Nenad Grujovic

A general beam finite element with deformable cross-section

A general beam finite element is proposed in the paper. The formulation of the element relies on the assumption that the beam-type structural response can be described by using the usual beam and 3D continuum theories. The beam behaviour is represented through the beam degrees of freedom of the global nodes on reference axis, and local effects are taken into account through the relative displacements of the cross-sectional nodes defining the in-plane and out-of-plane deformations of the cross-section. Consistent derivations for small and large displacements within incremental analysis are presented. The cross-section is modeled by segments (cross-sectional elements) and can be of arbitrary shape, including thick- and thin-walled types. The element is formulated as a superelement consisting of the isoparametric subelements (3D, shell, beam) with the relative displacements as the internal degrees of freedom of the element group, which is considered as a substructure. The relative displacements can be translations and rotations, depending on the type of the subelements. Continuity of the relative displacements is ensured within the element group, and connection of the beam elements with other finite element (FE) groups is realized through global (beam) degrees of freedom. Incompatible generalized displacements are implemented to subelements to improve their behavior. External loading of the element can correspond to global and cross-sectional nodes. The proposed beam superelement (BS) is easy for application within FE general purpose package (as our program PAK) in the preparation of input and in the postprocessing. A number of typical examples illustrate accuracy of results obtained by use of the beam superelement in linear and geometrically nonlinear analysis.

Effect of vacuum-treatment on deformation properties of PMMA bone cement.

Deformation behavior of polymethyl methacrylate (PMMA) bone cement is explored using microindentation. Two types of PMMA bone cement were prepared. Vacuum treated samples were subjected to the degassing of the material under vacuum of 270 mbar for 35 s, followed by the second degassing under vacuum of 255 mbar for 35 s. Air-cured samples were left in ambient air to cool down and harden. All samples were left to age for 6 months before the test. The samples were then subjected to the indentation fatigue test mode, using sharp Vickers indenter. First, loading segment rise time was varied in order to establish time-dependent behavior of the samples. Experimental data showed that viscous part of the deformation can be neglected under the observed test conditions. The second series of microindentation tests were realized with variation of number of cycles and indentation hardness and modulus were obtained. Approximate hardness was also calculated using analysis of residual impression area. Porosity characteristics were analyzed using CellC software. Scanning electron microscopy (SEM) analysis showed that air-cured bone cement exhibited significant number of large voids made of aggregated PMMA beads accompanied by particles of the radiopaque agent, while vacuum treated samples had homogeneous structure. Air-cured samples exhibited variable hardness and elasticity modulus throughout the material. They also had lower hardness values (approximately 65-100 MPa) than the vacuum treated cement (approximately 170 MPa). Porosity of 5.1% was obtained for vacuum treated cement and 16.8% for air-cured cement. Extensive plastic deformation, microcracks and craze whitening were produced during indentation of air-cured bone cement, whereas vacuum treated cement exhibited no cracks and no plastic deformation.

A General Orthotropic von Mises Plasticity Material Model With Mixed Hardening: Model Definition and Implicit Stress Integration Procedure

A general orthotropic von Mises plasticity model, with an extension of the Hill’s yield criterion to include mixed hardening, is introduced in the paper. Material constants and equivalent stress-equivalent plastic strain curves are defined in a way to suggest their experimental determination. The model represents a special case of a general anisotropic metal plasticity model proposed by the authors. An implicit stress integration procedure, representing an application of the governing parameter method (GPM) introduced by the first author, is presented. The GPM is briefly described, and the computational procedure, together with calculation of the consistent tangent moduli, are given in some detail for a general three-dimensional deformation, with direction of application to plane stress/shell conditions. Numerical examples illustrate applicability of the model and effectiveness of the computational algorithm.

