Mechanical Behavior Of Materials

Synthesis and characterization of neodymium doped ceria nanocrystalline ceramic structures

by Prof.Dr. İbrahim USLU

Ceramics International
First Online March 2012
İbrahim Uslu, Arda Aytimur, Mustafa Kemal Öztürk, Serhat Koçyiğit

In this study, a new method to synthesize neodymium doped ceria ceramic nanopowders by the electrospinning of the... more In this study, a new method to synthesize neodymium doped ceria ceramic nanopowders by the electrospinning of the hybrid polymers solution of their composite precursor was put forward. Calcined and sintered nanopowders were characterized by FT-IR, XRD, BET, SEM, and AFM techniques. According to the XRD analysis, the obtained powders are single phase and independent of the dopant concentration in the range investigated. The crystallite sizes were calculated using Scherrer equation. Moreover, lattice parameters, dislocation densities and microstrain values were calculated. BET results show that the increase of the neodymium doped content decrease the surface area of the composite powders, confirming the highly ordered micro and mesostructure. SEM and AFM results show that the samples have spherical grains. According to the surface roughness measurements, the increase in the amount of neodymium and the decrease in the amount of cerium decreased the surface roughness.

Synthesis and Characterization of Bi2O3–doped with Lanthanum by Electrospinning Method

by Prof.Dr. İbrahim USLU

International Journal of Material Science and Electronics Research, Vol. 1, No. 2, July-Dec. 2010, pp. 81-85
Ş. Durmuşoğlu, S. Keskin, ibrahim Uslu, A. Aytimur, A. Akdemir

Electrospinning process was utilized to fabricate lanthanum doped bismuth ultrafine nanofibers. PVA was used as... more Electrospinning process was utilized to fabricate lanthanum doped bismuth ultrafine nanofibers. PVA was used as precursor polymer solution with bismuth acetate/lanthanum acetate, followed by calcination at 800oC. The produced nanofibers and calcined crystal structures were characterized by using scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques. The results indicated that samples after calcination should be sintered more than one to obtain pure crystalline phase.

Preparation and Characterization of Neodymia Doped PVA/Zr-Ce Oxide Nanocrystalline Composites via Electrospinning Technique

by Prof.Dr. İbrahim USLU

Materials and Manufacturing Processes
Volume 26, Issue 11, 2011
10.1080/10426914.2011.551954S.
S. Keskin, ibrahim Uslu, T.Tunç, M.Öztürk, A. Aytimur

In this study, neodymia doped poly(vinyl) alcohol/zirconium–cerium acetate (PVA/Zr-Ce) nanofibers were prepared using... more In this study, neodymia doped poly(vinyl) alcohol/zirconium–cerium acetate (PVA/Zr-Ce) nanofibers were prepared using the electrospinning technique, and then calcined at 800°C for 2 hours. For this purpose, PVA/Zr-Ce polymer solutions doped with different concentrations of neodymia were prepared using electrospinning technique, and then calcined and sintered at 800°C for 2 hours. The effect of neodymia doping was investigated in terms of solution properties, morphological changes and thermal characteristics. The fibers were characterized by Fourier transform infrared (FT-IR), X-ray diffraction (XRD), and scanning electron microscope (SEM). The measurements showed that the conductivity, pH, viscosity, and surface tension of the hybrid polymer solutions have decreased with increasing Nd acetate content. The FT-IR spectra of the fibers were in good accordance with the literature. The average crystal size values for calcined and sintered samples which were obtained from precursor solutions were calculated. XRD analysis showed that the crystallite size was decreased with increasing Nd content. This result is verified by the calculation of the total areas of the peaks appeared in the XRD spectra. The very sharp and high intense peaks found in the diffraction patterns revealed the crystalline nature of the product. Moreover, the SEM micrograph of the fibers showed that the average fiber diameters decreased with increasing Nd content.

