Professor Mark Hoffman

Titanium alloys are widely used in parts of dental implants, such as screws and abutments. In practice, unwanted relative sliding between contacting implant parts can cause excessive wear, which may lead to early failure. The effect of a submicron diamond-like carbon (DLC) coating on the friction and wear of Ti6Al4V alloys under sliding contact in artificial saliva was investigated. Critically, the DLC film suppressed adhesion between contacting surfaces, significantly lowered the coefficient of friction and contact stress, and ultimately the wear rate, relative to uncoated Ti6Al4V, while maintaining good film-substrate bonding and undergoing a limited extent of fracture. Results are explained within the framework of contact mechanics. Implications for the development of durable dental implants are discussed.

Despite numerous published studies on the wear of dental composites, few have considered the influence of temperature on the two-body wear process. Additionally, no previous work has considered the influence of temperature on dominant wear mechanisms during the consumption of hot substances, hence the focus of this study. Reciprocating wear tests were carried out at varying artificial saliva lubricant temperatures (37 and 57°C) and material loss was quantified using profilometry. The wear tracks were analysed using FIB/SEM/TEM. Results reveal that the wear rate of a dental composite can significantly increase with temperature, with fatigue/delamination and ploughing acting as dominant mechanisms.

Relaxor ferroelectrics are technologically important materials for applications in, for example, high-temperature capacitors, transducers and nano-positioning systems. These materials have often been reported to exhibit surface or skin layers with distinct physical properties to the bulk. The control of formation and functionality of these skin layers has remained elusive and is becoming increasingly critical due to device miniaturization, where the surface contribution to overall material properties becomes significant. We recently demonstrated that the distinct structural distortion of the skin layer is intimately related to the internal chemical pressure applied by oxygen vacancies and the plane stress conditions at the surface. (S. Kong, N. Kumar, S. Checchia, C. Cazorla and J. Daniels, Adv. Funct. Mater., 2019, 29, 1900344) Here, we demonstrate a unique capability to control the formation and properties of the skin layer through the control of defect concentration. Most interestingly, the skin layer is polar and both electrically and optically active, making it functional and a new candidate for low operating voltage and/or optoelectronic devices. The surface domains in the skin could be altered by applying a small voltage bias (1000 times lower than bulk) or light illumination. A reversible optical change in surface domains provides a new non-contact external control to tune the material polarisation.

NBT-BT-based lead-free relaxors have attracted significant attention due to their excellent ferroelectric properties. They undergo a phase transformation to a ferroelectric state with domains, which plays an important role in their exhibited fatigue behavior. In case of non-ergodic relaxors (e.g. NBT-6BT) and ferroelectric compositions (e.g. NBT-12BT) domain wall pinning by oxygen vacancies leads to severe fatigue, while in case of ergodic relaxors (e.g. NBT-6BT-3(K1/2Na1/2)NbO3) domain wall pinning is relatively less effective. The importance of domain switching and concentration of oxygen vacancies has been further emphasized by comparing fatigue under unipolar and bipolar conditions, performing fatigue on NBT-6BT-3(K1/2Na1/2)NbO3 under non-ergodic (5 °C) and ergodic conditions (40 °C), and intentionally altering point defects concentration in NBT-6BT through non-stoichiometry. Furthermore, it has been shown that permanent fatigue due to electrode delamination and a blocking surface layer may commence at high numbers of fatigue cycles, depending upon the amplitude of fatigue waveform. Finally, the results obtained in this study have been summarized using a schematic.

This paper investigates the tribological and mechanical properties of silk-based nanocomposite coatings which are finding applications in optics, biomedicine and dentistry, thanks to the exceptional mechanical/optical properties and associated biocompatibility of silk. Three different nanocomposite formulations were synthesized, and thin films were prepared by spin coating at different thicknesses and with different post-deposition annealing processes. Ellipsometry, FTIR spectroscopy, AFM, nanoindentation, scratch testing, continuous/reciprocating wear testing, confocal microscopy and SEM were used to characterize the coatings. The results reveal that their hardness and elastic modulus are in the range 0.56¿1.30 GPa and 23.6¿55.4 GPa, respectively, which are much higher than those reported for other silk films in literature. Incorporation of titanate nanosheets also improved coatings¿ scratch resistance.

The occupationally disordered structures and associated local polar fluctuations in lead-free relaxors determine their electrical properties that are also sensitive to external stimuli. These stimuli can lead to phase transitions, and the associated enhancement in the electro-mechanical responses necessitate a better understanding of these transitions. Here we report a non-canonical spontaneous phase transition from a relaxor to a ferroelectric phase in (Na1/2Bi1/2)TiO3-BaTiO3 with temperature. With the help of experiments (dielectric permittivity, diffraction, differential scanning calorimetry, polarization and Raman spectroscopy), a complete picture of the temperature evolution of relaxor behavior leading to this spontaneous phase transition has been reported. Furthermore, it has been shown that internal chemical pressure from oxygen vacancies can be utilized to tailor these phase transitions. Finally, an electric field-temperature phase diagram has been proposed with an emphasis on the influence of the defect chemistry. This work provides new insights into the origin of these spontaneous phase transitions.

(Ba, Ca)(Ti, Zr)O3 ceramics have been considered as a potential lead-free alternative to commonly used lead-based piezoelectric ceramics due to their high piezoelectric performance at room temperature. In this study, the bipolar fatigue behavior of this material is investigated at room temperature. Two compositions were cycled with a bipolar electric field signal at 10 Hz with a maximum of three times the coercive field for up to approximately 107 cycles. Both investigated compositions exhibited high bipolar fatigue resistance compared to other ceramics reported in the literatures. The high fatigue resistance originates from the lack of mechanical damage and a weak domain wall pinning effect due to their location in the phase transition region. It was also found that pore morphology affected bipolar fatigue behavior.

(Ba,Ca)(Ti,Zr)O3 lead-free piezoelectric ceramics have been considered to be one of the most potential lead-free alternatives for PZT in the room-temperature range. The stability of the piezoelectric performance during unipolar cycling is investigated in this study. It is found that the unipolar fatigue behavior is similar to soft PZT. Developments of bias field, offset polarization, asymmetry in strain, and dielectric hysteresis loops are observed during bipolar measurements. The changes are mainly contributed to the migration of charge carriers to the grain boundaries driven by the unscreened depolarization field. Redistribution of the accumulated charge carriers by bipolar electric cycling or thermal annealing can significantly recover the unipolar fatigued state. The unipolar strain response stabilized after 1000 cycles at 0.053% for an electric field of 0.6 kV/mm (d33*= 883 pm/V), which is a good characteristic for actuator applications.

Obstructive Sleep Apnoea (OSA), characterised by repeated collapse of the pharyngeal airway during sleep, causes cessation of breathing followed by arousal, restoring normality. Continuous Positive Airway Pressure (CPAP) is a non-invasive, effective treatment for OSA where positive pressure is applied to the airway through a mask, maintaining patency. Nasal masks are commonly used, contacting the face across the upper lip, sides of the nose and the nasal bridge. Despite health benefits, therapy compliance is sub-optimal, often due to poor mask fit and discomfort. Masks have been designed to conform to the facial profile, but have not taken into account facial deformations. The nature of facial deformations while undergoing CPAP therapy is unknown. This study investigates the facial deformations experienced by a patient while undergoing nasal CPAP therapy. Magnetic Resonance Imaging (MRI) scans of the face were acquired of participants in the reference configuration and while undergoing CPAP therapy. The CPAP scan volume was registered onto the reference volume prior to surface generation for each state. Perpendicular deformation was measured from the reference facial profile to the deformed profile. Large facial deformations were measured at the sides of the nose (4.6±1.6mm) and the upper lip (4.9±1.8mm) with much smaller deformations at the nasal bridge (2.4±0.2mm). When normalised by applied load and tissue thickness, no difference was found. These findings and techniques can be used to consider facial deformation in the development of future nasal CPAP masks to improve comfort and compliance to therapy.

Electrical partial discharges were studied in different piezoelectric ceramics. Epoxy material with micro sized cavities was also tested and compared to the piezoelectric ceramics. It is found that compared to epoxy, partial discharge (PD) occurs at relatively lower electric fields for piezoelectric ceramics. The PD inception voltage was found to be lower for materials with higher relative permittivity. This indicates that the intensification of the electric field within the defects is the main cause for the differences in inception field observed for epoxy compared to piezoelectric ceramics. Furthermore, phase resolved PD pattern analysis was carried out for all materials at elevated electric fields. A broad distribution of the discharge event was observed for both epoxy and hard PZT samples, whereas for soft PZT discharge occurs concentrated at electric fields slightly above the coercive field. This intensification of PDs close to the coercive field suggests that PDs may be enhanced due to an increase of the internal field and electron emission rate induced by the domain switching process.

