Radoslav Aleksić

Stress analysis in hydroxyapatite/poly-L-lactide composite biomaterials

Abstract A three-dimensional (3D) finite element (FE) analysis of the stress concentration factor (SCF) in biocomposite model cell has been performed. The model composite consisted of a hydroxyapatite hard particle (HAp) embedded in poly- l -lactide soft matrix (PLLA). Two cases were considered, namely the shape of the particle was held constant while the volume fraction of HAp was varied and the particle shape was changed whilst the volume fraction was constant. For block shaped embedded particles, it was found that the SCF decreases with an increase of the HAp particle volume fraction. It was also found that the shape of reinforcing particles had little effect on the mechanical behaviour of material.

MECHANICAL AND MAGNETIC PROPERTIES OF COMPOSITE MATERIALS WITH POLYMER MATRIX

Many of modern technologies require materials with unusual combinations of properties that cannot be met by the conventional metal alloys, ceramics, and polymeric materials. Material property combinations and ranges have been extended by the development of composite materials. Development of Nd-Fe-B/polymer composite magnetic materials has significantly increased interest in research and development of bonded magnets, since particles of Nd-Fe-B alloys are proved to be very suitable for their production. This study investigates the mechanical and magnetic properties of compression molded Nd-Fe-B magnets with different content of magnetic powder in epoxy matrix. Mechanical properties were investigated at ambient temperature according to ASTM standard D 3039-00. The obtained results show that tensile strength and elongation decrease with an increase of Nd-Fe-B particles content in epoxy matrix. The modulus of elasticity increases, which means that in exploitation material with higher magnetic powder content, subjected to the same level of stress, undergoes 2 to 3.5 times smaller deformation. Scanning Electron Microscopy (SEM) was used to examine the morphology of sample surfaces and fracture surfaces caused by the tensile strength tests. The results of SQUID magnetic measurements show an increase of magnetic properties of the investigated composites with increasing content of Nd-Fe-B particles.

Wood-Thermoplastic Composites Based on Industrial Waste and Virgin High-Density Polyethylene (HDPE)

This article investigated the water resistance, mechanical and thermal properties, and the microstructure of wood–plastic composites, which were made by hot pressing using either virgin or high-density polyethylene (HDPE) as industrial waste with wood filler. The waste polyethylene was collected from residues of polymer production from Petrochemical plant, and wood particles were obtained from a local sawmill. The mechanical and thermal properties of the composites based on waste polyethylene were lower to those based on virgin HDPE, and the water resistance was shown to be higher. Adding ethylene maleated anhydride copolymer (MP) by 0.5–1.5 wt% in the composite formulation significantly improved both the stability and mechanical properties. Microstructure analysis of the fractured surfaces of MP modified composites confirmed improved interfacial bonding.

Magnetic and Dynamic Mechanical Properties of Nd-Fe-B Composite Materials with Polymer Matrix

At the end of the last century whole variety of technical-technological achievements occurred. Cars overflow the streets, air-conditions coated the facades, everyday life became unimaginable without computers and cell phones; kitchens are filled with assorted appliances, and industry has been improved and enforced by high technology in order to achieve higher manufacturing products, with less energy and manpower consumption. If a deeper look is taken, behind modern design exteriors, inside all of these appliances, it is noticeable that one of the key roles in their performance is engaged by permanent magnets. These materials have very important role as functional components within the wider spectra of contemporary devices in different industrial branches, as well as in the wider consumption. One of the most important applications of the permanent magnets are: spare parts in AC and DC engines production (Slusarek & Dudzikowski, 2002), as well as synchronized motors, transformers, actuators, magnetic buffers, stationary fields, etc. (Gutfleisch et al., 2011). Information storage (hard discs), communications, medical appliances and scanners, automobile and aircrafts industries, could also be emphasized as important applications (Matsuura, 2006; Brown et al., 2002). The nanocrystalline Nd-Fe-B alloys are one of the most superior magnetic materials with high value of maximum energy production (app 50 MGOe) (Herbst, 1991). Besides their high values of the remanence and coercivity, as well as relatively high Curie temperatures (app 312 °C) (Sagawa et al., 1984) this type of magnetic alloys are identified suitable for research and further development of magnetic composite materials with polymer matrix, so called bonded magnets (Brown et al., 2006; Ma et al., 2002). Contemporary research in the field of magnetic composite materials on the basis of Nd-Fe-B alloys are directed into four basic directions: increase of magnetic energy, meaning optimisation of magnetic capacities; improving corrosion resistance; optimisation production process of process parameters; and reduction of the subtle rare earth content (Nd), targeting decreasing the price of the final magnetic material, keeping high values of the maximum magnetic energy. Application of

Delamination Detection in Woven Composite Laminates with Embedded Optical Fibers

Woven glass fiber reinforced plastics (GRP) are being widely applied to advanced structures in aerospace, automobile, and marine industries. Optical fibers, originally developed for telecommunications, can be used as sensors that can monitor the state of damage and strain in advanced composite materials. In intensity-based sensors a parameter, such as strain, produces a change in the power of the light propagating in the fiber. The present study aimed to assess the effectiveness of using an optical fiber, stripped of the buffer and acrylate coated, for delamination detection in woven GRP three-point bend specimens.