Synthesis and characterization of boron doped alumina stabilized zirconia fibers

by Prof.Dr. İbrahim USLU

FIBERS AND POLYMERS
2011, Volume 12, Number 3, 303-309, DOI: 10.1007/s12221-011-0303-1
ibrahim uslu, M.K. Öztürk, T. Tunç, T.C. Karabulut, S. Keskin,

Boron doped PVA/Zr-Al acetate nanofibers were prepared by electrospinning using PVA as a precursor. The effect of... more Boron doped PVA/Zr-Al acetate nanofibers were prepared by electrospinning using PVA as a precursor. The effect of calcination temperature on morphology and crystal structure was investigated at 250, 500, and 800 °C. The study also establishes the effect of boron doping on the morphology of PVA/Zr-Al acetate nanofibers at various calcination temperatures. The measurements showed that the conductivity, pH, viscosity and the surface tension of the hybrid polymer solutions have increased with boron doping. In addition, the fibers were characterized by FTIR, DSC, XPS, XRD and SEM techniques. The addition of boron did not only increase the thermal stability of the fibers, but also increased the average fiber diameters, which gave stronger fibers. The DSC results indicated that the melting temperature (Tm) of the fibers was increased from 256 to 270 °C with the addition of boron. XRD peak patterns showed that after further heat treatment at 800 °C, zirconia exists in two phases of tetragonal and monoclinic modifications. Moreover, alumina does not transform into the γ-Al2O3 and θ-Al2O3 phase at 800 °C. The SEM appearance of the fibers showed that the addition of boron resulted in the formation of crosslinked bright surfaced fibers.

Boron-nitride coated nuclear fuels

by Prof.Dr. İbrahim USLU

Nuclear technology
1996, vol. 116, no1, pp. 78-90
G. Gündüz, ibrahim Uslu, H.H. Durmazuçar

Pure urania- and urania-gadolinia-containing fuel pellets were coated with boron nitride (BN) to improve the physical... more Pure urania- and urania-gadolinia-containing fuel pellets were coated with boron nitride (BN) to improve the physical and neutronic properties of the fuel. The BN coating seems to have a technological advantage over zirconium-diboride coating. The BN is chemically inert, corrosion resistant, withstands rapid temperature changes, and has a high thermal conductivity. Since gadolinia fuel has low thermal conductivity, the gadolinia content can be lowered in the fuel by coating it with BN. In fact, the existence of two burnable absorbers in a fuel introduces desired nuclear properties since gadolinia is a fast-burning and boron a slow-burning element. The BN was deposited on fuel from two different sources, (a) from the reaction of boron trichloride (BCl3) and ammonia (NH3) at 875 K and (b) from the decomposition of trimethylamine borate complex at 1200 K. The infrared and X-ray diffraction (XRD) spectra of BN from both precursors agreed with the available data in the literature. However BN powder from borane complex had a shifted XRD peak due to the presence of carbonaceous material in the structure. The BN powder-coated fuels were heated to 1400, 1525, and 1600K to sinter the BN. The examination under scanning electron microscope showed that grainy, rodshaped and layered BN coatings were achieved. Rodshaped structures were usually seen on gadolinia fuels. The increased thickness of coating favors the formation of a glassy looking layer. The BN from a borane complex seems to form a layered structure more easily than the BN from BCl3. The BN coated the surface of the fuels, and it did not penetrate into the fuels.

Temperature dependent current-voltage (IV) characteristics of Au/n-Si (1 1 1) Schottky barrier diodes with PVA (Ni, Zn-doped) interfacial layer

by Prof.Dr. İbrahim USLU

Materials Science in Semiconductor Processing
Volume 14, Issue 2, June 2011, Pages 139-145