The dielectric properties and electric-field-induced polarization and strain behavior of (1-x)PZT-xSBN (x ranged from 0 to 1.0 weight fraction) ceramics prepared by a conventional mixed-oxide method were investigated. The dielectric properties indicated that the dielectric constant of PZT could be enhanced with small addition of SBN (x¿=¿0.1). From the polarization hysteresis loop measurements, it was found that the ferroelectric properties of nominal PZT-SBN ceramics changed strongly from the normal ferroelectric in PZT-rich ceramics to the paraelectric character in SBN-rich compositions. The strain hysteresis loops of nominal PZT-SBN under bipolar electric field loading suggested that the butterfly curve was observed in some compositions (0¿=¿x¿=¿0.3 and pure SBN ceramic). This research clearly showed the significance of SBN in controlling the electrical properties of nominal PZT-SBN ceramics.

Fracture of silicon wafers is responsible for lower than desirable manufacturing yields in the photovoltaic industry. This study investigates the fracture response of polycrystalline silicon wafers under sliding contacts at different length scales, by means of macro and microscratch tests which simulate cutting processes. The dominant fracture modes were found to be partial cone cracking (macro) and radial cracking (micro). Statistical analysis of the critical loads for crack initiation showed that polycrystalline wafers are weaker than their single-crystal counterparts, that is, they crack at lower applied loads under comparable conditions. Moreover, the Weibull modulus of polycrystalline silicon was found to be the average of the relevant single-crystal directions. Subsequent microscopic observations and flexure tests reveal that the lower resistance of polycrystalline silicon to scratch fracture is due mainly to the presence of relatively large polishing defects, and not to the weakness of its grain boundaries. Alternatives are proposed to minimize damage during ingot cutting, with a view to minimizing wafer breakages during wafer handling and machining.

Large strain rolling - up to 97% thickness reduction - was carried out on pure copper with two different initial grain sizes: coarse-grained (CG, 24 µm) and ultrafine-grained (UFG, 360 nm). After rolling, classical copper-type rolling texture was obtained for the CG copper while a brass-type rolling texture was observed in the starting UFG copper, obtained by eight-pass equal channel angular pressing at room temperature prior to rolling. Using the Viscoplastic Self Consistent polycrystal model, it is found that the deformation mechanism at large rolling strain is grain size dependent and a change in the major slip mode has a large impact on the type of deformation texture. The brass-type texture in the UFG material can be simulated using {111} < 112¯> partial slip together with {111} < 11¯0 > slip and a small amount of twinning; for the CG material, only {111} < 11¯0 > slip is needed to reproduce the copper-type rolling texture. Further, it is found that the polycrystal deformation condition approaches Taylor behavior in the UFG material whereas much more strain heterogeneity is present in rolling of the CG copper. By orientation imaging electron microscopy the geometrically necessary dislocation (GND) density is found to be decreasing in the UFG rolled samples while increasing in the CG case. This tendency corroborates the texture simulation parameters concerning strain heterogeneity.

Structural efficiency is a common engineering goal in which an ideal solution provides a structure with optimized performance at minimized weight, with consideration of material mechanical properties, structural geometry, and manufacturability. This study aims to address this goal in developing high performance lightweight, stiff mechanical components by creating an optimized design from a biologically-inspired template. The approach is implemented on the optimization of rib stiffeners along an aircraft engine nacelle. The helical and angled arrangements of cellulose fibres in plants were chosen as the bio-inspired template. Optimization of total displacement and weight was carried out using a genetic algorithm (GA) coupled with finite element analysis. Iterations showed a gradual convergence in normalized fitness. Displacement was given higher emphasis in optimization, thus the GA optimization tended towards individual designs with weights near the mass constraint. Dominant features of the resulting designs were helical ribs with rectangular cross-sections having large height-to-width ratio. Displacement reduction was at 73% as compared to an unreinforced nacelle, and is attributed to the geometric features and layout of the stiffeners, while mass is maintained within the constraint.

Attrition-corrosion is a synthesized human enamel wear process combined mechanical effects (attrition) with corrosion. With the rising consumption of acidic food and beverages, attrition-corrosion is becoming increasingly common. Yet, research is limited and the underlying mechanism remains unclear. In this study, in vitro wear loss of human enamel was investigated and the attrition-corrosion process and wear mechanism were elucidated by the analysis of the wear scar and its subsurface using focused ion beam (FIB) sectioning and scanning electron microscopy (SEM). Human enamel flat-surface samples were prepared with enamel cusps as the wear antagonists. Reciprocating wear testing was undertaken under load of 5. N at the speed of 66 cycle/min for 2250 cycles with lubricants including citric acid (at pH 3.2 and 5.5), acetic acid (at pH 3.2 and 5.5) and distilled water. All lubricants were used at 37. °C. Similar human enamel flat-surface samples were also exposed to the same solutions as a control group. The substance loss of enamel during wear can be linked to the corrosion potential of a lubricant used. Using a lubricant with very low corrosion potential (such as distilled water), the wear mechanism was dominated by delamination with high wear loss. Conversely, the wear mechanism changed to shaving of the softened layer with less material loss in an environment with medium corrosion potential such as citric acid at pH 3.2 and 5.5 and acetic acid at pH 5.5. However, a highly corrosive environment (e.g., acetic acid at pH 3.2) caused the greatest loss of substance during wear.

A considerable body of knowledge now exists from studies involving the development of lead-free piezoelectric ceramics and a number of high potential alternatives to current lead-based materials have been identified. Stability under cyclic electric fields is an important property of piezoelectric materials. Here, we review the research to date which shows that fatigue under cyclic electrical loading is prevalent in many lead-free piezoelectric ceramic compositions. However, the variety of compositions and mechanisms for piezoelectric behavior in these materials corresponds to significant variances in the nature of fatigue degradation and the likely mechanisms thereof, which do not directly parallel those of well-studied lead-based materials. In particular, the use of field-induced phase changes as an actuation mechanism provides distinctive fatigue behaviors. Particular attention is given to fatigue of ferroelectric and relaxor (ergodic and nonergodic) structures and their dependence upon temperature and electric field and the potential design of materials with high fatigue resistance. © 2014 The American Ceramic Society.

Although piezoelectric ceramics are widely used, the structural reliability of these materials in long-term service is still a concern. In this study, a nondestructive method for assessing material strength reliability using partial discharge (PD) tests is proposed. The PD inception electric field and the flexure strength are both controlled by the defects in the ceramics which suggests that the PD test can be an alternative nondestructive method for evaluating the structural reliability of piezoelectric ceramics. PD and four-point flexure tests were carried out on piezoelectric ceramics. The influence of the PD test on samples was investigated by comparing the microstructure and electrical properties before and after the PD test. Samples which were not PD tested were also fractured and compared. It was found that no strength degradation was caused by the PD test. Weibull analysis revealed a correlation between the distribution in values for PD inception electric field and flexure strength. © 2014 The American Ceramic Society.

This paper describes the results of an in vitro investigation on the interrelations among microstructure, composition and mechanical properties of remineralizing human dental enamel. Polished enamel samples have been demineralized for 10 min in an acetic acid solution (at pH 3) followed by remineralization in human saliva for 30 and 120 min. Microstructure variations of sound, demineralized and remineralized enamel samples have been analysed using focused ion beam, scanning electron microscopy and transmission electron microscopy, while their compositions have been analysed using energy dispersive x-ray. Variations in the mechanical properties of enamel samples have been assessed using nanoindentation. The results reveal that, under the selected conditions, only partial remineralization of the softened enamel surface layer occurs where some pores remain unrepaired. As a result, while the nanoindentation elastic modulus shows an improvement following remineralization, hardness does not. © 2014 IOP Publishing Ltd.

Many coatings and films of interest for engineering applications and microelectromechanical systems (MEMSs) are brittle, and their use under applied stress is ultimately limited by fracture. Safe design and material selection therefore require detailed knowledge of the fracture strength. The present work reviews the state-of-the-art in its characterization and analysis. First, we review the experimental methods for strength measurement in small-scale specimens. The more conventional nanomechanical testing of micromachined free-standing films is compared with alternative techniques in which testing is conducted directly on as-deposited films. While the former is preferred for the analysis of MEMS components (e.g.: silicon, silicon oxide, silicon carbide), we argue that the latter is advantageous in order to accurately measure the strength of protective coatings containing residual stress (e.g.: carbon based films; titanium oxide, nitride and nanocomposites; thermal barrier coatings) and of ultra-thin films, and to assess microstructural effects. Given the stochastic nature of brittle fracture, we summarize the fundamentals of Weibull theory for the probabilistic analysis of fracture strength. Special attention is paid to critical variables which affect the measured strength, such as loading geometry and specimen size. Finally, extrapolation from laboratory tests to actual loading configurations during service is briefly discussed. © 2014 Elsevier B.V.

