Key Engineering Materials Vols. 293-294

Abstract: In several situations the failure of a reinforced concrete structure is preceded by a gradual deterioration of the materials which can be recognised by measuring the change of their physical properties [1]. The main purpose of the destructive tests described in the present paper was to obtain an evaluation of the damage evolution in a reinforced concrete beam, submitted to harmonic displacements imposed by means of a shaking table. A reinforced concrete beam, with two different spans, was designed to sustain a static load of a mass located at the centre of the longer span. Ten identical specimens were prepared and tested at the LNEC (National Laboratory for Civil Engineering, Lisbon) shaking table facility [2]. This paper presents tests, which have been performed in the frame of the European Commission programme ECOEST/PECO (European Consortium of Earthquake Shaking Tables / Central and Eastern European Countries extension). The beams were fixed to the shaking table and submitted to a sinusoidal displacement in the vertical direction, with a pre-established duration and constant amplitude. The tests were carried out by successive stages of increasing amplitudes, until the collapse of each beam was reached. The tests aimed to give an evaluation of the loading history influence on the occurrence of the critical state. During the tests, displacements and accelerations were continuously recorded at several points of the structures, along with ultrasonic measurements taken at different directions, before and after each successive stage. In the present paper the design of the specimens is given. The instrumentation plan, the test setup and the test procedure are also described. Finally, the most relevant results are shown followed by the formulation of a global damage law to predict the limit state of the beams.

Abstract: In structural engineering applications a sufficient quantity of experimental data to be able to achieve a consistent estimate of nonlinear quantities is seldom available: this applies in particular when the structures are to be tested in situ. This report discusses the definition of instantaneous estimators to be used in the dynamic identification of invariant nonlinear systems on the basis of Short-Time Fourier Transform representation of excitation and system’s response and within the framework of a Volterra series representation of the input/output relationship. An estimation of the parameters of a dynamic system can be worked out from the evolution of such instantaneous estimators.

Abstract: Past earthquakes indicate that pounding between inadequately separated structures may cause considerable damage or even lead to collapse of colliding structures. Intensive study has been recently carried out on mitigation of pounding hazards. The assessment of damage due to structural pounding, or its prediction under a particular ground motion, requires the knowledge of the maximum impact force value expected during the time of earthquake. The aim of the present paper is to consider the concept of impact force response spectrum for two closely-spaced structures, which shows the plot of the peak value of pounding force as a function of the natural structural vibration period. The spectrum can be used as a practical tool to assess the magnitude of the expected pounding-induced damage and, if necessary, to apply some damage reduction techniques. In the analysis, both interacting structures have been modelled by single-degree-of-freedom systems and pounding has been simulated by the non-linear viscoelastic model, which accounts for the energy dissipation during impact. The examples of response spectra show that the appropriate selection of the separation gap between structures as well as the dynamic structural parameters, such as the natural vibration period, mass and damping, might have a significant influence on the intensity of damage due to earthquake-induced pounding.

Abstract: Theoretical relations among local and nonlocal damage variables in localized band as well as global damage variable according to the measured stress-strain curve of quasi-brittle material subjected to shear and tensile failures in uniaxial compression and in three-point bending were presented. The nolocal damage variable depends on the local counterpart and its second spatial gradient based on nonlocal theory. Analytical solution for the local damage variable was derived by substitution of the nonlocal damage variable for averaged damage variable in shear band and by using boundary condition as well as by resorting to the assumption that the actual thickness of shear band corresponds to the maximum local damage variable. It is found that local damage variable depends on the internal length parameter, coordinate, flow shear stress, shear strength, shear elastic and softening moduli. Assuming that the shear localization is initiated just at the peak strength, and that afterwards the strain-softening behavior of specimen occurs. The relation among shear strength, uniaxial compressive strength, flow shear and compressive stresses was established. The global damage variable was defined according to the measured stress-strain curve whether the post-peak response is snap-back or snap-through, which depends on the uniaxial compressive strength and flow compressive stress. A relation among local, nonlocal, and global damage variables in uniaxial compression was proposed analytically, and then a relation applicable to three-point bending was directly presented. The latter relation can be simplified for uniaxial tension condition if tensile stress is assumed to be uniform. Two examples were presented to investigate the two- and three-dimensional distributions of local damage variable in shear and tensile localized bands. The present analytical solutions for the distributed damage in shear and tensile localized bands qualitatively agree with the previous numerical predictions.

