Key Engineering Materials

Published by Trans Tech Publications
Online ISSN: 1662-9795
Publications
Schematic of a sandwiched Brazilian disk specimen.  
Typical load-displacement curves at selected loading angles.  
Normalised stress intensity factors for isotropic and sandwich disks.  
Interfacial fracture toughness of the bone-cement interface.  
Conventionally, the bonding strength of bone-cement interface is obtained by mechanical strength testing which tends to produce large variability between specimens and test methods. In this work, interfacial fracture toughness of synthetic bone-cement interface has been determined using sandwiched Brazilian disk specimens. Experiments were carried out under selected loading angles from 0 to 25 degrees to achieve full loading conditions from mode I to mode II. Solutions for complex stress intensity factors as well as strain energy release rates were obtained for a sandwich disk with a finite interlayer using the finite element method. Phase angles were obtained at a fixed distance to the crack tip. The fracture loads were obtained from the load displacement curves and the values of interfacial fracture toughness were calculated from the fracture loads and the finite element J-integral solutions. The implication of this information on the assessment of fixation in acetabular replacements was discussed in the light of in-vitro fatigue testing of implanted acetabula.
 
In this paper we describe how to improve the workflow in forestry management using the increased capability of today's scanner-systems. We developed a new approach for single-tree delineation based on a volumetric analysis in laser-scanner data. The algorithm detects positions and dimensions of individual trees in a forest for use in a 3D-GIS. The measured data can be combined with other measured and statistical data giving the user a detailed view in the forestry units, leading to a database consisting of individual trees-the so called "Virtual Forest". The volumetric approach was adapted to use a combination of aerial photos and 3D laser-scanner data. While positions were only taken from the better resolved true-orthophotos dimensions and especially height-information are based on the 3D-data.The volumetric approach was implemented into a 3D-GIS and successfully tested using recorded data from a 150 km<sup>2</sup> test-area.
 
Non-contact pneumatic measurement technique is an important method in the dimension measurements of machine parts. In the recent years, the authors have developed the new type of air-pin pneumatic-electronic sensor. In this paper, Ultra-precision Air-pin Sensor and Its Dynamic Characteristic have been studied. The air-pin pneumatic-electronic sensor with 10 μm in the measurement range, and its measurement accuracy is better than 0.1μm, and its response time is less 10 ms.
 
In the past, a prediction equation based on the single nucleotide polymorphisms (SNP) is derived to calculate genomic breeding values (GEBV). However, the genome is very complex; a function could not reflect the relation between markers and phenotypes. Unlike the methods of regression, artificial neural networks (ANNs) could perform well for optimization in complex non-linear systems, however, artificial neural networks (ANNs) have not been used to calculate genomic breeding values (CEBV).In this paper, back-propagation neural network is used to predict the genomic breeding values (GEBV) or polygenic genotype value, and the different learning rate and hidden neurons number were used to discuss the influencing of the learning rate on estimating the polygenic genotype value. The result showed artificial neural networks could gather knowledge by detecting the relations between molecular marker polymorphism and phenotype value, and could predict the animal polygenic genotype value or breeding values as well as the molecular marker genotype being given. Training speed, prediction accuracy and stability could be improved along with enlargement of number of hidden neurons. The learning rate could not affect the prediction accuracy, and could almost affect the training speed. The training process was quite sensitive to the number of hidden neurons, even a hidden neurons change could lead to conspicuously training time prolong. It was necessary to have an applicable number of hidden neurons for predicting polygenic genotype value.
 
This paper describes a bipolar outputs DC-DC converter that uses a single inductor for size and cost reduction. We propose timing diagram for a charge pump circuit in the negative voltage generation part, and present its configuration, operation principle and simulation results. We also show that employing pseudo-continuous conduction mode improves cross-regulation between the two outputs.
 
The compensation law is a linear positive correlation between thermal activation parameters. It is usually reported in conjunction with certain types of cooperative and structural relaxations as, for example, that of a glass transition relaxation. In this communication we try to show that the compensation relationship is a consequence of three main effects (1) the characteristic temperature region in which a given relaxation presents itself (2) the way in which the experiment is performed (3) the existence a wide distribution of activation entropies characteristic of that type of relaxation.
 
The mechanical properties of copper thin films formed by cold-rolling and electroplating were compared using tensile test and nano-indentation. Both the Young's modulus and tensile strength of the films were found to vary drastically depending on the microstructure in the films. The Young's modulus of the cold-rolled film was almost same as that of bulk material. However, the Young's modulus of the electroplated thin film was about a fourth of that of bulk material. The micro structure of the electroplated film was polycrystalline and a columnar structure with a diameter of a few hundred-micron. The strength of the grain boundaries of the columnar grains seemed to be rather week. Such a columnar structure causes the cooperative grain boundary sliding and the film shows low elasticity and superplastic deformation. In addition, there was a sharp distribution of Young's modulus along the thickness direction of the film. Though the modulus near the surface of the film was close to that of bulk material, it decreased drastically to about a half within the depth of about 1 mum. There was also a plane distribution of Young's modulus near the surface of the film.
 