This paper presents modelling techniques for achieving convergent and accurate solutions in simulating quasi-static indentation of closed-cell aluminium foams and sandwich panels with aluminium foam core using the commercial finite-element software ABAQUS/Standard. Various indenters are utilised: hemispherical indenters of 10, 15 and 20. mm diameter, a 16-mm cylindrical flat punch, and a long flat punch. The material parameters are established by comparison with the previous experimental results in uniaxial compression. Experimentally-observed tearing of the cell walls is accounted for in the simulations by introducing element failure and deletion criteria. Stress and strain analysis of the results reveals that foam failure occurs where cell tearing takes place. This occurs at critical tear energy and shear strength, independent of indenter size. The resulting load-displacement curves correlate closely with experimental results. The quasi-static indentation of sandwich panels of aluminium foam cladded with aluminium skins is also simulated for hemispherical indenters of diameter 5-20. mm. Cell tearing does not occur until the point of failure and thus, is not accounted for in the simulations. The effect of the skin strength on the stiffness, strength and energy absorption of the panels is elucidated. A particular emphasis is placed on the material formulation for the foam core which would ensure convergence and reliable predictions. © 2014.

Many ferroelectric devices are based on doped lead zirconate titanate (PZT) ceramics with compositions near the morphotropic phase boundary (MPB), at which the relevant material's properties approach their maximum. Based on a synchrotron x-ray diffraction study of MPB PZT, bulk fatigue is unambiguously found to arise from a less effective field induced tetragonal-to-monoclinic transformation, at which the degradation of the polarization flipping is detected by a less intense and more diffuse anomaly in the atomic displacement parameter of lead. The time dependence of the ferroelectric response on a structural level down to 250 µs confirms this interpretation in the time scale of the piezolectric strain response.

The Taylor-Lin polycrystal elasto-viscoplastic model was employed to simulate the fatigue behavior of a strong basal textured Mg alloy (AZ31) at room temperature. Extensive f10-12g tensile twinning in a compression-compression cycling mode was shown though no strain hardening occurred in the stress-strain cyclic curves. Using a volume transfer scheme, twinning was simulated where the twin variants were predefined as grains. Parts of the volumes of the mother grains were transferred to the twins by employing the minimum elastic energy criterion for selecting the two least-energy twin variants. Massive detwinning was shown by simulating through transferring parts of the volumes of the twins back to the mother grains. The modeling approach led to fair agreement with experimental twin activity measured by electron backscatter diffraction. The texture changes due to twinning were well reproduced by the model. Additionally, the stress distribution was examined in the polycrystal in the loaded and unloaded (residual) states, separately for the mother and twin grain populations. It has been found that the twins had large residual stresses which deviated in sign and distribution considerably from the residual stresses in the mother grains. This deviation is caused by the different orientations of the twins and the kinematic hardening obtained naturally with crystal plasticity. The crystal plasticity-induced kinematic hardening offers a good interpretation of the large Bauchinger effect observed in low cyclic fatigue of polycrystalline magnesium alloys. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The response of aluminium sandwich panels comprising thin foam cores and thin face sheets of low and high yield strength was investigated under three-point bending load. The effect of skin strength, bending span and core thickness on the failure modes and loads was investigated. While the lower strength face sheet is associated with lower failure loads, that decrease is not proportional to the yield strength due to the additional failure mode of face yielding. Theoretical models enabled prediction of the failure loads and elucidated the contribution of each failure mode. Failure maps were subsequently constructed which can be used for the design of foam-cored sandwich panels with very thin face sheets. © 2014 Elsevier Ltd. All rights reserved.

This paper outlines measurement and analysis methodologies created for determining the structural responses of electroceramics to an electric field. A sample stage is developed to apply electric fields to ceramic materials at elevated temperatures during neutron diffraction experiments. The tested voltages and temperatures range from -20 kV to +20 kV and room temperature to 200 °C, respectively. The use of the sample environment for measuring the response of ferroelectric ceramics to an electrical stimulus is demonstrated on the instrument Wombat, a monochromatic neutron diffractometer employing a curved positive sensitive detector. Methodologies are proposed to account for the geometrical effects when vector fields are applied to textured materials with angularly dispersive detector geometries. Representative results are presented for the ferroelectric (Bi1/2Na1/2)TiO3- 6%BaTiO3 (BNT-6BT) which show both phase transformation and ferroelectric domain texturing under the application of an electric field. This experimental and analysis approach is well suited for time-resolved measurements such as stroboscopic and in situ studies on a variety of electro-active materials. © 2014 Springer Science+Business Media New York.

(1-x)PZT-xSBT ceramics were prepared by a solid-state mixed oxide method. Phase characteristics of (1-x)PZT-xSBT ceramics showed a PZT phase and new phase in the compositions with 0.1 = x = 0.3, while a mainly new phase was observed in the compositionwith x=0.5. The tetragonality (c/a) of PZT phase decreasedwith increasing in SBT content up to x = 0.3. SEM micrographs of the sample surface showed equiaxed grains of the PZT phase and irregularly-shaped grains of the new phase in samples with 0.1 = x = 0.3. The 0.5PZT-0.5SBT sample showed mainly irregularly-shaped grains of the new phase. The ferroelectric properties of these samples showed that the addition of small amounts of SBT (x = 0.1) into PZT increased the remanent polarization. Copyright © 2013 Taylor & Francis Group, LLC.

PVC/CaCO 3 polymer nanocomposites of differing compositions were produced using a two-roll mill and compression molding. In all formulations, 0.6 phr of titanate was incorporated to assist dispersion during processing. The morphology was observed using transmission electron microscopy, and the static and dynamic mechanical and fracture properties were determined. Fracture toughness examination was performed according to strain energy release test method. The presence of nanometer-sized CaCO 3 particles led to a slight decrease in the tensile strength but improved the impact energy absorption, storage modulus, and fracture toughness. The use of titanate coupling agent softened the polymer matrix and reduced the matrix's modulus. Fracture surface examinations by scanning electron microscopy showed that the coupling agent improved particle-matrix bonding and inhibited void formation around the particles. Finite element analysis suggested that the improved particle-matrix bonding reduced the matrix's plasticity around the particles, which decreased the toughening efficiency of the composites. © 2012 Wiley Periodicals, Inc.

Lead zirconate titanate (PZT) is one of the most commonly used piezoelectric ceramics. The major causes of its electrical fatigue are suggested to be domain pinning and cracking. However, their contributions to fatigue have never been quantitatively compared. This study focuses on the electrical fatigue-induced microstructure damage in the near-electrode regions of PZT and uses a refatigue method to determine quantitatively the contribution of the cracking mechanism to electrical fatigue. It is shown that during bipolar electrical cycling, a large number of cracks are initiated in the samples, and the cracking is particularly concentrated in the near-electrode regions. So the loss of piezoelectric properties can be partially restored by removing such regions. For a particular fatigue stage, the cracking mechanism contributes significantly more to the electrical fatigue than the domain pinning mechanism. © 2012 The American Ceramic Society.

The buckling of nanotubes embedded in an elastic matrix is modeled within the framework of Timoshenko beams. Both a stress gradient and a strain gradient approach are considered. The energy variational approach is adopted to obtain the critical buckling loads. The dependences of the buckling load on the nonlocal parameter, the stiffness of the surrounding elastic matrix, and the transverse shear stiffness of the nanotubes are obtained. The results show a significant dependence of critical buckling load on the nonlocal parameter and the stiffness of the surround matrix. The Euler beam model, which neglects the shear stiffness of the nanotubes, over-predicts the critical buckling load. It is also found that the strain gradient model provides the lower bound and the stress gradient model provides the upper bound for the critical buckling load of nanotubes. In addition to mechanical buckling, thermally induced buckling of the nanotubes embedded in an elastic matrix is also studied. All results are expressed in closed-form and therefore are easy to use by materials scientists and engineers for the design of nanotubes and their composites. © 2011 Elsevier Ltd. All rights reserved.

An improved third order shear deformation theory is employed to formulate a governing equation for predicting free vibration of layered functionally graded beams. The Ritz method is adopted to solve the governing equation for various types of boundary conditions and the frequency results are validated by some available and experimental results. A multi-step sequential infiltration technique is used to fabricate the layered functionally graded beams for vibration testing. For the first time, a simple mathematical model, based on a power law distribution, is introduced to approximate material volume fraction of the layered beams. The details of layered beam fabrication according to the infiltration technique, microstructure and volume fraction analysis as well as vibration experimental set up are included and described in this investigation. Aspects which affect natural frequencies, such as material compositions, thickness ratio, and boundary conditions, are then taken into consideration. The impact on frequency of added mass is presented and discussed. © 2011 Elsevier Ltd.

