Search

Ultrasonic fatigue testing machines and related research are continuously growing. They allow researchers to study fatigue in a time and energy reliable manner, fatigue strength and fracture mechanics between 10E06 to 10E10 cycles, the established Very High Cycle Fatigue (VHCF) regime. Almost all published research to date was conducted under uniaxial tension–compression, pure torsion or bending loading under ultrasonic fatigue\ud testing. The interest in multiaxial ultrasonic testing for VHCF has begun to evolve from the increasing available knowledge surrounding uniaxial fatigue in VHCF. Tension-Torsion and in-plane biaxial ultrasonic fatigue tests were first reached by adapting the tension–compression ultrasonic setup. The present work focuses on the crack path, critical initiation angle, fracture surface analysis and evaluation of different specimens, that failed\ud under different biaxial loading conditions under ultrasonic fatigue. Specimens were specially designed and machined to function under ultrasonic fatigue machines resonance concept. Three different biaxial loading paths were carried out, in-phase tension–torsion and tension–tension, and out-of-phase tension–compression. Crack initiation, crack path and crack surface morphologies were analyzed by optic microscope and scanning electron microscope. Experimental results were analyzed and discussed; conclusions concerning these types of failures under ultrasonic testing regime are drawn.

Mechanical and material testing and analysis are of indisputable importance. As the variety of materials and/or the complexity of products grows, so does the need for new material characterisation methods. In this study, experimental results from a new method for ultrasonic fatigue testing of equibiaxial and pure shear cruciform specimens is presented and validated. The method uses specially designed cruciform specimens’ geometries for ultrasonic fatigue testing machines. Three different cruciform geometries were tested. The responses from the specimens were first studied by an adapted modal calculation method FDD. The new method allows determining fatigue strength of cruciform specimens in the VHCF regime with very satisfactory results.

With the development of new materials, it is now known that there is no such thing as a fatigue endurance limit, i.e., materials do not have infinite life when the stress level is such that there is no fracture up to 10 million (1E7) cycles. The problem of testing materials above this number of cycles is that most testing equipment operates well below 150 Hz, making testing up to 1 billion (1E9) cycles or above an impracticality. The recent developments of ultrasonic testing machines where frequencies can go as high as 20 kHz or above enabled tests to be extended to these ranges in just a few days. This is now known as Very High Cycle Fatigue (VHCF). On the other hand, critical components used in Engineering applications are usually subjected to multi-axial loads, as is the case of the fuselage and wings of aircrafts which are subjected to biaxial states of stress. In this paper, VHCF cruciform test specimens purposely designed to develop orthogonal biaxial stresses with different biaxiality ratios will be analysed. The specimens are composed from Aluminium 6082-T651, a medium strength alloy used in many highly stressed engineering applications, including trusses, cranes, bridges and transportation. The specimens work as tuning forks with determined mode shapes at 20±0.5 kHz, where maximum principal stresses are developed at the centre of the specimen. Finite Element Analysis (FEA) is used to assess the dynamic behaviour of the specimens. The framework on how to design and manufacture cruciform specimens with different biaxiality ratios will be explained in a clear way so it can be used by other engineers in the field.

Fatigue behavior for any material or composite is an ever-evolving complex subject of big engineering importance. Within the very high 106 to 109 cycle regime the use of ultrasonic fatigue has been a big research topic in recent years, ranging from all metals and composite studies, new experimental methods and induced stress states, to the frequency effect and associated fracture mechanics. Following the associated cruciform ultrasonic fatigue testing research in development, new and improved cruciform specimens were acquired and subjected to experimental analysis. The conducted fatigue cruciform testing method induces an axial-axial in phase stress state through the use of a piezoelectric ultrasonic testing machine built at Instituto Superior Tecnico laboratories. A modal response analysis was performed within the frequency working range of the used piezoelectric transducer using a Polytec laser vibrometer. A modal calculation method termed Frequency Domain Decomposition (FDD) was adapted to the measured cruciform vibration displacements. The points of measure were in correlation to a previous published numerical research. The results were then compared to previous cruciform specimens’ behavior and numerical results. Strain gauges were also used for the strain and stress study at the specimen’s center. With both conducted experiments showing promising results, the new and improved specimens were led to fatigue failure and a preliminary fracture analysis made.