Reverse modeling and solid free-form fabrication of sternum implant

The paper presents a case where an implant for a part of the sternum (with costal cartilages) affected by cancer was created and implanted by using the specific reverse modeling method and solid free-form fabrication. The method provides surgeons with a fast and reliable tool for tissue engineering and implantation and therefore improves the quality of life for patients. Digital images of healthy sternum samples were used to develop a reverse modeling algorithm that semi-automatically generates a necessary and sufficient simplification of the tissue geometry to be fabricated in an inexpensive and applicable manner. In this particular case, the redesign of the missing part of the sternum in CAD software took three designer-hours. At the same time, the suitable simplification of the geometry affects the fabrication of simpler and less expensive casting molds. Furthermore, the core of the developed algorithm for the reverse modeling of sternum can be applied in the reverse modeling improvement of other tile (or plate-like) bones.

Elastic-Plastic Orthotropic Multilayered Pipe Deformation Under External Load and Internal Pressure

Elastic-plastic deformation of a thin-walled pipe, composed of layers with material orthotropic in the elastic and plastic domains, is analyzed. The first material direction for each layer is inclined for an angle (+a and -a successively) with respect to the pipe axis. The yield condition of the material represents a generalization of Hills criterion to include material hardening. The pipe is loaded by internal pressure and other external loads, and it is supposed that the pipe cross section is free to expand or contract. The derived incremental relations for stress integration, which take into account the stress-strain conditions in the pipe wall, are based on the governing parameter method, developed by the first author, where the problem of implicit integration of inelastic constitutive relations within a tune (load) step is reduced to solution of one governing nonlinear equation. Also, the expressions for the tangent elastic-plastic constitutive matrix are derived. One solved simple numerical example demonstrates the main characteristics of the developed algorithm, especially suited for a general elastic-plastic analysis of composite pipes (within the displacement-based finite element method). B, N C e, ein p S Nomenclature = material constants = tangent constitutive matrix = total and inelastic strains = governing parameter = deviatoric stress = internal variable = increment of plastic strain = stress

Custom design of furniture elements by fused filament fabrication

Additive manufacturing technologies enable rapid prototyping of different parts, according to the three-dimensional model software solution. This paper presents some aspects of fused deposition modeling technology and its application in the wood industry. The fused deposition modeling technology was initially developed for three-dimensional printing of plastic parts, whereas acrylonitrile butadiene styrene and polylactic acid plastics filament are commonly applied. Possibilities for application of different composites with fused deposition modeling in the wood industry are reviewed and presented in this paper. Several industrial applications were considered also, from aspects of material durability, mechanical strength, low cost, and customization. Directions of further research have been discussed, considering graphene and carbon nanotubes as composite reinforcement materials and bio-organic composites with wooden particles or fibers mixed into the polymer matrix. Development of new composites for use with the fused deposition modeling technology is promising area taking into account that new low-cost extruders are already commercially available, as a support to fused deposition modeling device. Three-dimensional printing is very convenient for investigation of different custom-made composite materials, as well as custom shapes of final parts, starting from powders, their mixing, drawing of composite material filament, which are further used for three-dimensional printing with solidification and fabrication of custom-made products. New composites for fused deposition modeling, made of wood and plastic combinations were experimentally investigated for use as functional and customized elements of furniture. Obtained results strongly indicated that both new composite materials and fused deposition modeling printing can be efficiently used for broad customization from aspects of material properties and product shapes, thus enabling low-cost fabrication of small series of complex furniture elements, especially fixtures or clamp tools.

Characteristics and Applications of Silver Nanoparticles

This review provides an insight into the level of knowledge about the properties of silver nanoparticles, their already existing applications and possible further developments, as well as their effects on different behaviour and properties of the products wherein they are used. This chapter reviews the application fields of nanosilver, starting from basic silver properties and influential parameters in definition of silver nanoparticles (Ag NPs). Toxicity of Ag NPs is observed from various aspects in relation to cell toxicity and relevant mechanisms of their behaviour within tissue environment. Range of sizes and surface chemistry of Ag NPs and possible effects, as well as known in vivo effects are reviewed, based on the already established research results. Antibacterial properties of Ag NPs and relevant mechanisms of action are presented. Application areas where commercialization of nanosilver has started are presented: medicine (wound dressings, drug delivery, biosensors and medical diagnostics, orthopedics), food and textile industries, and water disinfection systems. Environment related issues have been considered and important conclusions derived from established results are presented. Each application sector comprises descriptions of basic mechanisms related to Ag NPs, gained benefits, but also possible risks and recognised limitations in application of Ag NPs. Future directions as recognised in specific research groups dealing with some of the sectors are listed. Exquisite properties of Ag NPs, especially antibacterial and optical properties, along with availability and lower cost of fabrication, processing and storage, compared to other noble elements makes them very promising for numerous future applications. However, proven cell toxicity must be further studied and methods to overcome these adverse effects on tissue in general are subject of current research. One important application field is drug delivery for targeted cancer cell destruction that is expected to show particular results very soon. Even with all the limitations that need to be imposed on usage of Ag NPs, they are promising nanoagent for novel advancements in different areas.