A closed-form analytical model is presented for the deformation and fracture behaviour of sandwich panels with ductile foam cores and low yield-strength face sheets when subjected to quasi-static indentation with hemispherical indenters of various diameters and compared with an extensive experimental investigation. The aim is to predict the indentation strength and the influence of the indenter size on it. The face sheet is assumed an isotropic thin sheet undergoing large tensile deformation and bonded to a rigid-plastic foundation. The indentation strength of the sandwich panel is presented as a superposition of the force contributions of the skin, the foam core and the interfacial shear at the skin-foam interface at the point of skin fracture. To validate the model, sandwich panels with 0.32. mm thick sheets of AA 1100-O aluminium alloy and aluminium closed-cell foam core are quasi-statically indented with spherical indenters of 10, 15 and 20. mm diameters. It is established that failure of the sandwich panels is governed by radial skin fracture, and that the indenter size and the foam resistance greatly influence the indentation response as larger indenters are associated with higher failure loads. A simple parameter for system failure is ascertained, the subtended angle at the indenter-skin contact, which is dependent on the product of the indenter radius and the foam compressive strength. The developed model closely predicts the experimental load-displacement history and the failure loads and may be applied to any thin soft sheets bonded to a rigid-plastic foundation, and thus, to other sandwich constructions of similar constituents. © 2012 Elsevier B.V.

The effect of anisotropy on the response and fracture pattern on scratching {1 0 0} and {1 1 1} single crystal silicon wafers in two characteristic directions, i.e. <1 0 0> and <1 1 0> in a {1 0 0} wafer, and <1 1 0> and <1 1 2> in a {1 1 1} wafer, respectively, was studied. Predictions of the locations of the onset of fracture, as well as the fracture patterns on the wafer surfaces, were obtained applying a "minimum crack length" criterion assisted by numerical determination of the stress states using the finite element method. It was found that the first crack appears on the {1 1 1} or {1 1 0} cleavage plane. The <1 1 2> scratching direction on the {1 1 1} wafer is the weakest among the four directions studied, since it provides the highest resolved tensile stress and the shortest initial defect for crack propagation. The <1 0 0> scratching direction in the {1 0 0} wafer appears to be strongest. Experiments validated the approach and also showed a higher reliability in the {1 0 0}<1 0 0> and {1 1 1}<1 1 0> directions. The methodology used in this manuscript can be applied to the determination of fracture patterns in other single crystal materials, under scratching or other mechanical loading conditions. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

This paper reports a transmission electron microscopy (TEM) analysis of subsurfaces of enamel specimens following in vitro reciprocating wear tests with an enamel cusp sliding on a flat enamel specimen under hydrated conditions. The obtained results show that crack formation occurred in the wear scar subsurface. The path followed by these cracks seems to be dictated either by the histological structure of enamel or by the contact stress field. Moreover, the analysis of a set of enamel wear results obtained from the literature and application of fracture-based models, originally developed for ceramics, correlate well, confirming the similar wear processes taking place in these materials. This analysis also reveals a marked influence of coefficient of friction on the enamel wear rate: for a higher coefficient of friction value, enamel wear can be severe even under forces generated during normal operation of teeth. © 2011 Elsevier Ltd.

Hydroxyapatite coating of orthopaedic implant materials generally involves high temperature plasma spray, which can initiate recrystallization and/or grain growth in ultrafine-grained (UFG) titanium. Therefore, two alternative low-temperature hydroxyapatite coating methods (sol-gel spin coating followed by calcination (325-450°C) and anodization followed by hydrothermal processing (200-225°C)) are also investigated in terms of coating quality and thermal stability of the UFG titanium substrate. Plasma spray produced a crystalline coating ~38 µm thick with essentially no influence on the UFG titanium substrate. Sol-gel coating followed by calcination resulted in highly variable coatings with recrystallization and grain growth in the ultrafine grained titanium substrate. Anodization followed by a controlled hydrothermal process resulted in a homogeneous, roughened and porous coating integrating Ca and P ions into the Ti substrate. This work presents a unique hydrothermal processing route for conversion of the anodized precursor film. The underlying UFG Ti substrate was not influenced by the hydrothermal processing.

The microstructure, fatigue crack growth behaviour and hardness of ultra fine grained 6061 aluminium alloy obtained by equal angle channel processing was studied. ECAP resulted in significant grain refinement down to the sub micron level and corresponding increase in hardness. Results point to a similar fatigue threshold stress intensity range and fatigue crack growth rates for 1, 2, 4 and 6 passes of ECAP. © (2011) Trans Tech Publications.

The bipolar and unipolar fatigue behavior of the lead-free piezo- and ferroelectric 0.94Bi1/2Na1/2TiO3-0.06BaTiO 3 (94BNT-6BT) was investigated. To obtain a complete profile of the fatigue behavior, both large-signal (polarization, strain) and small-signal (d33, É33) properties were measured at room temperature. The results indicate that the material suffers a significant degradation in both large- and small-signal properties after cycling, with most of the fatigue occurring in the first 104 cycles of the fatigue test. © 2010 The American Ceramic Society.

The influence of temperature on the kinetics of domain switching in lead zirconate titanate was investigated by using in situ neutron diffraction. Samples were electrically loaded to 1 kV/mm at 30°C, 125°C, and 175°C, after which the diffracted patterns in the on- and off-state were compared. The results demonstrated that the degree of domain switching increases with increased temperature. Corroboration with hysteresis measurements showed that while the coercive field decreases with increasing temperature, the degree of saturation increased significantly. According to Merz's model, it is therefore apparent that, due to increased switching rate at high temperature, domain switchability increases with temperature. © 2011 The American Ceramic Society.

This article describes a study involving three-dimensional FIB tomographic characterization of Vickers microindentation sites in an alumina material containing pores. The study was carried out using a dual beam FIB instrument for loads in the range 50-200 g. The results obtained show that this technique can be used to obtain 3-D distributions of indentation-generated cracks. Although, compared to a dense material, presence of pores in alumina reduced the ability to generate and/or propagate radial cracks, the significant probability of crack-pore interactions indicates that pores assist in the generation/ propagation lateral cracks. The subsurface images show that pores act as sites for crack initiation and/or assist in crack propagation. In addition, a clearly reduced ability to generate and/or propagate radial cracks has been noted. However, considering that FIB tomography is destructive and invasive, microstructural changes seem to have occurred during the process and consequently it is not possible to identify all the generated cracks. Moreover, propagation of some of the cracks also occurred due to material removal in FIB milling. © 2011 The American Ceramic Society.

The mechanical properties and the scratch resistance of titanium oxide (TiO2) thin films on a glass substrate have been investigated. Three films, with crystalline (rutile and anatase) and amorphous structures, were deposited by the filtered cathodic vacuum arc deposition technique on glass, and characterized by means of nanoindentation and scratch tests. The different damage modes (arc-like, longitudinal and channel cracks in the crystalline films; Hertzian cracks in the amorphous film) were assessed by means of optical and focused ion beam microscopy. In all cases, the deposition of the TiO 2 film improved the contact-mechanical properties of uncoated glass. Crystalline films were found to possess a better combination of mechanical properties (i.e. elastic modulus up to 221 GPa, hardness up to 21 GPa, and fracture strength up to 3.6 GPa) than the amorphous film. However, under cyclic sliding contact above the critical fracture load, the amorphous film was found to withstand a higher number of cycles. The results are expected to provide useful insight for the design of optical coatings with improved contact-damage resistance. © 2011 Elsevier B.V. All rights reseved.

Bioactive hydroxyapatite (HA) coating on titanium (Ti) implant can be used as a drug delivery device. A controlled release of drug around the implant requires the incorporation of drug into the coating material during the coating process. HA coating was prepared using a two-step procedure in conditions suitable for simultaneous incorporation of the protein-based drug into the coating material. Monetite coating was deposited on Ti substrate in acidic condition followed by the transformation of the monetite coating to HA. X-ray diffraction (XRD) confirmed the formation of the monetite phase at the first step of the coating preparation, which was transformed into HA at the second step. Fourier transform infrared spectroscopy demonstrated typical bands of a crystallized carbonated HA with A- and B-type substitution, which was confirmed by the XRD refinement of the structural parameters. Scanning electron microscope was used to observe the morphology of monetite and HA coatings. Adhesion of the coatings was measured using a scratch tester. The critical shearing stress was found to be 84.20 ± 1.27 MPa for the monetite coating, and 44.40 ± 2.39 MPa for the HA coating. © 2010 Springer Science+Business Media, LLC.

