Short-Training Damage Detection Method for Axially Loaded Beams Subject to Seasonal Thermal Variations

Vibration-based damage features are widely adopted in the field of structural health monitoring (SHM), and particularly in the monitoring of axially loaded beams, due to their high sensitivity to damage-related changes in structural properties. However, changes in environmental and operating conditions often cause damage feature variations which can mask any possible change due to damage, thus strongly affecting the effectiveness of the monitoring strategy. Most of the approaches proposed to tackle this problem rely on the availability of a wide training dataset, accounting for the most part of the damage feature variability due to environmental and operating conditions. These approaches are reliable when a complete training set is available, and this represents a significant limitation in applications where only a short training set can be used. This often occurs when SHM systems aim at monitoring the health state of an already existing and possibly already damaged structure (e.g., tie-rods in historical buildings), or for systems which can undergo rapid deterioration. To overcome this limit, this work proposes a new damage index not affected by environmental conditions and able to properly detect system damages, even in case of short training set. The proposed index is based on the principal component analysis (PCA) of vibration-based damage features. PCA is shown to allow for a simple filtering procedure of the operating and environmental effects on the damage feature, thus avoiding any dependence on the extent of the training set. The proposed index effectiveness is shown through both simulated and experimental case studies related to an axially loaded beam-like structure, and it is compared with a Mahalanobis square distance-based index, as a reference. The obtained results highlight the capability of the proposed index in filtering out the temperature effects on a multivariate damage feature composed of eigenfrequencies, in case of both short and long training set. Moreover, the proposed PCA-based strategy is shown to outperform the benchmark one, both in terms of temperature dependency and damage sensitivity

Vibration control by means of piezoelectric actuators shunted with LR impedances: Performance and robustness analysis

This paper deals with passive monomodal vibration control by shunting piezoelectric actuators to electric impedances constituting the series of a resistance and an inductance. Although this kind of vibration attenuation strategy has long been employed, there are still unsolved problems; particularly, this kind of control does suffer from issues relative to robustness because the features of the electric impedance cannot be adapted to changes of the system. This work investigates different algorithms that can be employed to optimise the values of the electric components of the shunt impedance. Some of these algorithms derive from the theory of the tuned mass dampers. First a performance analysis is provided, comparing the attenuation achievable with these algorithms. Then, an analysis and comparison of the same algorithms in terms of robustness are carried out. The approach adopted herein allows identifying the algorithm capable of providing the highest degree of robustness and explains the solutions that can be employed to resolve some of the issues concerning the practical implementation of this control technique. The analytical and numerical results presented in the paper have been validated experimentally by means of a proper test setup

Passive multi-mode piezoelectric shunt damping: an approach based on matrix inequalities

Piezoelectric shunt damping is a well-known technique for suppressing vibrations in light mechanical systems. The method is based on the connection of a properly designed electrical network (shunt circuit) to a piezoelectric actuator bonded to the vibrating structure. This network can be either passive (i.e. made from resistances, capacitances and inductances) or active. When active shunts are used, possible problems related to instability of the system can raise. This paper addresses a new approach for designing shunt electrical circuits allowing to damp more than one mechanical mode of the structure at the same time with a single piezoelectric actuator. Moreover, the method assures to design passive shunt impedances, thus avoiding instability problems. Starting from a state space description of the electro-mechanical system, the definition of the shunt circuit is achieved using an approach based on matrix inequalities, which allows to design shunt circuits with different goals by expressing the desired target as a single or a system of matrix inequalities

A comparison between aeroacoustic source mapping techniques for the characterisation of wind turbine blade models with microphone arrays

Characterising the aeroacoustic noise sources generated by a rotating wind turbine blade provides useful information for tackling noise reduction of this mechanical system. In this context, microphone array measurements and acoustic source mapping techniques are powerful tools for the identification of aeroacoustic noise sources. This paper discusses a series of acoustic mapping strategies that can be exploited in this kind of applications. A single-blade rotor was tested in a semi-anechoic chamber using a circular microphone array. The Virtual Rotating Array (VRA) approach, which transforms the signals acquired by the physical static array into signals of virtual microphones synchronously rotating with the blade, hence ensuring noise-source stationarity, was used to enable the use of frequency domain acoustic mapping techniques. A comparison among three different acoustic mapping methods is presented: Conventional Beamforming, CLEAN-SC and Covariance Matrix Fitting based on Iterative Re-weighted Least Squares and Bayesian approach. The latter demonstrated to provide the best results for the application and made it possible a detailed characterization of the noise sources generated by the rotating blade at different operating conditions

