Dave Webb

A method for investigating the mechanical properties of intracoronary stents using finite element numerical simulation

The proliferation of stent designs poses difficult problems to clinicians, who have to learn the relative merits of all stents to ensure optimal selection for each lesion, and also to regulatory authorities who have the dilemma of preventing the inappropriate marketing of substandard stents while not denying patients the benefits of advanced technology. Of the major factors influencing long-term results, those of patency and restenosis are being actively studied whereas the mechanical characteristics of devices influencing the technical results of stenting remain under-investigated. Each different stent design has its own particular features. A robust method for the independent objective comparison of the mechanical performance of each design is required. To do this by experimental measurement alone may be prohibitively expensive. A less costly option is to combine computer analysis, employing the standard numerical technique of the finite element method (FEM), with targeted experimental measurements of the specific mechanical behaviour of stents. In this paper the FEM technique is used to investigate the structural behaviour of two different stent geometries: Freedom stent geometry and Palmaz-Schatz (P-S) stent geometry. The effects of altering the stent geometry, the stent wire diameter and contact with (and material properties of) a hard eccentric intravascular lesion (simulating a calcified plaque) on stent mechanical performance were investigated. Increasing the wire diameter and the arterial elastic modulus by 150% results in the need to increase the balloon pressure to expand the stent by 10-fold. Increasing the number of circumferential convolutions increases the pressure required to initiate radial expansion of mounted stents. An incompressible plaque impinging on the mid portion of a stent causes a gross distortion of the Freedom stent and an hour-glass deformity in the P-S stent. These findings are of relevance for future comparative studies of the mechanical performance of stents, in designing newer stents and also in clinical practice.

Use of FEM in performance assessment of perforated plates subject to general loading conditions

Abstract Perforated plates, also called tube plates, are widely used in power generating plants and chemical processing refineries as components in heat exchangers and steam generators (particularly in nuclear power stations). The integrity of these plates under general (from hydrostatic pressure differentials) and other loading conditions (such as those generated by tubes flexing or the influence of constraints) is vital to the safety of plant. In this paper a general methodology is developed which is applicable to any regular perforation (or penetration) pattern. The solution procedure isolates a unit module from which, by successive reflections and translations, the whole plate can be constructed. The unit perforation module, composed of isotropic material, is replaced by a unit solid brick-shaped module of an equivalent anisotropic plate with identical overall dimensions. Then, by comparing the responses of the isotropic perforated and the anisotropic solid models, the equivalent “effective” elasticity constants E∗, v ∗ and G ∗ are calculated for the equivalent plate, which enable a nominal stress field, for any loading state, to be evaluated. A series of curves, referred to as stress multiplier factors, are also compiled which, together with the nominal stresses, allow the behaviour of the plate on the periphery of the perforation, the most severely stressed zone, to be estimated. The finite element method (FEM) involves the calculation of the elasticity matrix [K] (from {σ ∗ } = [K]{e ∗ } ) and then the evaluation of the anisotropic engineering constants-by the inversion of [K] into [φ]. Two particular penetration patterns (square and triangular) are examined but the method employed is general and can be readily extended to deal with other perforation patterns of different ligament efficiencies.

From Space, No One Can Watch You Die

As soon as CIA Director Leon Panetta was appointed to his post earlier this year, he asked President Obama for a significant escalation in the number of armed ‘‘drone’’ flights, utilizing Unmanned Aerial Vehicles (UAVs) over both Afghanistan and Pakistan. These robot planes are flown by ground-based pilots, either in-country or from the United States, and represent the latest developments in war-fighting technology, separating the warfighter from the consequences of their actions by as much as several thousand miles. Young airmen and women are controlling planes over Afghanistan and Pakistan from computer terminals in Nevada, New Mexico, and California in the United States. They have also been used by Israel in Palestine, and are receiving increasing attention in the media as they are adding significantly to civilian death tolls in these regions.

FEM simulation of the pressing of a strip into a cylindrical die and of a circular plate into a hemispherical one and their comparison with experiment

Abstract The elastic-plastic bending of a copper strip into a cylindrical die and the bending-stretching of a circular plate into a hemispherical die by ‘matched’ cylindrical and hemispherical punches (where the punch is one plate thickness less than the die radius) are simulated using the Finite-Element Method (FEM). The deformation processes are assumed to take place using frictionless tools. The load-deflection characteristics and the detailed deformation response modes are monitored and plotted for both circumstances. In the case of the strip, the deformation process is divided into several stages, each associated with a distinct loading range. The forming process for the circular plate is also monitored in detail, the salient differences between the two models being highlighted. The procedure employed in the analysis solution is based on the use of a commercially available FE code. The program can be adapted easily to include the effects of friction, changes of material properties and a more complex geometry, under both static and dynamic loading conditions. This method of analysis presents an easy way of monitoring the various variables likely to be of interest-stress, strain, displacement - as functions of time or as functions of other variables. Comparison with many features in the experimental and theoretical work of Yu and Johnson in 1983 can be made, which well validates the work described herein.

