete. The methodology to develop ground models was based on extending the problem from a simple to a complex structure/ground interaction.
Testing of a novel propulsion system for micro air vehicles
Authors: Cheung W S , Tilston J R
Published: October 2002

The Combustion and Environment Group of the Propulsion and Performance Department at the Defence Evaluation and Research Agency proposed a bipropellant (H2O2 and hydrocarbon fuel) hybrid rocket-turbine engine for micro air vehicle (MAV) propulsion. A prototype bipropellant system was designed, built and tested to assess the full benefits of the rocket part of this type of engine. The operating parameters of the prototype engine were compared with predictions obtained in an earlier feasibility study. A considerable amount of knowledge on the usage and performance of H2O2 has been gained from these experiments. The test results show good agreement with predictions made in the feasibility study. Heat loss from the nozzle and ignition of the fuel are identified to be major problems which must be overcome before the engine can be developed into a viable propulsion system for MAVs.
Optimum wind tunnel regulation
Authors: Whalley R , Ebrahimi M
Published: October 2002

The regulation of a wind tunnel system using minimum control effort is considered. A theory enabling input-output models to be employed in the design process is outlined. The procedure is applied to the regulation of the airspeed within the tunnel working section enabling turbulent conditions to be replicated, using a Dryden wind gust generator. Optimum dynamic control with least control effort is achieved using a simple closed-loop structure. The penalties involved in selecting a non-optimum design are commented on.
Phosphor thermometry in an electron beam physical vapour deposition produced thermal barrier coating doped with dysprosium
Authors: Feist J P , Heyes A L , Nicholls J R
Published: October 2002

In the paper the production and testing of a prototype smart thermal barrier coating (TBC) consisting of yttria-stabilized zirconia (YSZ) doped with dysprosium is described. The coating was produced using the electron beam physical vapour deposition (EBPVD) technique and consisted of a 180 ím thick layer of YSZ doped with 10 per cent dysprosium layed down on an aluminized C263 substrate. The thermoluminescent properties of the coating were investigated with the sample placed in a temperature-controlled furnace and illuminated with UV light from a pulsed laser. Using an intensity ratio technique comparing emission at 455, 482 and 493 nm (the preferred ratio being 455 nm=482 nm), the coating was shown to be suitable for temperature measurement from 534 K to at least 950 K with an uncertainty of better than ±2.6 per cent. The research therefore demonstrates the possibility of producing smart TBCs with both thermal insulating and temperature sensing properties using a production coating technique. Further work is now required to optimize the concentration of the dysprosium dopant for maximum dynamic range and measurement precision and to investigate the thermomechanical properties of the coating.
An optimal approach for generation of time delay algorithms for anti-air missiles
Authors: Nyongesa H O
Published: October 2002

This paper sets out a new approach for generating time before detonation in anti-air missiles equipped with proximity fuses. Conventional algorithms for determining these delay times rely on human effort, knowledge and experience, and are generally deficient. It is demonstrated that, by applying an evolutionary learning process, genetic programming (GP), determination of these times can be automated and made near optimal.
Impact of aerodynamic surface tolerance on aircraft cost driver
Authors: Curran R , Kundu A , Raghunathan S , McFadden R
Published: October 2002

The cost impact of aerodynamic tolerances on the wetted surface of an engine nacelle is considered. Manufacturing tolerance data were obtained from Bombardier Aerospace Shorts, Belfast, while the corresponding costs were calculated according to various recurring elements such as basic and overtime labour, rework, concessions, redeployment, etc., along with amortized non-recurrent costs due to jigs and tooling, machinery, etc. The cost—tolerance relations were modelled statistically so that the influence of tolerance change could be ascertained. It was found that relatively small tolerance relaxations resulted in significantly reduced costs of production but that there was a large variation in the sensitivity, at current levels. The resultant drag penalty was also predicted in conjunction with the cost—tolerance estimation to show that the operational cost effectiveness of the nacelle was increased by 0.33 per cent.
Helicopter manoeuvre gaming simulation and mathematical inverse solution
Authors: Cao Y , Su Y
Published: October 2002