Stretchable electronics offer potential application areas in biological implants interacting with human tissue, while also facilitating increased design freedom in electronics. A key requirement on these products is the ability to withstand large deformations during usage without losing their integrity. Experimental observations show that delamination between the metal conductor lines and the stretchable substrate may eventually lead to short circuits while also the delaminated area could result in cohesive failure of the metal lines. Interestingly, peel tests show that the rubber is severely lifted at the delamination front caused by its high compliance. To quantify the interface in terms of cohesive zone properties, these parameters are varied such that the experimental and numerical peel-force curve and rubber-lift geometry at the delamination front match. The thus obtained interface properties are used to simulate the delamination behavior of actual three-dimensional stretchable electronics samples loaded in tension.
 
The disturbance observers have been used to estimate the disturbance in the plant. Several papers on design methods of disturbance observers have been published. The parametrization of all disturbance observers for plants with any output disturbance was clarified. However, no paper examines the parametrization of all disturbance observers for plants with any input disturbance. In this paper, we clarify existence conditions of a disturbance observer and a linear functional disturbance observers for plants with any input disturbance. Under these conditions, we propose the parametrizations of all disturbance observers and all linear functional disturbance observers for plants with any input disturbance.
 
Silicon carbide (SiC) materials have increasingly been needed in the wide range of industries, such as for structural components, automobile parts, space telescope, X-ray mirror, and next-generation semiconductors. However, SiC materials have difficulties in super-smooth finishing for their hard and brittle features. The authors have been investigating optimized conditions on their finishing by fine-grinding with the unique grinding process called ELID (electrolytic in-process dressing). When grinding conditions are changed to fine grinding from coarse grinding, each material has a transition phenomenon; from brittle to ductile removal. ELID has a stable grinding ability, so very detailed characteristics of their material-remove mechanisms were to be investigated. Surface analysis of each material has been discussed through the ELID, and this study proposes good finishing conditions for SiC. In this paper, the advantages of the applied fine-grinding are shown, and unique features on grinding characteristics of SiC through various grinding experimental parameters are described.
 
The method of laser Doppler visualization and measurement of 2D velocity vector field in flows with minimization of influence of a multi–particle scattering is discussed. The investigated section in the flow is illuminated with a laser sheet. Our approach is based on reception of a pair wise difference of normalized frequency–demodulated images of a laser sheet in light beams scattered in various directions. The 2D velocities field is formed from linear combinations of images of the laser sheet in frequency–demodulated light scattered in directions orthogonal to it.
 
Spherical alumina (Al2O3) nanoparticles were incorporated into diglycidyl ether of bisphenol A (DGEBA) epoxy resin. The objective of this study is to investigate the effect of water absorption on the mechanical and dielectric properties of the Al2O3/epoxy nanocomposites. The results from tensile tests indicate that the incorporation of the Al2O3 nanoparticles into the epoxy can improve the stiffness of the matrix. DMA results show that the stiffness improvement is more pronounced at the rubbery state of the matrix. The Al2O3 nanoparticles can increase the dielectric constant of the epoxy resin due to the increase in the total interfacial area. Upon water absorption, the mechanical properties of the Al2O3/epoxy nanocomposites decrease evidently, because of the damage of water on the epoxy resin. However, the ductility can be improved by the water absorption process. In addition, both dielectric constant and dielectric loss of the Al2O3/epoxy nanocomposites increases greatly after water absorption treatment.
 
Test methods are presented to determine failure modes and energy absorption properties of composite crash structural elements from quasi-static tests on chamfered carbon fabric/epoxy tube segment specimens under axial compression loads. High speed film and CT scans of failed specimens are used to identify trigger mechanisms, failure mode evolution at the crush front and failure processes during steady crushing. FE models of failure were developed which could be the basis for materials selection and design procedures for crashworthy composite structures. These are based on meso-scale composites ply damage models combined with cohesive interfaces to represent delamination failures, which damage and fail when the interface fracture energy is reached. The models are implemented in an explicit FE code and parameters for the ply damage and delamination models were obtained from related materials test programmes. The FE models were applied to simulate axial crushing in tube segments and C-channels, showing good predictions of measured peak forces at failure initiation, steady crush forces and total energy absorption.
 
An homogeneous introduction of sintering additives to silicon nitride powder compacts is of great importance in the fabrication of high strength silicon nitride ceramics. Inhomogenities and impurities brought into the compacts with addition of sintering additives may influence the microstructure and phase development and subsequently degrade the mechanical properties and reliability of silicon nitride ceramics. Sintering additives in the system of Sm2O3+Al2O3 as metaloxides and nitrates are intoduced to two different kinds of alpha-silicon nitride powder. Thereby, a more homogeneous distribution of additives through an intimate mixing is aimed. Advantages of this type of processing of silicon nitride powders against conventional method are discussed. The contribution of powder characteristics in determination of these factors are displayed.
 
Advanced metallic materials within the Al-base family are being developed for applications on current and future aerospace vehicles. These advanced materials offer significant improvements in density, strength, stiffness, fracture resistance, and/or higher use temperature which translates into improved vehicle performance. Aerospace applications of advanced metallic materials include space structures, fighters, military and commercial transport aircraft, and missiles. Structural design requirements, including not only static and durability/damage tolerance criteria but also environmental considerations, drive material selections. Often trade-offs must be made regarding strength, fracture resistance, cost, reliability, and maintainability in order to select the optimum material for a specific application. These trade studies not only include various metallic materials but also many times include advanced composite materials. Details of material comparisons, aerospace applications, and material trades will be presented.
 
