A. M. R. Ribeiro

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.

Transmissibility matrix in harmonic and random processes

The transmissibility concept may be generalized to multi-degree-of-freedom systems with multiple random excitations. This generalization involves the definition of a transmissibility matrix, relating two sets of responses when the structure is subjected to excitation at a given set of coordinates. Applying such a concept to an experimental example is the easiest way to validate this method.

Dynamical Response of a Multi-Laminated Periodic Bar: Analytical, Numerical and Experimental Study

This article presents a study on the use of the dynamical response of multi-laminated periodic bars to create resonance band gaps within useful frequency ranges. The objective is to control, in a passive form, the longitudinal vibration transmissibility in specific and wide enough frequency ranges of interest. This is achieved by the separation of two adjacent eigenfrequencies. A relation between the modal analysis, the harmonic analysis and the Bloch wave theory is proposed, for which no reference was found in the searched literature. As shown, the selection of appropriate material pairs is essential to obtain useful frequency ranges. The use of pairs of steel and cork agglomerate is proposed, since it allows the design of attenuators at lower frequencies through a prediction based on finite element analysis (FEA). This approach requires the storage modulus of cork for which analytical and numerical FEA models were verified and validated. A methodology to obtain experimentally the storage modulus of cork is presented. Regarding the structural improvement problem, we discuss a methodology to design periodic bars for a specific location of the first attenuations frequency range and illustrate the main results through several examples.

Natural fibre-reinforced composite parts for automotive applications

In this paper, we conduct a brief revision of the state-of-the-art on vegetable fibre-reinforced composite applications in the automotive industry. We then present some results for the dynamic characterisation of an auto part made of jute fibres and an equivalent one made of traditional glass fibres, where the increase in damping is most evident. An inverse characterisation method based on the free vibration response of composite plate specimens is also applied to estimate damped material parameters in vegetable fibre-reinforced composites. The obtained results are compared with the ones obtained with equivalent specimens made of glass fibre and future application of this technique to the study of environmental parameters on the material properties is proposed.

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.

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.

An Experimental Characterization of Cork Storage Modulus for Cork-Steel Applications in Vibration Attenuation

This study presents an experimental characterization of cork storage modulus used to model the vibration response of bars built using alternate layers of cork and steel. In the experimental setup, the specimen was suspended from a fixed support by two thin lines while a shaker was suspended from a mobile support by metallic chains. The shaker was connected to the bar specimen through a force transducer imposing a dynamical deformation that propagates through the specimen. An accelerometer in the opposite extremity of the bar measures the corresponding vibration response and the cork storage modulus is then obtained from the first peak of this frequency response. The proposed methodology successfully characterized the storage modulus of the cork material used in the multilaminated periodic bars. The results obtained illustrate a satisfactory correlation between

Road grades and tire forces estimation using two-stage extended Kalman filter in a delayed interconnected cascade structure

Intelligent vehicles sense their dynamics and the environment to make proper decisions. Some of this information are hard to be measured or need expensive sensors. This paper addresses the estimation of road grade angles, along with tire-ground interaction forces, in a delayed interconnected cascade observer structure. A new approach using a Two-Stage Extended Kalman Filter is proposed, allowing a robust simultaneous estimation of the slow and fast dynamics variables. Experimental data is used to validate the estimator.