The helicopter has evolved into one of the most powerful flight vehicles for air domination at low level. There are many factors that influence the success of a helicopter in manoeuvre gaming. Among those factors, the helicopter must have the manoeuvrability and agility to gain a favourable firing position first, and then the whole system must be able to achieve first round hits for successful manoeuvre gaming. In an attempt to address these factors, a general analytical model of helicopter manoeuvring flight and the projectile path is established in this paper. Taking helicopter manoeuvre gaming (blue helicopter versus red helicopter) at short range as an example, the inverse solution technique can be described with the following three stages. Firstly, this method calculates the kinematic parameters of the blue helicopter and then the flight attitudes and control law of the blue helicopter. At the final stage, the red helicopter position, projectile path, hit time and the firing position of the turreted gun are calculated.
A practical approach for the indirect prediction of structural fatigue from measured flight parameters
Authors: Azzam H
Published: October 2002

The perceived benefits of permanent usage monitoring equipment in helicopters include savings in the cost of helicopter maintenance and a favourable impact on safety and fleet management. Unlike indirect fatigue monitoring, direct load monitoring relies on a large number of sensors which requires high operational and maintenance costs. In this paper, a learning network has been developed, and during training sessions allowed to learn how to predict fatigue damage indirectly from flight parameters. Each training example consists of instantaneous fatigue damage induced during a small time step, and flight parameters measured during the time step. The instantaneous damage values are evaluated through a new approach called the progressive damage model. By considering the laws of aerodynamics and dynamics, the network combines the flight parameters in such a way that the resulting features can be mapped into fatigue. About 10.5 flying hours of data were used to train the network. After training, the network was blind tested using flight parameters from 5.8 flying hours. The network was found to predict the fatigue damage of two rotating components indirectly from the flight parameters with accuracy better than the accuracy of a strain gauge system with 5 per cent measurement error.
Coupling of sloshing and orbit dynamics-driven reaction force fluctuations in response to impulse with various rotating speeds
Authors: Hung R J , Long Y T , Chi Y M
Published: October 2002

Coupling of sloshing dynamics within a partially filled rotating dewar of superfluid helium II with orbit dynamics is investigated in response to lateral impulse. The study considers such points as how the rotating bubble of superfluid helium II reacts to the impulse in microgravity, how amplitudes of the slosh reaction force and torque fluctuate with growth and decay of disturbances, how these disturbances in fluid systems coupled with orbit dynamics with various rotating speeds differ in frequencies of sloshing modes excited in terms of differences in rotating speeds and how sloshing of orbit dynamics differ with and without a baffle. The numerical computation of sloshing dynamics is based on the non-inertial frame spacecraft-bound coordinate system. Results of the simulations are illustrated.
Design and performance of bonded patch repairs of composite structures
Authors: Soutis C , Hu F Z
Published: October 2002

The compressive behaviour of bonded patch repaired composite laminates is examined. A non-linear stress analysis is performed on a double-lap joint in order to identify critical joint parameters and design an efficient external patch repair. It is found that oversized patches not only increase the structure's weight but also increase the stress concentrations in the repaired region which can cause premature failure. Reducing the patch thickness near the edges of the overlap and increasing the local adhesive thickness decreases the stress concentration in both shear and peel stresses. A three- dimensional finite element analysis is then performed to determine the stresses in the optimum repaired configuration and is used with a stress failure criterion to predict the ultimate failure load. Experimental measurements show that carefully designed bonded patch repairs can recover almost 80 per cent of the undamaged laminate strength.
Reducing cost, time and risk in aero-engine compressor testing
Authors: Backhouse R , Ivey P
Published: October 2002

There is constant pressure to reduce engine development cycles to minimize costs and bring forward the revenue earning part of the engine's life. As cost and time are designed out, the risk of performance shortfall must be minimized by bidding efficiencies from demonstrated technology. This requires a large-scale ongoing advanced development program.

In compressor development, high-speed tests are unavoidable but are expensive; low-speed rigs are easier to instrument and provide a cost-effective filter for the high-speed designs. The large-scale, low- speed compressor at Cranfield is one of only four worldwide. Recent investment in data reduction software has greatly reduced the test/analysis cycle time.
Model-based reasoning for fault diagnosis of twin-spool turbofans
Authors: Baig M F , Sayeed N
Published: October 2002

A simplified rule-based expert system can be of great use to maintenance engineers involved in on-line health monitoring of aero-engines. The development of a rule base has been done on the following lines:

1 A design and off-design mathematical model of the turbofan has been developed. 2 Healthy and fault-implanted engines have been run at various off-design conditions to see the effect of faults on certain aero-thermodynamic performance parameters, with appropriate selection of independent parameter(s). 3 From these calculated values, fault matrices have been developed for sea-level static conditions taking the net thrust as an independent parameter. 4 From these fault matrices, rules have been developed which form the knowledge core of the expert shell.

