Issue 49

E.U.L. Palechor et alii, Frattura ed Integrità Strutturale, 49 (2019) 614-629; DOI: 10.3221/IGF-ESIS.49.56 615 I NTRODUCTION racks often occur in structural members and cause serious structural pathologies or even structural collapse. The effects of these damages influence the dynamic response (frequencies and mode shapes) of structural systems. Structural damages must be identified in its initial state before compromising the integrity and service life of the structure. However, in its initial state, a crack is relatively small making it difficult to detect changes in the dynamic properties. Moreover, there are situations where damages may be hidden due to external cover façade, finishing, or skinning of buildings, bridges, etc… turning the detection of damages a difficult process. Therefore auxiliary tools to indicate possible existence of damages are always welcome. One of the first concepts of structural identification was introduced in Civil Engineering almost two decades ago by Aktan et al [1]. However, methods for identifying cracks, based on the changes of dynamic properties of bridges and buildings, are not very well effective and practical. Moreover, depending on the size of the surface crack, a crack can be detected using traditional technique like the visual inspection. However, it is not possible to visually identify cracks when the structure surface is covered, insulated or located in unreachable locations. To overcome these difficulties, numerical/computational techniques have been under development [2, 3], thus enabling quick decision-making process concerning the minimization or elimination of damages. From the point of view of safety and economy, the detection of damages especially in bridges is an important issue. It is essential to perform periodic inspection to detect changes in the structural stability [4]. A recent research on the technical literature shows that many damage identification algorithms were developed using dynamic characteristics, especially in the frequency domain [5]. In general these techniques uses comparisons between the intact and damaged response of the structure. Techniques based on Wavelet Transforms can overcome this limitation. Such techniques have been applied over the years and have been presented satisfactory results [6,7,8,9,10,11,12]. However, the purpose of this paper is to use additional masses associated with wavelet transform in the processing of non-stationary signals and apply this methodology in experimental done in commercial profiles of steel beams under support conditions close to real situations. U SE OF ADDITIONAL MASSES amage identification may be seen as an inverse problem of identification of a system whose input signals and output signals are known, but the geometry of the damage location and shape are unknown [13]. This means that the purpose of damage detection is to describe a damage in an existing structural model, based on output data obtained experimentally (dynamic response) from specific input signals. It is often desirable to detect irregularities or changes in structure response, considering properties that have been altered by the presence of the damage in the structure. This work presents the application of an identification methodology based on the analysis of the dynamic properties of simply supported steel beams. Beams here are submitted to the action of additional masses that can generate progressive changes of the natural frequencies [14, 15]. The change in the structural stiffness due to the existence of damage in the beam may not be so evident. Therefore, in this research, the Wavelet Transform is used to help in the process of locating possible stiffness changes due to damages [16, 17, 18]. This research also presents experimental tests results on steel beams with simulated damages. The experimental tests were carried out in the Laboratory of Vibrations of the Department of Mechanical Engineering in the University of Brasília. Even though the experiments were conducted within the laboratory, the size of the beams tested corresponds to small commercialized steel beams available in the market. In this research, the natural frequencies, which act as sensitive indicators of the structural integrity were chosen as a measurement parameter of the dynamic properties of the structure. In this way, frequent inspections and measurement of the first frequency may be used to monitor the integrity state of the structure. The first frequency variation signals due to an additional mass at different locations may be processed with Wavelet Transform and may indicate the region where damages may be found [16]. Therefore, this paper presents a new methodology based on the measurements of just the fundamental frequency of a vibrating structure (in this research: beams) with an added mass positioned at different locations along the beam. Damage can be detected observing the peaks of the Discrete Wavelet Transformation of the signal of the variation of the fundamental frequency (DWT-f1) vs. Added Mass Positions (AMP). Two steel beams, or specimens, with different lengths and a variety of simulated damages are tested and the proposed methodology applied. Tab. 1 presents the geometric characteristics of the specimens. The additional mass placed on the beams were small steel plates fixed with braces and bolts. The assembly of the added masses is schematically shown in Fig. 1. C D