Modal Identification from Frequency Response Functions (FRFs) has been extensively investigated up to the point its research reached a stagnation state. Yet, a new approach to determine the modal damping factors from FRFs was recently proposed, showing that there still is scope for new findings in the field. Contrary to other modal identification methods which are based on the dynamic motion governing equations, the method used the dissipated energy per cycle of vibration as a starting point. For lightly damped systems with conveniently spaced modes, it produced quite accurate results, especially when compared to the well-known method of the inverse. The method used a plot of the sine of the phase of the receptance against its amplitude, whereby damping was determined from the slope of a linear fit to the resulting plot. In this paper, it is shown that this plot has other (perhaps more important) special properties that were not explored before. Near resonant frequencies, its shape is elliptical, whereby the real and imaginary parts of the modal constants can be determined from numerical curve-fitting. This finding allowed developing a new method which formulation is presented in this paper. The method is discussed through numerical and experimental examples. Although the intention is not to present a new modal identification method that is superior to other existing ones (like the method of the inverse or those based on the Nyquist plot), the authors believe that this new representation of the receptance and its properties may bring valuable insights for other researchers in the field.

The present work has used an alternating magnetic field treatment at ambient temperature to improve the wear resistance of nickel-aluminium bronze and aluminium alloy 2014-T6. Pin-on-disc wear tests under lubricating conditions using a AISI52100 steel ball bearing as the counter face material have shown reduction in the width and depth of wear scars as well as lower values of the coefficient of friction following the treatment. The improved wear properties have been attributed to increased precipitation of κIV in the case of the nickel-aluminium bronze and of GP zones as well as θ’’ for AA2014-T6. Transmission electron microscopy also revealed changes in the dislocation distribution, while X-ray diffraction showed changes in the residual stress for both alloys. The mechanism leading to these changes is discussed by consideration of the soft ferromagnetic properties of the nickel-aluminium bronze and the paramagnetic nature of AA2014.

The present work has used an alternating magnetic field treatment at ambient temperature to improve the wear resistance of nickel-aluminium bronze and aluminium alloy 2014-T6. Pin-on-disc wear tests under lubricating conditions using a AISI52100 steel ball bearing as the counter face material have shown reduction in the width and depth of wear scars as well as lower values of the coefficient of friction following the treatment. The improved wear properties have been attributed to increased precipitation of κIV in the case of the nickel-aluminium bronze and of GP zones as well as θ’’ for AA2014-T6. Transmission electron microscopy also revealed changes in the dislocation distribution, while X-ray diffraction showed changes in the residual stress for both alloys. The mechanism leading to these changes is discussed by consideration of the soft ferromagnetic properties of the nickel-aluminium bronze and the paramagnetic nature of AA2014.

Many applications in Experimental Modal Analysis (EMA) require that the sensors’ masses are known. This is because the added mass from sensors will affect the structural mode shapes, and in particular its natural frequencies. EMA requires the measurement of the exciting forces at given coordinates, which is often made using piezoelectric force transducers. In such a case, the live mass of the force transducer, i.e. the mass as ‘seen’ by the structure in perpendicular directions must be measured somehow, so that compensation methods like mass cancelation can be performed. This however presents a problem on how to obtain an accurate measurement for the live mass. If the system is perfectly calibrated, then a reasonably accurate estimate can be made using a straightforward method available in most classical textbooks based on Newton’s second law. However, this is often not the case (for example when the transducer’s sensitivity changed over time, when it is unknown or when the connection influences the transmission of the force). In a self-calibrating iterative method, both the live mass and calibration factor are determined, but this paper shows that the problem may be ill-conditioned, producing misleading results if certain conditions are not met. Therefore, a more robust method is presented and discussed in this paper, reducing the ill-conditioning problems and the need to know the calibration factors beforehand. The three methods will be compared and discussed through numerical and experimental examples, showing that classical EMA still is a field of research that deserves the attention from scientists and engineers.

In the drive towards a sustainable bio-economy, a growing interest exists in the development of composite materials using renewable natural resources. This paper explores the life cycle assessment of processing of Flax fibre reinforced polylactic acid (PLA), with a comparison of glass fibre triaxial fabric in the production process. The use of hydrocarbon fossil resources and synthetic fibres, such as glass and carbon, have caused severe environmental impacts in their entire life cycles. Whereas, Flax/PLA is one of the cornerstones for the sustainable economic growth of natural fibre composites. In this study, the manufacturing processes for the production of Flax/PLA tape and triaxial glass fibre were evaluated through a gate-to-gate life cycle assessment (LCA). The assessment was based on an input-output model to estimate energy demand and environmental impacts. The quality of the natural hybrid composite produced and cost-effectiveness of their LCA was dependent on their roving processing speeds and temperature applied to both the Flax/PLA tape and triaxial glass fabrics during processing. The optimum processing condition was found to be at a maximum of 4 m/min at a constant temperature of 170 &deg, C. In contrast, the optimum for normal triaxial glass fibre production was at a slower speed of 1 m/min using a roving glass fibre laminating machine. The results showed that when the Flax and PLA were combined to produce new composite material in the form of a flax/PLA tape, energy consumption was 0.25 MJ/kg, which is lower than the 0.8 MJ/kg used for glass fibre fabric process. Flax/PLA tape and glass fibre fabric composites have a carbon footprint equivalent to 0.036 kg CO2 and 0.11 kg CO2, respectively, under the same manufacturing conditions. These are within the technical requirements in the composites industry. The manufacturing process adopted to transform Flax/PLA into a similar tape composite was considerably quicker than that of woven glass fibre fabric for composite tape. This work elucidated the relationship of the energy consumptions of the two materials processes by using a standard LCA analytical methodology. The outcomes supported an alternative option for replacement of some conventional composite materials for the automotive industry. Most importantly, the natural fibre composite production is shown to result in an economic benefit and reduced environmental impact.