The success of implants in orthopaedic and dental load-bearing applications crucially depends on the initial biological fixation of implants in surrounding bone tissues. Using hydroxyapatite (HA) coating on Ti implant as carrier for bone morphogenetic proteins (BMPs) may promote the osteointegration of implants; therefore, reduce the risk of implant failure. The goal of this study was to develop an HA coating method in conditions allowing the incorporation of protein-based drugs into the coating materials, while achieving a mechanical stable coating on Ti implant. HA coatings were deposited on six different groups of Ti surfaces: control (no pretreatment), pretreated with alkali, acid, heat at 800°C, grit blasted with Al 2O 3, and grit blasted followed by heat treatment. HA coating was prepared using a two-step procedure. First step was the chemical deposition of a monetite coating on Ti substrate in acidic condition at 75°C and the second step was the hydrolysis of the monetite coating to HA. Coatings were characterized by scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). The roughness of substrates and coatings was measured using profilometry technique. The mechanical stability of the coatings deposited on the pretreated substrates was assessed using scratch test. The coatings deposited on the grit-blasted Ti surface demonstrated superior adhesive properties with critical shearing stress 131.6 ± 0.2 MPa. Copyright © 2011 Wiley Periodicals, Inc.

Bone has a complex hierarchical structure. Combined wide angle X-ray diffraction and small angle X-ray scattering were used together with in situ tensile testing to investigate the deformation and failure mechanisms of bovine cortical bone at three material levels: (1) the atomic level, corresponding to the mineral crystal phase; (2) the nano level, corresponding to the collagen fibrils; (3) the macroscopic level. It was found that deformation was linear at all three levels up to a strain of 0.2% in the longitudinal tensile direction. At this critical strain a sudden 50% decrease in the fibrillar and mineral strains was observed. This suggests the presence of partial local damage that leads to inhomogeneous strain distributions within the probed gauge volume. This also gives rise to diffraction peak broadening in the mineral phase, as well as strain relaxation at the nanoscale. Above the critical strain the longitudinally oriented strains below the nanoscale remained constant at a reduced level until failure. This suggests that the lateral orientation of the nanostructures toughens the bone, while a higher material level dominated the subsequent deformation process, either by sliding between the lamellar layers or by the growth of microcracks. Analysis of the diffraction data also shows that the bone has compressive residual stress in the crystal phase. A better understanding of the basic mechanics of the hierarchical bone structure could be the basis to enhance research in biomimetics, developing new advanced materials, and to find solutions for orthopedic problems.

A study of the influence of water environments on the cyclic fatigue crack behavior of polyvinyl chloride (PVC), with (PVC-M) and without (PVC-U) chlorinated polyethylene (CPE) impact modifier was undertaken and compared with corresponding results in air. Two frequencies of 1 and 7 Hz were applied to assess the influence of frequency on the fatigue behavior; a higher fatigue resistance and threshold were obtained with increasing frequency. This trend is more significant in water. However, in this environment, the fatigue resistance deteriorated under conditions of higher stress intensity factor amplitude (¿K) and frequency. The fatigue properties of PVC-U are the most affected by the presence of water, particularly at low frequency and higher ¿K. Examination of the fracture surface showed the interaction of water molecules and the PVC matrix with the formation of (1) a nodular structure, close to the fatigue threshold and (2) plasticized structures at high ¿K, which are associated with a greater threshold value and fatigue resistance. The absorption of the water retarded the fibrillation of craze and caused crack blunting effects. Water functions as a plasticizer, particularly at high ¿K, through the formation of the plasticized structures. Results are compared with those observed from an in-service failure. POLYM. ENG. SCI., 50:352-364, 2010. Copyright © 2009 Society of Plastics Engineers.

The effect of coating thickness on the deformation behaviour of diamond-like carbon (DLC) coatings on silicon substrates was investigated. Following nanoindentation of a 0.6 µm thick DLC coating, the subsurface microstructures were characterized and the data was compared to prior studies on a similar, but thicker coating. Indentation resulted in localized plastic compression in the coating without any through-thickness cracking. It was shown that the discontinuities in the load-displacement curves appeared at lower loads for the thinner coating. Accordingly, the silicon substrate exhibited cracking, plastic deformation and phase transformation at significantly lower loads than in the case of the thicker coating. Further, the widths, parallel to the interface, over which slip and the phase transformation zone are spread out in the substrate, was found to increase with the thickness of the coating. The mechanism responsible for the first pop-in was found to change from phase transformation in uncoated silicon to dislocation nucleation in the presence of the coating. Crown Copyright © 2009.

Three different functionally graded amorphous carbon (a-C) thin films were deposited on to aluminium substrates using a closed-field unbalanced magnetron sputtering ion plating method. The closed-field configuration prohibits the loss of secondary electrons and consequently enhances the plasma density significantly. The functional gradient of the a-C films was achieved by varying the bias voltage linearly during deposition. Three graded a-C systems possessing different variations in Young's modulus were deposited with the highest Young's modulus at the (i) top surface, (ii) interface or (iii) middle of the film. Of the three systems investigated, the one with the highest Young's modulus at the middle of the film thickness was found to exhibit significantly lower levels of cracking at higher indentation depths. Finite element models that included an embedded ring crack controlled by cohesive zone elements were developed to clarify the effect of ring cracks on the deformation of the films. This study provides guidance for the design of functionally graded coatings against contact damage. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.

The feasibility of the use of the scratch test to measure the fracture strength of brittle thin films has been proved. In particular, amorphous diamond-like carbon (DLC), SiC and TiO2 films deposited on a soda-lime glass substrate, in which damage initiated on the film surface in the form of partial cone cracks, have been considered. Optical inspection of the scratch tracks allowed the normal and lateral force at the point of fracture to be determined. The critical values were subsequently used to calculate the fracture strength. The great advantage of this method is that it allows the contribution of surface defects to strength to be obtained. Weibull analysis of the strength data was carried out, showing that the SiC film possess higher strength and reliability compared to DLC and TiO2 films. © 2010 Elsevier B.V. All rights reserved.

The effect of substrate and surface roughness on the contact fracture of diamond-like carbon coatings on brittle soda-lime glass substrates has been investigated. The average surface roughness (Ra) of the examined samples ranged from 15 nm to 571 nm. Contact damage was simulated by means of spherical nanoindentation, and fracture was subsequently assessed by focused ion beam microscopy. It was found that, in the absence of sub-surface damage in the substrate, fracture occurs in the coating in the form of radial, and ring/cone cracks during loading, and lateral cracks during unloading. Increasing the surface roughness results in a decrease in the critical load for crack initiation during loading, and in the suppression of fracture modes during unloading from high loads. When sub-surface damage (lateral cracks) is present in the substrate, severe spalling takes place during loading, causing a large discontinuity in the load-displacement curve. The results have implications concerning the design of damage-tolerant coated systems consisting of a brittle film on a brittle substrate. © 2010 Elsevier B.V.

The deformation mechanisms of a range of TiN coatings with different thicknesses, deposited on a V820 steel substrate following nanoindentation were characterized using focused ion beam (FIB) cross-sectioning and imaging, as well as cross-sectional transmission electron microscopy (TEM) of the indented region. Four TiN coatings were examined, including a cathodic arc evaporation (CAE) coating with a thickness of ~ 0.7 µm and low voltage electron beam (LVEB) evaporation coatings with thicknesses of ~ 2.0, ~ 3.7 and ~ 4.0 µm. Based on a model developed by Xie et al., the intercolumnar shear stresses were calculated to be approximately 2.20, 3.05, 3.50 and 3.55 GPa in the ~ 0.7, ~ 2.0, ~ 3.7 and ~ 4.0 µm thick TiN coatings respectively, that is, increasing as the coating thickness increases. Columnar cracking and shear steps at the coating/substrate interface were observed more frequently in the thinner TiN coatings indicated that these coatings deformed predominantly by shear along the columnar grain boundaries. In contrast, inclined cracking was the more dominant fracture type in the thicker TiN coatings. It is suggested that increased grain boundary strength occurs together with a lack of direct crack path along the grain boundaries through the thicker coatings due to the more equiaxed grain structure. Clearly, the grain structure and/or thickness of the TiN coating play a highly significant role in the deformation mechanisms. © 2009 Elsevier B.V. All rights reserved.

The effect of the presence of diamondlike carbon coatings deposited on (100) Si substrates on the deformation mechanisms operating in the silicon substrate during contact loading have been investigated by both cross-sectional transmission electron microscopy and modeling of the stresses generated beneath the indenter tip. The observed subsurface microstructures were correlated to the Tresca shear stress and the hydrostatic stress generated in the silicon substrate beneath the indenter tip. The presence of the coating altered the stresses generated in the substrate, and changed the deformation mechanism from one of principally phase transformation in uncoatcd Si to predominantly dislocation motion in the silicon substrate for the diamondlike C-Si system. The magnitude and distribution of the shear and hydrostatic stresses in the substrate were found to depend on both the indentation load and the thickness of the coating. Furthermore, the observed width of deformation, parallel to the interface, which was found to increase with coating thickness, was correlated to the wider distribution of the Tresca shear stress in the substrate brought about by the presence of the coating. © 2010 Materials Research Society.

