bluuebunny/crossref_metadata_2025_split huggingface dataset:
Determination of superplastic constitutive equations and strain rate sensitivities for aerospace alloys
Authors: Kim T W , Dunne F P E 
Published: October 2002

Constitutive equations for the superplastic behaviour of titanium and aluminium aerospace alloys have been presented and computational procedures developed for the determination of the material parameters arising in the equations. The equations include a description of grain growth kinetics with deformation enabling the influence of microstructure on superplastic deformation behaviour to be modelled. Three predictive methods have been developed for the determination of strain rate sensitivity. The models represent the dependence of the strain rate sensitivity on strain, strain rate, and grain growth, and therefore, provide useful information for process design and optimization. The models are validated by comparison of predictions with experimental data reported in the literature and obtained in this work. Finally, two conventional experimental testing methods for the determination of strain rate sensitivity, namely the constant strain rate and strain rate jump methods are appraised using the methods developed.
Risk aspects of a mission to a comet
Authors: Turner R F 
Published: October 2002

Space science missions are rarely a repeat of a predecessor and thus in the wider view they do not have the benefit of heritage-related statistics. This paper discusses the cost and risk factors in gaining access to space and describes the challenges of the forthcoming Rosetta mission, which has the objective of conducting studies of a comet and its increasing activity as it moves towards the sun. It will fly in formation with the nucleus and later send a lander to its surface to conduct in situ measurements. Reference will be made to the further challenges for the scientific instruments faced by an eight-year period of inactivity during the cruise phase after launch. Statistics have a part to play at the component and unit test levels but are increasingly less relevant at instrument, spacecraft and mission levels. Ultimately the designer is faced with a string of single-point failure possibilities, commencing with the launch vehicle and culminating with the unknown surface conditions that will greet the lander.
Essentially non-oscillatory schemes for the Euler equations on unstructured meshes
Authors: Zamecnik M , Hernández J A 
Published: October 2002

A numerical scheme to solve the two-dimensional Euler equations around aerodynamic profiles is presented. The space domain is discretized by an unstructured mesh where triangular elements are used. The scheme uses a first- and a second-order finite volume method where evolution equations for the mean values are obtained at each cell. The first-order approximation is obtained by a constant approximation at each cell while the second-order approximation is obtained by a linear ENO (essentially non-oscillatory) polynomial. Fluxes at cell interfaces are determined using an approximate solver for the Riemann problem. The solution of the Riemann problem introduces the upwind characteristic of the scheme. In this paper, particular emphasis is placed on the determination of fluxes at every cell interface, obtaining a compact expression for the resolution on unstructured meshes. The Burgers equation is analysed as a typical scalar case, and it is shown how the vectorial Riemann problem in unstructured meshes can be reduced to four scalar problems along four directions of propagation. This paper also shows how the use of a preconditioned technique based on local time step improves the convergence to the steady state solution. An anomalous behaviour in the convergence of the residuals in a second-order scheme is observed and it is related to the shock structure and the spatial approximation. Transonic and subsonic test cases of aerodynamic profiles are simulated as an application to advanced aerodynamic design.
A non-linear adaptive guidance algorithm for last-stage launcher control
Authors: Teofilatto P , De Pasquale E 
Published: October 2002

On-line adaptive guidance algorithms are currently difficult to implement since algorithms involving trial and error are needed to solve the two-point boundary value problems related to the optimization process. These routines may require an unpredictable number of iterations to converge to a solution; moreover convergence itself cannot be assured a priori. This problem can be circumvented by linearizing the control strategy about the nominal trajectory. However, since the system to be controlled is intrinsically non-linear, the linearization may have a limited accuracy. In this paper an adaptive guidance method is presented which drives the launcher to the target in an optimal way even if the deviations from the nominal trajectory are so large that the variations of the guidance parameter become non-linear. The adaptive guidance proposed avoids iterative algorithms or convergence routines, shows good accuracy and a computational time suitable for current on-board computers.
Boundary lubrication in vivo
Authors: Hills B A 
Published: October 2002

