Abstract: A statistical evaluation based on the Weibull method was performed to correlate the mechanical properties and the diameter of different lignocellulosic fibers. The sisal, rami and curaua fibers were found to have a hyperbolic correlation between their ultimate strength and diameter. This permitted to select thinner high strength fibers, with over 1000 MPa, as reinforcement for the strongest polymer composites ever fabricated with these fibers. A structural analysis was conducted by electron microscopy to identify the strengthening mechanism for both, the high performance fiber and their improved polymer composites.
Abstract: X-ray computed tomography is an important tool for evaluating the three dimensional microstructure of modern materials non-destructively. To resolve material structures in the micrometre range and below high brilliance synchrotron radiation has to be taken. But materials of low absorption or mixed phases show a weak absorption contrast at there interfaces. A Contrast enhancement can be achieved by exploiting the refraction of X-rays at interfaces. This technique was developed and applied at the NDT department of the Federal Institute for Materials Research and Testing (BAM) during the last decade. It meets the actual demand for improved non-destructive characterisation of high performance composites, ceramics and other low density materials and components. The technique is based on Ultra Small Angle Scattering (USAXS) by micro structural elements causing phase related effects like refraction and total reflection at a few minutes of arc as the refractive index of X-rays is nearly unity. The extraordinary refraction contrast of inner surfaces is far beyond absorption effects and hence especially useful for materials of low absorption or mixed phases, showing similar X-ray absorption properties. Crack orientation and fibre-matrix debonding in plastics, polymers, ceramics and metal-matrix-composites after cyclic loading and hydro thermal aging can be visualized. By combining the refraction technique with the computed tomography technique the three dimensional imaging of the micro structure of the materials is obtained. In most cases the investigated inner surface and interface structures correlate to mechanical properties. Recent results with a sub-micrometer resolution will be presented.
Abstract: Mo-Ti3SiC2 layered material was prepared by spark plasma sintering. Mixed Ti, Si, graphite and Al powder with molar ratio of 3Ti:1Si:2C:0.2Al was put into a graphite mould and pressed with a pressure of about 0.5 MPa, then, Mo powder was put on top of the mixed powder. Experimental results showed that Mo-Ti3SiC2 layered material could be fabricated successfully by sintering the above powder mixture at 1300°C for 20 minutes under a pressure of 50 MPa in vacuum. The surface and interfaces of the layered composite were tight and clear without any observable crack. In order to study the thermal stability at elevated temperature, the fabricated Mo-Ti3SiC2 layered composite was heat treated at 800°C for 5, 10, 20 and 40 hours. After 40 hours of annealing, the intermediate layers formed between the Mo and Ti3SiC2 matrix grew thicker. The interfaces are clean and tight with no obvious formation of voids and new phases. The initial 10 hours of annealing is the fast growing period, after that, the growth rate slowed down significantly.
Abstract: In the present work, the structure and sintering behaviour of mullite-zirconia composites were investigated. The composites were prepared by reaction sintering of Algerian kaolin, α-Al2O3, and stabilized zirconia (3Y-TZP). The raw materials were wet ball milled in a planetary ball mill followed by attrition. The green samples shaped using a uniaxial press were sintered between 1100°C and 1600°C for 2 hours. The density was measured by the water immersion method. Phases present and change of the average crystallite size of the mullite phase as a function of sintering temperature and ZrO2 content were characterized through X-ray diffraction. Mulite grains had whiskers' shape; however, the increase of ZrO2 content changed the morphology of grains to near spherical shape. The microstructure of the samples revealed uniform distribution of ZrO2 particles; also, aluminium was uniformly distributed on all grains exception on zirconia grains. At least a relative density of 95 % was achieved for all samples at a relatively lower sintering temperature of 1500°C. In composites containing up to 16 wt. % ZrO2, the zirconia phase retained its tetragonal structure and the transformed part did not exceed 3 %. However, with the addition of 32 wt. % ZrO2 around 66 % of the tetragonal structure changed to monoclinic structure.