In this work, two special aluminium cruciform specimens are designed and tested in an ultrasonic fatigue machine. They were designed based on Single-Input-Multiple-Output (SIMO) modal analysis to induce in-plane biaxial stress combinations (in-phase T-T and out-of-phase C-T) when at resonance at 20 kHz. The geometries were subjected to both numerical analysis and experimental testing to understand if they can indeed create the intended biaxial state of stresses. Both numerical and experimental results showed an impact of nearby resonant modes of non-interest on the correct functioning of the specimens, especially regarding the T-T specimen where a large deviation from the mode of interest was measured. This means that future work includes re-designing T-T specimens taking into account these mode shapes. Only out-of-phase specimens demonstrated to work properly and tests until failure were conducted. The first failure results showed to be consistent with literature when out-of-phase biaxial stress is applied cyclically.

Fatigue studies of materials in simple or complex loading systems for any given lifetime is object of continuous research. This is due to the advancements on mechanical and structural components, as well as for new and innovative materials, which implies the knowledge of a materials response to all dynamic loads. The fatigue failure regime beyond what was once considered to be the fatigue limit (infinite life) is characterized between 107 and 109, known as Very High Cycle Fatigue regime. Due to the time consuming and wide energy consumption of conventional fatigue testing for such regime, fatigue tests under ultrasonic actuators are being used, capable of applying the dynamic loads at around 20 kHz. Nowadays, several variants of ultrasonic fatigue tests were already proposed and tested but it is still a somewhat limited fatigue test if compared to the conventional servo-hydraulic fatigue testing machines of general use. In this study, biaxial in plane stresses are induced in specially designed cruciform specimens with ultrasonic fatigue testing resonant principals. Two geometries were numerically analysed, manufactured and experimentally tested, the in-phase tension-tension (T-T) specimen and the out-of-phase compression-tension (C-T) specimen. All specific designed geometries go under a thorough numerical and several experiments analysis for their validation. The specimens showing a correct and as intended behaviour are led to failure.

With the introduction of new materials and advances in medical science, the endodontic files have changed since the early days of root canal treatments. In the late days, we have seen an increasing use of Nickel-Titanium (NiTi) alloys, to the detriment of more conventional alloys. At body temperature, NiTi alloys present a superelastic behaviour, which allows to be more effective in the removal of the tooth pulp tissue, and in the protection of the tooth structure. Anyhow, these NiTi instruments will eventually fracture, usually without any visual signal of degradation. Thus, there is a need of studying these alloys, as they present a high hysteresis cycle and non-linearities in the Elastic domain. Currently, there is no international standard to test NiTi endodontic files, so various authors have attempted to design systems that can test them under fatigue loads, usually based on empirical setups. Following a systematic approach, this work presents the results of rotary fatigue tests for two Alfa Aesar® Nitinol wires with different diameters (0.58mm and 0.25mm).The formulation is presented, where the material strength reduction can be quantified from the determination of the strain and the number of cycles until failure, as well numerical FEM simulation to verify the analytical model predictions.

This study presents a development on a time efficient approach to measure the Acoustic Impedance of Industrial Insulating materials using an impedance tube with single moveable microphone and a white noise generator. The standing wave method is used to measure the acoustic properties (absorption coefficient and acoustic impedance) of sound absorbing materials. In order to use a burst of white noise instead of individual stationary sine waves, a signal processing technique was developed. The algorithm is based on the equation of simple harmonic motion, but uses distance as a variable, instead of time. This innovative method allows measuring at frequency resolutions as low as 5 Hz in a reasonably short amount of time. This is an advantage, as the classical standing wave method uses frequency resolutions of one-third of octaves for measurements, as otherwise time would be prohibitive. Experimental results are validated with those measured with the sine wave generator using one-third of octave frequencies by comparing their behaviours.