Evidence is reviewed for the concept that the body employs essentially the same lubrication system in many sites in the body where tissues slide over each other with such ease. This system consists of fluid adjacent to surfaces coated with an oligolamellar lining of surface-active phospholipid (SAPL) acting as a back-up boundary lubricant wherever the fluid film fails to support the load—a likely event at physiological velocities. Particular attention is paid to the load-bearing joints, where the issue of identifying the vital active ingredient in synovial fluid is reviewed, coming down—perhaps predictably—in favour of SAPL. It is also explained how Lubricin and hyaluronic acid (HA) could have ‘carrier’ functions for the highly insoluble SAPL, while HA has good wetting properties needed to promote hydrodynamic lubrication of a very hydrophobic articular surface by an aqueous fluid wherever the load permits. In addition to friction and wear, release is included as another major role of boundary lubricants, especially relevant in environments where proteins are found, many having adhesive properties. The discussion is extended to a mention of the lubrication of prosthetic implants and to disease states where a deficiency of boundary lubricant is implicated, particular attention being paid to osteoarthritis.
Investigation into the tribological condition of acetabular tissue after bipolar joint replacement hip surgery
Authors: Pickard J , Fisher J , Ingham E , Egan J , Hallett J 
Published: October 2002

The aim of this research was to investigate the tribological condition of acetabular tissue before and after bipolar hip surgery. Articular cartilage was taken from the femoral head of patients undergoing primary joint replacement as a control. Tissue was also taken from the acetabular cups of patients undergoing revision hip surgery after primary bipolar surgery and compared with the control cartilage. The biomechanical characteristics of the two tissue types were tested using friction and compression tests. The friction tests were carried out on a sliding friction rig under nominal contact stresses of 0.5 and 4 MPa. The compression tests were carried out under a 0.8 MPa contact stress. The majority of the bipolar patients produced friction coefficients that were significantly higher than those produced by the control group, and the compression tests highlighted that the tissue from the bipolar patients produced a much greater rate of increase in displacement compared with the control cartilage. Histology showed major differences between the control cartilage and the bipolar tissue. The control cartilage showed a healthy collagen structure with a good distribution of proteoglycan whereas the majority of the bipolar tissue had lost tissue architecture and had a sparse fibrous structure. The high friction coefficients with the bipolar tissue imply that the frictional torque at the outer head of the bipolar prosthesis would be large compared with the inner bearing frictional torque. It was therefore predicted that the motion of the bipolar prosthesis should occur at the inner bearing.
A method for the evaluation of the change in volume of retrieved acetabular cups
Authors: Raimondi M T , Sassi R , Pietrabissa R 
Published: October 2002

Studies on retrieved hip prostheses are currently performed in order to assess the wear mechanisms and the overall wear rate of such artificial joints. Several reported studies on the survival of artificial hips have been based on the measurement of the amount of worn material directly on retrieved acetabular cups. The estimation of the change in volume, V, of the cup cavity is particularly difficult in the case of slight wear due to several factors of which the most critical is the lack of information on the unworn geometry of the cup. This paper presents a new measuring technique, which is described in detail and has been applied to estimate the wear of 65 acetabular cups harvested from revised hip arthroplasty. The coordinate data of several points on the articular surface are sampled using a coordinate measuring machine (CMM). The value of V is calculated mathematically from the measurements by using the hypothesis that the actual shape of both slightly worn and highly worn surfaces has a small departure from a truly spherical shape. The wear volume is estimated with reasonable accuracy mainly on those cups showing penetration depths greater than 0.2 mm, corresponding to an amount of wear greater than 100 mm3 for a 32 mm cup. The uncertainty in the results is estimated for each cup. The repeatability of the technique is studied for a case showing very slight wear. The advantages, disadvantages and limitations of the method are presented and discussed.
Friction in orthopaedic zirconia taper assemblies
Authors: Macdonald W , Aspenberg A , Jacobsson C M , Carlsson L V 
Published: October 2002

The torque resistance of zirconia ceramic heads/titanium taper trunnion junctions was tested in accordance with ISO 7206-9:1994(E); using twelve modified heads of 32 mm diameter under representative physiological conditions. Test parameters studied included assembly force, vertical load during test (test load) and head length. Mean torque resistances measured were 8.9 N m for a 1 kN test load and 15 N m at 4 kN test load. Coefficients of friction calculated for the torsional stability ranged from 0.06 to greater than 1.0. Multiple regression analysis confirmed that the failure torques measured were significantly dependent on test load (β = 0.77; P < 0.001) whereas assembly force and head length played a lesser, insignificant, part in the variation. Data from push-on/pull-off tests were used to calculate coefficients of friction under axial loading, which were significantly correlated with taper angle and material. Torque testing shows greater variability than push-on/pull-off tests for similar combinations, and for zirconia heads on other tapers. The coefficients of friction measured (0.16-0.31) are significantly different from values typically used in stress analyses.
Simplified motion and loading compared to physiological motion and loading in a hip joint simulator
Authors: Smith S L , Unsworth A 
Published: October 2002

