Cecil Armstrong

Variations in the intrinsic mechanical properties of human articular cartilage with age, degeneration, and water content.

UNLABELLED In a series of 103 specimens from the lateral facet of the human patella, the intrinsic mechanical properties of articular cartilage were measured using a confined compression creep test. By considering the cartilage as a porous, permeable solid filed with fluid, this experimental procedure allowed the determination of the intrinsic equilibrium modulus of the cartilage matrix and its permeability to fluid flow. The intrinsic equilibrium modulus and the permeability both were highly correlated with the water content of the tissue; as water content increased, the matrix of the tissue became softer and more permeable. There was only a marginal decrease in the equilibrium modulus of the tissue with increasing age and surface degeneration. The permeability of the cartilage matrix was not significantly correlated with age or degeneration. CLINICAL RELEVANCE We concluded that the visual or histological appearance of a cartilage specimen may be a poor indicator of its ability to function as the bearing material in the intact joint. A more reliable indicator of the functional properties of a specimen can be obtained either by direct mechanical testing or by biochemical analysis of its composition.

In vitro measurement of articular cartilage deformations in the intact human hip joint under load.

Using a new roentgenographic technique for measuring cartilage deformation in intact joint specimens, twenty-eight normal human hip joints from subjects twenty-five to eighty-five years old were loaded with a force of five times body weight in a testing machine. The initial unloaded thickness of the articular cartilage of the femoral head and the changes in thickness of this cartilage under load were measured roentgenographically at seven to twelve sites on each femoral head. These measurements showed that the deformations of femiral-head articular cartilage under load in the intact joint are non-uniform and increase greatly with age. In twelve specimens measurements were also made of the increase in cartilage deformation with time when the load of five times body weight was maintained on the joint. A single osteoarthrotic joint was also studied. The experimental findings imply changes in the fundamental mechanical properties of the cartilage with age, which probably result from age-related alterations in cartilage microstructure and chemical composition.

2D finite-element mesh generation by medial axis subdivision

Abstract An algorithm for the automatic generation of two-dimensional finite element meshes using quadrilateral elements has been demonstrated. The technique uses information derived from the medial axis of a 2D region, the locus of the centre of an inscribed disc of maximal diameter as it rolls around the region interior. Using this information, an arbitrarily complex object can be subdivided into a series of meshable subregions. Within these subregions relatively conventional meshing patterns are then generated. The resulting meshes are well structured and flow smoothly round the object boundary with minimum mesh irregularity.

Shape description by medial surface construction

The medial surface is a skeletal abstraction of a solid that provides useful shape information, which compliments existing model representation schemes. The medial surface and its associated topological entities are defined, and an algorithm for computing the medial surface of a large class of B-rep solids is then presented. The algorithm is based on the domain Delaunay triangulation of a relatively sparse distribution of points, which are generated on the boundary of the object. This strategy is adaptive in that the boundary point set is refined to guarantee a correct topological representation of the medial surface.

Hexahedral meshing using midpoint subdivision and integer programming.

Abstract The generation of finite element meshes of 3D hexahedral elements can be performed by subdividing the model into a number of simple bodies like hexahedra, triangular prisms, etc. Within these bodies, regular or simple transition meshes can be generated. In this paper, a method called midpoint subdivision will be discussed which enables the user to build the model using a number of high level subregions, which satisfy certain simple topological conditions. These subregions can be automatically subdivided into meshes of hexahedral elements for analysis. The distribution of elements within the mesh is typically controlled by specifying the division numbers of certain edges. However, for a given set of subregions there are a number of constraints on the edge division numbers that will yield a compatible mesh. The use of integer programming will be discussed: this finds the compatible division numbers which are nearest to, but not less than, the target values specified.

Mixed-dimensional coupling in finite element models

In many finite element analysis models it would be desirable to combine reduced- or lower-dimensional element types with higher-dimensional element types in a single model. In order to achieve compatibility of displacements and stress equilibrium at the junction or interface between the differing element types, it is important in such cases to integrate into the analysis some scheme for coupling the element types. A novel and effective scheme for establishing compatibility and equilibrium at the dimensional interface is described and its merits and capabilities are demonstrated. Copyright

Modelling requirements for finite-element analysis

Abstract Efficient modelling of many problems in the finite-element analysis of structural and other continuum problems requires substantial simplification of the design geometry. Tools are needed for the efficient transformation of the detailed design geometry into an appropriate analysis model. It is argued that the medial axis and surface transform of geometric models provide an alternative representation that has many attractive properties for analysis feature recognition and simplification. Strategies for identifying possible idealizations, controlling their application, and estimating the associated errors appear feasible. Some requirements for geometric-modelling tools are identified.

Mixed Dimensional Coupling in Finite Element Stress Analysis

Abstract.Many analysis models utilize finite elements of reduced dimension. However, to capture stress concentrations at local details, it would be desirable to combine the reduced dimensional element types with higher dimensional elements in a single finite element model. It is therefore important in such cases to integrate into the analyses some scheme for coupling the element types that conforms to the governing equations of the problem. In this paper, a novel method that can correctly couple beams to solids, beams to shells and shells to solids for elastic problems is presented. The approach adopted is to equate the work done on either side of the interface between dimensions, and this leads to multi-point constraint equations, thus providing a relationship among nodal degrees of freedom between the differing element types. Example results show that the proposed technique does not introduce any spurious stresses at the dimensional interfaces.

A small feature suppression/unsuppression system for preparing B-rep models for analysis

CAD technology plays an ever more central role in todays multidisciplinary simulation environments. While this has enabled highly complex and detailed models to be used earlier in the design process it has brought with it difficulties for simulation specialists. Most notably CAD models now contain many details which are irrelevant to simulation disciplines. CAD systems have feature trees which record feature creation but unfortunately this does not capture which features are relevant to which analysis discipline. Many features of little significance to an analysis only emerge during the construction of the model. The ability to selectively suppress and reinstate features while maintaining an audit trail of changes is required to facilitate the control of the idealisation process. Features suppressed for one analysis can be retrieved for use in another.This work uses combinatorial topology concepts to outline the necessary conditions so that CAD model simplification operations can be designed as continuous transformations. Irrelevant features can then be suppressed and subsequently reinstated, within defined limitations, independently from the order in which they were suppressed. The implementation of these concepts provides analysts with a mechanism for generating analysis models with different levels of detail, without having to repeat the simplification process from the original CAD geometry. Most importantly, the information recorded during the suppress operations forms an essential audit trail of the idealisation process and can be presented in a feature-tree like structure allowing analysts to review their modelling decisions retrospectively. The approach also facilitates the generation of local, detailed models encapsulating a feature of interest. The proposed system follows a Find and Fix paradigm; where different algorithms for feature finding and fixing can be utilised in a common cellular modelling framework.

Simulation of the stretch blow molding process of PET bottles

Simulations of various stretch blow and blow moldings of axisymmetric PET bottles has been carried out using ABAQUS. A creep constitutive model with material data developed for a thermoforming process was used in the finite element analysis. Simulations using shell and solid elements were compared with experimental moldings. The creep material model, when combined with solid elements and a very high coefficient of friction, provided the best predictions for bottle side wall thickness, strains, blowing pressure, and general material movement. It was found that, using ABAQUS, the predicted wall thickness distribution of the material in an injection blow molded bottle agreed well with the values obtained using commercial process conditions.