A Contribution to the Modal Identification of the Damping Factor based on the Dissipated Energy

ABSTRACT: 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. For lightly damped systems with conveniently spaced modes, it produced quite accurate results in comparison to the direct application of the method of the inverse, both in the numerical and in the experimental examples. It also is a simple technique that can be used to produce quick estimates of the modal damping factors. Furthermore, this is also a contribution to further developments on modal analysis and identification methods as, up to today, the developed technique has not yet been proposed.Final Published versio

Students’ views and correlation regarding performance and attendance for a first year engineering cohort

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 R2=0.78 when plotting final marks vs overall attendance. Student view, which was also considered as an important aspect of this study, convey 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

Computational analysis of a car chassis frame under a frontal collision.

This paper aims at studying the frontal collision of a car frame using non-linear FEA (Finite Element Analysis). Three frontal crash situations are evaluated: a full frontal impact against a rigid barrier and two frontal impacts with 40% overlap against an ODB (Offset Deformable Barrier). These three simulations are intended to mimic the FMVSS no.208, the 96/79/EC and the EURONCAP tests. The model of the chassis used in the simulations – a Ford F150 - is based on one that has previously been published in another paper. However, in that paper, the simulation only considers a static load on the bumper (a pressure) and the conclusions do not reflect what would happen during a real impact with dynamic loads. Several results are presented and discussed: the dissipated energy during the impact, the acceleration time history and the HIC (Head Injury Criterion) are evaluated from the set of results so obtained. Furthermore, different test situations and initial conditions have been applied, aiming at better understanding the frame’s response in a real impact situation

Comparação de limas ProFile GT Série X com ProFile GT através da caracterização por DRX e de um Estudo por Elementos Finitos

The emergence of NiTi alloys and the development of their properties, namely superelasticity, motivated the inclusion of such materials in files used in the clinical practice of Endodontics. These files present several advantages when compared to Stainless Steel files (still widely used today), especially their larger flexibility, whereby the improvement of these alloys over the last few years has deserved a growing attention by both the medical industry and medical community. Flexibility in endodontic files is paramount for the preparation of anatomically complex root canals, since it preserves the dental structure, limits the apical transport, reduces the risk of iatrogenic mistakes and allows the irrigant to flow and reach the apical foramen. Flexibility in NiTi files due to their superelastic behaviour mainly dependends not only on the crystallographic phases present in the alloy and the thermal, mechanical and chemical treatments to which the alloy has been subjected but also due to files’ geometry. Recently, the market has seen introduced the M-Wire, a NiTi alloy used in GTX files that, according to its manufacturer, has been treated in order to have a larger flexibility at body temperature than files built with conventional NiTi wire, for instance the one used in GT files. This study aims at contributing to the better understanding of the mechanisms behind the properties each kind of file presents and to find out supporting grounds that substantiate the manufacturer pretension, through the metallurgic characterization of GT and GTX files by X-Ray Diffraction (XRD) and through finite element models of the files under bending. Some experimental results are presented and discussed, being shown that, at body temperature, the GTX files accuse the presence of R Phase, contrary to the GT files, being their geometry very alike; as such, they are expected to be more flexible and clinically superior

Development of a Time-Efficient Approach to Measure the Acoustic Impedance of Industrial Insulating Materials

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

And yet another method for the identification of modal constants in experimental modal analysis

Modal Identification from Frequency Response Functions (FRFs) is a chapter of Experimental Modal Analysis (EMA) that many would consider being closed in a book and covered in dust. Yet, in a previous paper from the same authors, a new approach to determine the modal damping factors from FRFs was proposed. Contrary to other modal identification methods which are based on the dynamic motion governing equations, the method used as a starting point the dissipated energy per cycle of vibration. For lightly damped systems with conveniently spaced modes, it produced quite accurate results, especially when compared to the 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. In this paper, it will be shown that this plot has other special properties, whereby the real and imaginary parts of the modal constants can be determined from the plot through numerical extrapolation

A study on the influence of Ni-Ti M-Wire in the flexural fatigue life of endodontic rotary files by using Finite Element Analysis.

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

A Review of Computational Fluid Dynamics Simulations on PEFC Performance

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 mount. 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

A generalised multiple-mass based method for the determination of the live mass of a force transducer

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

An alternative representation of the receptance: the ‘elliptical plane’ and its modal properties.

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