Local damage detection from dynamic SOFO ...

Local damage detection from dynamic SOFO experimental data Casciati S.1, Domaneschi M.1 and Inaudi D.2 1

Department of Structural Mechanics, University of Pavia, Via Ferrata 1 , 27100 Pavia, Italy 2 SMARTEC, Via Pobiette 11, TI 6928 Manno, Switzerland

ABSTRACT Two goals are pursued in this paper. The first goal consists of comparing the performance of the innovative SOFO dynamic system, which uses long-gauge fiber-optic sensors, with the traditional monitoring method based on accelerometers. For this purpose, a dynamic laboratory test was carried out, and measurements were taken from a single-storey threedimensional steel frame model excited at the base by a shaking table. The SOFO dynamic system was installed on one column of the frame structure, while two accelerometers were mounted on the base and on the frame storey, respectively, for comparison. The use of fiber-optic sensors allows to overcome the difficulties associated with the traditional dynamic measurement methods, such as the limitations in the number and in the locations of the monitoring devices. Furthermore, the long-gauge fiber-optic strain sensors show a very high sensitivity and extend the frequency range (1mHz-1KHz). The second goal is to investigate the sensitivity to local damage of a recently proposed method for damage detection and localization. Indeed, the use of better performing long-gauge strain sensors allows the detection of local damage that is hardly visible in the global response of the structure. Damages of increasing intensities are therefore gradually introduced in the structure, and the measurements acquisition is repeated for each of the damaged cases. The SHM-RSM method, which is based on the idea of using a response surface model to approximate the relationship between the measurements collected by different sensors during the same test, is finally applied to the collected data to detect and locate the damages of different intensities. Keywords: fiber-optic sensors, traditional accelerometers, dynamic-laboratory tests, damage detection, damage localization

1. INTRODUCTION New and emerging opportunities in the field of building control systems and the need for a reduction of structural maintenance costs have increased the interest on structural monitoring and assessment systems. An innovative technique of structural monitoring through the usage of long-gauge fiber-optic sensors was presented in [1], and its theoretical basis is given in [2], [3], and [4]. The combination of efficient algorithms and innovative technology can increase the ability of global methods to capture damages of different time and length scales. To allow real-time, online assessment of the health of a structure, the damage diagnosis algorithm should be computationally simple and able to discard the information related to an unchanged situation, while a transfer of information should be regarded as an alarm of structural modification. Furthermore, it should be insensitive to changes in the operational and environmental conditions in which the structure is operated, since they are not significant of damage. To meet these requirements, a statistical method based on the response surface approximation theory was developed by the first author in [5]. In particular, this method presents the advantage of not requiring either a full understanding of the underlying physical mechanism governing the structural response (which is never possible without introducing model errors), or data from damaged structures which are unlikely available in practical situations (unsupervised learning mode). The capability of the method to correctly identify and locate the damage has already been tested on structural models where the damage was introduced by the removal of a bracing system from one or more floors of a multi-storeys frame [6]. However, a clear interpretation of the results related to the damages of smallest entities was prevented by the limitations in the sensors number and in their locations. In this study, instead, the employment of the SOFO dynamic system allows to apply the proposed method also for local damage detection. The measurements collected for different damaged situations are used to investigate the sensitivity of the method to the damage intensity. 2005 SPIE Smart Structures Conference, San Diego, CA. March 7-10,2005

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2. TEST SET-UP AND MEASUREMENTS ACQUISITION The tests were carried out at the Vibration Laboratory of the University of Pavia. The testing environment consists of a shaking table, which was used to induce the external forces on a structural system (Figure 1). The structure is a physical model of a single-storey three-dimensional steel frame. 2.1 Sensing devices Two monitoring systems were installed on this structure. The traditional one sees two mono-axial accelerometers placed on the shaking-table (base) and on the storey slab, respectively. The innovative SOFO dynamic system consists of two longgauge fiber optic sensors (Sensors 1 and 2) fixed to the upper part and to the lower part, respectively, of a column of the physical model. Figure 1 provides the details of the different monitoring systems. The acquisition and processing unit of the SOFO dynamic system is shown in Figure 2. The next section briefly describes the functioning principles of this device. 2.2 Description of the SOFO dynamic system The SOFO dynamic system [3] relies on interferometric demodulation as represented in Figure 3. The emission from a coherence-collapsed Laser diode is injected in the passive part of the sensor through a coupler. The light is then passed in the Michelson interferometer, composed of two fibers in the SOFO sensors, and is reflected back to the demodulation system. A Mach-Zehnder interferometer with an active phase modulator is used as demodulation interferometer. The modulator is driven at about 50 KHz frequency. Finally, the light intensity is collected by a photodiode and digitalized. The resulting fringe pattern is analyzed using an Optiphase DSP board that provides the corresponding cumulated phase in digital and analog formats. In the final system the light from the same laser is split with a 1x8 coupler to allow simultaneous demodulation of 8 channels, each with a dedicated DSP board. The performance parameters of the system are given in Table 1. The SOFO sensors used in the tests consist of two optical fibers, each of them pre-packed in a thin composite tape [4] and symmetrically bonded to the column of the model as shown in Figure 4. Practically, the sensors work in a push-pull manner, and measure the difference in deformation between two sides of the column (differential sensors), i.e., they measure the curvature of the column. The influence of axial strain and temperature is self-compensated by the architecture of the sensors. Sensor 1 was installed at the upper part and Sensor 2 at the lower part of the column, as shown in Figure 4. 2.3 Measurements acquisition The measurements were taken by switching on the shaking table (see Figure 1) and by applying vibrations of different amplitudes to the base of the frame structure. In particular, the responses of the structure to vibration amplitudes of 1 mm and 20 mm, respectively, were separately measured. As an example of the acquired data, Figure 5 shows the signal recorded by Sensor 2 during a test in which an excitation of amplitude 20 was applied to the undamaged structure. The considered signal is made of 10000 points, and it includes the decay following

the shaking table arrest.

Figure 1. Structural model on the shaking table.

Figure 2. Acquisition and processing unit of SOFO dynamic system.

2005 SPIE Smart Structures Conference, San Diego, CA. March 7-10,2005

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Figure 3. Setup of the SOFO dynamic system

Figure 4. Architecture of differential sensors installed on the column

Table 1. Performances of the SOFO dynamic system Bandwidth Resolution

0 to 10 kHz 0.01 µm

Measurement Range

± 5mm

Velocity range

Max. 10 mm/s

Drift Max number of channels Acquisition