Tuncay Tunc, şemsettin Altindal, iIbrahim Uslu, ilbilge Dökme, Habibe Uslu

Current–voltage (I–V) characteristics of Au/PVA/n-Si (1 1 1) Schottky barrier diodes (SBDs) have been investigated in... more Current–voltage (I–V) characteristics of Au/PVA/n-Si (1 1 1) Schottky barrier diodes (SBDs) have been investigated in the temperature range 80–400 K. Here, polyvinyl alcohol (PVA) has been used as interfacial layer between metal and semiconductor layers. The zero-bias barrier height (ΦB0) and ideality factor (n) determined from the forward bias I–V characteristics were found strongly dependent on temperature. The forward bias semi-logarithmic I–V curves for different temperatures have an almost common cross-point at a certain bias voltage. The values of ΦB0 increase with the increasing temperature whereas those of n decrease. Therefore, we have attempted to draw ΦB0 vs. q/2kT plot in order to obtain evidence of a Gaussian distribution (GD) of the barrier heights (BHs). The mean value of BH and standard deviation (σ0) were found to be 0.974 eV and 0.101 V from this plot, respectively. Thus, the slope and intercept of modified vs. q/kT plot give the values of and Richardson constant (A⁎) as 0.966 eV and 118.75 A/cm2K2, respectively, without using the temperature coefficient of the BH. This value of A* 118.75 A/cm2K2 is very close to the theoretical value of 120 A/cm2K2 for n-type Si. Hence, it has been concluded that the temperature dependence of the forward I–V characteristics of Au/PVA/n-Si (1 1 1) SBDs can be successfully explained on the basis of the Thermionic Emission (TE) theory with a GD of the BHs at Au/n-Si interface.

Preparation of Gadolina Stabilized Bismuth Oxide Doped with Boron via Electrospinning Technique

by Prof.Dr. İbrahim USLU

JOURNAL OF INORGANIC AND ORGANOMETALLIC POLYMERS AND MATERIALS
J Inorg Organomet Polym (2012) 22:105–111

Tuncay Tunc¸ ibrahim Uslu, Senol Durmusoğlu, Selda Keskin, Arda Aytimur, Ahmet Akdemir

In this study, boron doped and undoped poly (vinyl) alcohol/bismuth–gadolina acetate (PVA/Bi–Gd) nanofibers were... more In this study, boron doped and undoped poly (vinyl) alcohol/bismuth–gadolina acetate (PVA/Bi–Gd) nanofibers were prepared using electrospinning technique then calcinated at 800 °C for 2 h.
The originality of this study is the addition of boron to metal acetates. The effects of boron doping were investigated in terms of solution properties, morphological changes and thermal characteristics.
The characteristics of the fibers were investigated with FT-IR, XRD, SEM and BET. The addition of boron did not only increase the thermal stability of the fibers, but also their diameters, which yielded stronger fibers. XRD analyses showed that boron doping increased the peak intensities and indicated that the boron doping enhanced the crystallite size. Moreover, no shifts were noticed in diffraction angles for boron doped and undoped samples. Therefore, boron doping did not significantly alter the lattice spacing. The SEM micrograph of the fibers showed that the addition of boron resulted in the formation of cross-linked bright-surfaced fibers. The average fiber diameter for boron doped and undoped fiber mats were 204 and 123 nm, respectively. Also, grain diameters of boron doped and undoped nanocrystalline sintered powders were measured as 140 and 118 nm, respectively. The BET results showed that boron undoped and doped Bi2O3–La2O3 nanocrystalline powder ceramic structures sintered at 800 °C have surface areas of 59.72 and 39.80 m2/g, respectively.

Conductance of graphene nanoribbons under mechanical deformation

by Rajib Chowdhury

Physica E: Low-dimensional Systems and Nanostructures

Graphene is a material system of increasing technological importance with excellent mechanical and electrical... more Graphene is a material system of increasing technological importance with excellent mechanical and electrical properties. Depending on the edge configuration, graphene may be electrically conducting, semiconducting, or insulating, so deformation is believed to have strong effects on electrical properties. In this letter, ab initio approach is used to demonstrate the effect of torsional and strain induced deformation on the electrical conductance characteristics. These nanostructures are described using a single-band tight-binding Hamiltonian. Important observations on the connection between mechanical and electrical behavior are made based on the transport calculations. In particular, the conductance behavior shows interesting features on deformed graphene.

Nonlinear aspects of energy dissipation in wood-panel joints

by Sara Casciati

Casciati S. (2007). “Nonlinear aspects of energy dissipation in wood-panel joints”. Earthquake Engineering and Engineering Vibration, 6(3), 259-268. ISSN: 1671-3664.

