Showing papers in "Key Engineering Materials in 2001"

Assessment of the Exit Defects in Carbon Fibre-Reinforced Plastic Plates Caused by Drilling

Abstract: This paper investigates the formation of the exit defects in carbon fibre-reinforced plates and characterizes their features in terms of drilling conditions. It was found that spalling and fuzzing are the major mechanisms of exit defects. The spalling, consisting of a main region and a secondary region, is caused by chisel and cutting edge actions, in which the former plays a key role. The fuzzing, however, exists in the cutting region where the included angle between the fibre direction of the surface layer and that of the cutting speed is acute. A severer spalling damage corresponds to a high spindle speed, a large feed rate and a great thrust force. Some empirical relationships, including a dimensionless formula, were developed for assessing the characteristic dimension of the spalling damage based on the known drilling conditions.

Ceramic Foams: Fabrication, Properties and Applications

Abstract: This paper reviews the current fabrication methods of ceramic foams and discusses their properties and most relevant applications. (orig.)

Crystallisation and Tetragonal-Monoclinic Transformation in ZrO2 and HfO2 Dielectric Thin Films

Abstract: The crystallisation and the tetragonal-to-monoclinic phase transformation in ZrO2 and HfO2 thin films prepared by atomic layer chemical vapour deposition (ALCVD) are studied using high-temperature grazing-incidence X-ray diffraction (HT-XRD). These films are developed for applications as high-k dielectric gate in CMOS transistors. HT-XRD shows that all the tested samples have a crystallisation onset temperature below 600°C. The crystallisation onset temperature depends not only on the material, but also on the film thickness. The thinner a film is, the higher is the crystallisation onset temperature. For a film with the same thickness, HfO2 crystallises at a higher temperature than ZrO2. The phase composition of the crystallised films depends also on the material and film thickness. In ZrO2, the tetragonal phase is more stable than in HfO2. The t-m transformation during annealing has been observed.

Built-In Diagnostics for Monitoring Crack Growth in Aircraft Structures

Abstract: A built-in cost-effective diagnostic system is being developed to monitor fatigue crack growth in aircraft structures. The proposed system consists of a SMART layer with an embedded network of distributed piezoelectric sensors/actuators, a diagnostic unit, and software. The layer is surface-mounted on the critical area where crack initiation and growth are suspected. Using the software, pre-selected diagnostic signals from a designated piezoelectric actuator to its neighboring sensors are generated by the diagnostic unit. The corresponding sensor signals are recorded and compared to a baseline reference, which was previously recorded. Based on the changes in the sensor signals, the software interprets the crack growth condition at the time of measurement. The results of this technique have been verified by experiments.

Laser Microtexturing of Implant Surfaces for Enhanced Tissue Integration

Abstract: Dental and orthopaedic implants rely on combinations of bone and soft tissue integration for mechanical support. While polished metal surfaces have been shown to encourage fibrous tissue encapsulation, roughened surfaces encourage bone integration. It is an accepted practice to roughen or texture regions of implants to encourage bone integration and produce smooth regions to promote soft tissue integration. We have developed a laser microtexturing technique that can be used to produce precision microtextured surfaces. Several techniques are commonly used to produce texturing. These include machining, surface blasting, acid etching, and plasma spraying techniques. These techniques produce a range of surface microstructures of varying sizes and shapes, from sub-micron to more than 50 /an in size. None of these techniques allow micron-tolerance control of microstructure size and placement, and blast texturing, which is usually produced using glass bead for matte finishes and alumina grit for surface roughening, can cause significant surface contamination with embedded fragments of blast medium. Newer blast methods use resorbable or soluble blast medium to avoid this problem. Blasting is often followed by acid etching, which removes the embedded material and produces a fine microtexture of its own. We have developed a computer-controlled Excimer laser micromachining method for producing surface microtexturing on metal implants. This method produces clean, controlled microstructural patterns in defined regions, with micron resolution and reproducibility. We have tested bone response to these surfaces in three animal models, an implantable chamber system, an intramedullary rod model, and a dental implant model. The results indicate that controlled microstructural patterns such as microgrooves in an 8-12 micron size range suppress fibrous encapsulation, microintegrate directly with bone, and control regional integration of bone and soft tissue. These surfaces organize attached cells, causing production of oriented extracellular matrix. The end result is surface microstructural control of the organization of attached tissue. We have applied these surfaces to dental implants and are conducting human clinical trials. We are also developing surfaces for guided tissue regeneration membranes, transcutaneous prosthetic fixation systems, and other biomedical applications. This project was supported by grants from the Orthopaedic Research and Education Foundation, the New Jersey Center for Biomaterials, The New Jersey Commission on Science and Technology, NSF grant DMI-9304020, and NIH grant 1R41 NS351551-01 to Orthogen Corporation. Dental implants were produced in collaboration with BioLok International.