Since two decades technological progress at lasers, video techniques, optoelectronics, computers and evaluation algorithms allows to extract quantitative information from images of flows, even in complex environments. Continuous improvement of such image based measurement techniques and decreasing costs of equipment enabled many research groups to exploit these techniques for extraction of 2-dimensional or even 3-dimensional data mainly for fundamental research. Since a decade ago many image based measurement techniques have found interest in aerodynamics and are even used as a matter of routine in industrial applications, especially in large wind tunnels or at in-flight testing. Application is mainly performed in the scope of large industrial projects in European co-operation. For this purpose mobile measurement systems have been developed, which can be flexibly adjusted to particular testing environments. All data is acquired non-intrusively so that no interference of the flow field by the measurement is to occur. In consequence, the methods developed are particularly suited for the aero-dynamical and aeroacoustical analysis of complex, unsteady three-dimensional flow fields. The paper will report on the state-of-the-art of the application of image based measurement techniques in aerodynamics and will describe some of the current problems and future needs.
 
The aim of this research is to assess the possible use of Italian zeolitic rocks for the production of lightweight aggregates. In particular, both the expansion at high temperature and the technological features of fired products were investigated. Fifteen zeolite-bearing volcanoclastites from Northern Sardinia and three zeolitized tuffs from Campania and Tuscany (Sorano and Campanian ignimbrites and Neapolitan Yellow Tuff) were taken into account. The firing expansion turned out to be mainly dependent on the chemical composition (especially SiO2 and fluxing oxides such as Fe2O3, Na2O, K2O, MgO and CaO) and the water content (largely related to the zeolite amount) of the raw materials. Other relevant parameters were the temperature of maximum expansion (ranging between 1350 and 1500 °C, without additives) and soaking time (between 2 and 5 min). Some products are highly impervious to water (water absorption below 1%) and exhibit a considerable firing expansion (>100% in volume), a low bulk density (0.5–0.7 g·cm−3) and fair technical properties (loose weight and strength of particles). These encouraging results make some of the investigated tuffs interesting raw materials for the production of lightweight aggregates.
 
Different models are used to predict the tensile strength distribution of single fibre composites (SCS-6-fibre coated with Ti-6Al-4V matrix). The strength distribution of uncoated fibres is analysed with the aid of Weibull statistics. The fibre and matrix stresses are calculated in single fibre composites that are cooled down from fabrication temperature to test temperature under subsequent axial loading. Calculations are based on nonlinear analysis. Good agreement between measured Weibull distribution and a predicted one is shown.
 
The formation of amorphous and other metastable alloys is critically affected by nucleation kinetics. Experimental studies of nucleation on cooling the liquid and on heating the glass are reviewed and related to theory. The control of nucleation to produce useful microstructures by devitrification is discussed. The extension of nucleation studies to solid state amorphization is considered.
 
A compliance function is used to quantify the shielding capacity of grain bridging, the degradation of which is the main cyclic fatigue mechanism in alumina. Materials with different grain sizes were processed and the fatigue experiments were performed using the double torsion test. Significant degradation is observed in the coarse grain material and a marked sensitivity to the loading level is outlined. At moderate loads, bridging degradation occurs prior to fatigue crack growth during an incubation period. At low cyclic loads, the shielding capacity can be entirely degraded, leading to a cyclic fatigue threshold equivalent to that of the fine grain material.
 
A new amorphous ceramic material consisting of Si, N, C, and B can be synthesized by pyrolysis of a preceramic polyborosilazane (PBS-Me) made of the "single source" precursor Cl3Si-NH-BCl2 (TADB). Novel ceramic fibers consisting of borosilicon carbonitride (SiBN3C) can be synthesized from this polymer by using a simple melt spinning process, followed by an intermediate curing step and successive pyrolysis of the obtained infusible green fibers. In the present paper we report on the preparation of the ceramic fibres, as well as their thermal stability, and their mechanical behaviour, including high temperature creep data. The reported mechanical data will be correlated to the microstructure of the fibers.
 
The phonon thermal conductivity kappa **p**h(T) at low temperatures is measured on a series of amorphous Pd-Cu-Si based alloys. In detail, the changes of the phonon scattering by low-energy excitations (LEE) at temperatures 0. 3 K less than T less than 1 K are systematically investigated, when the glass temperature T//g and the amount of frozen-in free volume v//f is altered by different methods. On the basis of calculations of the free volume it will be shown that the density of LEE as inferred from the kappa **p**h(T) measurements correlates strongly with the amount of free volume. In contrast to the irreversible changes in kappa **p**h observed upon annealing the samples, also reversible changes occur which are attributed to effects in chemical short range ordering in the Pd-Cu-Si glasses.
 