FEM simulation of the static and dynamic forming of circular plates

Abstract A commercially available finite-element package (ABAQUS) has been used to analyse the forming of a circular plate subjected to simultaneous stretching and bending processes. The method is used to study the plate response under both static and dynamic loading conditions, retaining the non-linearities involved in the large changes in geometry, the continuous change in contact surface conditions and the isotropic material work-hardening characteristics. An axisymmetric model is studied and discussed in detail. For the dynamic case, the plate is assumed to be deformed by the impact of a cylindrical projectile. In the dynamic analysis response the material strain-rate dependence via the Cowper-Symonds power law is included in the analysis. An algorithm to deal with the contact between the cylindrical projectile and plate has been developed by means of the penalty approach. The interaction of the sliding surfaces is modelled with a modified Coulomb friction law. A qualitative comparison betwee the computed results and experiments is referred to in the text.

A Novel FEA Model for the Integral Analysis of a Machine Tool and Machining Processes

Thermal deformations of machine tool structures and the heat resulting from the machining processes have a significant effect on the dimensional accuracy and surface finish generated in precision machining. However, thermal deformations of machine tool structures and the machining process have been studied with FEA models separately, although the two aspects influence each other. In this paper, a novel FEA model is proposed to take account of the above two aspects simultaneously. The model takes a specific machine produced by the industrial collaborator as a case study. Firstly, a thermal FEM analysis was conducted to simulate the temperature distribution of the machine tool. The temperature distribution obtained by the analysis was validated by the temperature rises of some points as measured on the machine. The thermally-induced deformation errors of the machine tool structure were then estimated by the FEM structural analysis. Also, according to thermal deformation of the machine tool structure, the machining process simulation was established using an explicit dynamics finite element code. The temperature distribution obtained from the machining process simulation is was turn input into the FEA model as a heat source. The preliminary simulation results produced by the model and simulations are very promising.

Experimental investigation of the machined surface waviness, vibrations and cutting forces in peripheral milling

Abstract. This paper presents an experimental investigation on the intrinsic relationship among the machined surface waviness, vibrations of both the spindle head and workpiece, and cutting forces in peripheral milling. A series of well designed cutting tests were carried out with different helical end mills under different cutting parameters on a vertical machining centre. The 3D cutting forces were recorded by a Kistler dynamometer, and the vibrations of the spindle head and workpiece were monitored by accelerometers during the cutting process. The surface topography and surface waviness were measured by a Form Talysurf PGI after the cutting process. All of the recorded signals were carefully analyzed and compared to each other. The results demonstrate that the amplitude of the surface waviness is correlated to the amplitude of the vibrations or chatter of both the spindle head and the workpiece, and the surface waviness is dominant at the spindle frequency, regardless of chatter occurrence in the cutting process.

Cumulative Fading and Rainfall Distributions for a 2.1 km, 38 GHz, Vertically Polarized, Line-of-Sight Link

Between February 1,1997 and January 31,1998, a 2.1 km vertically polarized 38 GHz communications link was studied in the subtropical city of Brisbane, Australia. According to the current ITU fading prediction method the link would be expected to experience a rainfall intensity of approximately 50 mm/h for 52.6 min during the year, when averaged over a 60 s integration time. This should have caused approximately 20 dB of fading for 52.6 min [1]-[3]. However the link actually experienced 20 dB of fading for 150.5 min, with fading of 37 dB exceeded for 53.5 min during the year. A rainfall intensity of 84 mm/h was exceeded for 64 min at the receive end, and a rainfall intensity of 72 mm/h was exceeded for 52 min at the transmit end of the hop.

FE investigation of a spirally slotted tube under axially compressive static and dynamic impact loading

In this study, helically gouged Steel tubes of relatively small thickness are discussed as a new design configuration for energy absorption, using the results of numerical simulations. The tubes are loaded in two different modes: (1) Static compressive axial loading applied via a displacement and the kinematic coupling option of the FE code, which constrains the nodes at each end of the tube to the respective reference nodes. (2) Dynamic axial impact loading via a mass element attached to the reference node of the rigid base platen which is energised by an initial velocity while appropriately maintaining its load sustaining resistance. One of the salient features of the deformation mechanism is that in both cases the axial shortening of the tube is accompanied by twisting. The boundary constraints of the reference nodes at both the loaded and supported ends are of significant importance and care has been exercised to specify the appropriate degrees of freedom for movement in both rotation and translation. The contact that occurs between various parts of the tube has been correctly identified and targeted. In the present study attention has been focused on the energy storage capacity of the model under both loading modes and in providing an explanation for the major differences in response that exist between the two cases. The material properties of steel are specified as linear elastic followed by non-linear work hardening in plastic range with moderate and high degree of sensitivity to strain rate effect. The solution reveals several important features, which are discussed in this paper. The proposed device could be used in clusters to limit damage in the event of cable failure in hoist & lift compartments. Although the procedure is applied to axial loading, the method is equally applicable to lateral static and impact loading as cited in the introduction.

The Ethical Use of Outer Space

The night sky in all its glory is undoubtedly an inspiring sight, and the origins of much of our scientific and philosophical thought lie in our ancestors’ experiences of observing and questioning heavenly displays. Unfortunately, urban light pollution is making it increasingly difficult for city dwellers to wonder at the splendour of a starry night, and it is even possible that access to outer space will soon be denied. The commercial and military uses of the space environment are increasing rapidly and, whether we realise it or not, our lives are becoming increasingly influenced by, and dependent on, the use of space technology. The thousand or more operational satellites that currently orbit the Earth collect and broadcast enormous amounts of information worldwide, making important contributions to mapping and communications, environmental monitoring, agriculture, weather forecasting and an ever-growing range of human activities.