DATA E LUOGO DI PUBBLICAZIONE: September 2007; Springer, New York, NY 10013, USA.

ABSTRACT: The joints connecting vertical and horizontal elements are the “weak link” in structural systems assembled... more ABSTRACT: The joints connecting vertical and horizontal elements are the “weak link” in structural systems assembled from wood panels. If they are too weak, local failures may occur, resulting in performance that is significantly below expectations. If they are too resistant, the joints may be unable to dissipate energy during vibrations, thus possibly initiating a fast progressive failure. This paper re-processes and re-elaborates the results of shaking table tests previously carried out by the author and other co-workers. The goal is to assess the feasibility of a joint which is able to dissipate energy during vibration, without degrading the connection performance.

KEYWORDS: dynamic excitation - energy dissipation - wood panels joint - monitoring system - shaking table tests

Fatigue characterization of a Cu-based shape memory alloy

by Sara Casciati

Casciati F., Casciati S., and Faravelli L. (2007). “Fatigue characterization of a Cu-based shape memory alloy”. Proceedings of the Estonian Academy of Sciences – Physics Mathematics, 56(2), 207-217. ISSN: 1406-0086.

DATE AND PLACE OF PUBBLICATION: June 2007; Estonian Academy Publishers, Tallinn 10143, Estonia.

ABSTRACT: Applications of a Cu-based shape memory alloy in monuments retrofitting were conceived, designed, and... more ABSTRACT: Applications of a Cu-based shape memory alloy in monuments retrofitting were conceived, designed, and tested. The polycrystalline nature of the material requires a preliminary discussion in order to allow the extension of properties which are well known for monocrystalline specimens. Furthermore, one of the main drawbacks of the investigated alloy is its brittleness. Therefore, fatigue tests were carried out and the results of those conducted under cycles of torsion loading–unloading are reported. In particular, the identification of the response range, which is most suitable for structural engineering applications, is pursued.

KEYWORDS: compression tests, fatigue tests, shape memory alloy, tension tests, torsion tests, thermal treatment.

Random imperfection fields to model the size effect in laboratory wood specimens

by Sara Casciati

Casciati S. and Domaneschi M. (2007). “Random imperfection fields to model the size effect in laboratory wood specimens”. Structural Safety, 29(4), 308-321. ISSN: 0167-4730.

DATA E LUOGO DI PUBBLICAZIONE: October 2007; Elsevier Science Bv, 1000 AE Amsterdam, Netherlands.

ABSTRACT. The composite nature of a wood continuum prevents one from extrapolating the results of laboratory tests on... more ABSTRACT. The composite nature of a wood continuum prevents one from extrapolating the results of laboratory tests on standard wood specimens to structural elements of significant size. Therefore, these elements are usually tested under standardized loading conditions in order to detect a sort of average material behaviour.
In this paper, the initial step consists, instead, of testing the material specimens. The extension of the results to structural elements is then pursued by introducing a random field, or, in a discretized model, a random array of imperfections.
The calibration of the suitable spatial distribution of the imperfections is then investigated by a mixed experimental–numerical approach, for a reference beam. The analyses on the relative finite elements model are iterated to match the response of the full scale laboratory tests.

KEYWORDS: Biaxial tests; Finite element model; Imperfections; Laboratory tests; Random field; Wood specimens

Cohesive Crack Propagation in a Random Elastic Medium

by Sara Casciati

Bruggi M., Casciati S., and Faravelli L. (2008). “Cohesive crack propagation in a random elastic medium”. Probabilistic Engineering Mechanics, 23(1), 23-35. ISSN: 0266-8920.

DATA E LUOGO DI PUBBLICAZIONE: January 2008; Elsevier Sci Ltd, Kidlington, Oxford OX5 1GB, Oxon, England.