Endodontic rotary file instruments used to treat root canals in dentistry suffered breakthrough transformations in recent years when stainless steel was replaced by Nickel-Titanium (NiTi). NiTi alloys used in Endodontics possess superelastic properties at body temperature (37C) that bring many advantages on the overall performance of the root-canal treatment. They can follow curved root canals more easily than stainless steel instruments and have been reported to be more effective in the removal of the inflamed pulp tissue and protection of the tooth structure. However, these instruments eventually fracture under cyclic bending loading due to fatigue, without any visible signals of degradation to the practitioner. This problem brought new challenges on how new instruments should be tested, as NiTi alloys are highly non-linear and present a large hysteresis cycle in the Elastic domain. Current existing standards are only available for Stainless Steel testing. Thus, many authors have attempted to design systems that can test NiTi endodontic files under fatigue loads. However, no approach has been universally adopted by the community yet, as in most cases they are based on empirical set ups. Following a more systematic approach, this work presents the results of rotary fatigue tests for several NiTi wires from different manufacturers (Memry™ and Euroflex™). The formulation is presented, where the material strength reduction can be quantified from the determination of the strain and the number of cycles until failure, as well numerical FEM simulation to verify the analytical model predictions.

Fatigue is one of the main causes for in service failure of mechanical components and structures. With the development of new materials, such as high strength aluminium or titanium alloys with different microstructures from steels, materials no longer have a fatigue limit in the classical sense, where it was accepted that they would have ‘infinite life’ from 10 million (107) cycles. The emergence of new materials used in critical mechanical parts, including parts obtained from metal additive manufacturing (AM), the need for weight reduction and the ambition to travel greater distances in shorter periods of time, have brought many challenges to design engineers, since they demand predictability of material properties and that they are readily available. Most fatigue testing today still uses uniaxial loads. However, it is generally recognised that multiaxial stresses occur in many full-scale structures, being rare the occurrence of pure uniaxial stress states. By combining both Ultrasonic Fatigue Testing with multiaxial testing through Single-Input-Multiple-Output Modal Analysis, the high costs of both equipment and time to conduct experiments have seen a massive improvement. It is presently possible to test materials under multiaxial loading conditions and for a very high number of cycles in a fraction of the time compared to non-ultrasonic fatigue testing methods (days compared to months or years). This work presents the current status of ultrasonic fatigue testing machines working at a frequency of 20 kHz to date, with emphasis on multiaxial fatigue and very high cycle fatigue. Special attention will be put into the performance of multiaxial fatigue tests of classical cylindrical specimens under tension/torsion and flat cruciform specimens under in-plane bi-axial testing using low cost piezoelectric transducers. Together with the description of the testing machines and associated instrumentation, some experimental results of fatigue tests are presented in order to demonstrate how ultrasonic fatigue testing can be used to determine the behaviour of a steel alloy from a railway wheel at very high cycle fatigue regime when subjected to multiaxial tension/torsion loadings.

This paper presents a case study to assess the students’ views and correlation between attendance to lectures, laboratories and seminars and their performance in terms of final results. The population is composed by a group of first year undergraduate students at the department of Design and Engineering, Bournemouth University, in an Engineering Design unit. Attendance was monitored for a number of 19 students over one academic year (2016-2017). Students’ views regarding the impact of some factors - such as clear expectations, content easy to understand, student collaboration and interaction, peer-pressure, and to be seen by the lecturer – on assessment performance have been surveyed. This data was obtained from a Likert scale survey ran over a population of 10 students in the 2017-2018 academic year. The data that correlates final marks with attendance (laboratory, lectures and seminars) was analysed and indicates a strong least-squares fit correlation between attendance and final marks, with a coefficient of correlation $\mathrm {R}^{2}=0.78$ when plotting final marks vs overall attendance. Student view, which was also considered as an important aspect of this study, conveys the benefits of attending the lectures and quantify some of the factors mentioned above including a minimum-maximum attendance rate for a good grade (first-class) or just a pass mark. Students perceive that the two main reasons for attending lectures are that, firstly, it makes it easier to understand the content and, secondly, they can get clear expectations on what they need to prepare and focus on.

Peer assessment has been a subject of great debate in recent years. The way students perceive assessment and what motivates them when assessing may differ significantly from the tutor. This paper discusses a study designed to correlate students’ marking with the marks awarded by their tutors when peer assessing one another from in-class oral presentations. A new and alternative approach to correlate results is presented, which is based on the normalisation of the quantitative judgements based on determined criteria. The methodology was blind and holistic, as described in previous works: some guidelines were provided to the students on what is considered acceptable without getting into detail ( holistic marking), and peer-assessment marks were made confidential ( blind approach). It was observed that students have a tendency to overrate fellow students – especially where lower marks might be awarded. There is, however, direct agreement with the tutor’s marking in terms of qualitative judgements, which is highlighted by the presented correlation method used to adjust students’ marks. The presented methodology to correlate marks between the students and tutor showed to be a promising one. After processing the data with this simple and straightforward algorithm, peer and tutor assessment practically showed a perfect match.