These rules have been developed for Garrett TFE 731-2, a moderate bypass and overall pressure ratio, generic twin-spool turbofan and so the package (off-design code plus expert system) can serve as a pedagogical tool for training of engineers in the aero-engine industry and academic institutions.
Non-linear multivariable flight control of aircraft
Authors: Lyshevski S E
Published: October 2002

Innovative design methods are needed for advanced aircraft in response to requirements towards substantial performance improvements. Functionally and operationally, the aircraft must be considered as the highly coupled non-linear multi-input multi-output system, i.e. the aerodynamics have to be mapped by non-linear differential or difference equations. To improve flying and handling qualities, to increase manœuvrability and to expand the operating envelope, an innovative optimization procedure is developed to design the constrained controllers for multi-input multi-output aircraft. In particular, a bounded control law is synthesized by employing the Hamilton-Jacobi theory, and the admissibility concept is used to study the stability of the resulting closed-loop system. The developed optimization procedure is applied to a non-linear ninth-order model of an AFTI/F-16 aircraft. A bounded controller is designed, and modelling results are presented to demonstrate the dynamic performance of the resulting closed-loop system.
Design for success
Authors: Hockley C J
Published: October 2002

The idea of specifying reliability and maintainability (R&M) requirements in ways that accept the notion of random failure (i.e. MTBF) has outlived its usefulness. Design engineers do not design random failures and faults into products yet they occur and play havoc with the operational availability of military equipment. New ways must be found to deliver both mission reliability and guaranteed availability at the front line. Under the banner ‘Designing for Success’, the RAF is proposing a paradigm shift in the delivery of more certain and higher levels of R&M and these requirements are now progressively being specified for new equipments where appropriate. Equipment being procured under the philosophy of Integrated Logistic Support (ILS) will allow significant trade-offs between availability, maintainability and supportability. Yet it is the design process using the latest technology and concepts such as Systems Engineering which will give the ability to deliver guaranteed maintenance-free operating periods (MFOPs) for systems and platforms. Through parallel initiatives such as the Society of British Aerospace Companies (SBAC) Foresight Action Programme, industry has embraced the idea of a paradigm shift in reliability improvement under a programme called the Ultra Reliable Aircraft (URA). The Ministry of Defence (MoD) fully supports this initiative through specific activities as it will be a prime customer for its outputs.
Onset of petalling in a thin spacecraft wall perforated by an orbital debris particle
Authors: Schonberg W P
Published: October 2002

All spacecraft in low earth orbit are subject to hypervelocity impacts by meteoroids and orbital debris. Recent studies have shown that the nature of a spacecraft wall perforation can range from a flat hole with a jagged edge (i.e. a so-called cookie-cutter hole) to a hole accompanied by bulging, cracking and petalling. If a cracking event were to occur on-orbit, unstable crack growth could develop, which could lead to an unzipping of an impacted pressurized spacecraft module. It is therefore imperative to be able to determine whether or not a spacecraft wall perforation will be accompanied by either petalling or cracking. This paper presents the results of a study whose objective was to develop an empirical model that could be used to determine whether a spacecraft wall perforation would be in the form of a petalled hole or a cookie-cutter hole. The model consists of a petalling limit function that predicts the onset of petalling in terms of impact conditions and spacecraft wall system geometry. The model is used to show that the perforation of a spacecraft wall with an enhanced shield will always be accompanied by petalling and cracking, while the perforations of a Whipple-type spacecraft wall may or may not be petalled and cracked, depending on impact conditions.
Historical rotorcraft restoration: The C-30 Autogiro
Authors: López-Díez J , Cuerno-Rejado C , López-Ruiz J L
Published: October 2002

Seventy-five years ago, on 17 January 1923, the first successful flight of a rotary wing aircraft took place. The first gyroplane, patented as Autogiro, was invented by Juan de la Cierva Codorniú. For this reason, the Juan de la Cierva Foundation in Spain has promoted a project to build a replica, authorized to fly, of the Cierva's C-30, the most famous of his Autogiros. It is planned to fly the replica only in exhibitions. In this paper an historical review of the works of Juan de la Cierva is presented and the C-30 restoration is illustrated and described.
The effect of grid topology and density on inviscid hovering rotor solutions
Authors: Allen C B
Published: October 2002

