Key Engineering Materials Vol. 446

Abstract: Recently peaceful use of the nuclear energy and radioactive nuclides has increased the radiation pollution factor in the world and radiation safety problems have become actual ones. The development of the nuclear power engineering, protection and processing of radioactive wastes of nuclear reactors, the use of radioactive isotopes in national economy, nuclear explosion, industrial wastes and etc. may cause the radioactive pollution of the environment. In the case of such pollution the environment as well as living organisms are exposed to radioactive radiation (a-, b-, γ- etc.). Radioactive substances emit radioactive rays and as their decay time varies for natural and artificial radioactive substances the time of their environmental effect ranges from some years to million years.

Abstract: In order to determine the fracture toughness of the materials presenting high hardness values in the superficial layers, the Vickers micro-indentation was imposed as a reliable procedure. That method became attractive because of the relative simplicity of the experimental technique and because of its low cost. There are several calculus relationships that could be applied using the data provided by that method, in order to determine the material fracture toughness. The determination of fracture toughness using the Vickers indentation method is based on the analysis of radial cracks propagation, from the corners of the indentation trace. The length of these cracks is connected with the material fracture toughness, on the basis of some semi-empirical calculus relations that are taking into account the indentation load and some physical characteristics of the test material, as Young’s modulus and Poisson’s coefficient. In the present paper, fracture toughness was determined on a series of ceramic samples, made of the same material, but with different geometrical shapes and obtained by applying different technological procedures. The influence of some technological parameters on the fracture toughness was evaluated. The material fracture toughness was determined, into the vicinity of the propagated cracks (in a sample that could be a final product), on an area with a specified geometric contour. As a preliminary stage, a step by step FEM analysis was made, into the Vickers indentation material region, for different values of indentation load. In this manner, it was proved that the maximum stress value, on the perpendicular direction, as related to the crack diagonal plane, is always located at the peak of the indentation trace, and that is the effective start-point of cracking, for this type of indentation.

Abstract: Carbonaceous materials have been obtained by pyrolysis of composites based on olive stones biomass, novolac resin as binding agent, with or without an aromatic compound (naphthalene). The pyrolysis residue at 1000 °C is 40 w% and its electrical conductivity, σ, is 0.13 S/cm. Small cylindrical specimens have been manufactured and pyrolyzed at 1000 °C in order to be used as electrodes. Platinum was electrodeposited by cyclic voltammetry on these specimens using them as working electrodes or on commercial carbon fibres, respectively, for correlating purposes. The morphology of both carbon materials, used as electrodes, was characterized by SEM images and the presence of Pt was determined based on EDS analysis. The crystallographic planes of Pt–carbon of Pt deposited on carbon materials were characterized with XRD. The oxidation of ethanol from a proper solution using the carbonaceous specimen and the carbon fibres as working electrodes was examined by cyclic voltammetry.

Abstract: The aim of this study is to elaborate electro-rheological fluids based on kaolinite. the scientific characterization made it possible the identification of the composite nature and the checking of the intercalation of the polymer among the clay particles The rheological behaviour of the fluid depends on the electric field. A yield stress of the suspensions is observed, which increases with the applied electric field. An interpretation based on the different modes of association between the clay particles is proposed to account qualitatively for the observed behaviour.

Abstract: The origin of this work takes place in the framework of an industrial collaboration in order to study the behavior of a mechanical joint, used in the industry of railway structures. One problem in using the structural adhesive in the industry is hygrothermal ageing. In these papers; the effect of the adhesive cure temperature on the mechanical properties of a commercial bi-component epoxy bonding joint will be studied. Usually the bi-component epoxy is cured at room temperature. The industrial goal in this study is to increase the mechanical performances of adhesively bonded structures. To achieve this goal the curing process of the adhesive will be optimized.

Abstract: The industrial application (not described for confidential reasons) needs the assembly of a ceramic piece and a steel one. This PhD research work aims to provide a reliable industrial bonding between ceramic and steel substrate using structural adhesive. This industrial joint shall withstand a wide range of temperature with pretty brutal thermal changes and also severe humidity conditions. As prerequisite to this bonding assembly qualification, the purpose of this work is the study of the durability, in a hot-wet environment. Depending on the formulations, we observe different behaviours in term of weight uptake, morphological, thermomechanical and chemical modifications of the epoxy network.

Abstract: The effect of strain rate on the mechanical behavior of thermoplastic polymers (Polymethyl methacrylate, Polycarbonate and Polyamide 66) has been studied. Deformation tests in tension were conducted over the range of strain rate varying between 2.6 10-4s-1 to 1.3 10-1s-1. The Young’s Modulus E and Yield stress σST evolutions have been identified and modelled as a function of the strain rate. It has been established that, in the range of the considered strain rates, the yielding behavior of PMMA and PC is well described by the Eyring theory while for PA66 the Ree-Eyring theory is successfully used to illustrate the yielding behavior. During tensile tests the specimen surface temperatures were monitored using an infrared camera. Results reveal a significant temperature rise at large deformations for PA66 and PC. As the strain rate increases the temperature is steadily increased with deformation due to plastic work. Hence, for PC and PA66, a significant thermal softening is observed after yielding which affects the stress-strain behavior. Thermo-mechanical coupling during polymer deformation can be considered in the modeling of the mechanical behavior of polymers. No self-heating has been detected for PMMA.

Abstract: The mechanical properties of E-glass/epoxy composite at high strain rates are important in evaluating this kind of composite under dynamic and impulsive loading. The in-plane and out-of-plane compressive properties at strain rates from 300 to 2500 s-1 were tested with split Hopkinson pressure bar. Samples were tested in the thickness as well as in-plane direction for seven fibre orientations: 0°, 20°, 30°, 45°, 60°, 70° and 90°. The kinetics of damage and the failure modes were identified using a high-speed photography, infrared camera, optical techniques and a scanning electron microscope. Results of the study were analyzed in terms of maximum stress, Strain at maximum stress, failure modes, damage history and fibres orientation effects. From the experimental data, the stress-strain curves, compressive stiffness, and compressive strain of the composite are rate-sensitive in in-plane and out-of-plane compressive directions. The failure and damage mechanisms are implicitly related to the rise in temperature during static and dynamic compression.

Abstract: Even if the mechanical performances of composite materials give new perspectives for the aircraft and space design, the variability of their behavior, linked to the presence of initial microscopic defects or led in service, constitute however a still important brake in their development. As regards particularly the response to fatigue loads or ageing, the behavior of these materials is affected by several sources of uncertainties, notably on the nature of the physical mechanisms of degradation, which are translated by a strong dispersion in life time. In aerospace industry, low energy impact phenomenon is not well known concerning composite materials and composite structures. Many manufacturers use important safety factors to design structures. The aim of this work is to define the most predominant parameters which permit a good response of damage using experiences plans. The differences of these parameters by using Resin Transfer Molding (RTM) or Liquid Resin Infusion (LRI) process than prepreg one is also studied in this work.