Abstract: Titanium-alloy matrix composites (TMC) are nowadays one of the material class with the highest specific resistance from room temperature up to 800° C. Centro Sviluppo Materiali SpA (CSM) efforts have been focused on the developing of an innovative solution to reduce the process costs. The new approach consists in an experimental “diffusion bonding” plant for co-rolling at high temperature sheets of titanium alloy and silicon carbide monofilaments fabrics. The result is a process cost reduction of about 40% respect to HIP process. The experimental pilot plant has been proposed for patent with n° 2006A000261 on may 2006. This paper describes the pilot plant and the process results. The metallographic examination on products shows full bonded samples (100 mm wide and 1500 mm long) obtained in a work field that is at least 100 times faster than that of HIP. High temperature tensile tests have been carried on Roll Diffusion Bonded specimens and the results are reported in comparison with those obtained by Isostatic Pressing (HIP) and Thermal- Spraying (TS) processes on the same composite.
Abstract: We have recently developed ceramic eutectics, which are named Melt Growth Composites (MGCs). The binary MGCs (Al2O3/YAG and Al2O3/GAP binary systems) have a novel microstructure, in which continuous networks of single-crystal Al2O3 phases and single-crystal oxide compounds (YAG or GAP) interpenetrate without grain boundaries. To characterize the entangled structure of the typical MGCs, the X-ray computerized tomography (micro X-ray CT) was performed at a synchrotron radiation facility Spring8. The micro X-ray CT showed that the Al2O3 and the GAP are entangled with each other. Therefore, the MGCs have excellent high-temperature strength characteristics, creep resistance, superior oxidation resistance and thermal stability in the air atmosphere at very high temperatures. To achieve higher thermal efficiency for gas turbine systems, MGC bowed stacking nozzle vanes have been fabricated on an experimental basis.
Abstract: This paper investigates the combined effects of relative density and material distribution on the elastic constants and the yield strength of metallic honeycombs. Periodic regular hexagonal cell is employed as the structural model. Bending, transverse shearing and axial stretching (or compression) are taken into account in the analysis. Closed-form solutions for the yield strength and for all the five independent elastic constants are obtained for honeycombs with cell walls of uniform thickness. For honeycombs with cell walls of non-uniform thickness, the closed-form solutions would be too lengthy to use in practical applications. We instead provide the numerical results to show the combined effects of relative density and material distribution on the yield strength and on all the five independent elastic constants of metallic honeycombs. The results can serve as a guidance for the optimal design of metallic honeycombs.
Abstract: A novel design for a Fabry-Perot Interferometric Sensor (FPIS) consisting of a Fabry-Perot cavity formed between two bonded surfaces is discussed. The Fabry-Perot cavity and the optical fiber to which it is coupled are used as the sensing element and interconnect, respectively. The Fabry-Perot cavity is fabricated using the Micro Electro Mechanical Systems (MEMS) technology. The introduction of a center rigid body diaphragm gives this sensor considerable advantage when compared with previous Fabry-Perot cavity based sensors.
Abstract: In engineering applications, simulations involving heterogeneous materials where it is necessary to capture the local response coming from the heterogeneities is very difficult. The use of homogenization techniques can reduce the size of the problem but will miss the local effects. Homogenization can also be difficult if the constituents obey different constitutive laws. Additional complications arise if inelastic deformation occurs. In such cases a two-scale approach is preferred and this work addresses these issues in the context of a two-scale Finite Element Method (FEM). Examples of using two-scale FEM approaches are presented.
Abstract: The mechanical behavior of ceramics under high pressures and temperatures is a subject of considerable interest. Since high pressures can be generated under static or dynamic conditions, it is necessary to measure mechanical properties of the materials under both. In the present work, compression and shear strength of titanium diboride measured under plane shock wave compression is revealingly compared with the recent measurement of compression and shear strength of titanium diboride obtained under static high pressures.