Grid dependence is examined for Euler simulations of multibladed rotors in hover. The numerical simulation of rotor flows poses a unique problem for flow solvers. The solution is extremely sensitive to the accuracy of capture of the vortical wake over several turns; this means that a much finer grid density is required away from the blade than for a fixed-wing case, resulting in excessive run-times. An attempt is made to determine the number of grid points required to obtain practical results, by performing grid convergence tests for O-H and O-C grid topologies and determining the optimum ratio of grid densities in each parametric direction, for two- and four-bladed test cases. An upwind Euler solver is used on O-H and O-C structured grids, generated by transfinite interpolation along with a periodic transformation. It is shown that O-H grids produce more accurate solutions than O-C grids with the same number of points, and that 3 3 105 grid points produce inviscid solutions of sufficient accuracy for initial design. It is also shown that solutions can be improved by choosing optimum ratios of grid density in each parametric direction.
Statistical discrete gust analysis of tower-based atmospheric turbulence data for aircraft response studies
Authors: Anandakumar K , Kailas S V
Published: October 2002

Turbulence modelling for aircraft response studies requires adequate representation of atmospheric turbulence to provide for realistic reproduction of turbulence loads on an airframe. The power spectral density method, the traditional tool based on Fourier analysis that has been extensively used, does not account for coherent structures or the gusts so predominant in atmospheric turbulence. The statistical discrete gust (SDG) model was developed to overcome this problem, and the coherent structures are introduced in the form of discrete ramp-gusts. The model defines the associated probability distributions in terms of the amplitude and scale of such discrete gusts with refinements to take care of deviation of real turbulence from self-similar behaviour and scale-dependent intermittency, and provides a more realistic basis for predicting aircraft response to atmospheric turbulence. In the present work, the possibility of using tower-derived atmospheric turbulence data, instead of the commonly used aircraft measured data, for SDG analysis is investigated. The discrete gusts in the data have been detected using two methods, namely the wavelet transform method and the smoothing-differencing method, and the associated amplitudes and scales are extracted. The SDG analysis carried out by varying both the similarity parameter and the fractal dimension of turbulence to account for, respectively, the deviation of atmospheric turbulence from self-similarity and scale-dependent intermittency shows that tower-based data are highly useful to determine the model parameters. The values of the similarity parameter estimated from the data (from both the power spectra and wavelet transform) indicate the need for the incorporation of deviation of atmospheric turbulence from self-similarity and scale-dependent intermittency in the SDG model. The intermittency and intensity parameters involved in the SDG model are estimated from the tower data and show good comparison with those derived from aircraft measured data. The variation of the parameters with height and time is discussed and the relation between the intensity of fluctuations in the data and their intermittency with the corresponding model parameters is elucidated.
Dynamic behaviour of open-loop systems of intelligent structures
Authors: Chen S , Cao Z , Wen B
Published: October 2002

A finite element formulation for a bending plate element with piezoelectric sensors and actuators (S/As) is presented by using Hamilton's principle. This element not only saves much memory and calculation time but also increases the accuracy of solutions. Using this method, the natural frequencies of the open-loop systems are studied and the influence of variations in structural parameters on eigenvalues and eigenvectors of structures is analysed in detail. Finally, two numerical examples are presented. The first is to illustrate the validation of the present method. The second is to study the problem of natural frequencies of the structures with S/As. The influence of the number and location of S/As is analysed and the mode shapes and associated voltage distributions are obtained.
Thermal sensitivity analysis of a 22 N bipropellant thruster in a spacecraft
Authors: Suresha S , Katti R A
Published: October 2002

Transient thermal sensitivities of a 22 N bipropellant thruster in a spacecraft environment are studied in the on-orbit non-firing, firing and soak-back modes. The system has conductive and radiative modes of heat transfer and also for some time heat dissipation on valves. Temperatures of valves of the thruster assembly are analysed for their sensitivities to some of the design parameters, such as conductance between the thruster flange/mounting bracket, thruster flange/combustion chamber and thruster flange/ valve and conductance of the multilayer insulation blanket. Similarly, the influence of optical properties such as absorptance and emittance of the combustion chamber and heat shield are investigated. Studies relating to the heater power on the valves are also conducted. The lumped parameter approach is adopted and the governing equations are solved using a numerical technique. It was demonstrated that flange/ bracket conductance is the controlling parameter of the valve temperature during the sunlit period of the non-firing mode. In the soak-back condition, conductance across the flange and the valve dictates the valve temperature.
The phenomenon of helicopter rotor blade sailing
Authors: Newman S
Published: October 2002