Scanning electron image and energy dispersive x-ray analysis of the titania infiltrated PYSZ TBCs displaying titanium distribution at the column edges without filling the intercolumnar space (left) and EDS analysis showing composition of infiltrated TBCs at different heights (right). 
The new generation gas turbines are to generate higher gas temperatures which may seriously affect the thermal stability of the state-of-the-art TBCs (e.g. Partially Yttria Stabilized Zirconia, PYSZ). The reasons for that is the occurrence of considerable phase transformation and/or sintering-induced volume changes which drastically degrade the columnar structure of EB-PVD coatings and raise the modulus of elasticity, and as a result the internal stresses, thus restricting the favorable strain tolerance of TBCs. There are two suggested strategies for the improvement of TBCs to be used in the new generation of gas turbine engines; (1) infiltrate coatings with another oxide and hence reduce the diffusion rate at the nanostructured feather-like features and avoid intracolumnar pore coalescence, (2) use new TBC-compositions with alternative crystal structures. This context deals mainly with the first strategy, i.e. infiltration of these structures with a liquid-phase sol based on titania to inhibit the sintering. The applied infiltration method is dip-coating via sol-gel-process. As-coated and infiltrated EB-PVD-TBCs are heat-treated at 1000° and 1100°C and characterized by SEM/EDX. A substantial improvement in the thermal stability of morphology was observed by infiltration of EB-PVD PYSZ TBCs with titania.
 
This paper is concerned with the dynamic tensile characteristics of transformation-induced plasticity (TRIP)-type and dual phase (DP)-type steel sheets at intermediate strain rates ranging from 0.003 to 200 s−1. The dynamic responses of TRIP600, TRIP800, DP600 and DP800 steel sheets are investigated with the evaluation of stress–strain curves, the strain rate sensitivity, the fracture elongation and the effect of pre-strain. The dynamic responses were acquired from dynamic tensile tests at the intermediate strain rates with a high-speed material testing machine developed. Experiments were carried out with specimens whose dimensions were carefully determined by finite element analyses and experiments to induce uniform deformation in the gauge section at the intermediate strain rates. The tensile tests provide stress–strain curves and the strain rate sensitivity. Experimental results show two important aspects for TRIP-type and DP-type steel sheets quantitatively: The flow stress increases as the strain rate increases and the fracture elongation and the formability of TRIP-type sheets are better than those of DP-type sheets at the intermediate strain rates. The pre-strain effect was also investigated for two types of metals at the intermediate strain rates. TRIP600 and DP600 steel specimens pre-stained by 5% and 10% were elongated at the strain rate of 0.003 s−1 for quasi-static loading, and then tested at strain rates of 0.003, 1, 10 and 100 s−1. The results demonstrate that the mechanical properties of TRIP600 and DP600 steels are noticeably influenced by the pre-strain when the strain rate is over 1 s−1. The ultimate tensile strength as well as the yield stress increases due to the pre-strain.
 
The aircraft industry strives for significantly reduced development and operating costs. Reduction of structural weight at safe design is one possibility to reach this objective which is aimed by the running EU project COCOMAT. The main objective of COCOMAT is a future design scenario for composite curved stiffened panels which are understood as parts of real aircraft structures. This design scenario exploits considerable reserve carrying capacities in fibre composite fuselage structures by accurate simulation of collapse. The project results will comprise an experimental data base, improved slow and fast computational tools as well as design guidelines. A reliable simulation of the collapse load requires also taking degradation into account. For the validation of the tools a sound database of experiments are needed which give information about the progress of damage during the loading process. This paper focuses on experimental results of four nominally identical CFRP panels tested within the COCOMAT project at the buckling test facility of the Institute of Composite Structures and Adaptive Systems (DLR). In a first step, three of the four panels were loaded several thousand times. Each time the panel was loaded beyond global buckling and was unloaded to zero. Finally, all panels were tested until collapse. During the tests, advanced measurement systems such as High-Speed-ARAMIS, thermography and Lamb-waves were applied. The test results given in this paper may be used as benchmarks.
 
Partially Yttria Stabilized Zirconia (PYSZ) based Thermal Barrier Coatings (TBC) manufactured by EB-PVD process are a crucial part of a system, which protects the turbine blades situated at the high pressure sector of aero engines and stationary gas turbines under severe service conditions. These materials show a high strain tolerance relying on their unique coating morphology, which is represented by weakly bonded columns. The porosity present in ceramic top coats affects the thermal conductivity by reducing the cross sectional area through which the heat flows. The increase in thermal conductivity after heat-treatment relates to the alteration of the shape of the pores rather than the reduction of their surface-area at the cross section. The studies carried out by the authors demonstrate that the variation of the parameters during the EB-PVD processing of PYSZ based top-coats alters the columnar morphology of the coatings. Consequently, these morphological changes affect primarily the thermal conductivity and eventually the Young' Modulus which are the key physical properties of this material group. New ceramic compositions covering zirconia coatings stabilized with alternative oxides, pyrochlores and hexaluminates are addressed. Failures occurring in ceramic top coats mark the lifetime of TBC system and therefore, it is necessary that their performance should go beyond that of the-state-of-the-art materials. This context summarizes the research and developments devoted to future generation ceramic top coats of EB-PVD TBCs.
 