ABSTRACT. The issue of generating non-Gaussian, multivariate and correlated random fields, while preserving the... more ABSTRACT. The issue of generating non-Gaussian, multivariate and correlated random fields, while preserving the internal auto-correlation structure of each single-parameter field, is discussed with reference to the problem of cohesive crack propagation. Three different fields are introduced to model the spatial variability of the Young modulus, the tensile strength of the material, and the fracture energy, respectively. Within a finite-element context, the crack-propagation phenomenon is analyzed by coupling a Monte Carlo simulation scheme with an iterative solution algorithm based on a truly-mixed variational formulation which is derived from the Hellinger–Reissner principle. The selected approach presents the advantage of exploiting the finite-element technology without the need to introduce additional modes to model the displacement discontinuity along the crack boundaries. Furthermore, the accuracy of the stress estimate pursued by the truly-mixed approach is highly desirable, the direction of crack propagation being determined on the basis of the principal stress criterion. The numerical example of a plain concrete beam with initial crack under a three-point bending test is considered. The statistics of the response is analyzed in terms of peak load and load–mid deflection curves, in order to investigate the effects of the uncertainties on both the carrying capacity and the post-peak behaviour. A sensitivity analysis is preliminarily performed and its results emphasize the negative effects of not accounting for the auto-correlation structure of each random field. A probabilistic method is then applied to enforce the auto-correlation without significantly altering the target marginal distributions. The novelty of the proposed approach with respect to other methods found in the literature consists of not requiring the a priori knowledge of the global correlation structure of the multivariate random field.

KEYWORDS: Multivariate non-Gaussian random fields; Auto-correlation; Cohesive crack propagation; Truly-mixed finite element method; Monte Carlo simulations

Experimental and numerical studies toward the implementation of shape memory alloy ties in masonry structures

by Sara Casciati

Casciati S. and Hamdaoui K. (2008). “Experimental and numerical studies toward the implementation of shape memory alloy ties in masonry structures”. Smart Structures and Systems, 4(2), 153-169. ISSN: 1738-1584.

DATA E LUOGO DI PUBBLICAZIONE: March 2008; Techno-Press, Daejeon 305-600, South Korea.

ABSTRACT: The use of pre-tensioned shape memory alloy (SMA) wires to retrofit historic masonry structures is... more ABSTRACT: The use of pre-tensioned shape memory alloy (SMA) wires to retrofit historic masonry structures is investigated. A small wall, serving as a prototype masonry specimen, is constructed to undergo a series of shaking table tests. It is first studied in its original state, and its dynamic characteristics (in terms of modal frequencies) are extracted from the recorded signals. The results are then compared with those obtained when an increasing number of couples of pre-stressed SMA wires are introduced in the specimen to link the bricks together. A threedimensional finite element model of the specimen is developed and calibrated according to the modal parameters identified from each experimental test (with and without SMA wires). The calibration process is conducted by enhancing the masonry mechanical behaviour. The results and the effectiveness of the approach are presented.

KEYWORDS: masonry; numerical modelling; shaking table tests; shape memory alloys; system identification.

Experimental studies on the fatigue life of shape memory alloy bars

by Sara Casciati

Casciati S. and Marzi A. (2010). “Experimental studies on the fatigue life of shape memory alloy bars”. Smart Structures and Systems, 6(1), 73-85. ISSN: 1738-1584.

DATE AND PLACE OF PUBLICATION: January 2010; Techno-Press, Daejeon 305-600, South Korea.

ABSTRACT: The potential offered by the thermo-mechanical properties of shape memory alloys (SMA) in structural... more ABSTRACT: The potential offered by the thermo-mechanical properties of shape memory alloys (SMA) in structural engineering applications has been the topic of many research studies during the last two decades. The main issues concern the long-term predictability of the material behaviour and the fatigue lifetime of the macro structural elements (as different from the one of wire segments). The laboratory tests reported in this paper are carried out on bar specimens and they were planned in order to pursue two objectives. First, the creep phenomenon is investigated for two different alloys, a classical Ni-Ti alloy and a Cu-based alloy. The attention is then focused on the Cu-based alloy only and its fatigue characteristics at given temperatures are investigated. Stress and thermal cycles are alternated to detect any path dependency.

KEYWORDS: creep; fatigue; hysteresis; shape memory alloys; thermo-mechanics; viscosity.