Endodontic rotary file instruments used to treat root canals in dentistry suffered breakthrough transformations in recent years when stainless steel was replaced by Nickel-Titanium (NiTi). NiTi alloys used in Endodontics possess superelastic properties at body temperature (37C) that bring many advantages on the overall performance of the root-canal treatment. They can follow curved root canals more easily than stainless steel instruments and have been reported to be more effective in the removal of the inflamed pulp tissue and protection of the tooth structure. However, these instruments eventually fracture under cyclic bending loading due to fatigue, without any visible signals of degradation to the practitioner. This problem brought new challenges on how new instruments should be tested, as NiTi alloys are highly non-linear and present a large hysteresis cycle in the Elastic domain. Current existing standards are only available for Stainless Steel testing. Thus, many authors have attempted to design systems that can test NiTi endodontic files under fatigue loads. However, no approach has been universally adopted by the community yet, as in most cases they are based on empirical set ups. Following a more systematic approach, this work presents the results of rotary fatigue tests for several NiTi wires from different manufacturers (MemryTM and EuroflexTM). The tests were done on a versatile fully automatic rotary bending testing machine. The formulation is also presented, where the material strength reduction can be quantified from the determination of the strain and the number of cycles until failure.

Fuel cell technology can be traced back to 1839 when British scientist Sir William Grove discovered that it was possible to generate electricity by the reaction between hydrogen and oxygen gases. However, fuel cell still cannot compete with internal combustion engines although they have many advantages including zero carbon emissions. Fossil fuels are cheaper and present very high volumetric energy densities compared with the hydrogen gas. Furthermore, hydrogen storage as a liquid is still a huge challenge. Another important disadvantage is the lifespan of the fuel cell because of their durability, reliability and maintainability. Prognostics is an emerging technology in sustainability of engineering systems through failure prevention, reliability assessment and remaining useful lifetime estimation. Prognostics and health monitoring can play a critical role in enhancing the durability, reliability and maintainability of the fuel cell system. This paper presents a review on the current state-of-the-art in prognostics and health monitoring of Proton Exchange Membrane Fuel Cell (PEMFC), aiming at identifying research and development opportunities in these fields. This paper also highlights the importance of incorporating prognostics and failure modes, mechanisms and effects analysis (FMMEA) in PEMFC to give them sustainable competitive advantage when compared with other non-clean energy solutions.

In this study, the design aspects of optically accessible pressure vessels are investigated via a case study of a High Pressure Combustor experimental rig. The rig was designed to take optical measurements of combustion, simulating the conditions found in internal combustion engines and turbines. Although, it is not new to equip chambers and reactors with sight windows, important aspects of design and relevant information regarding optical access is missing or are insufficiently explored or not readily accessible in the existing literature. A comprehensive review of requirements for optical access to such high-pressure, high-temperature systems has been conducted. It is shown in a readily-navigable format as function of application and precision, with data and technical correlations hitherto not found in a ‘user-friendly’ style. The material selection procedure is detailed and supported by a complete comparison of optical materials and relevant properties. The review revealed a significant inconsistency in mechanical properties claimed in the literature for optical materials. As a response to this, increased safety factor values are suggested as function of level of uncertainties and effects of failure, typically three to four times higher than the industrial standard. Moreover, newly developed equations are presented linking performance analysis to the design criteria. Keywords: Optically-accessible reactor, Optical engine, Pressure vessel, Sapphire, Window design, Combustion

The aim of this paper is to analyze the cyclic performance of two different Ni-Ti endodontic rotary files made from different alloys under bending using Finite Element Analysis (FEA). When experimentation is not available, this is not a trivial task and most papers on the subject rely on static analysis only. Two Ni-Ti rotary instruments are selected, ProFile GT and a GT Series X (GTX). The latter file is made from M-Wire, which has been thermo-mechanically processed to have larger flexibility, according to its manufacturer. The mechanical response was studied by considering different scenarios in the FEA package, in which the material properties were introduced according to existing literature. The method and results are presented and discussed so that this paper can be used as a guideline for future works. Although not fully reflective of the instrument's behavior in a dynamic rotation intra-canal system, the models used constitute a good approximation when a comparison between two instruments is at stake. It is shown that the GTX file has a lower risk of fatigue fracture during its clinical use when compared to the GT file, especially when the root canal makes the file deform into an extreme geometry. However, if the root canal does not make the file deform more than a certain amount, the GT file is equally good from the point of view of mechanical endurance.