CVI-R Gas Phase Processing of Porous, Biomorphic SiC-Ceramics

Abstract: Natural pine wood was converted into biomorphic SiC-ceramics by CVI-R processing (chemical vapour infiltration - reaction). The wood samples were first pyrolyzed in inert atmosphere at temperatures of 800 C to yield biocarbon-derived template structures. Subsequently, the biocarbon preforms were infiltrated with silicon by isothermal CVI processing with MTS (methyltrichlorosilane) in excess of hydrogen at temperatures between 800 and 850 C, then converted into SiC-ceramic by annealing in inert atmosphere at temperatures between 1200-1600 C. During processing, the inherent open porous structure of the pine wood is retained down to the submicrometer level, yielding a highly porous SiC-ceramic with a unique microcellular morphology. (orig.)

Fatigue Dissipation and Failure in Unidirectional and Angle-Ply Glass Fibre/Carbon Fibre Hybrid Laminates

Abstract: The tensile fatigue behaviour of unidirectional 0degrees, [+/-10](4S) and [+/-45](4S) carbon fibre/glass fibre hybrid composite has been investigated. The dissipation was measured by the stiffness, hysteresis loss and temperature field of the specimen surface in an insulated testing chamber. The hysteresis loss correlates well with the increase temperature. Microscopic studies show frictional sliding of longitudinal crack faces between carbon and glass fibre bundles to be the main source of dissipation for on-axis specimens. With increasing off-axis angle the primary loss mechanism became cyclic shear deformation of the polymer matrix. With a finer dispersion of the constituents of the hybrid, the growth of these longitudinal cracks or of zones of inelastic matrix shear deformation. would be suppressed, which would result in a more fatigue resistant material. A localisation of heat generation sets in just prior to final failure. Damage and heat localisation lead to impending failure. If the parameters that control localisation were better understood, it would be possible to improve the fatigue resistance of the material by sensible microstructural design.

Developing a New Processing Route to Manufacture Honeycomb Ceramics with Negative Poisson's Ratio

Abstract: Materials with negative Poisson's ratios (auxetic materials) have several advantages compared to conventional materials, including increased plane strain fracture resistance and increased shear modulus. A method for the manufacture of honeycomb ceramics with a structure, which yields negative Poisson's ratio is described. Ceramics with cellular or honeycomb structures have applications as substrates in catalytic converters for the emissions from internal combustion engines. In such devices due to the extreme temperature variations, the structure has to provide high thermal shock resistance. It is possible to increase the thermal shock resistance by incorporating specific web geometries. Such structures can absorb or compensate for compressive and tensile stresses generated in the body during thermal expansion and contraction. It is shown that such structures can be produced by the extrusion of ceramic pastes through polymer dies produced by rapid prototyping (selective laser sintering), CAD technology was used to design the die. A ceramic paste based on a zirconia powder was used to make the cellular extruded tube. The negative Poisson's ratio behaviour was confirmed by employing a simple test. (orig.)

Ceramic Based Solar Cells in Fiber Form

Abstract: The way of assembly and utility of a photoelectrochemical solar cell in fiber form has been investigated. A long, flexible tubular solar cell is assembled by building up the solar cell from the inside out. A conductive stainless steel is used as internal electrode. This electrode is coated with semiconductive nanosized TiO{sub 2} powder. The efficiency of different organic dyes used to sensitize the TiO{sub 2} powder is illustrated. The assembly of electrode, ceramic powder and electrolyte is incorporated into a transparent polymer tube that has been coated with conductive polymer on the inside. Considerations for the use of these cells in the textile area will be described. (orig.)

