E. A. Patterson

Tightening characteristics for screwed joints in osseointegrated dental implants

The significance of tightening abutment screws and gold cylinders to osseointegrated fixtures with the correct torque is demonstrated, and a simple relationship between applied torque and screw preload is derived by use of mechanical engineering principles. The principles of a number of tightening methods are outlined and assessments made of their accuracy. The difference between optimum and design torque is highlighted. The necessity and means of achieving optimum torque to ensure a reliable joint in clinical practice is discussed.

A review of the general theory of thermoelastic stress analysis

Thermoelastic stress analysis (TSA) is now a well-known experimental technique providing information on the surface stress field in structures. Many studies have assessed the potential of the technique for a number of applications and some useful and detailed reviews of these investigations are available, focusing mainly on the experimental aspects related to the measurement of the thermoelastic signal. In this work, instead, a complete and detailed insight into the origins of the various forms of the equations describing the thermoelastic effect is given with reference to the concepts of the thermodynamic theory of a continuum. A discussion on the theory leading to the thermoelastic effect law is intended to give a useful overview of the applications and real limitations of TSA.

A THREE-DIMENSIONAL ANALYSIS OF A BIOPROSTHETIC HEART VALVE

A three-dimensional finite element model of the leaflets of a bicuspid bioprosthetic heart valve is presented. The model is based on a non-linear elastic representation of the tissue behaviour which closely simulates that found in experiments. The geometry of the model is based on measurements from a real valve. Shell elements which permit bending have been used in the analysis. The results indicate that bending stresses in the leaflets make a significant contribution to their deformation. This confirms earlier two-dimensional work which had suggested that analyses, where only membrane stresses were modelled, were likely to produce significant errors in the stress states. The analysis also predicts peak stresses close to, but not at, the attachment of the leaflet to the stent post.

Simultaneous observation of phase-stepped images for automated photoelasticity

Abstract A novel instrument is described for the simultaneous observation and capture of four phase-stepped photoelastic images. A theoretical description of the optics of the instrument is presented for the first time. Three examples are given of the use of the instrument in reflection photoelasticity to generate full-field maps of isochromatic and isoclinic parameters. The results from these experiments show close correlation to results from both theoretical analyses and manual measurements. The instrument can be used in either reflection or transmission mode and it is concluded that the new instrument significantly enhances the potential for real-time studies using reflection photoelasticity.

Use of phase-stepping with demodulation and fuzzy sets for birefringence measurement

Abstract The application of phase-stepping and image processing to photoelastic stress analysis is restricted by the periodic nature of the birefringence data generated and the interaction between the isoclinic parameter and relative retardation. These difficulties have been investigated using signal analysis and fuzzy set theory. A new strategy for processing phasestepped data from birefringent materials is proposed and involves demodulation of intensity signals and the classification of periods of data using fuzzy sets. The procedure works well in a wide range of applications.

The stress intensity of mixed mode cracks determined by digital image correlation

A generalized approach for determining the stress intensity factors (SIFs) K I and K II for any mode mixity directly from displacement fields obtained by digital image correlation is presented using a centre fatigue cracked aluminium plate as an example problem. It was found that the crack-tip position could be determined on average to within 50 per cent of the displacement vector spacing (60 μm). The approach has been shown to be fairly robust, both in terms of the stability of the SIFs thus obtained and their sensitivity (less than 0.07 MPa

A comparative study of linear and nonlinear simulations of the leaflets in a bioprosthetic heart valve during the cardiac cycle

Two geometrically identical models of the leaflets of a bicuspid bioprosthetic heart valve have been constructed using finite elements. The boundary conditions applied to the models were also identical but a linear material model has been used in one and a nonlinear elastic model in the other. The models were fullscale and contained 2600 Belytschko-Lin-Tsai shell elements which allowed the variation of stress through the thickness of the leaflet to be modelled. A time-varying, spatially-uniform pressure differential was applied across the leaflets to model their behaviour during a complete cardiac cycle. The simulation was performed using a dynamic, explicit, time-stepping, finite element code. A comparison of the two models showed that the nonlinear model was more responsive to the time-varying pressure wave, and deformed into more complex shapes during the opening and closing phases which induced lower compressive but higher tensile stresses in the leaflets.

Completely automated photoelastic fringe analysis

Abstract The integration of two automated systems of photoelastic analysis has been performed. This combines the results obtained with a full-field polariscope based on phase-stepping techniques with those from spectral contents analysis. The latter technique identifies the absolute value of the isochromatic parameter at a point. This information is used to calibrate maps of relative retardation produced by the phase-stepping method. Thus, completely automatic determination of calibrated isochromatic and isoclinic patterns in photoelastic specimens is achieved. The evaluation of the whole system, establishing limits of validity and influences of parameters, is performed. Results show that the analysis carried out with the combined system, incurred errors no longer than those of each system working independently.

Full-field separation of principal stresses by combined thermo- and photoelasticity

The combined use of thermoelastic stress analysis and full-field reflection photoelasticity based on the phase-stepping technique has been developed for twodimensional problems. The first method determines the sum of the principal stresses, the latter evaluates the difference of the principal stresses. Thus the principal stresses were separated at each point in the field of view without reference to neighboring points. An evaluation of this approach has been performed using a tensile plate with a central circular hole. The results show that the analysis carried out combining thermo- and photoelasticity incurred errors no larger than those of each system working independently.

A two-dimensional finite element analysis of a bioprosthetic heart valve

A finite element scheme has been developed using total Lagrangian techniques for the two-dimensional analysis of bioprosthetic heart valve leaflets undergoing large deformation. Two models of a leaflet, namely a radial and a circumferential slice, have been analysed. The attachment of the slice to the stent was simulated by progressive contact on a circular former and the coaptation of the leaflets in the centre of a heart valve by a straight line of contact. For the circumferential model, different initial configurations have been considered. The prolapse pressure under which the heart valve closes has been shown to be small in comparison with the normal pressure a heart valve sustains. The regions of the valve that are most heavily stressed are subjected to a strong component of bending. The amount is sensitive to the details of the boundary conditions and to the initial configuration of the valve. These observations are likely to be significant in the use of this kind of stress analysis to improve the design of this type of valve.