Abstract: This article deals with a method for seismic damage identification in buildings with steel moment-frame structure. The damage identification is based on artificial neural networks and natural frequencies. A simplified finite element model is used to obtain the data needed for training the nets. The method is simulated on a four-storey building under conditions as close as possible to reality. The robustness of the method and its sensitivity to the variations of the mass with time and the influence of the data errors is addressed. The statistical analysis of the results is successful, but it reveals that the predictions are quite sensitive to the data errors.

Abstract: The purpose of this paper is to report selected results of an experiment in which two, natural size r/c frames were put on shaking table and subjected to a sequence of seismic excitations with increasing intensity interlaced with low level, diagnostic tests. The shaking table experiment aimed at working out new methodology for monitoring vibrations of r/c structures to assess their state. Characteristic decrease of natural frequencies and increase of structural damping was observed and analyzed in detail. It was interesting to note 20 per cent drop in natural frequencies prior to visual detection of any cracks.

Abstract: The University of Trento is promoting a research effort aimed at developing an innovative distributed construction system based on smart prefabricated concrete elements that can allow real-time assessment of the condition of bridge structures. So far, two reduced-scale prototypes have been produced, each consisting of a 0.2×0.3×5.6m RC beam specifically designed for permanent instrumentation with 8 long-gauge Fiber Optics Sensors (FOS) at the lower edge. The sensors employed are FBG-based and can measure finite displacements both in statics and dynamics. The acquisition module uses a single commercial interrogation unit and a softwarecontrolled optical switch, allowing acquisition of dynamic multi-channel signals from FBG-FOS, with a sample frequency of 625 Hz per channel. The performance of the system is undergoing validation in the laboratory. The scope of the experiment is to correlate changes in the dynamic response of the beams with different damage scenarios, using a direct modal strain approach. Each specimen is dynamically characterized in the undamaged state and in different condition states, simulating different cracking levels. The location and the extent of damage are evaluated through the calculation of damage indices which take into account changes in frequency and in strain-modeshapes. This paper presents in detail the results of the experiment as conducted on one of these prototypes and demonstrates how the damage distribution detected by the system is fully compatible with the damage extent appraised by inspection.

Abstract: Empirical mode decomposition (EMD) method is introduced, and a new EMD based approach for damage detection of rolling bearings is presented. In this approach, the characteristic high-frequency signal with amplitude modulation of rolling bearings with local damage is separated from the mechanical vibration signal as an intrinsic mode function (IMF) by using EMD, and an envelope signal can be obtained by using Hilbert transform. Then, the characteristic frequency of damage of rolling bearings is extracted by applying Fourier transform to the envelope signal. The presented approach is used to analyse experimental signals collected from rolling bearings with outer race damage or inner race damage, and the results indicate that the EMD based approach can detect damage of rolling bearings more effectively comparing with traditional envelope analysis method.

Abstract: Traction drive elevator installations employ ropes of variable length as a mean of car and counterweight suspension. The inertial and elastic characteristics of elevator suspension systems depend on the rope construction and vary slowly during the elevator travel. The system suffers from vibrations caused by various sources of excitation. This paper presents the analysis of the dynamic response of the suspension system employing traditional steel wire ropes as well as ropes constructed of aramid fibers. The equations describing the lateral response of the system subjected to a boundary periodic excitation are solved numerically. The results show that the entire rope is subjected to repetitive low frequency transient resonances. Consequently, the structural integrity of the suspension ropes is compromised. The issue of active vibration control and the feasibility of the integration of shape memory alloy elements within the suspension rope design are discussed.

Abstract: This paper presents an investigation into the sharpness of a surgical scalpel blade. An experiment was carried out in which a surgical scalpel blade was pushed through an elastomeric substrate at a constant velocity. The force-displacement characteristics were examined by plotting the stiffness as a function of blade displacement and it was found that this curve could clearly identify the point where the material separates to form a cut. A blade sharpness measurement was defined as the energy required to initiate an opening or cut in the substrate. A finite element model was developed to examine the stress state in the substrate at the point where the opening initiates. The development of this model is described. The model was validated against the experiment and close agreement was obtained. The von-Mises stress distribution under the blade tip was plotted and it was shown that the peak stress actually occurs away from the blade tip, suggesting that material separation would initiate away from the substrate surface.