Enhanced In Vivo Response to Silicate-Substituted Hydroxyapatite

Abstract: This paper describes an investigation into the effect of silicate ion substitution in hydroxyapatite on the rate of bone healing in vivo. Slotted cylinders composed of stoichiometric hydroxyapatite and 1.2 wt% silicate substituted hydroxyapatite (S-HA) were implanted in the distal end of the femur of 6 month old New Zealand white rabbits for a period of 3 weeks. Tetracycline and alizarine red fluorochrome labels were administered 7 and 14 days post-operatively, respectively. After retrieval the implants were sectioned and examined using optical and fluorescence microscopy. Optical microscopy demonstrated that significant amounts of bone formation was only consistently found at the center of the machined slots of the silicate substituted implants. Moreover, fluorescence microscopy demonstrated that of the new bone laid down around and in contact with the implants, significant quantities of tetracycline labeled bone (i.e. bone laid down seven days after implantation) was only present in S-HA implants. These results indicate that the bioactivity of hydroxyapatite is significantly enhanced by the incorporation of silicate ions into its lattice

On the long-term stability of normal condition for damage detection in a composite panel

Abstract: Novelty detection provides an effective strategy for carrying out the lowest level of damage identification. The idea is to create a model of normal condition based on data measured when the system is known to be undamaged and then examine subsequent data to see if there are statistically significant deviations from normality. Problems arise with this approach if the normal condition data varies with the environmental conditions. If the object is simply to detect significant change, variation between different normal conditions with differing environments must be factored out. The current paper examines the problem of environmental change for the situation where a composite panel is monitored for damage under changing conditions of humidity. A previous investigation was concerned with variation in temperature and, for the purpose of comparison, a set of temperature cycling tests was included at the end of the humidity experiment. The features used for detection are measurements of Lamb wave profiles and the detection method is outlier analysis. The exercise is carried out experimentally and the controlled conditions are provided by an environmental chamber.

Deformation and Failure in Stochastic Fibrous Networks: Scale, Dimension and Application

Abstract: There is an extensive literature on identification and analysis of representative volume elements (RVE’s) in order to bound intrinsic materials properties in porous and nonporous solids. However, in such analyses there is often an implicit assumption made that solution of several large classes of linear scientific problems can be simultaneous achieved simply via a single solution of Laplace's equation for the domain. We find, however, that for an extremely technologically significant class of disordered fibrous/particular structures, the transport properties and the mechanical properties cannot simultaneously be found using a single field solution. Specifically, alterations in microstructure during loading of the material can produce different degrees of effects on mechanical load transfer and conductivity. The details of load transfer within stochastic, porous arrays are critical in understanding their likely properties, which we have shown to exhibit large variability. Nonetheless, through thoughtful use of stochastic finite element simulation, we are able to provide technologically useful guidance on material synthesis, construction, and estimations of lifetime in several key contexts. In doing so, we have addressed several mathematical issues of solution of field equations around singularities produced by phase contrast, boundary condition choice and material contrast, and geometric features inherent in fused structures. This work is a summary of some of our key findings on the subject, and suggests a roadmap for new areas of study, based both on our simulations and on our experiments on materials used in battery systems.

Porous Ceramics Formed Using Starch Consolidation

Abstract: Porous alumina bodies have been prepared with between 15 and 45% porosity using rice, corn or potato starch as a combined consolidating agent and pore former. Rice starch produces the finest pores but agglomerates more easily and produces less robust green bodies. Porosity is primarily dependent upon starch content, but is also influenced by processing parameters. The strength drops with increasing porosity, but remains as high as 100 MPa at 40% porosity. (orig.)

Multi-Scale Microstructural Characterization of Micro-Textured Ti-6Al-4V Surfaces

Abstract: This paper presents the results of a multi-scale microstructural characterization of microtextured Ti-6Al-4V surfaces that are used in biomedical implants. The hierarchies of substructural and microstructural features associated with laser micro-texturing, polishing and surface blasting with alumina pellets are elucidated via atomic force microscopy (AFM), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and optical microscopy (OM). The nano-scale roughness profiles for the different surface textures are characterized via AFM. Submicron precipitates and dislocation substructures associated with wrought processing and laser processing are revealed by TEM. OM and SEM microand meso-scale images of the groove structures and then described before discussing the implications of the result for the optimization of laser processing schemes. The implications of the results are examined for the fabrication of micro-textured surfaces that will facilitate the self organization of proteins, and the attachment of mammalian cells to the Ti-6Al-4V surfaces in biomedical implants.