This study aims at developing lightweight and high performance composite bipolar plates for use in polymer electrolyte membrane fuel cells (PEMFCs). The thin polymer composite bipolar plates (the thickness <1.5 mm) containing of vinyl ester resin, graphite powder, organoclay have been fabricated by bulk molding compound (BMC) process. Organoclay was prepared by ionic exchange of montmorillonite (MMT) with three different molecular weight (Mw) of poly(oxypropylene)-backboned diamine intercalating agents. Results indicate that the basal spacing and content of MMT varied with Mw of POP-diamines are critical in determining the resultant mechanical properties for bipolar plates. Flexural strength of MMT composite plates was increased from 30.21 to 45.66 MPa by adding 2 phr of MMT. The flexural strength of the plate was also ca. 38% higher than the pristine graphite plate as the basal spacing of MMT was increased from 1.71 to 5.43 nm. Meanwhile, the unnotched impact strength of the composite plates was increased from 58.11 to 80.21 J m−1. The unnotched impact strength of the plate was ca. 30% higher than that of the original graphite plates as the basal spacing of MMT was increased from 1.71 to 5.43 nm. The limiting oxygen index (LOI) and the UL-94 test revealed that the bipolar plate possesses excellent flame retardant with LOI >50 and UL-94-V0. The thermal decomposition temperature of each MMT composite plate is also higher than 250 °C. In addition, the bulk electrical conductivity of the bipolar plate with different MMT contents and basal spacing of MMT is higher than 100 S cm−1. The corrosion current is less than 10−7 A cm−2. Results confirm that the addition of MMT leads to a significant improvement on the performance of the composite bipolar plate.
 
This report describes the reinforcement of RBSN by carbon-coated SIC continous fibers (SCS-6, Textron). The comparison of conventional long-time nitridation with a newly developed short-time nitridation procedure shows that the former procedure causes partial fiber degradation whereas the fibers of the latter remain stable. Composites produced via short-time nitridation exhibit favourable stress-strain behaviour due to optimized fiber/matrix bonding. Stress values corresponding to the failure of short-time nitridized composites increased linearly with the fiber content up to about 850 MPa at of 19 vol.%.
 
In current highly integrated microelectronic devices including system-in-package and stacked-die solutions, system reliability is strongly influenced by reliability of the gold and copper wire bond interconnections. Especially in state-of-the-art ICs containing mechanically sensitive low- K dielectric materials, controlling the mechanical properties of the free air ball (FAB) is of utmost significance due to chip damage risks during the bond process. Because of an extreme change in microstructure when forming the FAB, the material properties change significantly. Consequently, it is necessary to determine the properties of the FAB itself, when analyzing chip damage risks via finite element simulations. We present a micro-compression test that allows the determination of the hardening behavior of typical gold and copper FABs with diameters between 45 μm and 75 μm. In this test a FAB is placed on a diamond support or a test capillary and loaded by a diamond flat punch in a microindenter. The hardening was determined from force/displacement behavior via parameter identification using finite element simulations. The identified yield stresses correlate very well with the microstructure which was determined by electron backscatter diffraction method; this means that the yield stress decreases with increasing mean grain diameter in analogy to the Hall- Petch correlation. Compared to unprocessed wires the initial yield stresses are 50% to 60% lower. Considering these material properties, the damage risk during bonding on complex bond pad layouts can be predicted more realistically. This can be shown by results of real bond structures.
 
Mean characteristics of the instantaneous estimators
Instantaneous estimators of the equivalent polynomial system
Restoring force surface: comparison between the exact surface and the surface identified 
This paper presents a technique for identification of non-linear hysteretic systems subjected to non-stationary loading. In the numerical simulations, a Bouc-Wen model was chosen for its ability to represent the properties of a wide class of real hysteretic systems. The parameters of the model are computed instantaneously by approximating the internal restoring force surface through an "ad hoc" polynomial basis. Instantaneous estimates result from time-varying spectra of the response signals. A numerical application of interest to earthquake engineering is finally reported.
 
Sketch of rouleaux Fig. 2: Free energy per unit Fig. 3: Phase diagram for 
Sketch of rouleaux formation by erythrocytes.
Free energy per unit area as a function of separation distance.
Phase diagram for rouleaux formation.
We discuss materials which owe their stability to external fields. These include: 1) external electric or magnetic fields, and 2) quantum vacuum fluctuations in these fields induced by suitable boundary conditions (the Casimir effect). Instances of the first case include the floating water bridge and ferrofluids in magnetic fields. An example of the second case is taken from biology where the Casimir effect provides an explanation of the formation of stacked aggregations or "rouleaux" by negatively charged red blood cells. We show how the interplay between electrical and Casimir forces can be used to drive self-assembly of nano-structured materials, and could be generalized both as a probe of Casimir forces and as a means of manufacturing nanoscale structures. Interestingly, all the cases discussed involve the generation of the somewhat exotic negative pressures. We note that very little is known about the phase diagrams of most materials in the presence of external fields other than those represented by the macroscopic scalar quantities of pressure and temperature. Many new and unusual states of matter may yet be undiscovered.
 
A survey is presented of the principles and practice of tailoring sintering liquid composition and processing cycle to enable crystallisation of intergranular phases in silicon nitride and sialon ceramics. Critical features in sialon ceramics are the O/N balance in residual glasses and post-sintering heat-treatment temperatures to enable nucleation of either intermediate phases at constant composition or oxide phases with re-partitioning of non stoichiometric components in β’ or α’ solid solutions. Crystallisation of disilicate phases in non-sialon compositions exemplifies a problem in control of polymorphs with differing atomic volumes. Crystallisation of intergranular phases has an influence mainly on high-temperature mechanical and environmental behaviour of these ceramics. Key wordssilicon nitride–sialon–crystallisation–microstructure–grain-boundary phase–mechanical properties
 