Keratin–polyethylene oxide bio-nanocomposites reinforced with ultrasonically functionalized graphene

Polyethylene oxide (PEO) functionalized graphene (f-G) was prepared by ultrasonication of pristine graphene in PEO aqueous solution. The feasible sonication protocol of PEO degradation and graphene functionalization enabled fabrication of solvent cast nanocomposites. Additionally, the steps to form new bio-nanocomposite films have been described. Taking the advantage of the combination of graphene, PEO and keratin fibers from poultry feather waste, the aforementioned bio-nanocomposite films were designed with extraordinary properties allowing the films to have promising applications as eventual packaging materials and enabling bio-waste keratin to be converted into value-added materials. Compared to neat PEO, addition of only 0.3 wt% f-G provided an increase of 92% to the storage modulus. These findings are similar to the nanoindentation results, which yielded increases in the reduced modulus of the same composition by about 92%. Nanoindentation testing shows that the incorporation of 0.3 wt% f-G increased the reduced modulus and hardness of the keratin–PEO blend by about 155 and 99%, respectively.

Dynamic mechanical and impact properties of composites reinforced with carbon nanotubes

This study presents dynamic mechanical and impact properties of the new form of hybrid thermoplastic composites. The six composites of polyurethane/p-aramid multiaxial fabric forms (Kolon fabrics) were impregnated with 10 wt% poly(vinyl butyral) (PVB)/ethanol solution with the addition of pristine multiwalled carbon nanotubes (MWCNT) where the PVB/fabric ratio was 20 wt%. All the composites consisted of four layers of the impregnated fabrics. The MWCNT/PVB content was 0, 0.5 and 1 wt%. The surface of the three composites with different MWCNT/PVB content was modified with γ-aminopropyltriethoxy silane (AMEO silane)/ethanol solution. The physical and dynamic mechanical properties of the prepared composite samples were analyzed by dynamic mechanical analysis (DMA) and high speed puncture impact tester. The structures of the composite samples were investigated by scanning electron microscopy (SEM). The results showed that the Kolon/AMEO/PVB/1 wt% MWCNT sample yielded a 60 % improvement in the storage modulus and a 73 % improvement in the impact absorbed energy compared to the Kolon/PVB sample. The carbon nanotubes (MWCNT) were added to improve the dynamic mechanical and impact properties of the materials for ballistic protection.

Effect of Silane Coupling Agents on Mechanical Properties of Nano-SiO2 Filled High-Density Polyethylene Composites

Composites with nano-SiO2 particles and high density polyethylene (HDPE) matrix were produced by hot pressing with various particle contents and particle surface treatment using commercially available silane coupling agents: γ-methacryloxypropyltrimethoxy silane and γ- glycidyloxypropyltrimethoxysilane. The influence of the particle treatment on the mechanical properties of composites was determined by compression and indentation tests. Additionally, numerical analysis was performed in order to calculate Young’s modulus and stress concentrations for various particle contents in order to provide reference data by simulating micro- and macro particle composites with perfect bonding to the matrix.

Low Energy Impact Damage Detection in Laminar Thermoplastic Composite Materials by Means of Embedded Optical Fibers

The possibility of applying optical fibers as sensors for investigation of real time low energy impact damage in laminar thermoplastic composite materials has been studied. For that purpose intensity based optical fibers were embedded in composite material specimens. Kevlar 129 (DuPont’s registered trade-mark for poly (p-phenylene terephthalamide)) woven fabric was used as reinforcement. Impact toughness testing by the Charpy impact pendulum was conducted in order to investigate low energy impacts. Transient intensity of optical signal during the impact, were compared with material crack initiation energy and crack propagation energy. Following this approach, development of damage in material was monitored. Obtained results show that intensity based optical fibers could be used as detectors for material damage appearance, and also, for level evaluation of its degradation caused by low energy impacts.

Ductile fracture prediction of steam pipeline steel

Considering the conditions to which steels used for the manufacture of steam pipelines are exposed, the micromechanism of their destruction in exploitation is exclusively the ductile one. In order to make an estimation of the level of the damage that occurs in exploitation, in this paper a combined experimental and numerical procedure has been developed based on micromechanical or local approach to the fracture mechanics of metallic materials. After the analysis of the results obtained for micromechanical criterion of failure for virgin steel and that used in the steam pipeline, a proposal for prolongation of the working life of tested steel for steam pipelines until the next overhaul is given.