The application of an alternating magnetic field (0.54 T) was observed to lead to an improvement in the fatigue endurance and an increase in Vickers microhardness and tensile strength of both EN8 steel and AA2014-T6 alloy. Fractography using scanning electron microscopy showed evidence of more ductile fracture features after treatment in contrast to untreated samples. The results of X-ray diffraction indicated formation of more compressive residual stresses following treatment, while examination by transmission electron microscopy showed evidence of fewer dislocations. In the case of the AA2014-T6 alloy, Guinier-Preston (GP) zones were also generated by the alternating magnetic field. However, the temperature increase during the treatment was too low to explain these observations. The results were attributed to the non-thermal effect of the alternating magnetic field treatment that led to depinning and movement of dislocations and secondary precipitation of copper.

When considering structural loss in vibration problems, the Young’s modulus is a complex value that may be defined in the Argand plane as the vectorial sum of the real part of the Young’s modulus itself and the loss modulus. By performing simple quasi-static force–displacement tests in a universal testing machine it is possible to experimentally measure the complex elastic modulus of a material, even if the loss factor is very small. In this work, the authors present the experimental results of the complex Young’s modulus measurements on a specimen of a quasi-isotropic carbon fibre reinforced laminate, considering the constant hysteretic damping model. The setup is fully described and results are presented in detail, giving a special attention to the experimental problems so that this work can become a useful contribution for further studies.

Among the number of fuel cells in existence, the proton exchange fuel cell (PEFC) has been favoured because of its numerous applications. These applications range from small power generation in cell phones, to stationary power plants or vehicular applications. However, the principle of operation on PEFCs naturally leads to the development of water from the reaction between hydrogen and oxygen. Computational fluid dynamics (CFD) has played an important role in many research and development projects. From automotive to aerospace and even medicine, to the development of fuel cells, by making it possible to investigate different scenarios and fluid flow patterns for optimal performance. CFD allows for in-situ analysis of PEFCs, by studying fluid flow and heat and mass transfer phenomena, thus reducing the need for expensive prototypes and cutting down test-time by a substantial amount. This paper aims at investigating the advances made in the use of CFD as a technique for the performance and optimisation of PEFCs to identify the research and development opportunities in the field, such as the performance of a novel PEFC, with focus on the underlying physics and in-situ analysis of the operations.

Nickel-Titanium (NiTi) alloys with superelastic properties have been increasingly introduced as a substitute to more conventional alloys, such as stainless steel, in a variety of applications. In Dentistry, NiTi alloys are used in tools such as Endodontic rotary files, allowing the file to follow teeth root canals more easily than their stainless steel counterparts. Nevertheless, during surgery, the file is subjected to cyclic bending loading, since it is rotating while being deformed inside the curved canals and is prone to fracture due to fatigue, without showing any visible signals of degradation. Following a systematic approach, this work presents the results of rotary fatigue tests for several NiTi wires from different manufacturers (Memry and Euroflex). The formulation is presented, where the material strength reduction can be quantified from the determination of the strain and the number of cycles until failure. Experimental tests as well as numerical Finite Element Analysis (FEA) simulations are presented to better understand the fatigue fracture mechanisms present in NiTi alloys, showing that there is good agreement between the predicted strains (difficult to measure in such small wires) and the cycles to failure.

Introduction This article aims to evaluate how different superelastic nickel-titanium (NiTi) alloys determine the static mechanical performance of endodontic files during bending and torsion. Methods Two NiTi rotary instruments with similar geometries and equal cross sections, ProFile GT (GT) and a GT Series X (GTX), were selected. The latter file is made from M-Wire, a NiTi alloy that, according to its manufacturer, has been thermomechanically processed to have larger flexibility at body temperature. The mechanical response was studied for a series of static bending and torsional loads by using finite element (FE) models. The materials were characterized according to previously published stress-strain curves. Results For the same load and boundary conditions, the GTX material significantly increased the instrument’s performance. For instance, the deflection for a 1N force at the tip of the file was found to be 28.5% larger for the GTX file, whereas the maximum stress decreased 13.2%. Conclusions Although not fully reflective of the instrument’s behavior in a dynamic rotation intracanal system, the static results showed that the GTX file is more flexible and capable of stress relief at the most critical sections than the GT file, suggesting that it has a lower risk of fracture inside the root canals during its clinical use.