Fatigue laboratory tests toward the design of SMA portico-braces

by Sara Casciati

Carreras G., Casciati F., Casciati S., Isalgue A., Marzi A., and Torra V. (2011). “Fatigue laboratory tests toward the design of SMA portico-braces”. Smart Structures and Systems, 7(1), 41-57. ISSN: 1738-1584

DATE AND PLACE OF PUBLICATION: January 2011; Techno-Press, Daejeon 305-600, South Korea.

ABSRACT: A deeper understanding of the effectiveness of adopting devices mounting shape memory alloy (SMA) elements in... more ABSRACT: A deeper understanding of the effectiveness of adopting devices mounting shape memory alloy (SMA) elements in applications targeted to the mitigation of vibrations is pursued via an experimental approach. During a seismic event, less than 1000 loading unloading cycles of the alloy are required to mitigate the earthquake effects. However, the aging effects during the time of inactivity prior to the oscillations (several decades characterized by the yearly summer-winter temperature wave) should be considered in order to avoid and/or minimize them. In this paper, the results obtained by carrying out, in different laboratories, fatigue tests on SMA specimens are compared and discussed. Furthermore, the effects of seismic events on a steel structure, with and without SMA dampers, are numerically simulated using ANSYS. Under an earthquake excitation, the SMA devices halve the oscillation amplitudes and show re-centering properties. To confirm this result, an experimental campaign is conducted by actually installing the proposed devices on a physical model of the structure and by evaluating their performance under different excitations induced by an actuator.

KEYWORDS: damping; fatigue life; passive control systems; shape memory alloys; vibration mitigation

Fatigue tests on SMA bars in span control

by Sara Casciati

Casciati S. and Marzi A. (2011). “Fatigue tests on SMA bars in span control”. Engineering Structures, 33(4), 1232-1239. ISSN: 0141-0296.

DATE AND PLACE OF PUBLICATION: April 2011; Elsevier Sci Ltd, Kidlington, Oxford OX5 1GB, Oxon, England.

ABSTRACT: The estimation of the fatigue lifetime of copper based shape memory alloy (SMA) specimens in the form of... more ABSTRACT: The estimation of the fatigue lifetime of copper based shape memory alloy (SMA) specimens in the form of bars is investigated in view of their use in passive devices for structural control applications. In the envisaged application, the pre-stress value assigned to the SMA bars is selected so that it is only slightly modified when strain variations occur within an operative range. This condition requires that the fatigue cycles are performed in span control, conversely to the common practice of driving the tests in load control to study the fatigue behaviour of traditional metals such as steel. Several experiments are carried out at different temperatures for different strain ranges and the results are arranged with the aim of building suitable fatigue models.

KEYWORDS: Dampers; Energy dissipation; Fatigue; Copper-based shape memory alloy; Vibration mitigation

Fatigue damage accumulation in a Cu-based shape memory alloy: preliminary investigation

by Sara Casciati

Casciati F., Casciati S., Faravelli L., and Marzi A. (2011). “Fatigue damage accumulation in a Cu-based shape memory alloy: preliminary investigation”. CMC-Computers Materials & Continua, 23(3), 287-306. ISSN: 1546-2218.

DATE AND PLACE OF PUBLICATION: June 2011; Tech Science Press, Norcross, GA 30071, USA.

ABSTRACT: The potential offered by the main features of shape memory alloys (SMA) in Structural Engineering... more ABSTRACT: The potential offered by the main features of shape memory alloys (SMA) in Structural Engineering applications is object of attention since two decades. The main issues concern the predictability of the material behavior and the fatigue lifetime of macro structural elements (as different from wire segments). In this paper, the fatigue characteristics, at given temperatures, of multigrain samples of a specific Cu-based alloy are investigated. The results of laboratory tests on bar specimens are discussed. The target is to model the manner in which the effects of several loading-unloading cycles of different amplitude cumulate.

KEYWORDS: Damage accumulation, Fatigue, Shape memory alloy, SMA Training.

