V. Fuis

Experimental Study of Lubricant Film Thickness Behavior in the Vicinity of Real Asperities Passing through Lubricated Contact

This article presents an experimental study of the influence of real surface micro-geometry on the film thickness in a circular elastohydrodynamic (EHD) contact formed between a real, random, rough surface of steel ball and smooth glass disk. Phase shifting interferometry was used to measure in situ initial undeformed rough surface profiles, whereas thin film colorimetric interferometry provided accurate information about micro-EHD film thickness behavior over a wide range of rolling speeds. Two real roughness features were studied in detail—a 56-nm-high ridge and a 90-nm-deep groove, both transversely oriented to the direction of surface motion. It was shown that the ridge is heavily deformed in a loaded contact and its height increases with increasing rolling speed. The asperity tip film thickness behavior is quite similar to the contact average film thickness when the film thickness is higher than the undeformed ridge height. However, below this limit the film is thicker than what the EHD theory predicts. For the groove, a local reduction in film thickness at the leading edge was observed. When the groove is passing through the EHD conjunction, it maintains its undeformed shape. The behavior of both roughness features studied shows good agreement with previous experimental observations conducted using an artificially produced ridge and groove.

Reliability of the Ceramic Head of the Total Hip Joint Endoprosthesis Using Weibull’s Weakest-link Theory

The paper deals with calculation of the parameters of ceramic material from a set of destruction tests of ceramic heads of total hip joint endoprosthesis. The standard way of calculation of the material parameters consists in carrying out a set of 3 or 4 point bending tests of specimens cut out from parts of the ceramic material to be analysed. In case of ceramic heads, it is not possible to cut out specimens of required dimensions because the heads are too small (if the cut out specimens were smaller than the normalised ones, the material parameters derived from them would exhibit higher strength values than those which the given ceramic material really has). On that score, a special testing jig was made, in which 40 heads were destructed. From the measured values of circumferential strains of the head’s external spherical surface under destruction, the state of stress in the head under destruction was established using the final elements method (FEM). From the values obtained, the sought for parameters of the ceramic material were calculated using Weibull’s weakest-link theory

Finite element simulation of mechanical tests of individual cells

The threedimensional finite element (FE) model of eucaryotic cell presented in the paper is based on similar models published recently; it comprehends elements representing cell membrane, cytoplasm and nucleus, and a complex tensegrity structure representing cytoskeleton. In contrast to the previous models, this tensegrity structure consists of several parts. External and internal parts count 30 struts and 60 cables each and their corresponding nodes are interconnected by 30 radial members; these parts represent cortical, nuclear and deep cytoskeletons, respectively. This arrangement enables us to simulate the load transmission from the extracellular space via membrane receptors (focal adhesions) to the central part of the cell (nucleus, centrosome); this ability of the model was tested by simulation of some mechanical tests of isolated cells, in particular tension test with micropipettes, indentation test and magnetic tweezer test. Although material properties of components have been defined as realistic as possible on the base of the mechanical tests with vascular smooth muscle cells, they were not identified in fact and are not unique probably. However, simulations of the tests have shown the ability of the model to simulate the global load-deformation response of the cell under various types of loadings, as well as several substantial global features of the cell behaviour, e.g. “at a distance effect”, non-linear stiffening with increasing load, or linear dependence of stiffness on increasing prestrain.

Stress And Reliability Analyses Of Damaged Ceramic Femoral Heads

Today, the problem of reliability of the hip joint ceramic heads becomes important, since world-wide the destruction of the ceramic head has generally been stated in a non-negligible number of patients. Therefore stress and reliability analyses of the ceramic heads were made using experimental and computational modelling aimed at finding the causes of the head destruction. The deformation and stresses are obtained by computational methods (finite elements) under testing conditions, in accordance with 1S0 7206-5. Head reliability is based on the Weibull weakest link theory. The head’s stress and reliability are significantly influenced by manufacturing inaccuracies of the cone contact areas, The considered manufacturing inaccuracies are modelled as axisymmetric or 3D. The effects of manufacturing inaccuracies on the head’s hoop stress and the head’s contact pressure along the contact taper are analysed in detail, because the maximum tensile stress acts in this region, Finally, the effect of the mentioned manufacturing inaccuracies on the head’s failure probability during the loading is shown. The results obtained are used in clinical practice and in the technological processes of stem and ceramic head manufacturing.