Blade sailing is an aeroelastic phenomenon affecting helicopter rotors when rotating at low speeds in high wind conditions. This is a potentially dangerous blade motion and the excessive flapwise tip deflections generated endanger the airframe, the flight crew and any personnel working close to the aircraft. This phenomenon is particularly applicable to naval helicopters or those operating off exposed sights such as oil rigs. The research covered an experimental investigation into the effect of an abeam wind flow over a simulated flight deck of a Rover Class Royal Fleet Auxiliary vessel. Blade flexibility and rotor hub mechanical features were introduced into the theory and the resulting method applied to the Westland Lynx and Sea King aircraft. The semi-rigid rotor of the Lynx is relatively well controlled, but the rotor hub construction of the articulated rotor of the Sea King and the interaction with the flexing blades allow blade tip deflections to be generated of an order to strike the fuselage.

The research described in the paper has led to an improved understanding of the blade sailing phenomenon. The results concur with reports of blade sailing occurrences and the operational conditions that are likely to trigger this phenomenon. The development of theoretical models may be used to assess the importance or otherwise of this effect on future rotor designs or the operational limitations of the aircraft.
State-space multivariable non-linear identification and control of aircraft
Authors: Lyshevski S E
Published: October 2002

In the design of commercial and military aircraft, flying and handling qualities, as well as performance (agility and manoeuvrability, controllability and stability) and safety, have received increased emphasis. The aircraft must exhibit acceptable handling qualities while performing manoeuvres, and different flight management systems have been designed. For advanced aircraft, the assumption that longitudinal and lateral dynamics can be decoupled leads to poor flying and handling characteristics owing to longitudinal-lateral cross-coupling. Aircraft stability severely degraded at low speed, high-angle-of-attack flight and gain-scheduled linear control algorithms cannot provide the desired performance (gain-scheduled control laws are used because they are certifiable). These have resulted in the critical need for significant improvements in flying and handling qualities as well as expansion of the flight envelope at low-speed, high-angle-of-attack flight by using robust non-linear controllers. Military aircraft must be capable of performing a diverse range of operational missions, which include air-to-ground and air-to-air combat. Mission effectiveness relies on flight management systems and control algorithms. Control system effectiveness can be achieved as the designer integrates non-linear flight dynamics and advanced optimization methods to design multivariable control algorithms. In this paper a study is made of non-linear identification and control methods which can be applied to multivariable aircraft using measured variables. By performing non-linear modelling, it is shown that the methods reported lead to very encouraging results.
Design forms of total knee replacement
Authors: Walker P S , Sathasivam S
Published: October 2002

The starting point of this article is a general design criterion applicable to all types of total knee replacement. This criterion is then expanded upon to provide more specifics of the required kinematics, and the forces which the total knee must sustain. A characteristic which differentiates total knees is the amount of constraint which is required, and whether the constraint is translational or rotational. The different forms of total knee replacement are described in terms of these constraints, starting with the least constrained unicompartments to the almost fully constrained fixed and rotating hinges. Much attention is given to the range of designs in between these two extreme types, because they constitute by far the largest in usage. This category includes condylar replacements where the cruciate ligaments are preserved or resected, posterior cruciate substituting designs and mobile bearing knees. A new term, ‘guided motion knees’, is applied to the growing number of designs which control the kinematics by the use of intercondylar cams or specially shaped and even additional bearing surfaces. The final section deals with the selection of an appropriate design of total knee for specific indications based on the design characteristics.
Evaluation of four blood pump geometries: The optical tracer technique
Authors: Rose M L J , Mackay T G , Martin W , Wheatley D J
Published: October 2002

Artificial blood pump assistance of the failing human heart can allow it to recover. Analysis of blood pump fluid flow is a useful tool for design development and thrombosis minimization. The aim of this study was to investigate fluid flow, particularly ventricular clearance rate and stagnation areas, in four different blood pump geometries and to determine the best design. The blood pumps consisted of a polyurethane ventricle, and combinations of inlet/outlet pipe angles and compression plate shapes. A video camera recorded the motion of fluid labelled with an optical tracer (Methyl Blue histological dye). A novel processing method was developed to produce colour maps of tracer concentration, experimentally calibrated. An overall picture of fluid flow in each pump geometry was generated by considering clearance curves, tracer concentration maps and inflow jet animations. Overall and local mixing coefficients are calculated for each pump. The best geometry featured straight inlet/outlet pipes and a domed compression plate.