The present paper studies the deformation response of closed-cell aluminium foam panels (ALPORAS brand) to local contact damage, experimentally simulated by indentation tests with indenters of various shapes and sizes. The foam density varies between 0.184g/cm3 and 0.351g/cm3; panels of various thicknesses are also considered. Analytical models are developed to estimate the crushing force, the tearing resistance, and the shearing resistance in indentation with cylindrical and spherical indenters. Of particular interest are the failure mechanisms, the foam ligament tear energy and the foam shear strength in all indentation types. The tear energy, as well as the shear strength, is deduced by fitting the experimental load-displacement curves and using the developed analytical models. The influence of the panel thickness, indenter shape and size, and foam microstructure on the deformation mechanisms and material properties is also discussed. © 2010 Elsevier B.V.

Substrate effects on the mechanical properties and contact damage of diamond-like carbon (DLC) films have been investigated. To that end, a DLC film of 1 µm thickness was deposited on two different substrates: soda-lime glass (compliant and soft), and single-crystal silicon (stiff and hard). The elastic modulus and hardness were measured by means of nanoindentation. Quasi-static and sliding contact configurations were simulated by means of ultra-micro indentation, and surface and cross-sectional damage were assessed using a focused ion beam (FIB) miller. It was found that a compliant and soft substrate enhances crack initiation on the film surface in the form of ring/cone cracks. On the other hand, a stiff and hard substrate delays crack initiation in the film, but is more susceptible to fracture in the form of median and lateral cracks due to increased brittleness. The results have implications for the reliability of DLC-coated systems. © 2010 Elsevier B.V. All rights reserved.

The nanoindentation-induced deformation behaviour of a ta-C (tetrahedral amorphous carbon) coating deposited on to a silicon substrate by a filtered vacuum cathodic vapour arc technique was investigated. The 0.17-µm-thick ta-C coating was subjected to nanoindentation with a spherical indenter and the residual indents were examined by cross-sectional transmission electron microscopy. The hard (~ 30 GPa) ta-C coatings exhibited very little localized plastic compression, unlike the softer amorphous carbon coatings deposited by plasma-assisted chemical vapour deposition. However, neither through-thickness cracks nor delamination was observed in the coating for the loads studied. Rather, the silicon substrate exhibited plastic deformation for indentation loads as low as 10 mN and at higher loads it showed evidence of both phase transformation and cracking. These microstructural features were correlated to the observed discontinuities in the load-displacement curves. Further, it was observed that even a very thin coating can modify the primary deformation mechanism from phase transformation in uncoated Si to predominantly plastic deformation in the underlying substrate. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.

The performance of ferroelectric ceramics is governed by the ability of domains to switch. A decrease in the switching ability can lead to degradation of the materials and failure of ferroelectric devices. In this work the dynamic properties of domain reorientation are studied. In situ time-of-flight neutron diffraction is used to probe the evolution of ferroelastic domain texture under mechanical cyclic loading in bulk lead zirconate titanate ceramics. The high sensitivity of neutron diffraction to lattice strain is exploited to precisely analyze the change of domain texture and strain through a full-pattern Rietveld method. These results are then used to construct a viscoelastic model, which explains the correlation between macroscopic phenomena (i.e. creep and recovered deformation) and microscopic dynamic behavior (i.e. ferroelastic switching, lattice strain). © 2009 Acta Materialia Inc.

Anisotropy effects on the reliability of single-crystal silicon were investigated by means of scratch tests along [1 1 0] and [1 0 0] crystallographic directions. It was found that fracture (partial cone cracks) starts along favoured {1 1 0} and {1 1 1} cleavage planes, with crack orientation varying upon the scratching direction. Moreover, the [1 0 0] direction was found to be twice as reliable as the [1 1 0] direction. Stress and phase analyses were carried out to explain this effect, which has implications for the design of silicon-based devices. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

This paper studies the problem of a line inclusion in a medium possessing coupled piezoelectric, piezomagnetic and magnetoelectric effects. Based on the extended Stroh formalism, the general two-dimensional solutions to the magnetoelectroelastic problem are obtained, involving five analytic functions of different variables. The magnetic, electric and elastic fields around the inclusion are given. Expressions of the stresses, electric field and magnetic field in the vicinity of the inclusion tip are derived and the field intensity factors are provided. The energy release rate is written in terms of those field intensity factors. Solutions for some special cases, including the electrically and/or magnetically insulated/conductive inclusions/cracks, are also established. Explicit algebraic results are given for a special case of an anti-plane crack in a magnetoelectroelastic medium. © 2010 - IOS Press and the authors.

There is a scarcity of investigation into the mechanical properties of subdermal fat. Recently, progress has been made in the determination of subdermal stress and strain distributions. This requires accurate constitutive modelling and consideration of the subdermal tissues. This paper reports the results of a study to estimate non-linear elastic and viscoelastic properties of porcine subdermal fat using a simple constitutive model. High-resolution magnetic resonance imaging (MRI) was used to acquire a time series of coincident images during a confined indentation experiment. Inverse finite element analysis was used to estimate the material parameters. The Neo Hookean model was used to represent the elastic behaviour (µ = 0.53 ± 0.31 kPa), while a single-element Prony series was used to model the viscoelastic response (a = 0.39 ± 0.03, t = 700 ± 255 s). © 2010 Springer-Verlag.

The mechanisms and controlling factors in human enamel wear are not fully understood yet. To address this problem, we have used focused ion beam (FIB) milling and field emission scanning electron microscopy (FESEM) to investigate the processes taking place below the wear surface of enamel specimens from in vitro wear tests. These reveal the generation of subsurface cracks during wear of enamel. An analysis of published qualitative and quantitative experimental wear results for enamel as well as for ceramics shows the similarities of the wear processes taking place under similar contact conditions, despite the differences that exist between these two materials. It is shown that, for the considered conditions, fracture under elastic contact is responsible for enamel wear. © 2010 Elsevier Ltd. All rights reserved.

The fracture strength of polycrystalline silicon wafers has been investigated by means of twist and four-point bending tests. Under a twisting configuration, which generates high tensile stresses within the middle of the wafers, a unimodal distribution in strength is obtained. The characteristic strength and Weibull modulus are 131.0 MPa, and 14.4, respectively. Under a four-point bending configuration, which generates high stresses both at the surface and at the edges, an additive bimodal distribution is obtained. The first mode of the distribution has a characteristic strength of 76.0 MPa and a Weibull modulus of 1.6; the second mode has a characteristic strength of 161.2 MPa and a Weibull modulus of 11.5. Fractographic observations confirm that the first mode (lower strength) corresponds to wafers which failed from large edge chips (sizes up to 90 µm). Weibull analysis suggests that the second mode (higher strength) corresponds to wafers which failed from smaller surface chips (sizes up to 50 µm). The results obtained point to large edge chips as the most dangerous defects degrading the fracture strength of the wafers. This is of great relevance for the photovoltaic industry, as fracture of silicon wafers limits both the performance and lifetime of the solar cells, and production yields. © 2009 The American Ceramic Society.

The ultraviolet and visible cathodoluminescence (CL) emitted at room temperature from bulk hard lead-zirconate-titanate polycrystalline perovskite has been systematically collected before and after an annealing cycle conducted in a reducing atmosphere. Spectroscopic assessments have been made of the in-depth stoichiometric profile developed upon annealing from the sample surface toward the subsurface. Trapping of electronic charge and local atomic scale distortions in the perovskite oxygen octahedron influences the variation observed in visible CL emission, while lattice distortions upon annealing directly arise from the formation of oxygen vacancies. © 2009 American Institute of Physics.

Tooth enamel is the stiffest tissue in the human body with a well-organized microstructure. Developmental diseases, such as enamel hypomineralisation, have been reported to cause marked reduction in the elastic modulus of enamel and consequently impair dental function. We produce evidence, using site-specific transmission electron microscopy (TEM), of difference in microstructure between sound and hypomineralised enamel. Built upon that, we develop a mechanical model to explore the relationship of the elastic modulus of the mineral-protein composite structure of enamel with the thickness of protein layers and the direction of mechanical loading. We conclude that when subject to complex mechanical loading conditions, sound enamel exhibits consistently high stiffness, which is essential for dental function. A marked decrease in stiffness of hypomineralised enamel is caused primarily by an increase in the thickness of protein layers between apatite crystals and to a lesser extent by an increase in the effective crystal orientation angle. © 2009 Elsevier Ltd. All rights reserved.