Tensile Stress Analysis of the Ceramic Head with Micro and Macro Shape Deviations of the Contact Areas

The paper deals with the problems of ceramic head of hip joint endoprosthesis destructions in vivo, and with assessing the impact of shape deflections of conical surfaces on the tensile stress in the head. There are assumed two material characteristics of the stem – the first is linear elastic and the second is bilinear elasto-plastic modelled austenitic steel used in the bio-implants. Concerned are shape deviations from the ideal conical surfaces of the stem and the head of the endoprosthesis. The shape deviations may be modelled at the macro-level - this concerns model shape inaccuracies such as deviation from the nominal degree of taper, at the micro-level - when the stochastic distribution of unevenness on the contact areas is respected. The problem of stress in ceramic heads was solved using the finite element method – system ANSYS under ISO 7206-5 loading. In the paper are presented and analysed the results of solution of the macro shape deviations and micro shape deviations, obtained from measurements made on the cones of stems and heads. There are analysed three variants of the sizes of the measured micro shape deviations (measured, doubled and halved).

Analyses of the Shape Deviations of the Contact Cones of the Total Hip Joint Endoprostheses

Complex reliability analyses of the bioimplants are very important today, because the failures of them cause traumatic consequences for the patients. We are interested in the calculations of the reliability of the ceramic heads of hip joint endoprosthesis. Reliability of the component made of the brittle material depends on the material parameters and on the tensile stress distribution in the component (bioimplant – hip joint ceramic head). Tensile stresses in the heads are very significantly influenced by the shape deviations of the cone contact areas of the head and the stem. Concerned are shape deviations from the ideal conical surfaces of the stem and the head of the endoprosthesis. The shape deviations may be modeled at the macro-level - this concerns model shape inaccuracies such as deviation from the nominal degree of taper, at the micro-level - when the stochastic distribution of unevenness on the contact areas is respected. The problem of stress in ceramic heads was solved using the finite element method – system ANSYS under ISO 7206-5 loading. There are presented and analysed the results of solution of the macro shape deviations and micro shape deviations, obtained from measurements made on the cones of stems and heads. The simulations of the loading of the one head pressed on the 5 different stems cones with macro and micro shape deviations (measured) is solved and analysed.

Development of Experimental Devices for Testing of the Biomechanical Systems

The paper is focused on the description of the development of single-purpose devices for determining me- chanical properties of the segments of human body. The re- search were focused on the development and design of a device for establishing the abrasion wear of the THR of the hip joint, and for testing spinal segments. On the basis of the gained knowledge, the attention has also been focused on the devel- opment and design of a multi-purpose experimental biome- chanical device. Its basic idea was derived from Stewart plat- form, for the modification of which an animation 3D model was developed.

Stress-Deformation Analysis of an Elbow Articulation with Radial Head Replacement

this contribution speaks about the progress of forming a geometric elbow model, generating the FEM network in the created volumes; stress-deformation analysis of the Final Element Model (contact task) and suggests possible geometric models of partial alloplasty of the radius’s proximal part.

STRESS ANALYSIS OF THE RADIAL HEAD REPLACEMENT IN AN ELBOW ARTICULATION

The paper is focused on computational modeling of elbow articulation with radial head replacement. The main part of the project is aimed towards the creation of computational model of suitable partial endoprothesis of proximal part of the radial bone tissue which would keep the function of the elbow articulation while replacing the distance created by resection. The geometrical and computational models of radial head replacements and elbow joint were created. Computational models with these implants were compared with given physiological state of elbow articulation. The influence of friction and material characteristics of bone tissue and cartilage on changes in contact pressure (and therefore to the abrasion) were analyzed using those models.

Tensile Stress Analyses of the Hip Joint Endoprosthesis Ceramic Head with Real Shape Deviations

In vivo fracture of ceramic heads of total hip joint endoprostheses has been stated in a not negligible number of patients in the Czech Republic hospitals, some time ago. The reliability of the ceramic head is based on the Weibull weakest link theory. The probability of the failure depends on the tensile stress values in the head, on the volume in which stress acts and on the material parameters of the used bioceramics. The stress analyses of the ceramic heads were realized using computational modelling - FEM system ANSYS under ISO 7206-5 loading. The maximum values of the tensile stress in the ceramic head are significantly influenced on the shape deviations of conical surfaces of the head and the stem cones. There are two types of the shape deviations – the first is macro deviations (different angle of the taper of the stem cone and the head cone), the second is micro shape deviations (the stochastic distribution of unevenness of cone contact surfaces in order micrometers). The macro and micro shape deviations of the real cones were carried out using the IMS-UMPIRE measuring equipment. By computational modelling it has been proved that the character and the size of shape deviations on a conical contact area between the stem and the head of hip joint endoprosthesis have a pronounced influence on the character and the value of stress in the head and, hence, on the probability of the head’s failure. Ignorance of this influence can lead to unforeseen failures of ceramic heads and thus to cast doubt on the trust-worthiness of a certain type of total hip joint endoprosthesis and of surgical treatment.