Sally E Clift

Fracture prediction in plastic deformation processes

This paper describes the use of the finite-element technique to predict fracture initiation in a range of simple metalforming operations. These cover typical processes and enable deformation and fracture initiation to be examined under several different loading conditions. Three types of metalforming operation are considered, simple upsetting, axisymmetric extrusion, and strip compression and tension

Determination of orthotropic bone elastic constants using FEA and modal analysis

Finite element models have been widely employed in an effort to quantify the stress and strain distribution around implanted prostheses and to explore the influence of these distributions on their long-term stability. In order to provide meaningful predictions, such models must contain an appropriate reflection of mechanical properties. Detailed geometrical and density information is now readily available from CT scanning. However, despite the use of phantoms, a method of determining mechanical properties (or elastic constants) from bone density has yet to be made available in a usable form. In this study, a cadaveric bone was CT scanned and its natural frequencies were measured using modal analysis. Using the geometry obtained from the CT scan data, a finite element mesh was created with the distribution of density established by matching the mass of the FE bone model with the mass of the cadaveric bone. The maximum values of the orthotropic elastic constants were then established by matching the predictions from FE modal analyses to the experimental natural frequencies, giving a maximum error of 7.8% over 4 modes of vibration. Finally, the elastic constants of the bone derived from the analyses were compared with those measured using ultrasound techniques. This produced a difference of <1% for both the maximum density and axial Youngs Modulus. This study has thereby produced an orthotropic finite element model of a human femur. More importantly, however, is the implication that it is possible to create a valid FE model by simply comparing the FE results with the measured resonant frequency of the CT scanned bone.

Finite element biphasic indentation of cartilage: A comparison of experimental indenter and physiological contact geometries

Abstract In experimental cartilage indentation studies, the indenter is typically a plane-ended or hemispherically ended cylinder and can be either porous or non-porous. Joints such as the hip and knee, however, have much higher radii of curvature than those used in experimental indentation testing. In order to interpret the results from such testing in a physiological context it is therefore useful to explore the effect of contact geometry on the pore pressure and strain distribution generated in the cartilage layer. Articular cartilage can be described as a saturated porous medium, and can be considered a biphasic material consisting of a porous, permeable solid matrix, and an interstitial fluid phase. This behaviour has been predicted in this study using the ABAQUS soils consolidation procedure. Four contact geometries were modelled: two typical experimental configurations (5 mm radii cylindrical indenter and hemispherical indenters) and two effective radii representative of the hip and knee (20 and 100mm). A 10 per cent deformation, or a load of 0.9kN, was applied over a ramp time of 2 s, which was then maintained for a further 100 s. The porous indenter generated less pore pressure compared with the equivalent non-porous indenter and produced higher values of compress-ive strain in the solid matrix. The predictions made using the cylindrical indenters, porous and non-porous, were dominated by the concentrations at the edge of the indenter and overestimated the peak compressive strain in the tissue by a factor of 21 and a factor of 14 respectively when compared with the hip model. The hemispherical indenter predicted peak strains in similar positions to those predicted using physiological radii, however, the magnitude was overestimated by a factor of 2.3 when compared with the knee and by 5.7 when compared with the hip. The pore pressure throughout the cartilage layer reduced significantly as the radius of the indenter was increased.

Application of finite elements to the stress analysis of articular cartilage

A common effect of arthritic disease processes in synovial joints is deterioration of the articular cartilage. Therefore, an improved understanding of the relationships between the composition and structure of articular cartilage and the mechanical behaviour is a subject of considerable interest. The numerical modelling tool of finite element (FE) analysis has been widely applied to analyse the behaviour of articular cartilage under compressive stress. FE analysis enables parameters and boundary conditions to be investigated which are not accessible experimentally or analytically. The biphasic theory describes the constitutive behaviour of soft hydrated biological tissues, such as articular cartilage, and has been successfully implemented using FE analysis. The development of successively more comprehensive biphasic models is described here detailing the use of FE analysis in modelling experimental configurations such as indentation. Key work in the area is reviewed in this paper.