Tooth enamel is the hardest tissue in the human body and is directly responsible for dental function. Due to its non-regenerative nature, enamel is unable to heal and repair itself biologically after damage. We hypothesized that with its unique microstructure, enamel possesses excellent resistance to contact- induced damage, regardless of loading direction. By combining instrumented indentation tests with microstructural analysis, we report that enamel can absorb indentation energy through shear deformation within its protein layers between apatite crystallites. Moreover, a near-isotropic inelastic response was observed when we analyzed indentation data in directions either perpendicular or parallel to the path of enamel prisms. An "effective" crystal orientation angle, 33°-34°, was derived for enamel micro- structure, independent of the loading direction. These findings will help guide the design of the nanostructural architecture of dental restorative materials.

PVC/CaCO3 polymer nanocomposites of differing compositions were produced using a two-roll mill and compression molding. The morphology was observed using transmission electron microscopy, and the static and dynamic mechanical and fracture properties determined. The presence of nanometer-sized CaCO3 particles led to a slight decrease in the tensile strength but improved the impact energy, the storage modulus and the fracture toughness. Fracture surface examination by scanning electron microscopy indicated that the enhanced fracture properties in the nanocomposites were caused by the assisted void formation at the particles. This hypothesis is supported by a microstructure-based finite element modeling based upon elastic-plastic deformation around a weakly bonded particle. Hence, this provides an explanation of both the uniaxial tensile behavior and enhanced toughness of the nanocomposites. © 2009 Elsevier Ltd.

The cyclic fatigue crack behavior of polyvinyl chloride (PVC), with (PVC-M) and without (PVC-U) chlorinated polyethylene (CPE) impact modifier, was studied. The effect of impact modifier upon fatigue crack growth rate and threshold was evaluated at frequencies of 1, 7, and 20 Hz. It was shown that the addition of CPE lowered the threshold stress intensity factor amplitude for crack growth (¿Kth) of PVC-M compared to that of PVC-U at lower frequencies, and that the effect became more pronounced at lower frequency. At lower stress intensity factor amplitudes (below ¿K = 1 MPa·m 1/2), there was a slight difference between the crack growth rates of U- and M-PVC. The crack advance mechanism is investigated by microscopic observation of the crack tip process zone. Although the zone is relatively large in PVC-M, associated with higher toughness, it did not improve the fatigue crack growth resistance significantly. Fracture surface observations reveal a higher density of fibrils on the fatigued surface of PVC-M with the density, relative to that observed in PVC-U, reducing with frequency. It is therefore hypothesized that accelerated fibril failure is a mechanism of fatigue. © 2009 Society of Plastics Engineers.

Structure, hardness, and elastic modulus of nanolayered aluminium/palladium thin films, with individual layer thickness varying from 1¿nm to 40¿nm, were investigated using transmission electron microscopy (TEM) and nanoindentation. TEM micrographs indicated a sharp but not flat Al-Pd interface. With just 6.5% (v/v) Pd a hardness enhancement of ~ 200% was observed for nanolayered Al/Pd compared to the hardness of pure Al film. A maximum hardness enhancement of up to 350% was observed for nanolayered Al/Pd samples compared to the hardness of pure Al film when bilayer thickness was 2¿nm and Pd was 50% (v/v). Modulus enhancement was also observed for the nanolayered thin films. © 2009 Elsevier B.V. All rights reserved.

Three tin(II) butoxides, namely: tin(II) n-butoxide, Sn(n-OBu)2; tin(II) i-butoxide, Sn(i-OBu)2; and tin(II) t-butoxide, Sn(t-OBu)2 were synthesized for use as coordination-insertion initiators in the bulk ring-opening polymerisation of e-caprolactone (CL) at 120 °C. Two different methods of synthesis were compared: an old well-established method which gave solid products and a new modified method which, in the case of Sn(n-OBu)2, gave a novel liquid product. The liquid Sn(n-OBu)2 had the advantage for initiation purposes of being much more soluble than the solid Sn(n-OBu)2 due to its lower degree of molecular aggregation, a common characteristic of tin(II) alkoxides which limits their usefulness. Kinetic studies of CL polymerisation by dilatometry showed that the liquid Sn(n-OBu)2 initiator gave a much faster reaction with a higher first-order rate constant (kp = 8.25 l mol-1 min-1) than the solid Sn(n-OBu)2 initiator (kp = 2.96 l mol-1 min-1). The molecular weight of the polymer formed was also hi her. Increasing the bulkiness of the OBu group resulted in solid products with decreased solubility and initiating efficiency (for Sn(i-OBu)2, kp = 2.20 l mol-1 min-1; for Sn(t-OBu)2, kp = 0.69 l mol-1 min-1). It is concluded that the new modified method of initiator synthesis is capable of producing soluble tin(II) alkoxides which have the potential to offer improved kinetic and molecular weight control in the ring-opening polymerisation of cyclic esters.

The present study focuses on the structural response of sandwich panels consisting of a commercial closed-cell foam core and thin aluminium sheet skins under static three-point bending loading. Panels of different thicknesses and span lengths were tested, and the influence of the foam density, core thickness and skin type on the response was revealed. The failure modes in bending were greatly dependent on the span length but independent on the foam thickness. For short spans, the deformed shape at failure was asymmetric, as opposed to a symmetric mode for long spans. The density and thickness of the foam core, the presence of reinforcing face sheets and the beam span determined the failure load and bending strength of the sandwich panels. © World Scientific Publishing Company.

We have investigated the fracture behavior of tetrahedral amorphous carbon films, with thicknesses 0.15 (ultrathin), 0.5 (thin), and 1.2 (thick) microns on silicon substrates. To that end, the systems were progressively loaded using a nanoindenter with a spherical tip, and surface and cross sections were subsequently examined using a focused ion beam miller at different loads. A transition was found as a function of film thickness: for ultrathin and thin films, cracking (radial and lateral) initiated in the silicon substrate and followed a similar path in the films. Thicker films, on the other hand, provided a higher level of protection to the substrate, and cracking (lateral and radial at the interface) was constrained to the film. The damage modes and the transition obtained differ from those that occur in thick coatings. Lateral cracks are highly dangerous, leading to delamination of thick films and to spallation when thinner films are used. The results have implications concerning the mechanical reliability of microelectromechanical systems. © 2009 Materials Research Society.

Thin and thick panels of two types of aluminium closed-cell foams of ~0.20-0.31 g/cm3 (ALPORAS) and ~0.32-0.57 g/cm3 (ALULIGHT) density were tested in uniaxial compression and the mechanical properties of the panels, plastic collapse strength, plateau stress and absorbed energy, were extracted. Their dependence on the foam density and specimen thickness was also ascertained. Micro-indentation tests were used to estimate the yield stresses of the parent materials. It was found that the higher density panels of ALULIGHT were also more anisotropic than the lower density panels of ALPORAS, and the collapse strength and the absorbed energy of both foams increased with density. More significant scatter the results was observed in the thin and higher density panels. © 2009 Elsevier B.V. All rights reserved.

Current educational thinking promotes a student-centred approach to teaching as more engaging and challenging for students, leading to improved learning outcomes. But what is 'student-centred' learning, and how can it be achieved in a higher education setting with very large classes and content-rich courses? In a materials engineering course for 300 first-year engineers, an online group project was introduced to add authenticity and collaborative activity into the course, and to improve student engagement. We explore the design, development and implementation of the project, and see if the intended outcomes were achieved. © 2009 TEMPUS Publications.

The Weibull size effect, consisting of an increase in fracture strength with decreasing specimen size, constitutes the basis of the probabilistic design of brittle structures. Here we present evidence of a reverse size effect in submicron amorphous carbon films. Underlying this effect is the presence of critical defects of comparable size in all films. This leads to higher stress intensity factors, and concomitant decrease in strength, with decreasing thickness. The results have implications concerning the mechanical reliability of submicron scale devices. © 2009 Acta Materialia Inc.

Three calcium a-SiAlON microstructures-namely, fine-grained, bimodal, and large elongated-were developed using powders of the same composition and then characterized. The evolution of grain size and morphology was determined to be a process of nucleation and growth that could be controlled with a two-step sintering technique. The extent of texture was identified in the as-hot-pressed materials as a function of sintering conditions. Samples with different microstructures exhibited different hardness and fracture toughness. The true hardness was derived from the intrinsic relation between applied loads and indent sizes. The effect of microstructure on hardness and fracture toughness was analyzed.

The single-edge V-notched-beam (SEVNB) testing geometry was used to measure the crack growth resistance (R-curve) behavior of multilayered alumina-zirconia composites. Fracture mechanics weight function analysis was applied to predict the R-curve behavior of multilayered composites having a stepwise change in composition. These results were then used to differentiate the influence of residual stresses from crack-bridging stresses on the measured R-curve behavior.

The single-edge-V-notched-beam testing geometry was used to measure the crack growth resistance (R-curve) behavior of multilayer graded alumina-zirconia composites for crack extensions parallel to the graded direction. Fracture mechanics weight function analysis was applied to explain the R-curve behavior of a compositional and grain-size graded microstructure. The results were then used to differentiate the influence of residual stress from other closure stresses, attributed to crack bridging, on the measured R-curve behavior. © 2002 Elsevier Science Ltd. All rights reserved.