Ceramic matrix composite (CMC) materials based on 2D-carbon fibre preforms show high heat-absorption capacities and good tribological as well as thermomechanical properties. To take advantage of the full lightweight potential of these new materials in high performance automotive brake discs, the thermal conductivity transverse to the friction surface has to be high in order to reduce the surface temperature. Experimental tests showed, that lower surface temperatures prevent over-heating of the brake's periphery and stabilizes the friction behaviour. In this study different design approaches with improved transverse heat conductivity have been investigated by finite element analysis. C/C-SiC bolts as well as SiC coatings and combinations of them have been investigated and compared with an orthotropic brake disc, showing a reduction of temperature of up to 50%. Original sized brake discs with C/C-SiC have been manufactured and tested under real conditions which verified the calculations. Using only low-cost CMC materials and avoiding any additional processing steps, the potential of C/C-SiC brake discs are very attractive under tribological as well as under economical aspects.
 
A new microcrack-based continuous damage model is developed to describe the behavior of brittle geomaterials under compression dominated stress fields. The induced damage is represented by a second rank tensor, which reflects density and orientation of microcracks. The damage evolution law is related to the propagation condition of microcracks. Based on micromechanical analyses of sliding wing cracks, the actual microcrack distributions are replaced by an equivalent set of cracks subjected to a macroscopic local tensile stress. The principles of the linear fracture mechanics are used to develop a suitable macroscopic propagation criterion. The onset of microcrack coalescence leading to localization phenomenon and softening behavior is included by using a critical crack length. The constitutive equations are developed by considering that microcrack growth induces an added material flexibility. The effective elastic compliance of damaged material is obtained from the definition of a particular Gibbs free energy function. Irreversible damage-related strains due to residual opening of microcracks after unloading are also taken into account. The resulting constitutive equations can be arranged to reveal the physical meaning of each model parameter and to determine its value from standard laboratory tests. An explicit expression for the macroscopic effective constitutive tensor (compliance or stiffness) makes it possible, in principal, to determine the critical damage intensity at which the localization condition is satisfied. The proposed model is applied to two typical brittle rocks (a French granite and Tennessee marble). Comparison between test data and numerical simulations show that the proposed model is able to describe main features of mechanical behaviors observed in brittle geomaterials under compressive stresses.
 
EB-PVD thermal barrier coatings (TBCs) are used on advanced turbine blades to increase the engine efficiency and improve the blade performance. Partially Yttria Stabilized Zirconia (PYSZ) is the standard material for current TBC applications. Lower thermal stability of the PYSZ-based TBCs, however, seriously affects the performance at demanding service temperatures. For the new generation turbines where higher operating gas temperatures (> 1200°C) are to expect, the performance of turbine blades can be improved by replacing the state-art-of-material PYSZ with superior thermal barrier coatings which belong to different crystal structures such as magnetoplumbite. Magnetoplumbite structure through its interlocking grain morphology and unique crystal structure provides essentially a sintering resistant, low thermal conductive layer, but also imparts a catalytic layer to reduce the environmentally harmful substances produced during propulsion and increase the catalytic performance. The complex structures of these compounds make it difficult to realize by conventional methods and requires careful adjustment of process parameters. The morphology and crystallographic aspects of these coatings as well as the mechanisms controlling the improvement are highlighted.
 
Pressure-assisted infiltration with a 2014 Al alloy of plain and Cu-coated single crystal platelets of alpha silicon carbide was used to study particulate wettability under dynamic conditions relevant to pressure casting of metal-matrix composites. The total penetration length of infiltrant metal in porous compacts was measured at the conclusion of solidification as a function of pressure, infiltration time, and SiC size for both plain and Cu-coated SiC. The experimental data were analyzed to obtain a threshold pressure for the effect of melt intrusion through SiC compacts. The threshold pressure was taken either directly as a measure of wettability or converted to an effective wetting angle using the Young-Laplace capillary equation. Cu coating resulted in partial but beneficial improvements in wettability as a result of its dissolution in the melt, compared to uncoated SiC.
 
The basic requirement for the use of a ceramic coating is sufficient adhesion to its substrate. A measure of the adhesive properties of a coating is the interfacial fracture toughness. The test method applicable for interfacial fracture toughness measurements depends on the mechanical properties of the material system and the geometry of the test piece. In this work, indentation methods have been evaluated for the estimation of the fracture toughness of ceramic thermal barrier coatings on metallic substrates. Coatings of 100 to 300μm thickness were applied by electron beam - physical vapour deposition. The performed test types were Vickers indentation at the interface of polished cross sections of the coating system and Rockwell indentation with a brale C indenter, penetrating the coating perpendicular to the surface. Both tests generate delamination, in which the delamination crack length corresponds to the interfacial fracture toughness. Fracture surfaces and cross sections of the fractured coatings were investigated by optical and scanning electron microscope. Determined fracture toughness values are discussed with respect to the loading conditions in the test and the fracture process - i.e. interaction between indenter and coating system and the crack propagation path.
 
Ceramic joining has been recognized as one of the enabling technologies for the successful utilization of ceramic components in a number of demanding, high temperature applications. Various joint design philosophies and design issues have been discussed along with an affordable, robust ceramic joining technology (ARCJoinT). A wide variety of silicon carbide-based composite materials, in different shapes and sizes, have been joined using this technology. This technique is capable of producing joints with tailorable thickness and composition. The room and high temperature mechanical properties and fractography of ceramic joints have been reported. These joints maintain their mechanical strength up to 1200 C in air. This technology is suitable for the joining of large and complex shaped ceramic composite components and with certain modifications, can be applied to repair of ceramic components damaged in service.
 