Crack Growth in the Creep-Fatigue Regime Under Constrained Loading of Thin Sheet Combustor Alloys

by Richard Johnston

Article in Press

Damage tolerant lifing methodologies are an essential requirement to support the safe and reliable operation of... more Damage tolerant lifing methodologies are an essential requirement to support the safe and reliable operation of combustors in the modern gas turbine aero-engine. These static components experience a wide range of temperature superimposed upon a complex stress field resulting from their geometrical form. Features such as seam welds, injector ports and cooling hole arrays could potentially initiate and subsequently interact with fatigue cracks, while the thin sheet structure of the combustion annulus locally imparts plane stress dominant conditions. However, on the macro-scale the stress system is highly constrained, ensuring that cracks usually arrest before achieving a critical size, thereby avoiding failures. These service observations must now be supplemented by a detailed scientific understanding of fatigue behaviour in combustor alloys under representative conditions.

The technical challenge of evaluating crack growth rates in thin sheet laboratory scale specimens under strain controlled fatigue at high temperatures is addressed. The alloy system of interest was the nickel based superalloy Haynes 230. A matrix of novel fatigue crack growth tests is reported at temperatures ranging from 816oC to 982oC under a variety of strain ratios (εmin/εmax) with crack growth monitored via an automated pulsed DCPD system. Contemporaneous measurements of constitutive behaviour will demonstrate the significant degree of stress relaxation that occurs under these constrained conditions, continually modifying the crack tip driving force. This was best quantified by the corresponding stress ratio, which generally fell throughout the period of the test and under dwell loading cycles often resulted in a highly negative value (e.g. stress ratio lower than minus fifteen) incorporating a minimal tensile stress contribution. Crack growth can clearly continue under such highly compressive regimes. The contribution from creep damage mechanisms will be illustrated via post test fractographic and microstructural examination.

Finite single wall capped carbon nanotubes under hydrostatic pressure

by Jose Luis Rodriguez Lopez

Co-authored with: S E Baltazar, A H Romero, and R Martoňák
J. Phys.: Condens. Matter 18, 9119 (2006).
doi: 10.1088/0953-8984/18/39/037

We report a classical molecular dynamics isothermal–isobaric ensemble (NPT) implementation for the simulation of... more We report a classical molecular dynamics isothermal–isobaric ensemble (NPT) implementation for the simulation of pressure effects on finite systems. The method is based on calculating the enclosed surface area by means of the Delauney triangulation method, which results in a fairly accurate description of the surface and the system volume. The external pressure is applied to the system by external forces acting on the triangulated surface covering the nanostructure. Pressure is exerted perpendicularly to every one of the Delauney triangles, by equally distributing the force to every corner of a triangle. We applied the method to finite single wall capped carbon nanotubes (SWCNTs) with different chiralities and different tube lengths ranging from 4 nm up to 30 nm. Pressure effects are studied as a function of the radii and the nanotube length, as well as as a function of temperature. Our results are in very good agreement when compared with both experimental and other theoretical results.

Freestanding abradable coating manufacture and tensile test development

by Richard Johnston

Abradable coatings are used extensively within gas turbines. Abradable material is applied to the inside surface of... more Abradable coatings are used extensively within gas turbines. Abradable material is applied to the inside surface of the compressor and turbine shroud sections using thermal spray methods, coating the periphery of the blade rotation path. The function of an abradable seal is to wear preferentially when rotating blades come into contact with it, while minimising the over-tip clearance, and improving the overall efficiency of the engine. There is a distinct lack of established materials property data for all abradable materials, due to the difficulty of testing this very unique class of materials. Abradables understanding is historically limited, with the field often described as a ‘black art’, and component/material improvements habitually the result of ‘firefighting’ actions. This work is part of a wider programme in partnership with Rolls-Royce plc to gain a greater understanding of abradable materials, how they perform, and ultimately how to improve their performance in-service. The paper describes a novel method, devised in tandem with Rolls-Royce plc, of producing free-standing abradable tensile test specimens via thermal spray. The specimen mould is composed of a dissolvable polymer composite which maintains its integrity during spraying and cooling, and is then ‘washed’ away in an ultrasonic water bath. This results in near-net shape specimens, which are machined to a specific geometry. The paper details the iterative testing development that contributed to a final working design and testing methodology for a previously untested class of materials.