The influence of the stem-cement interface in total hip replacement—a comparison of experimental and finite element approaches:

Abstract Experimental and finite element investigations were carried out on axisymmetric models of the femoral component of a total hip replacement. In one instance, the interface between the stem and the surrounding bone cement was assumed to be rigidly bonded; in a second, it was allowed to slip. For the latter case, a friction coefficient of 0.2 was determined experimentally. The predictions of the finite element models demonstrated excellent agreement with the results from the experimental tests at all sites where comparisons were made, thus validating these models. The effect of stemcement slip was shown to reduce the maximum shear stress in the cement mantle by approximately 30 per cent.

Biomimetics of Campaniform Sensilla: Measuring Strain from the Deformation of Holes

We present a bio-inspired strategy for designing embedded strain sensors in space structures. In insects, the campaniform sensillum is a hole extending through the cuticle arranged such that its shape changes in response to loads. The shape change is rotated through 90° by the suspension of a bell-shaped cap whose deflection is detected by a cell beneath the cuticle. It can be sensitive to displacements of the order of 1 nm. The essential morphology, a hole formed in a plate of fibrous composite material, was modelled by Skordos et al. who showed that global deformation of the plate (which can be flat, curved or a tube) induces higher local deformation of the hole due to its locally higher compliance. Further developments reported here show that this approach can be applied to groups of holes relative to their orientation.The morphology of the sensillum in insects suggests that greater sensitivity can be achieved by arranging several holes in a regular pattern; that if the hole is oval it can be “aimed” to sense specific strain directions; and that either by controlling the shape of the hole or its relationship with other holes it can have a tuned response to dynamic strains.We investigate space applications in which novel bio-inspired strain sensors could successfully be used.

Surface heating by transvaginal transducers

This safety study was designed to investigate tissue heating close to the surface of transvaginal ultrasound transducers, with the objective of assessing the validity of manufacturing safety standards set by the International Electrotechnical Commission (IEC).

Bone Remodelling of a Proximal Femur with the Thrust Plate Prosthesis: An In Vitro Case

The key to the development of a successful implant is an understanding of the effect of bone remodelling on its long-term fixation. In this study, clinically observed patterns of bone remodelling have been compared with computer-based predictions for one particular design of prosthesis, the Thrust Plate Prosthesis (Centerpulse Orthopedics, Winterthur, Switzerland). Three-dimensional finite-element models were created using geometrical and bone density data obtained from CT scanning. Results from the bone remodelling simulation indicated that varying the relative rate of bone deposition/resorption and the interfacial conditions between the bone and the implant could produce the trend towards the two clinically observed patterns of remodelling.

Wear of articular cartilage: the effect of crystals

An investigation of the effect of crystals in a lubricant on the wear of articular cartilage in vitro was carried out in order to examine the hypothesis that crystals present in synovial fluid could cause abrasive damage of the articular surface. Plugs of cartilage were worn against a stainless steel counterface in a pin-on-disc wear rig. The concentration of cartilage debris present in the lubricant was assessed by measuring the bound sulphate originating from the glycosaminoglycans by ion chromatography. Results indicated that the presence of crystals in the lubricant significantly increased the concentration of wear debris and that the crystal size and morphology influenced the type of damage sustained by the cartilage. Other experimental evidence suggested that cartilage scratched in vivo was no more susceptible to further in vitro damage in this experimental model than normal cartilage. These results implied that crystals present in the synovial fluid of arthritic joints have the potential to cause excessive wear of the articular surface, but that if such crystals are removed the scratched cartilage may not be susceptible to any further damage by abrasive wear.

Parametric sensitivity analyses for FEA of hot steel forging

Abstract This paper reports a system for quantifying and comparing the sensitivity of a thermomechanical finite element analysis (FEA) of forging to variations in different input parameters. The results of applying the method to analyses of simple upsetting, impression-die forging and backward extrusion of hot steel are also described. The number of parameters in a thermomechanical FEA of forging make it impractical to investigate all of them, so the investigations were restricted to the parameters that define the flow stress of the forged steel and heat transfer and friction at the die–workpiece interface. Theoretical forging investigations of the kind described should be compared with the results of physical forging trials. This comparison would indicate whether or not more work characterising parameters for FEA of forging were justified.