Diogo Montalvão

A review of vibration-based structural health monitoring with special emphasis on composite materials

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.

Numeric Comparison of the Static Mechanical Behavior between ProFile GT and ProFile GT Series X Rotary Nickel-Titanium Files

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 instruments 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 instruments 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.

Estimation of the Rotational Terms of the Dynamic Response Matrix

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.

Using the Detection and Relative Damage Quantification Indicator (DRQ) with Transmissibility

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.

Automation in Strain and Temperature Control on VHCF with an Ultrasonic Testing Facility

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 machines 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.

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 instruments 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.

An experimental study on the evolution of modal damping with damage in carbon fiber laminates

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.

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 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.

Redesigning axial-axial (biaxial) cruciform specimens for very high cycle fatigue ultrasonic testing machines

The necessity to increase performances in terms of lifetime and security in mechanical components or structures is the motivation for intense research in fatigue. Applications range from aeronautics to medical devices. With the development of new materials, there is no longer a fatigue limit in the classical sense, where it was accepted that the fatigue limit is the stress level such that there is no fracture up to 1E7 cycles. The recent development of ultrasonic testing machines where frequencies can go as high as 20 kHz or over enabled tests to be extended to ranges larger than 1E9 in just a few days. This area of studies is now known as Very High Cycle Fatigue (VHCF). On the other hand, most of the existing test equipment in the market for both classical and VHCF are uniaxial test machines. However, critical components used in Engineering applications are usually subjected to complex multi-axial loading conditions. In this paper, it is presented the methodology to redesigning existing cruciform test specimens that can be used to create an in-plane biaxial state of stress when used in ‘uniaxial’ VHCF ultrasonic testing machines (in this case, the term ‘uniaxial’ is used not because of the state of stress created at the centre of the specimen, but because of the direction at which the load is applied). The methodology is explained in such a way that it can be expanded to other existing designs, namely cruciform designs, that are not yet used in VHCF. Also, although the approach is presented in simple and logical terms, it may not be that obvious for those who have a more focused approach on fatigue rather than on modal analysis. It is expected that by contributing to bridging the gap between the sciences of modal analysis and fatigue, this research will help and encourage others exploiting new capabilities in VHCF.

Correlating peer and tutor assessment on a low-stakes engineering assignment

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.