Browse Articles

In this paper we determine a field of stress, which verifies the Cauchy equations of equilibrium, the conditions of continuity on the surfaces between layers, and boundary conditions, for a sandwich bar, with symmetrical distribution of layers, subject to traction. We have considered the relationships, previously obtained, for a composite bar built of three layers. Using a mediation formula for deformations and stress, we obtained a new formula for calculating the longitudinal elasticity modulus, in the case in which the constituent materials have different coefficients of transversal contraction. We realized the experimental measurements for test samples from polyesteric resin, reinforced with woven of fibers glass, carbon and glass-carbon. Keywords: composite materials, characteristic curve, elasticity modulus

The major interest of practitioners for FRC dental restorations derives from the biological traits of this technique. The study concerned the biomechanical behaviour of polyethylene fiber reinforced composites. The experimental determination of the maximum pressure force that can be supported by a FRC fixed prosthetic restoration has also been targeted (Construct - Premise Indirect). For this purpose two samples have been used, each been made of a FRC restoration resin bonded to two lateral teeth which are fixed in a plaster block. The 2 samples differ by the fiber width and by the thickness of the veneer composite. The 2 samples have been tested with static short duration loads on a testing machine. It has been observed that both samples had a perfect elastic behaviour for force under 280N (sample 1) and 360N (sample 2). The maximum flexural stresses as a result of the load are 102.77MPa for the first pontic and 92.5MPa for the second one. The persistence of the fiber allows maintaining the bridge under certain conditions and for a certain period of time. If the stress increases the restoration is destructed (the force has to be almost doubled before the important displacement could lead to the fracture of the bridge). In a clinical situation this reversible and cost effective procedure, which is minimally invasive, aesthetically and biologically desirable offers a viable alternative to conventional replacement of lost teeth. Keywords: FRC, polyethylene fiber, prosthetic restoration

This paper presents a numerical simulation for metal sheet bending with elastic pads made of silicone rubber reinforced with different ratios of very small glass fibers. Experimental tests were performed (traction and compression), in order to determine some characteristics for the materials and the results were used in the simulation program. A correction for the geometry of the punch, a prediction diagram for springback angle and the evolution of the force developed by the punch are also presented Keywords: numerical simultation, silicon rubber, glass fibers, finite elements

The work presents the experimental determination of the transverse contraction coefficient í (Poissson’s ratio) for three kinds of composite materials. For this purpose, it was used the digital image correlation method that is an optical method for the measuring of the deformations. The composite materials were made of the same epoxy resin but the reinforcement was different: reinforcement with glass fabric; reinforcement with oak wood flour; reinforcement both with oak wood flour and with glass fabric (hybrid composite material). The specimens were subject to tensile test and during the test, it was used the system Aramis 2M for image acquisition and their post-processing to determine both the longitudinal strain ex (axial strain) on the direction of tensile loading and transversal strain ey on the direction of the specimen width. The variations of the transverse strains ey as a function of the axial ones for each specimen were graphycal lyrepresented. Then, the experimental data were approximated by linear regression that is by linear functions. The slopes of the straight lines represent Poisson’s ratio n. It was found that the adding of the oak wood flour as filling material in case of the composite materials reinforced with glass fabric, leads to the increasing of the Poisson’s ratio n . Keywords: composite material, wood flour, tensile test, Poisson’s ratio

The aim of the present study was to synthesize and characterize some bioconjugated systems based on dendritic polymers and paclitaxel. Three types of polyamidoamine dendrimers (PAMAM) with amine terminated groups and different generations (PAMAM-G1-NH2, PAMAM-G4- NH2, PAMAM-G6-NH2) were used as polymeric hosts. The bioconjugated systems were characterized using FT-IR Spectroscopy and thermogravimetrical analysis. Keywords: dendrimers, paclitaxel, FTIR spectroscopy

The objective of this paper was to review the acrylic based resins biodegradation phenomena. The general objectives are based on multidisciplinary research of biomaterials used in dental medicine. For this analysis, different types of polymers were evaluated for water absorption and mechanical strength degradation caused by exposure to saliva. Two sets of 10 specimens were prepared from the analyzed materials according to manufacturer recommendations. The mechanical properties determined on wet samples were lower than those of samples from the same materials, tested dry. Difference between the mechanical properties determined in wet and dry was higher for those materials in which the degree of water absorption was higher. This indicates a clear influence of saliva on the biodegradation of these materials. Key words: dental polymer, mechanical properties, saliva

This paper presents the results of the experimental researches regarding the influence of the composition of some film-forming polymeric products for the protection of concrete on certain of their characteristics. Thus, there have come into prominence the film-forming products based on acrylic resins with superior characteristics by comparison to the film-forming products based on acrylic-styrene resins. Keywords: film-forming product, acrylic polymers, concrete, covering system, acrylic-styrene polymers

Aromatic polyimides containing side azobenzene groups have been synthesized by low-temperature solution polycondensation of certain aromatic dianhydrides with aromatic diamines containing preformed side azobenzene units followed by chemical imidization at 100°C in the presence of pyridine and acetic anhydride. The obtained polyimides were characterized and evaluated by FT-IR, UV-Vis, DSC and TGA analyses. These polymers show a good thermal stability with starting decomposition temperature above 300oC and glass transition temperature in the range of 111-230°C. These polyimides show a good solubility in various solvents. The photochromic behaviour, determined by the trans-cis isomerization process of azo-groups, in solution and in solid state, was also evaluated. Keywords: azopolyimides, photochromic behavior, thermal stability

The present work provides new insights about the micro and nanostructure of native starch granules from potato and maize by scanning electron microscopy (SEM) and atomic force microscopy (AFM). SEM images revealed the shape and the size distribution of the granules as well as some features, such as holes and wrinkles, on the surface of granules. These data were confirmed by AFM observations which showed the presence of numerous protrusions (nodules) on the granules surface of both starches. The structure of granules surface consisted of small spherical particles of about 30 nm in diameter, identified particularly for potato starch. These nanoparticles might be related with highly branched amylopectin molecules in substantial agreement with amylopectin blocklets (of about 20 nm) model. Larger particles of about 60 nm up to 80 nm were also visualized especially on the surface of maize starch granules representing different associations of amylopectin and amylose. The largest elongated particles of about 100 nm to 200 nm found randomly on granules of both starches might be assigned to arise from the granule-surface components, such as starch carbohydrates attached to granule proteins and phospholipids, in general agreement with starch granule surface composition data. This investigation also supports the complex structural network for the starch granule surface (periphery) and its role in maintaining the integrity of starch granules and in the starch gelatinization process. Keywords: starch granules, potato starch, maize starch, granule surface, supramolecular structuring, scanning electron microscopy (SEM), atomic force microscopy (AFM)

In this paper, the behaviour of polyamide 66 with 30% short glass fibers (PA66 GF30) is investigated using instrumented indentation technique combined with the dynamic mechanical analysis (DMA). The visco-elastic-plastic behaviour of PA66 GF30 is described by a rheological model which incorporates a spring, a plastic element and Kelvin-Voigt elements. The time-temperature superposition (TTS) principle was used to investigate the effect of time and temperature on the behaviour of PA66 GF30. Keywords: indentation, polyamide 66, creep, dynamic mechanical analysis