Two wear tests were conducted using the Durham Hip Joint Wear Simulator to investigate the effects of simplified motion and loading on ultra-high molecular weight polyethylene (UHMWPE) acetabular cup wear rates. Bovine serum was used as a lubricant and a gravimetric technique was used to measure wear. The first wear test duration was 7.1 × 106 cycles and investigated the effect of simplified loading. This was achieved by using full physiological motion and loading for the first 5 × 106 cycles of the test, then physiological motion with simplified loading for the final 2.1 × 106 cycles of the wear test. The UHMWPE acetabular cup wear rates using full physiological motion and loading were 32.2 and 51.7 mm3/106 cycles against zirconia and CoCrMo femoral heads respectively. Using simplified loading the cup wear rates were 30.1 and 49.2 mm3/106 cycles against zirconia and CoCrMo respectively which was not significantly different from wear rates with physiological loading. The effect of simplified motion was investigated in a second wear test of 5.0 × 106 cycles duration. Physiological loading was applied across the prosthesis with physiological motion in the flexion/extension plane only. Mean wear of the acetabular component dropped to 0.197 mm3/106 cycles. The surfaces of all the actabular cups were subject to gross examination, optical microscopy and scanning electron microscopy. No notable difference was observed between the cups subjected to physiological motion and loading and those subjected to simplified loading. The cups worn with a single plane of motion had a much smaller worn area and a notable difference in surface features to the other cups. Simplifed loading is therefore an acceptable simplification in simulator testing but simplifying motion to the flexion/extension plane axis only is unacceptable.
Temperature and ageing condition effects on the characterization of acrylic bone cement
Authors: Baleani M , Cristofolini L , Toni A 
Published: October 2002

This study investigates the effect of the environmental temperature and ageing condition on the characterization of acrylic bone cement. The tests were performed according to ISO 5833. The testing parameters were allowed to vary within the limits defined by the standard, in order to assess their effect on the results of the test. In certain cases the tests were also performed under conditions which the standard does not provide for but which are likely to occur clinically. This investigation showed that the cement behaviour may also change in the temperature range specified in the standard. Therefore, it is deemed appropriate to correlate the curing parameters of the bone cement to the environmental temperature, performing the test at different temperatures. In this way the effect of temperature on the duration of the phases in the cement curing could be assessed. The resultant graphical representation of the effect of temperature on the duration of the phases in cement curing has direct clinical relevance. Furthermore, this study showed that the ageing conditions of the mechanically tested specimens affected the results. Hence, it is deemed advisable to modify the ageing conditions of the specimens, fixing them closer to the in vivo conditions.
The design of a five-degree-of-freedom powered orthosis for the upper limb
Authors: Johnson G R , Carus D A , Parrini G , Marchese SScattareggia , Valeggi R 
Published: October 2002

In response to the need for a sophisticated powered upper-limb orthosis for use by people with disabilities and/or limb weakness or injury, the MULOS (motorized upper-limb orthotic system) has been developed. This is a five-degree-of-freedom electrically powered device having three degrees of freedom at the shoulder, one at the elbow and one to provide pronation/supination. The shoulder mechanism consists of a serial linkage having an equivalent centre of rotation close to that of the anatomical shoulder; this is a self-contained module in which power transmission is provided by tensioned cables. The elbow and pronation/supination modules are also self-contained. The system has been designed to operate under three modes of control: 1. As an assistive robot attached directly to the arm to provide controlled movements for people with severe disability. In this case, it can be operated by a variety of control interfaces, including a specially designed five-degree-of-freedom joystick. 2. Continuous passive motion for the therapy of joints after injury. The trajectory of the joints is selected by ‘walk-through’ programming and can be replayed for a given number of cycles at a chosen speed. 3. As an exercise device to provide strengthening exercises for elderly people or those recovering from injury or surgery. This mode has not been fully implemented at this stage. In assistive mode, prototype testing has demonstrated that the system can provide the movements required for a range of simple tasks and, in continuous passive motion (CPM) mode, the programming system has been successfully implemented. Great attention has been paid to all aspects of safety. Future work is required to identify problems of operation, and to develop new control interfaces.
Composite hydrogels for implants
Authors: Ambrosio L , De Santis R , Nicolais L 
Published: October 2002