Biodegradable Metals as Biomaterials for Clinical Practice: Iron-Based Materials

This review presents the state-of-the-art in the development of iron-based degradable medical implants. Basic properties demanded by the new concept of degradable implants are elaborated, along with the work devoted to understand the underlying mechanism and to improve the properties towards best fitted to the natural tissue. Three application areas are considered: vascular stents, orthopedic implants and tissue engineering scaffolds. Each of these has its own specific demands imposed upon the artificial substitution materials. Biocompatibility is an essential feature that each medical implant must have, but different aspects can be considered depending on the end application. Furthermore, adequate mechanical properties and various characteristics related to the fabrication and in vitro and in vivo testing are presented for pure iron, alloys and composites, as well as joint structures. Corrosion control is a foundation in the development of these materials development and different aspects are also given. Iron-based materials need increased degradation rate because they are still more similar to the permanent implants, due to the slow corrosion process and various methods to overcome this issue have been tried. Porosity and its relation to material structures, mechanical properties, degradation behaviour, magnetic properties, and fabrication technologies, as well as methods of numerical simulations as a supporting tool have been elaborated. Porous structures represent one way to enhance corrosion, while maintaining intact other necessary properties of the biomaterial. Economic impact of the biomaterials sector in general is significant and justifies large investments in research. Iron-based materials for degradable implants are not in clinical practice yet, but the research results achieved so far promise the future applications.

Differences Between Adopters and Non-adopters of Innovation: Case Study of New Technologies|Adoption by Small and Medium Enterprises in Serbia

This article explores the factors that govern the process of innovation adoption by companies. We have studied the factors that promote the willingness or resistance of companies to adopt new research results or novelty, in general. As one case study, we will elaborate factors that influence a website adoption by smaller companies that mainly do not have their own ICT departments. The study investigated the attitudes of managers towards Internet-mediated business development and promotion, comprising a case study of small and medium-sized enterprises (SMEs) in Serbia. Special focus was placed on low income micro-SMEs. The results showed that, in the case of larger organisations, the models proposed by Flanagin (Hum Commun Res 26(4):618–646, 2000) and Davis (MIS Q 13(3):319–339, 1989) can be effectively used as guidance for website adoption. However, in the case of very small companies, with limited budgets and locally oriented markets, a few of those factors are more prominent, while some have very little influence on the adoption process. Education background of the owners essentially determines decision making and the adoption process, and competitive pressures have no significant influence. The annual income of the company has a certain influence, but, even with the available budget, micro-SMEs will not consider introduction of a website if they cannot perceive any benefits, often due to a lack of awareness of innovations in ICT and marketing. If the owners can perceive website benefits, the cost of introduction and maintenance has little influence. Specific cultural aspects of the region should be considered as the significant factor influencing innovation introduction.

Introduction—The Current Status and Momentum in Nanotechnology Commercialisation

Everybody is talking about nanoscience and nanotechnology. Nowadays ‘nano’ is very popular in science and technology, as well as in everyday news in journals, magazines and newspapers. If you ask non-scientific people “What is nano?”, the majority would probably respond “something small and new”… We can use some of the definitions, starting from Feynman’s famous sentence, “There’s Plenty of Room at the Bottom” or some officially adopted such as “Nanotechnology is science, engineering, and technology conducted at the nanoscale, which is about 1 to 100 nm.”, USA National Nanotechnology Initiative. Today, the term nano has become more than this simple definition. It is probably also becoming a kind of philosophy, as once in ancient Greece, philosophy was the science comprising mathematics, biology and other fields of natural science.