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CONTENTS
Volume 40, Number 5, December10 2011
 

Abstract
Impact experiments have been carried out on concrete slabs. The first group was traditionally manufactured, densely reinforced concrete targets, and the next were ordinary Portland and calcium aluminate cement based HPSFRC (High performance steel fiber reinforced concrete) and SIFCON (Slurry infiltrated concrete) targets. All specimens were hit by anti-armor tungsten projectiles at a muzzle velocity of over 4 Mach causing destructive perforation. In Part I of this article, production and experimental procedures are described. The first group of specimens were ordinary CEM I 42.5 R cement based targets including only dense reinforcement. In the second and third groups, specimens were produced using CEM I 42.5 R cement and Calcium Aluminate Cement (CAC40) with ordinary reinforcement and steel fibers 2 percent in volume. In the fourth group, SIFCON specimens including 12 percent of steel fibers without reinforcement were tested. A high-speed camera was used to capture impact and residual velocities of the projectile. Sample tests were performed to obtain mechanical properties of the materials. In the companion Part II of this study, numerical investigations and simulations performed will be presented. Few studies exist that examine high-velocity impact effects on CAC40 based HPSFRC targets, so this investigation gives an insight for comparison of their behavior with Portland cement based and SIFCON specimens.

Key Words
high-velocity impact; projectile; calcium aluminate cement; steel fibers; reinforced concrete; SIFCON

Address
H. Korucu: Turkish Naval Forces Command, 06100 Bakanl klar, Ankara, Turkey
P. Gulkan: Department of Civil Engineering, Cankaya University, 06810 Ankara, Turkey

Abstract
We present the numerical implementation, simulation, and validation of the high-velocity impact experiments that have been described in the companion article. In this part, numerical investigations and simulations performed to mimic the tests are presented. The experiments were analyzed by the explicit integration-based software ABAQUS for improved simulations. Targets were modeled with a damaged plasticity model for concrete. Computational results of residual velocity and crater dimensions yielded acceptable results.

Key Words
high-velocity impact; projectile; calcium aluminate cement; steel fibers; simulation; SIFCON; ABAQUS explicit

Address
P. Gulkan: Department of Civil Engineering, Cankaya University, 06810 Ankara, Turkey
H. Korucu: Turkish Naval Forces Command, 06100 Bakanllklar, Ankara, Turkey

Abstract
The present paper studies the variation of the natural frequencies and mode shapes of rectangular plates carrying a three degree-of-freedom spring-mass system (subsystem), when the subsystem changes (stiffness, mass, moment of inertia, location). An analytical approach based on Lagrange multipliers as well as a finite element formulation are employed and compared. Numerically reliable results are presented for the first time, illustrating the convenience of using the present analytical method which requires only the solution of a linear eigenvalue problem. Results obtained through the variation of the mass, stiffness and moment of inertia of the 3-DOF system can be understood under the effective mass concept or Rayleigh\'s statement. The analysis of frequency values of the whole system, when the 3-DOF system approaches or moves away from the center, shows that the variations depend on each particular mode of vibration. When the 3-DOF system is placed in the center of the plate, \"new\" modes are found to be a combination of the subsystem\'s modes (two rotations, traslation) and the bare plate\'s modes that possess the same symmetry. This situation no longer exists as the 3-DOF system moves away from the center of the plate, since different bare plate\'s modes enable distinct motions of the 3-DOF system contributing differently to the \"new\" modes as its location is modified. Also the natural frequencies of the compound system are nearly uncoupled have been calculated by means of a first order eigenvalue perturbation analysis.

Key Words
3-DOF spring-mass system; rectangular plate; frequencies; mode shapes; linear eigenvalue problem

Address
M. Febbo: Institute of Applied Mechanics, (IMA), Department of Physics, Universidad Nacional del Sur (UNS), CONICET Avda. Alem 1253, 8000-Bahia Blanca, Argentina
D.V. Bambill and R.E. Rossi: Institute of Applied Mechanics, (IMA), Department of Engineering, Universidad Nacional del Sur (UNS), CONICET Avda. Alem 1253, 8000-Bahia Blanca, Argentina

Abstract
Asbestos-cement is a material with valuable strength and durability. It was extensively used for water distribution pipes across the world from the 1950s until the early 1980s. The network of pipes in this case study dates from the 1970s, and after more than 30 to 40 years of service, some pipes have been found to break under common service pressure with no apparent reason. A set of mechanical tests was performed including bending, compression, pressure and crushing tests. Microscopy analysis was also used to understand the material behaviour. Tests showed that there was a clear loss of strength in the pipes and that the safety factor was under the established threshold in most of the specimens. Microscopy results showed morphological damage to the pipes. The loss of strength was attributed to a leaching effect. Leaching damages the cement matrix and reduces the frictional interfacial shear stress.