Damage detection based on strain transmissibility for beam structure by using distributed fiber optics

Structural damage identification is a coral and challenging research topic. Research mainly focuses on identification and detection of linear damage in structures by using modal parameters such as change of natural frequency, frequency response function, mode shape, etc. Transmissibility is conventionally defined as the spectra ratio of two measurement points, which has been utilized for damage identification as a powerful damage indicator. In this paper, strain transmissibility, defined as ratio of strain response spectra, is proposed as a new damage indicator. In order to achieve more precise sensing information, distributed fiber optics has been applied to damage detection on a beam structure, which adds new capability of sensing with its combination of high spatial density sensing and dynamic acquisition over a single optical fiber sensor. A numerical simulation has been conducted to investigate the feasibility of strain transmissibility for damage detection which has revealed a better performance compared to traditional transmissibility. The applicability of the proposed method has been confirmed by applying distributed fiber optics on a clamped-clamped beam. Both simulation and experiment validate the effectiveness of damage detection approach based on strain transmissibility by using distributed fiber optics

Vibration control with piezoelectric elements: The indirect measurement of the modal capacitance and coupling factor

The knowledge of the modal capacitance and electro-mechanical coupling factor is essential for a proper design of systems with embedded piezoelectric transducers and materials. In light of this, this paper presents two indirect methods for measuring the piezoelectric modal capacitance and a method to estimate the modal electro-mechanical coupling factor. All methods rely on simple vibration measurements of the structure with the piezoelectric transducer connected to a proper shunt impedance, thus avoiding measurements of piezoelectric current and voltage by expensive equipment. For the modal electro-mechanical coupling factor, the proposed method guarantees reduced uncertainty compared to traditional experimental estimation procedures. Upon introduction of the underlying theory, the paper experimentally demonstrates the reliability and effectiveness of the methods by comparison with well-established procedures

Comparison of different SMA-based adaptive tuned mass dampers

The use of smart materials has proven to be an effective strategy in the development of Adaptive Tuned Mass Dampers (ATMD). Among different possible materials, Shape Memory Alloys (SMA) show specific features which make their use advantageous for building ATMDs. More in detail, by heating/cooling the SMA components, it is possible to change the natrural frequency of the ATMD, thus allowing maintaining the ATMD tuned on the primary system to be damped. The heating/cooling is obtained by changing the amount of electrical current flowing through the SMA elements and, consequently, the amount of heat produced through Joule’s effect. This paper compares the two main layouts for designing ATMDs based on SMAs in case the primary system to be damped is excited by a disturbance of random nature: cantilever beams and tensioned wires with a central mass. The two layouts, which can be described through detailed and experimentally validated models, are compared in terms of adaptation capability, exerted force and electrical power consumption. It results that the wire-based layout, despite being characterised by a more complicated set-up, is less demanding in terms of power consumption and shows much better adaptation capability

Long term operational modal analysis of a stadium grandstand to structural health monitoring purposes

In the last years an increasing interest has been devoted to all the topics related to security and safety of people. Particular attention has been paid to health monitoring of large civil structures hosting lot of people, like high-rise buildings and stadia. The vast scientific literature confirms the possibility to relate structural health to the evolution of modal parameters, often reaching the aim of localizing any eventual damage, a task otherwise impossible with different techniques. This paper shows a part of the long lasting project involving Politecnico di Milano in the setting up of a permanent health monitoring system at the G. Meazza stadium in Milan. As damage identification is related to changes of the modal parameters, the evaluation of their normal spread is fundamental to fix a threshold in order to identify possible worrying situations. This paper deals with the identification of the spread in the modal parameter estimation of one of the grandstands of the so-called 3° ring of the G. Meazza stadium in Milan, setting up an automated Operational Modal Analysis algorithm and analyzing a first set of data. Some ideas are gathered in order to identify the minimum number of identifications needed to have a robust estimation of the modal parameters. ©2009 IEEE