Matrix crack propagation in ceramic matrix composites (CMCs) has been investigated with the aid of a finite element analysis. The fibre/matrix bond strength is considered to be zero, so that shear stress transfer is only possible through friction. Compressive and shear stresses at the fibre/matrix interface are modelled with the aid of contact elements, which obey Coulomb friction law. The dimensions of the required three-dimensional models, based on a hexagonal distribution of fibres, are estimated with the aid of calculated stress distributions in single fibre composites consisting of a single fibre surrounded by a cracked concentric ring of matrix material. Evaluating a first 3-D model with a matrix crack over half the fibre spacing (with a single fibre bridging the crack) shows that for the selected material properties (representing SIC fibre-reinforced glass) and in the range of considered coefficients of friction 0 less than or equal to eta less than or equal to 0.9 matrix crack propagation rather than fibre failure occurs.
 
A fiber-reinforced ceramic composite was developed using matrix-infiltrated and non-infiltrated fiber layers. Processed layers were hot-pressed together resulting in an oxide-fiber (Nexte1720), oxide-matrix (mullite) composite. Matrix slurries were prepared using submicron-sized α-Al 2O 3 powder coated with amorphous SiO 2, along with additives to decrease mullite-formation temperature. Room temperature 3-point bend tests were conducted on as-received samples and samples exposed to long-term, high-temperature oxidation. Composites demonstrate both reasonable fracture strength and damage tolerance. The favorable thermo-mechanical behavior can be explained by the composite's laminate-type structure. Matrix-infiltrated fiber layers provide strength but behave in a brittle, quasi-monolithic manner. Non-infiltrated fiber layers connect infiltrated layers, and provide damage tolerance through in-plane crack deflection and crack bridging. Varying matrix-to-fiber ratio allows tailoring of composite properties, trading off strength for damage tolerance. These composites provide damage tolerant behavior in the absence of fiber interface coatings, and when using a matrix which bonds strongly to fibers.
 
Multi-phase SiAION ceramics offer great scope for property design by controlling the types of phases present and the microstructure development. The most common SiAION composites are the pressureless sintered, glassy-phase containing β or α-β SiAION ceramics, where the presence of α SiAION acts as a means of controlling the hardness and the growth of elongated β SiAION crystals in the microstructure improves the toughness. The SiAION ceramics can also be reinforced by separate additions of other hard, refractory compounds such as SiC and TiN. The best effect is obtained if different toughening mechanisms are combined. The phase composition and microstructure of SiAION composites are discussed and related to some observed mechanical properties.
 
Low temperature sintering of lead magnesium niobate (PMN) has been studied. The sintering temperature of PMN ceramics could be reduced from about 1200°C to about 800–900°C by addition of a small amount of SrO. Dense composite ceramics PMN–xSr (x=0, 0.01, 0.02 and 0.04) with fine grains were obtained. With the results of XRD and EMPA, the sintering mechanism, during which Sr2+ substituted Pb2+ at A site of ABO3 and low-melting substance appeared, has been analyzed. Doping with SrO could reduce the size of crystal grain of PMN. The effects of SrO doping on dielectric properties have also been investigated. The maximum dielectric constant of PMN–0.02Sr sintered at 850°C was 12 000, and its loss tangent at room temperature was only 0.36%, which make it a very promising material for multilayer capacitors.
 
Organic–inorganic nanocomposites were prepared using epoxy and silane modified isocyanuric acid triglycidyl ester (SM-IATE). Novel epoxy nanocomposites containing silicon, nitrogen and phosphorus were fabricated successfully via the sol–gel method to increase thermal stability and flame retardant property. The epoxy was modified by a coupling agent to improve the compatibility of the organic and inorganic phases. Results revealed that T3 are the major environments forming a network structure. XRD showed the structures of hybrids were amorphous. DSC presented that the hybrids possessed a good thermal stability. LOI implied that nanocomposites matched the self-extinguishing property. TGA/MS showed that hybrids liberated less toxic gases than that of pure epoxy. UV/vis spectrum of epoxy hybrid shows the hybrids are transparent. The hybrids possess excellent optical transparency and can be used in protecting coating.
 
The Liquid Silicon Infiltration (LSI) process allows the joining of CMC parts during manufacturing, i.e. C/C-SiC joints can be produced in-situ. Porous C/C components are prepared and fixed together and molten silicon is caused to flow between the surfaces and react with the carbon material to convert it to silicon carbide and bond the surfaces together. Basic investigations have been carried out in terms of surface preparation, gap geometry, addition of paste, and its composition, and infiltration parameters. Lap joint specimens have been manufactured and tested with standardised geometry under compression. The mechanical and microscopical tests allowed the selection of an appropriate joining method which was transferred to real components. This paper deals with first results of this novel liquid phase bonding of CMC components.
 