The low specific weight of composite materials, together with their excellent mechanical properties, make them suitable to be widely used in many modern engineering structures. However, composite materials are quite sensitive to impacts: a specific kind of damage, called Barely Visible Impact Damage (BVID), may occur, constituting an unsafe failure of difficult assessment. In the past few years several methods have been developed aiming at assessing this type of damage. In this paper, a vibration-based technique that combines both the natural frequencies and the modal damping factors as damage sensitive features is tested for locating impact damage in carbon fibre reinforced laminates. The method is intended to be used for locating damage in real laminated composite structures that undergo in-service impacts, such as an airplane's fuselage or wings. Assessing a minimum of one response coordinate is the strict requirement during each inspection, because it uses the dynamic global parameters of the structure as damage features. This is possible because the method assumes that, at least for BVID, the mode shapes remain practically unchanged. The theory is summarized and the method is tested using experimental setups where damage is introduced at different locations. Additionally, the hypothesis that different damage morphologies on composite materials have different contributions to the damage features is addressed.

The Elliptical Plane has been recently introduced as a modal identification method that uses an alternative plot of the receptance. The method uses the dissipated energy per cycle of vibration as a starting point. For lightly damped systems with conveniently spaced modes, it produces quite accurate results, especially when compared to the well-known method of the inverse. When represented in the Elliptical Plane, the shape of the receptance is elliptical near resonant frequencies. The modal damping factor can be determined from the angle of the ellipse’s major axis with the horizontal axis, whereas the real and imaginary parts of the modal constants can be determined from numerical curve-fitting (as in the method of the circle - Nyquist plot). However, the lack of points that can be used near the resonance (due to limitations in the frequency resolution, and effects from other modes near each resonance) and the fact that measurements are polluted by noise, bring uncertainty to the numerical curve-fitting. This paper aims at providing the first steps on the improvement of the quality of the modal identification of the receptance in the Elliptical Plane. The method and results are discussed with a multiple degree-of-freedom numerical example.

The Detection and Relative Damage Quantification Indicator (DRQ) was presented previously as a reliable damage detection indicator when used with Operational Deflection Shapes (ODS). The DRQ was computed from the Response Vector Assurance Criterion (RVAC) between the damaged and the initial ODS and the resulting value proved to be a good indicator of the presence of damage. The use of the ODS implies that the loads applied to the structure with and without damage are either known or, at least, the same. If the forces are not deterministic but still ergodic, the power spectrum could be used to evaluate the ODS, but still the above conditions hold, in a statistical sense. When a structure is subjected to ambient excitation, those conditions can hardly be assured. The loads may vary quite significantly and the ODS changes may be due to those changes instead of the presence of damage. To avoid this handicap, the authors explore here the use of the Transmissibility functions. If properly defined, the Transmissibility is invariant with respect to the amplitude of the loads. Since the Displacement Transmissibility is load invariant, a picked set of responses can be measured in service and used to predict another set; the result will then be correlated to the actual values using the RVAC and the DRQ will be computed. Numerical and experimental examples illustrate the proposed technique.

Many of the techniques developed to assess structural damage are based on experimental modal analysis. This paper presents a study to extend the current understanding of how increasing damage in a carbon fiber reinforced plastic affects the modal damping factor of a laminated structure. Damage is introduced and quantified in terms of the dissipated energy. It is shown that there is a tendency for the overall damping to increase whereas there is a tendency for the overall stiffness to decrease. While these results are not novel, the former is quite relevant, since the authors are not aware of many other experimental studies on the evolution of the modal damping factor with damage in carbon fiber reinforced plastic. At the same time, a modal-based damage location technique that combines both the natural frequencies and the modal damping factors as damage sensitive features is discussed. The hypothesis that different damage morphologies on composite materials have different contributions to the damage features is drawn. The methods are illustrated with both numerical and experimental examples. One of the problems observed is that, although damping is consistently found to increase globally with damage, the determination of the individual changes of the modal damping factors is still very uncertain. This may be due to concurrent damage types being present at the same time, but most certainly due to uncertainties involved in the identification of the modal damping factors.

Structural health monitoring and damage detection techniques are tools of great importance in the off-shore, civil, mechanical and aeronautical engineering communities, both for safety reasons and because of the economic benefits that can result. The need to be able to detect damage in complex structures has led to the development of a vast range of techniques, of which many are based upon structural vibration analysis. In the present article, some of the latest advances in Structural Health Monitoring and Damage Detection are reviewed, with an emphasis on composite structures on the grounds that this class of materials currently has a wide range of engineering applications. FOREWORD-It should be noted that this review is not intended to be a general, all-encompassing review covering the whole range of structural health monitoring (SHM); it was planned as the starting point for a study focusing on damage detection, localization and assessment for certain kinds of structure. Thus, the line of thought behind the search and the structure of this review is a result of objectives beyond the scope of the paper itself. Nevertheless, it was considered that, once the above was understood, an updated synopsis such as this could also be useful for other researchers in the same field. ©2006 SAGE Publications.