This optical tracer technique has proven convenient, economical, sensitive to low concentrations of tracer and provides instantaneous pictures of tracer distribution in a ventricle.
Design of an occipito-cervical fixation device
Authors: Shepherd D E T , Bolger C M , Leahy J C , Mathias K J , Hukins D W L
Published: October 2002

Metal plates may be used to stabilize the cervical spine. The plates are attached to the posterior of the vertebra by placing screws into the lateral masses. The plating may be extended, in the form of rod or plate, to connect with and support the occiput. Several problems, such as screw loosening and the plate obscuring the surgeon's view as a screw is being inserted, have been identified with present plate systems. This paper describes the initial design for a cervical fixation device to overcome these problems, and the design and development that was undertaken to enable a prototype device to be manufactured.
The relationship between surface topography and contact in the elbow joint: Development of a two-dimensional geometrical model in the coronal plane
Authors: Waide D V , Lawlor G J , McCormack B A O , Carr A J
Published: October 2002

One of the first stages in developing an accurate biomechanical representation of the elbow joint is to model realistically the geometry of the joint. In particular, given the complex anatomy of the articular surfaces, the relationship between surface topography and joint contact must be fully understood in order to model the contact conditions. As the joint articulates, the location and size of the contacts between the mating surfaces change, altering the distribution and magnitude of load transmission.

In this paper, a geometric model of the anatomical elbow joint in the coronal plane is described. The contours of the articulating surfaces are represented algebraically by a series of connecting lines and circular arcs. It is shown that the location and size of the contact between the surfaces change significantly due to small changes in the topography of one or more of the mating surfaces. The surface topography-joint contact relationship is modelled for a number of different clinical conditions for the joint. The model is relevant to clinical studies of joint degeneration and to the design of prosthetic components for the elbow joint.
External ring fixators: An overview
Authors: Watson M A , Mathias K J , Maffulli N
Published: October 2002

External fixation is widely used in the fixation of fractures and limb deformities. The mechanical characteristics of a specific external fixator are major factors in determining the biomechanical environment at a fracture/osteotomy site and, hence, affect the healing process. Although the optimal biomechanical environment for healing of a fracture or an osteotomy is unknown, a specific range of interfragmentary motion exists which promotes healing. It is therefore desirable that the mechanics of an external fixator can be manipulated to enable the surgeon to control the range of interfragmentary motion. The characteristics of an external fixator are defined by a large number of variables. Therefore, to gain control over the degree of interfragmentary motion, an understanding of the effect of each variable and how it interacts with the others to determine the overall characteristics of the device is required. For the past two decades, individual components and whole-frame configurations have been studied in depth. This article provides a summary of previous work concerning the mechanics of external ring fixators and how they affect the biomechanical environment at the fracture/osteotomy site.
Influence of gelatin and bovine serum lubricants on ultra-high molecular weight polyethylene wear debris generated in in vitro simulations
Authors: Bell J , Besong A A , Tipper J L , Ingham E , Wroblewski B M , Stone M H , Fisher J
Published: October 2002

Ultra-high molecular weight polyethylene (UHMWPE) wear debris induced osteolysis has a major role in the late aseptic loosening and ultimate failure of total hip replacements (THR). Clinically relevant in vitro simulations of wear are essential to predict the osteolytic potential of bearing surfaces in artificial hip joints. Newborn calf or bovine serum has been accepted as a boundary lubricant for such in vitro tests, but its biological stability has been questioned. This study compared the wear factors, number of wear particles and levels of microbial contamination produced in bovine serum and a gelatin-based lubricant. The wear factors produced by the two lubricants were not significantly different, however the wear debris morphology produced was substantially different. The bovine serum became contaminated with micro-organisms within 28 h, whereas the protein-based lubricant remained uncontaminated.