Oxide based fiber-reinforced composites are attractive materials in applications where long-term oxidation and high temperature stability are of primary importance. In such materials strong interfacial reactions are unavoidable due to the high diffusion rates of oxides. An interphase material which provides crack deflection or crack bridging, and thereby fiber pull-out, needs to be developed. Lanthanumhexaluminate (LaAl 11O 18) is thus an interesting interphase material and the sol-gel process is an effective way to coat fiber yarns with a thin (< 500 nm) layer. Sols can be prepared from both organic and inorganic precursors. In this study, sols were prepared using various combinations of La(NO 3) 3, Al(NO 3) 3, Al-sec-butylate, Lanthanum oxalate, Lanthanum-2,4-pentanedionate and Lanthanum acetate. Subsequent gelation, drying and crystallization behavior were observed. The formation of LaAlO 3 could be suppressed by changing the starting materials. An all-organic precursor route resulted in the formation of LaAl 11O 18 at significantly lower temperatures. LaAl 11O 18 coatings on fibers were microstructurally characterized.
 
Although the autoclave technique produces composite parts of high-quality, the process is time consuming and has intrinsically high-capital and operating costs. Quickstep™ is a novel polymer composite manufacturing technique designed for the out-of-autoclave processing of high-quality, low-cost components with a reduction in cure cycle times. This paper assesses the use of the Quickstep method for the processing of an epoxy/carbon fibre aerospace material and compares this to equivalent composites produced using an autoclave process. Higher process ramp rates, achievable using Quickstep, have been shown to reduce resin viscosity thus facilitating void removal. Manipulation of the Quickstep cure cycle, while the resin is at low-viscosity, has significant effects on the mechanical properties of the product whilst simultaneously reducing the cure cycle time. Using Quickstep curing, samples were produced exhibiting comparable interlaminar properties but lower flexural strength as compared to those produced using the autoclave. However, normalisation of the data to a common fibre volume fraction showed that better interlaminar shear strengths could be obtained using Quickstep. This improvement in specific interlaminar shear strength was postulated to be due to the lowering of the resin viscosity over the duration of the cure, resulting in better wet through of fibres by resin and improved interfacial adhesion between fibre and matrix. This study identifies key parameters associated with the Quickstep process, providing a basis for further optimisation.
 
IRT flash scan of a composite plateFig. 2. IRT flash scan of a composite plateglass fibre (roving fabric)/polyester, impacted glass fibre (roving fabric)/polyester, impacted in the centre (E= 20 J). in the centre (E= 15 J).
IRT flash scan of a composite plateFig. 4. GFRP calibration sample, made of glass fibre(roving fabric)/polyester, impacted glass fibre(satin fabric)/polyester. in the centre (E= 10 J).
C-scans of a composite specimenFig. 18. C-scans of a composite specimenglass fibre(roving fabric)/polyester, glass fibre(roving fabric)/polyester, impacted at E = 30 J. impacted at E = 40 J.
Polymeric layered composites exhibit a variety of damages following in service loading conditions, like delamination, matrix cracking or even fibre breaking. Detection of such damages and assessing their extension and severity is vital during maintenance cycle, in view of keeping the normal operational reliability. For local inspections, IR thermography and ultrasonic scanning are among the best valued NDT methods. The paper describes the inspections performed by IR active thermography, in different variants, and pulse-echo ultrasonic scanning on GFRP. A variety of layered composites and defects/damages were inspected and the results are evaluated independently, in some cases being compared each other, with valuable conclusions for the users of the mentioned NDI techniques
 
Mullite/ZrO2 laminate composites were studied as a model for fiber/matrix system for a better understanding of interfacial relations. The laminate composites were fabricated by tape casting and hot-pressing. It was demonstrated that thermal mismatch stresses were closely related to the formation of cracks and the crack deflection behavior in the mullite/ZrO2 laminate composites. Two kinds of cracks were observed in zirconia layers; one is channel cracks in layers subjected to tensile stress, another is the edge cracks in layers subjected to compressive stress. Among three forms of ZrO2(monoclinic, tetragonal, cubic), tetragonal ZrO2 was effective to deflect the cracks at the interface due to the stress induced terra-mono phase transformation.
 
The paper outlines the manufacturing process of C/C-SiC materials via liquid silicon infiltration. It will focus on suitable fibers and precursors, on the three processing steps to produce the material as well as on the possibilities to tailor the microstructure and consequently the mechanical properties of the material.
 
We begin by examining the diverse connotations of the term quality. The desirable shape traced by the failure rate of the entire life of a good product, which might be called a hockey-stick line rather than a bathtub curve, is introduced. From the hockey-stick line and the definition of reliability, we extract two measurements. The terms r-reliability (failure rate) and durability (product life) are then explained. The conceptual analysis of failure mechanics explains that reliability technology pertains to the design area.The desirable shape of the failure-rate curve of electronic items, the hockey-stick line, clarifies that mean time to failure (MTTF) as the inverse of failure rate can be regarded as nominal life. We then discuss the BX life which is different from the MTTF, and reliability relationships between components and the set product. We recommend reshaped definitions of r-reliability and durability.We clarify the procedure to improve reliability and to identify the failure mode in order to find for right solutions and recommend a generalized life-stress failure model.
 
Top-cited authors
A. Erman Tekkaya
  • Technische Universität Dortmund
Marion Merklein
  • Friedrich-Alexander-University of Erlangen-Nürnberg
Takashi Nakamura
  • Kyoto University
Dieter Herlach
Joost Duflou
  • KU Leuven