In this work contact pressure and sliding distance-induced changes in wear rate and mechanism, and friction behaviour of alumina are investigated by consideration of sub-surface damage. Sub-surface cracking and the formation of tribochemical reaction products are observed through the use of a novel sectioning technique; focused ion-beam milling. Initially wear rate experiments with three different grain sizes and different contact loads were undertaken. These were followed by extensive sub-surface microscopic investigations. Wear experiments, undertaken on a pin-on-disc tribometer over a range of contact pressures, revealed that wear mechanisms changed as contact pressure decreased. Both wear rate and friction were monitored. Debris and surface layers which formed were characterised using X-ray photoelectron spectroscopy (XPS). This identified an oxide/hydroxide layer which formed during wear and which was found to significantly affect friction. The nature of this layer varied significantly with contact pressure and sliding distance. It was found that mild wear of alumina in air involves the continual formation and removal of a surface layer at differing rates. © 2002 Elsevier Science B.V. All rights reserved.

This paper presents an automated system for parameter identification of inelastic constitutive models. The system can find good approximate parameters for various identification problems under a user-friendly environment. In order to identify parameters efficiently and in a robust manner, an optimisation method is first proposed. The paper then describes the generalisations applied of modelling, simulation and identification for its various identification uses. Finally, a system, which is developed in conjunction with the generalisations, is described. The performances of the proposed optimisation method and the developed system were investigated with actual material data, and their effectiveness was consequently confirmed. © 2002 Published by Elsevier Science B.V.

AIN whiskers or dendrites were synthesized with a sublimation-recrystallization method by using Al, AIN powders and some additives as raw materials. Whiskers with different sizes that featured high purity and good crystallinity were obtained by controlling temperature and gas supersaturation in the reaction container. The whiskers were described as long and straight single crystals of approximately 1-30 µm in diameter by the centimeter range in length. However, AIN dendrites were about 1 mm in diameter by 0.5 cm in length, and showed an obviously preferential growth orientation, i.e., perpendicular to [2 1¯ 1¯ 1] and [1 0 1¯ 1] planes. It is concluded that the whiskers or dendrites grow via the vapor-solid mechanism.

The microstructure of Ca a-sialon ceramics can be tailored via the control of sintering conditions. This provides an excellent opportunity for studying the effect of microstructure on mechanical properties of ceramics. In this paper, a new approach to tailoring the microstructure of a selected Ca a-sialon composition is firstly presented. The process of microstructural evolution was analyzed in light of the relationship between the sintering parameters and the nucleation and growth rates. Secondly, mechanical properties of the Ca a-sialon composition, including wear resistance, were investigated in terms of microstructural variation. Fine-grained microstructure exhibited higher hardness, but lower fracture toughness and wear resistance, compared to large elongated microstructure. Crack bridging was the main toughening mechanism in the Ca a-sialon composition. Both hardness and fracture toughness played important roles in the control of Ca a-sialon wear resistance.

The design and preparation of a series of interactive computer-based tutorials for first year mechanical engineering students allowed them to become actively involved in their learning as a means of overcoming the problems with large tutorial groups. The students were taking an introductory subject in materials engineering, and the tutorials led the students through a number of modules providing information, simulations and examples. The student response to these tutorials was overwhelmingly positive, although some negative responses suggested that some tutorials were too long. Some revision of these courses resulted in their length being reduced as a consequence.

A series of modified PVC pipes, with different concentrations of chlorinated polyethylene, was manufactured under carefully controlled processing conditions. The strength and toughness of each pipe was measured by a variety of methods including uniaxial tensile strength, notched C-ring, essential work of fracture, strain energy release rate, fracture toughness, and impact tests. Over the range of compositions tested, all of the quantitative measures of toughness and strength suggest an approximately linear relationship with modifier concentration. For pipe of the dimensions used in this study, the yield strength is overestimated when determined via the net section stress on the notched C-ring.

In this work the development of cavities in spherical metal inclusions within a metal-ceramic composite and the subsequent influence upon the strength of the composite is investigated. A model experiment is undertaken whereby 100 µm diameter spherical aluminium inclusions are placed within an Al2O3-Al interpenetrating network composite. The samples are then fractured at temperatures ranging from room temperature to just below the melting temperature of aluminium. It is found that fracture originates from the aluminium inclusions and that there is clear evidence of cavitation in the ductile inclusions which also shows that these cavities formed as a result of high triaxial stress during cooling as a part of the fabrication process. This observation is supported by a numerical model of the stress formation and cavity growth process within an inclusion during cooling. The model also provides information on the effect of initial cavity size and location. Comparisons of experimental f racture strength and toughness data indicate that the observed high strength of these composites is explained by crack growth resistance due to ductile ligament bridging. © 2001 Published by Elsevier Science Ltd on behalf of Acta Materialia Inc.

The effects of residual stress in modified poly(vinyl chloride) pipes upon the relationship between pressure tests and C-ring tests have been studied. A series of pressure tests were carried out at different hoop stresses using both fixed-end and free-end enclosures. The C-ring test was used to determine the yield strength of the material as a function of time. The difference in behavior between the fixed end and free end configurations was less than predicted by the von Mises analysis assuming the only stresses in the pipe wall were generated by the applied internal pressure. Both configurations produced lower results than predicted on the basis of the C-ring tests. The residual stresses in the hoop and longitudinal directions were measured. Taking into account the residual and applied stresses, the von Mises criterion for yield was used to calculate the uniaxial yield stress of the material for both end enclosure configurations. The results were similar to the yield stress measured in uniaxial tension. The von Mises criterion for yield has been shown to provide an acceptable method of analysis of pressure tests with fixed and free end enclosures and the C-ring, when residual stress is taken into account.

Computer simulations have been designed to elucidate the evolution of microcracking in a nanocomposite using appropriate material values for alumina and silicon carbide. These are compared to a single-phase material using elastic and thermal expansion coefficients for alumina. It is found that the region and the fracture mode where microcracking ensues are determined by the intensity and the length scale of the residual stress fields, which interact. Of specific interest are the region, fracture mode, and length of ensuing microcracks for materials with different inclusion locations (at the grain boundary or within the grain) and with different grain size to inclusion size ratios. © 2000 Materials Research Society.

Examination of the published procedure for performing calculations on data obtained from the C-ring fracture toughness test for unplasticised poly(vinyl chloride) reveals a major flaw. The dimensions of the specimens do not satisfy the ASTM criteria for plane strain conditions and correction factors are therefore introduced. However, there is an error in the calculation procedure, which results in variations in the severity of the test with pipe wall thickness. An improved approach is proposed and recommendations are made for changing the ISO/DIS 11673·2 test standard, to allow for variations in the yield strength of unplasticised poly(vinyl chloride) and to identify the calculated data as Kc rather than K1c. © 2000 IoM Communications Ltd.

Fracture from artificial spherical pores, as well as natural defects, in alumina in a grain-size range of 0.8-9.2 µm has been studied experimentally and compared with a fracture-mechanics model. Results from fracture-strength measurements have been combined with detailed fractographic analysis to elucidate the ensuing crack instability. Two existing models of possible crack configurations have been extended and contrasted. The semicircular crack as well as the circumferential crack both are described as flaws in the stress-concentrating field of a spherical pore. Surface correction terms afforded by the presence of the pore have been incorporated. A comparative computation shows that fracture occurs more likely from the semicircular crack configuration than the circumferential crack configuration.

An improved third order shear deformation theory derived from elasticity theory is presented to analyse free vibration of functionally graded beams. A power law distribution is used to describe the variation of volume fraction of material compositions. The Ritz method is adopted to solve the governing equation in the form of an eigenvalue problem. All types of general boundary conditions are investigated in this paper. Convergence studies of this solution method are presented in order to achieve the results and define the accuracy. The numerical results are compared with the published results based on the first order shear deformation theory. © Civil-Comp Press, 2010.

Aluminium foam-cored sandwich panels were subjected to indentation and subsequently tested in four-point bending. The bending strength, failure modes and the deformation behavior were studied and compared with those of undamaged panels. It was found that the localised damage does not have a considerable effect on the failure modes and limit loads.

Nanoparticulate calcium carbonate was mechanically combined with PVC in varying volume fractions. The bonding strength between the particulate phase and the matrix was also varied. It was found that increasing the particulate content improves the toughness of the composite but this effect is reduced when interfacial bonding between the particles and matrix in improved; strength and stiffness show inverse behaviour. Modelling reveals that toughening is associated with hysteretic energy absorption processes associated with cavitation. Interestingly, the addition of nanoparticulate phase has no noticeable effect upon cyclic fatigue crack growth.