Hydrophilic composite structures are designed to mimic the transport and mechanical properties of natural soft tissue such as tendons, ligaments and intervertebral discs. Mechanical and viscoelastic behaviour of a soft composite material based on a hydrogel matrix reinforced with bundles of polyethylene therephthalate (PET) fibres is analysed. The typical J-shaped stress-strain behaviour, displayed by natural tendons and ligaments, is reproduced. The mechanical characteristics, such as the extent of the ‘toe-in region’ and the elastic modulus in the linear region, can be controlled by varying the winding angle of the fibres and the matrix composition. Dynamic mechanical analysis showed the dual behaviour of the composite systems due to the progressive contribution of the PET fibres. Different poly (2-hydroxyethylmethacrylate)/polycaprolactone (PHEMA/PCL) semi-interpenetrating polymer networks (IPNs) hydrogel composite systems reinforced with PET fibres have been investigated for potential use as intervertebral disc prostheses. Compression properties have been evaluated by static and dynamic tests. Uniaxial compression tests on the swollen samples showed an increase of the modulus and maximum stress with increasing content of PCL and PET fibres. Creep behaviour is also dependent on the hydrogel composition. The composite PHEMA/PCL hydrogels showed compression properties similar to those expressed by canine intervertebral discs in different spinal locations.
Flow characteristics of curing polymethyl methacrylate bone cement
Authors: Dunne N J , Orr J F 
Published: October 2002

During polymerization, polymethyl methacrylate bone cements have complex viscoelastic characteristics. Within a short working time they transform from dough-like consistencies to solid cements. Therefore, the time at which a cement is introduced to cancellous bone surfaces and subjected to pressure is important, to achieve optimum flow and mechanical interdigitation. Achieving adequate mechanical interlock increases the area for load transfer and reduces localized bone-cement interface stresses. The aim of this study was to measure the flow characteristics for commercial bone cements as a function of time and calculate the apparent viscosities for the curing bone cements The capillary extrusion method was used to measure the rate of flow of the curing cement, by means of a melt flow index apparatus, which was manufactured in-house. The tests were conducted using nozzles of different lengths and under two loads. This enabled the power index value, n, and the pressure at the die entry, Po, to be calculated for each material with respect to time. Once the flow characteristics were determined, a series of formulae were used to calculate the shear rates, y, the shear stresses, r, and the apparent viscosities, na, of the curing bone cements. The results indicated that acrylic bone cements are non-Newtonian, pseudoplastic materials, since the power index values are less than 1.0 during the curing stage. The consistency indices, K, were calculated from the shear stress versus shear rate data. The apparent viscosities of the cements were found to increase with respect to increases in time. Clinically, it was considered desirable to inject and pressurize the cement into the medullary canal while its viscosity is relatively low in order to obtain maximum interdigitation into cancellous bone, provided adequate containment and a means of pressurization can be achieved. The pseudoplastic character of bone cements is responsible for their reduction in viscosity with increased shear rate, a property that may be exploited to enhance penetration with appropriate delivery.
Acoustic and ultrasonic tissue characterization—assessment of osteoporosis
Authors: Langton C M , Njeh C F 
Published: October 2002

Osteoporosis, often termed the ‘silent epidemic’, has been defined as ‘a decrease in bone mass and architectural deterioration of bone tissue, leading to enhanced bone fragility and consequent increase in fracture risk’. In the United Kingdom alone, the annual health costs are in excess of £750 million, with 60 000 patients suffering a hip fracture each year. A quarter of these will die within 12 months of their fracture, half of the remainder will never regain independent living. The established procedure for assessing the risk of osteoporotic fracture is via bone mineral density (BMD) assessment using dual-energy X-ray absorptiometry (DXA). However, DXA is an expensive technique and is not widely available. Within the past 15 years, ultrasound assessment of bone has rapidly advanced in scientific understanding, technical development and clinical utility. Measurements of cancellous bone (particularly at the calcaneus) are generally performed in preference to those of cortical bone (tibial cortex). There are currently 15 commercial systems available and over 3500 systems are in use world-wide. The low cost and portability offered by ultrasound systems should enable an integrated community-based screening programme to be established in the near future. Ultrasound measurements of bone are generally obtained using transmission rather than pulse-echo techniques owing to its highly attenuating nature. Ultrasound velocity and attenuation measurements are utilized. For velocity, there are well-defined fundamental relationships describing the dependence upon the elasticity and density of bone.
Estimation of loads in the lumbar spine during slow symmetrical lifts and a method for their reduction
Authors: Thomson K D 
Published: October 2002

During slow, symmetrical lifts the anterior abdominal wall buckles at about the level of L3 vertebra, causing a local reduction in the cross-sectional area of the abdominal cavity. Calculations are made on the effect this buckling has on both lumbar vertebral compression and erector muscle tension. A simple exercise in statics shows that the size of the diaphragm is not relevant in determining the load relief due to intra-abdominal pressure, nor are the tensile loads in the abdominal muscles arising from this pressure. Finally a description is given of a simple harness worn on the torso, which is predicted to reduce substantially the loads in both the erector muscles and the lumbar vertebrae.