The results showed that bovine serum was not a stable boundary lubricant. They also showed that although the wear factors for the two solutions were not significantly different, the protein-based lubricant was not a suitable alternative to bovine serum because the wear debris produced was not clinically relevant.
Assessment of the non-linear behaviour of plastic ankle foot orthoses by the finite element method
Authors: Syngellakis S , Arnold M A , Rassoulian H
Published: October 2002

The stiffness characteristics of plastic ankle foot orthoses (AFOs) are studied through finite element modelling and stress analysis. Particular attention is given to the modelling and prediction of non-linear AFO behaviour, which has been frequently observed in previous experimental studies but not fully addressed analytically. Both large deformation effects and material non-linearity are included in the formulation and their individual influence on results assessed. The finite element program is subsequently applied to the simulation of a series of tests designed to investigate the relation between AFO trimline location and stiffness for moderate and large rotations. Through careful consideration and identification of key modelling parameters, the developed finite element solution proves to be a reliable and effective alternative means of assessing variations of a typical plastic AFO design so that particular patient requirements could be met, in the long term.
Analysis and modelling of wear of cobalt-chrome alloys in a pin-on-plate test for a metal-on-metal total hip replacement
Authors: Jin Z M , Firkins P , Farrar R , Fisher J
Published: October 2002

The complex interaction between wear and bearing surfaces of two contacting solids was investigated in this study, with particular reference to the use of metal-on-metal material combinations for artificial hip joint replacements. The contact mechanics model was coupled with the wear model and solved simultaneously as a function of time for a simple case of a uniaxial pin-on-plate wear test. Both a spherical pin and a flat-ended spherical pin were considered. It was shown that the contact pressure between the pin and the plate was substantially reduced by the wear process, particularly during the initial running-in period and for the spherical pin. The theoretical prediction of the worn profiles of the pin and the plate was found to be in good agreement with previous experimental measurements by Tipper et al. in 1999.
A moment-based technique for the automatic spatial alignment of plantar pressure data
Authors: Harrison A J , Hillard P J
Published: October 2002

Plantar pressure measurements have many potential clinical applications in the field of orthopaedics. However, the only current routine clinical application of pedobarography is for the identification of high pressure zones in diabetic patients with neuropathy who are at risk from foot ulceration. For pedobarography to become a routine clinical tool in orthopaedics, a thorough knowledge of normal plantar pressure distributions would be required. To date, no large scale studies to define population norms have been conducted, probably because of the high cost associated with analysing large quantities of plantar pressure data. Plantar pressure distributions display a high step-to-step variation and, therefore, it is necessary to conduct multiple trials. Comparison between these trials is difficult, somewhat subjective, and slow, because it must be conducted manually. The present authors have developed a technique to align automatically the spatial components of a series of datasets. The technique then allows predefined parameters to be automatically extracted from the data, hence greatly reducing analysis time. The authors believe that not only does the technique make large scale studies economically viable but that it could also form the basis for expert systems for the diagnosis of foot pathologies and the prescription of orthoses.
Limitations in the use of median frequency for lung sound analysis
Authors: Whittaker A R , Lucas M , Carter R , Anderson K
Published: October 2002

The aim of this paper is to investigate methods of standardizing lung sound analysis, with a view to supplementing traditional spirometric air flow measurements to help in the diagnosis of asthma and to provide a measure of the effectiveness of treatment. Lung sounds were measured in nine patients with asthma and five control subjects, alongside air flow measurements of forced expiratory volume (FEV1) and forced vital capacity (FVC). The patients were administered the broncho-dilator, salbutamol, to assess how effective these measurement techniques were for quantifying its effect. The results agree with previous studies, that analysis of lung sounds is a potentially useful tool for indicating air flow changes. The results, however, also demonstrate that the emerging standard of ‘F50’ or ‘median frequency’ should be treated with great caution because of its high sensitivity to the measurement frequency range. F50 is very unlikely to provide a reliable single indicator of lung condition.
The variation in the orientations and moment arms of the knee extensor and flexor muscle tendons with increasing muscle force: A mathematical analysis
Authors: Imran A , Huss R A , Holstein H , O'Connor J J
Published: October 2002

The orientations and moment arms of the knee extensor and flexor muscle tendons are evaluated with increasing values of muscle force during simulated isometric exercises. A four-bar linkage model of the knee in the sagittal plane was used to define the motion of the joint in the unloaded state during 0–120° flexion. The cruciate and collateral ligaments were represented by arrays of elastic fibres, which were recruited sequentially under load or remained buckled when slack. A bi-articular model of the patello-femoral joint was used. Simple straight-line representation was used for the lines of action of the forces transmitted by the model muscle tendons. The effects of tissue deformation with increasing muscle force were considered.