Key Words
asbestos-cement; failure

Address
Lluis Gil, Marco A. Perez, Ernest Bernat and Juan J. Cruz: Dept. of Strength of Materials and Engineering Structures, Universitat Politecnica de Catalunya, Barcelona Tech., ETSEIAT Campus Terrassa, c/ Colom, 11 08222 Terrassa, Spain

Abstract
This paper presents a theoretical analysis for the free vibration of rectangular tanks partially filled with an ideal liquid. Wet dynamic displacements of the tanks are approximated by combining the orthogonal polynomials satisfying the boundary conditions, since the rectangular tanks are composed of four rectangular plates. The classical boundary conditions of the tanks at the top and bottom ends are considered, such as clamped, simply supported, and clamped-free boundary conditions. As the facing rectangular plates are assumed to be geometrically and structurally identical, the vibration modes of the facing plates of the tanks can be divided into two categories: symmetric and antisymmetric modes with respect to the planes passing through the center of the tanks and perpendicular to the free liquid surface. The liquid displacement potentials satisfying the Laplace equation and liquid boundary conditions are derived, and the wet dynamic modal functions of a quarter of the tanks can be expanded by the finite Fourier transform for compatibility requirements along the contacting surfaces between the tanks and liquid. An eigenvalue problem is derived using the Rayleigh-Ritz method. Consequently, the wet natural frequencies of the rectangular tanks can be extracted. The proposed analytical method is verified by observing an excellent agreement with three-dimensional finite element analysis results. The effects of the liquid level and boundary condition at the top and bottom edges are investigated.

Key Words
hydroelastic vibration; rectangular tanks; liquid-coupled vibration; Rayleigh-Ritz method; orthogonal polynomials; Gram-Schmidt process

Address
Kyeong-Hoon Jeong: SMART Development Department, Korea Atomic Energy Research Institute, 1045 Daedeokdaero, Yuseong, Daejeon 305-353, Korea

Abstract
A new perturbation method is introduced to study the undamped free vibration of a nonprismatic Timoshenko beam for its natural frequencies and vibration modes. For simplicity, the natural modes of vibration of its corresponding prismatic Euler-Bernoulli beam with the same length and boundary conditions are used as Ritz base functions with necessary modifications to account for shear strain in the Timoshenko beam. The new method can transform two coupled partial differential equations governing the transverse vibration of the non-prismatic Timoshenko beam into a set of nonlinear algebraic equations. It significantly simplifies the solution process and is applicable to non-prismatic beams with various boundary conditions. Three examples indicated that the new method is more accurate than the previous perturbation methods. It successfully takes into account the effect of shear deformation of Timoshenko beams particularly at the free end of cantilever structures.

Key Words
Timoshenko beams; Euler-Bernoulli beams; perturbation; eigenvalue; natural mode; vibration

Address
Danguang Pan: Department of Civil Engineering, University of Science and Technology Beijing, Beijing, 100083, P. R. China; Department of Civil, Architectural, and Environmental Engineering, Missouri University of Science and Technology, Rolla, MO, 65409, USA
Genda Chen: Department of Civil, Architectural, and Environmental Engineering, Missouri University of Science and Technology, Rolla, MO, 65409, USA
Menglin Lou: State Key Laboratory for Disaster Reduction in Civil Engineering, Tongji University, Shanghai, 200092, P. R. China

Abstract
The aim of the investigation described in this paper is to study the nonlinear parametric vibrations and stability of a simply-supported viscoelastic beam with an intra-span spring. Taking into account a time-dependent tension inside the beam as the main source of parametric excitations, as well as employing a two-parameter rheological model, the equations of motion are derived using Newton\'s second law of motion. These equations are then solved via a perturbation technique which yields approximate analytical expressions for the frequency-response curves. Regarding the main parametric resonance case, the local stability of limit cycles is analyzed. Moreover, some numerical examples are provided in the last section.

Key Words
parametric vibrations; stability; viscoelastic materials; perturbation techniques

Address
Mergen H. Ghayesh: Department of Mechanical Engineering, McGill University, Montreal, Quebec, Canada H3A 2K6

Abstract
A hybrid damage monitoring scheme using parallel acceleration-impedance approaches is proposed to detect girder damage and support damage in steel plate-girder bridges which are under ambient train-induced excitations. The hybrid scheme consists of three phases: global and local damage monitoring in parallel manner, damage occurrence alarming and local damage identification, and detailed damage estimation. In the first phase, damage occurrence in a structure is globally monitored by changes in vibration features and, at the same moment, damage occurrence in local critical members is monitored by changes in impedance features. In the second phase, the occurrence of damage is alarmed and the type of damage is locally identified by recognizing patterns of vibration and impedance features. In the final phase, the location and severity of the locally identified damage are estimated by using modal strain energy-based damage index methods. The feasibility of the proposed scheme is evaluated on a steel plategirder bridge model which was experimentally tested under model train-induced excitations. Acceleration responses and electro-mechanical impedance signatures were measured for several damage scenarios of girder damage and support damage.

Key Words
acceleration; impedance; hybrid; structural health monitoring; steel plate-girder; girder crack; support failure

Address
D.S. Hong: Department of Ocean Eng, Pukyong National University, Nam-gu, Busan 608-737, Korea
H.J. Jung: Department of Civil and Environmental Eng., Korea Advanced Institute Science and Technology, Daejeon, Korea
J.T. Kim: Department of Ocean Eng, Pukyong National University, Nam-gu, Busan 608-737, Korea


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