The dynamic response of a structure can be described by both its translational and rotational receptances. The latter ones are frequently not considered because of the difficulties in applying a pure moment excitation or in measuring rotations. However, in general, this implies a reduction up to 75% of the complete model. On the other hand, if a modification includes a rotational inertia, the rotational receptances of the unmodified system are needed. In one method, more commonly found in the literature, a so called T-block is attached to the structure. Then, a force, applied to an arm of the T-block, generates a moment together with a force at the connection point. The T-block also allows for angular displacement measurements. Nevertheless, the results are often not quite satisfactory. In this work, an alternative method based upon coupling techniques is developed, in which rotational receptances are estimated without the need of applying a moment excitation. This is accomplished by introducing a rotational inertia modification when rotating the T-block. The force is then applied in its centroid. Several numerical and experimental examples are discussed so that the methodology can be clearly described. The advantages and limitations are identified within the practical application of the method.

The identification of the modal parameters from frequency response functions is a subject that is not new. However, the starting point often comes from the equations that govern the dynamic motion. In this paper, a novel approach is shown, resulting from an analysis that starts on the dissipated energy per cycle of vibration. Numerical and experimental examples were used in order to assess the effectiveness of the proposed method. It was shown that, for lightly damped systems with conveniently spaced modes, it produced quite accurate results when compared to those obtained from the method of the inverse. The technique also proved to be simple enough to be used for quick estimates of the modal damping factors. Finally, this paper is a contribution to modal analysis and identification methods, as the developed technique has never been proposed before.

Increased safety and reliability in mechanical components has become a subject of prime importance in recent years. Therefore, a proper understanding of damage and fracture mechanics in materials and components designed to withstand very high cycle fatigue (VHCF) loadings is extremely important nowadays. However, the use of conventional machines for fatigue testing is very time consuming and costly for VHCF tests. Ultrasonic machines have been introduced as a way to increase the number of cycles in fatigue testing up to IE8 to IE10 cycles within a considerably reduced amount of time. Nevertheless, the accurate measurement of the parameters that influence fatigue life at ultrasonic frequencies (e.g., stress, displacement, strain rate, temperature, and frequency) is still a matter of concern and ongoing development. Because of the high frequencies involved in VHCF testing, a huge amount of heat is generated over the specimen, which greatly affects the variables determining the fatigue behavior. This paper describes the design and instrumentation of an ultrasonic fatigue testing machine that operates at a working frequency of 20 kHz. Among other features, it incorporates automated strain and temperature control. In order to run automated tests, a closed-loop monitoring and control system was developed based on the measured temperature and displacement amplitudes. Temperature readings are made with a pyrometer and thermography camera, and displacement is monitored at the free end of the specimen with a high-resolution laser. The machine's power output is continuously adjusted from the displacement readings, so that the stress variations within the specimen are as flat as possible. When the temperature increases above a certain set value, a cooling function is triggered and the test is interrupted until the specimen is cooled down. Data are acquired, managed, and processed with a data acquisition device working at a 400 kHz sampling frequency. The advantages and limitations of metal fatigue testing at very high frequencies are discussed in this paper, with special emphasis on strain and temperature-control issues. Comparisons are made of tests carried out with and without both displacement and temperature control on two metallic alloys, copper 99 % and carbon steel, with the determination of strength-life (S-N) curves.

In the past 25 years, the scientific and industrial communities have made great efforts on the fields of damage detection and structural health monitoring (SHM). However, no single approach has proven appropriate for all situations. Composite materials, which are receiving an increasing attention in the aeronautical industry, namely carbon fibre reinforced plastics (CFRP), are very sensitive to impacts of medium and low energy. Typically, barely visible impact damage (BVID) will occur, constituting an unsafe failure of difficult assessment. To assess (detect, locate and quantify) damage in this kind of material is still a challenge, especially if a huge amount of sensors or expensive equipments at hand are not used. In this work, a methodology that makes use of a reduced amount of conventional sensors is explored, with the aim of locating damage for a low cost on components that are subjected to impacts during service. This represents a considerable benefit, namely for the assessment of damage in aeronautical components, compared to most methods used today. This work can also give a major contribution to the research community since uncommon approaches will be used to model damage in composite materials, namely the modal damping factor as the main feature for damage localization.

An investigation of the behaviour of Nylon and Polyester textiles subjected to wind loading was undertaken in an experimental study. Variable parameters of wind speed, incidence and excess length of the textiles were studied. As expected, increasing wind speeds and amount of slackness led to higher deflections above 4 degrees incidence. Below this angle large instabilities are found. Polyester with a higher air permeability, experienced greater levels of \ud eflection at low slackness when compared to the Nylon. Only small differences in deflection were found between the textiles when using higher slackness ratios.