Vasile Iancu

A NEW APPROACH FOR SEVERITY ESTIMATION OF TRANSVERSAL CRACKS IN MULTI-LAYERED BEAMS

NOWADAYS, THE DAMAGE SEVERITY EVALUATION IN MECHANICAL STRUC-TURES MADE OF BEAMS IS MOSTLY PERFORMED BY ANALYZING THE NATURAL FREQUENCY SHIFT. THE NON-ISOTROPIC MATERIALS, AS THE MULTI-LAYERED ONES, ARE WIDE-SPREAD IN INDUSTRIAL APPLICATIONS, FOR INSTANCE DUE TO THEIR INTERESTING PHYSIC-MECHANICAL PROPERTIES. THUS, A DEEPER APPROACH OF MULTI-LAYERED BEAMS BECOMES AN IMPORTANT REQUEST IN THE RESEARCH DOMAIN. IN THIS PAPER, THE ACCOMPLISHMENT OF DAMAGE SEVERITY IN MULTI-LAYERED BEAMS IS MORE COMPREHENSIVELY CHARAC-TERIZED BY INTRODUCING A NEW WAY TO HIGHLIGHT THE CRACK EVOLUTION AS A FUNCTION OF STORED ENERGY. IT IS WELL KNOWN THAT THE ENERGY STORED IN A BEAM WITHOUT DAMAGE IS GREATER THAN THE ENERGY OF THAT DAMAGED BEAM. AS A CONSEQUENCE, THE BEAM DEFLECTION CAN BE RELATED TO THE STORED ENERGY. IN THIS REGARD, THE POSSIBILITY TO SPLIT THE DAMAGE LOCALIZATION AND THE DAMAGE SEVERITY ASSESSMENT HAS BEEN PROVEN, AND ALSO THE GRAPHICAL EVOLUTION OF THE NATURAL FRE-QUENCY SHIFT HAS BEEN ACHIEVED AS A FUNCTION OF THE CRACK DEPTH. THE RESULTS ACHIEVED BY THE FINITE ELEMENT METHOD (FEM) AND EXPERIMENTAL TESTS ARE GIVEN IN TABLES AND GRAPHICS. FOR THE FIRST FIVE VIBRATION MODES, A COMPARISON WAS MADE BETWEEN FREQUENCIES ACCOMPLISHED BY ANALYTICAL, NUMERICAL AND EXPERIMENTAL ANALYSES, IN ORDER TO GIVE MORE CREDIBILITY TO THE ACCURACY OF THE RESEARCH DATA PRESENTED IN THIS PAPER.

Some Models of Elastomeric Seismic Isolation Devices

For the study and design of the elastomeric seismic devices is essential to know the mathematical relation between the horizontal displacement and the force leading it. In this paper we present mathematical models for three types of devices: (i) natural rubber bearings, (ii) lead rubber bearings and (iii) hybrid device combining the two first mentioned bearings. For all devices the specific domains of operation are determined and for each domain the relations connecting horizontal displacement and stiffness is contrived, highlighting the hysteretic behaviour in respect to ground excitation. Finally we present numerical results and a comparison between the three devices, defining the opportunity to involve them in specific applications, in function of the type and nature of the isolated structure.

The Velocity Evolution of an Isolated Element in a Friction Pendulum for Different Sliding Surfaces

In this paper, the velocity evolution for a structure isolated by friction pendulums (FP) with diverse sliding surfaces is presented. The surfaces are characterized by the curvature; as reference a cylindrical surface is considered and compared against surfaces generated by several polynomials. The position and velocity of the isolated element in different time moments is derived analytically for all surfaces, in order to evaluate the energy loss due to friction. This aspect is important because the materials used in manufacturing of seismic isolation devices can be sensitive to temperature variations, manifested by changing of the physical and mechanical properties. Findings of the study presented in this paper have practical importance, constituting guidelines for friction pendulum design.

Damage identification in rectangular plates using spectral strain energy distribution

In this paper, a vibration-based damage detection method designed for thin plates is proposed. The method bases on the relation existing between the strain energy stored in the plate in several vibration modes and the respective natural frequencies. For any mode, the strain energy results as sum of the energies stored in all plate elements in that mode. It depends on the square of the mode shape curvature achieved in each element location for the given vibration mode. As a result, the energy distribution along the plate is different for each mode. By reducing the rigidity of one plate element due to a damage, the frequencies will drop in a different manner, depending on the damaged element location. This permits to define patterns that characterize the dynamic behavior of the plated for any damage location. Actually, the patterns are derived from the normalized frequency shifts attained by numerical simulations. Herein the patterns that characterize a centrally located damage of different extent are consequently derived by means of the finite element analysis and used as a benchmark in the damage detection process. These patterns are successfully used to recognize, localize and quantify damages from measurements on in real plates.

On the Use of Higher-Order Frequencies in Crack Identification in Columns Subjected to Important Tip Mass

The paper present a novel non-destructive evaluation method designed to assess damage in structural elements subjected to important axial loads, as columns are. In the prior research a reliable damage detection method was developed for beams subjected to own mass, that consider the relation existing between the energy stored in the beam in certain vibration modes and the related natural frequencies. First we found the mathematical relations expressing the healthy pillar mode shapes and frequencies with respect to the top mass. Afterward, by means of FEM, we derived the natural frequencies for the numerous damage cases, in order to define the frequency shift curves. Analogous to the case of beams, the damage location is characterized by patterns that are derived from the mode shape curvatures square of the healthy beam. The damage location becomes an inverse problem while the damage position is found by interpreting frequency measurements made on the healthy and damaged beam.