During quadriceps contraction resisted by an external flexing load, the maximum change in moment arm of the patellar tendon was found to be 2 per cent at 0° flexion when the quadriceps force was increased tenfold, from 250 to 2500 N. The corresponding maximum change in orientation of the tendon was 3° at 120° flexion.

During hamstrings contraction resisted by an external extending load, the maximum change in moment arm of the hamstrings tendon was 8 per cent at 60° flexion when the hamstrings force was increased tenfold, from 100 to 1000 N. During gastrocnemious contraction, the corresponding maximum change for the gastrocnemious tendon was 3 per cent at 0°. The orientations of the flexor muscle tendons in this range of force either remained constant or changed by 1° or less at any flexion angle. The general trend at any flexion angle was that, as the muscle force was increased, the moment arms and the orientations approached nearly constant values, showing asymptotic behaviour.

It is concluded that experimental simulations of knee muscle action with low values of the externally applied load, of the order of 50 N, can provide reliable estimates of the relationships between muscle forces and external loads during activity.
The effect of kinematic conditions on the wear of ultra-high molecular weight polyethylene (UHMWPE) in orthopaedic bearing applications
Authors: Cornwall G B , Bryant J T , Hansson C M
Published: October 2002

It is known that wear mechanisms differ between the ultra-high molecular weight polyethylene (UHMWPE) components of total hip replacement (THR) and total knee replacement (TKR). The difference in relative contact position or ‘kinematic conditions of contact’ between the metal and polymer components is thought to contribute to the contrast in observed wear mechanisms.

A reciprocating wear tester was used to evaluate three basic kinematic contact conditions: sliding, in which the relative contact position on the polymer remains stationary; gliding, where the contact position on the polymer reciprocates; and rolling, where the contact position on the polymer varies and the relative velocities of both components are equal.

All static load tests used cast Co—Cr alloy and irradiated Chirulen® UHMWPE in a 37 °C environment lubricated with bovine serum albumin. UHMWPE test sample wear was measured gravi-metrically at intervals of 600000 cycles. The results indicated a difference in wear factor (volume lost due to wear per unit load per unit sliding distance) between the three groups with varying relative motion. The study indicates that screening tests which evaluate wear properties of new materials for total joint replacement should reflect the different kinematic contact conditions.
Effect of contact stress on friction and wear of ultra-high molecular weight polyethylene in total hip replacement
Authors: Wang A , Essner A , Klein R
Published: October 2002

This paper studies the effect of contact stress on friction and wear of ultra-high molecular weight polyethylene (UHMWPE) acetabular cups by means of friction and wear joint simulator testing under serum lubrication. For a given applied load, increasing the contact stress by increasing the ball/socket radial clearance decreased both the coefficient of friction and the wear rate. Friction and wear were highly correlated. The dependence of friction on contact stress for the UHMWPE socket under serum lubrication was similar to that of semi-crystalline polymers under dry sliding. This finding indicates the occurrence of partial dry contact at asperity levels for the metal-polyethylene ball-in-socket joint under serum lubrication.
Elastohydrodynamic lubrication analysis of ultra-high molecular weight polyethylene hip joint replacements under squeeze-film motion
Authors: Jagatia M , Jalali-Vahid D , Jin Z M
Published: October 2002

Elastohydrodynamic lubrication was analysed under squeeze-film or normal approach motion for artificial hip joint replacements consisting of an ultra-high molecular weight polyethylene (UHMWPE) acetabular cup and a metallic or ceramic femoral head. A simple ball-in-socket configuration was adopted to represent the hip prosthesis for the lubrication analysis. Both the Reynolds equation and the elasticity equations were solved simultaneously for the lubricant film thickness and hydrodynamic pressure distribution as a function of the squeeze-film time was solved using the Newton-Raphson method. The elastic deformation of the UHMWPE cup was calculated by both the finite element method and a simple equation based upon the constrained column model. Good agreement of the predicted film thickness and pressure distribution was found between these two methods. A simple analytical method based upon the Grubin -Ertel-type approximation developed by Higginson in 1978 [1] was also applied to the present squeeze-film lubrication problem. The predicted squeeze-film thickness from this simple method was found to be remarkably close to that from the full numerical solution. The main design parameters were the femoral head radius, the radial clearance between the femoral head and the acetabular cup, and the thickness and elastic modulus for the UHMWPE cup; the effects of these parameters on the squeeze-film thickness generated in current hip prostheses were investigated.