C.P. Providakis

Dynamic analysis of elasto-plastic flexural plates by the D/BEM

Abstract A direct domain/boundary element method is developed for the dynamic response analysis of thin elasto-plastic flexural plates of arbitrary geometry and boundary conditions subjected to any lateral loading history. The method employs the elastostatic fundamental solution of thin flexural plates in a time domain integral formulation. Thus, the plasticity effect, the inertial load and the external lateral load appear in domain integrals in the boundary integral formulation of the problem. This requires a boundary as well as an interior discretization of the plate. Quadratic isoparameteric boundary and interior elements are employed for increased accuracy. The solution is obtained by an explicit time integration scheme employed on the incremental form of the matrix equations of motion. The incremental plastic moments needed to evaluate the plasticity effect are calculated by a finite element methodology to avoid the evaluation of highly singular terms. Numerical examples are presented to illustrate the proposed method and compare it with the finite element method.

Free and forced vibrations of plates by boundary elements

Abstract A direct boundary element methodology is developed for the dynamic analysis of thin elastic flexural plates of arbitrary planform and boundary conditions. The formulation employs the frequency domain dynamic fundamental solution of the problem and this essentially creates only boundary integrals. Thus only the plate perimeter has to be discretized and this is accomplished with the aid of quadratic isoparametric boundary elements for increased accuracy. Both free and forced vibration problems are considered. The free vibration problem is reduced to a matrix eigenvalue problem wherein the matrix coefficients are complex Bessel functions of the frequency parameter. The forced vibration problem is solved with the aid of the Laplace transform with respect to time and this requires a numerical inversion of the transformed solution to obtain the plate dynamic response to arbitrary transient loading. Numerical examples are presented to illustrate the proposed methodology and demonstrate its advantages.

A general and advanced boundary element transient analysis of elastoplastic plates

A direct boundary element approach is developed for the determination of the transient response of thin elastoplastic flexural plates of arbitrary shape, and simple or double connectivity. All kinds of boundary conditions (uniform or mixed) are treated, and the effects of corners and internal supports are considered. The method employs the static fundamental solution of the problem and this creates not only boundary integrals but surface integrals as well owing to the presence of the plasticity effect, the inertial and external lateral load. Thus, boundary as well as domain (interior) elements are used in the space descretization of the problem. Linear isoparametric elements with a Hermitian interpolation for the deflection and quadratic isoparametric elements are employed for the boundary and interior descretization, respectively. Subsequently, using an explicit time integration scheme employed on the incremental form of the matrix equation of motion, the solution is obtained. Several examples are presented to illustrate the efficiency and accuracy of the proposed method.

Dynamic analysis of inelastic plates by the D/BEM

A direct boundary element method is developed for the dynamic analysis of thin inelastic flexural plates of arbitrary planform and boundary conditions. It employs the static fundamental solution of the associated elastic problem and involves not only boundary integrals but domain integrals as well. Thus boundary as well as interior elements are employed in the numerical solution. Time integration is accomplished by the explicit algorithm of the central difference predictor method. A viscoplastic constitutive theory with state variables is employed to model the material behaviour. Numerical results are also presented to illustrate quantitatively the proposed method of solution.

Damage detection in concrete structures using a simultaneously activated multi-mode PZT active sensing system: numerical modelling

In this study, a structural health monitoring approach that integrates both electromechanical admittance (EMA) and guided wave (GW) techniques is presented. More specifically, the EMA technique is used for local damage identification, by employing a piezoelectric transducer (PZT) as admittance sensor. Simultaneously, the same admittance sensor is disturbed by selected elastic GWs launched by another PZT to monitor the damages located beyond the sensing area of the admittance sensor. The validation of the integrated approach is achieved by identifying the changes in electrical admittance signatures as measured on the surface electrodes of PZTs. These changes occur when damage alters the mechanical impedance of the examined concrete structure and when propagating GWs encounter structural damage. Finite element models of damages occurring in conventional unreinforced, steel-reinforced or fiber reinforced plastics-reinforced concrete specimens are investigated. Results illustrate that the proposed integrated technique is an efficient approach for damage identification of concrete structures.

Dynamic analysis of beams by the boundary element method

Abstract Free and forced flexural vibrations of beams are numerically studied with the aid of the direct boundary element method. The free vibration case is treated as an eigenvalue problem, while the forced vibration one is treated with the aid of the Laplace transform. The structural dynamic response is finally obtained by a numerical inversion of the transformed solution. The effects of a constant axial force, external viscous or internal viscoelastic damping, and an elastic foundation on the response are also considered. Various numerical examples serve to illustrate the method and demonstrate its advantages and disadvantages.

Free and forced vibrations of shallow shells by boundary and interior elements

Abstract A direct boundary element method is developed for the dynamic analysis of thin elastic shallow shells of arbitrary geometry and boundary conditions. The integral formulation employs the static fundamental solution of thin elastic flat plates under combined flexural and membrane deformation. This creates not only boundary integrals, but surface integrals as well due to the presence of the inertia forces and the coupling between the flexural and membrane action in the shell. Thus the discretization involves boundary as well as interior elements, both of the quadratic isoparametric type. Both free and forced vibrations are considered. The free vibration problem is reduced to a generalized eigenvalue problem involving matrices with entries independent of frequency. The general forced vibration problem is solved with the aid of the Laplace transform with respect to time and this requires a numerical inversion of the transformed solution to obtain the time domain response. The effect of the external viscous or internal viscoelastic damping on the response is also taken into account. Numerical examples are presented to illustrate the method and demonstrate its advantages.

Damage Evaluation in Shear-Critical ReinforcedConcrete Beam using Piezoelectric Transducers asSmart Aggregates

Abstract Damage detection at early cracking stages in shear-critical reinforced concrete beams, before further deterioration and their inevitable brittle shear failure is crucial for structural safety and integrity. The effectiveness of a structural health monitoring technique using the admittance measurements of piezoelectric transducers mounted on a reinforced concrete beam without shear reinforcement is experimentally investigated. Embedded “smart aggregate” transducers and externally bonded piezoelectric patches have been placed in arrays at both shear spans of the beam. Beam were tested till total shear failure and monitored at three different states; healthy, flexural cracking and diagonal cracking. Test results showed that transducers close to the critical diagonal crack provided sound and graduated discrepancies between the admittance responses at the healthy state and thedamage levels.Damage assessment using statistical indices calculated from the measurements of all transducers was also attempted. Rational changes of the index values were obtained with respect to the increase of the damage. Admittance responses and index values of the transducers located on the shear span where the critical diagonal crack formed provided cogent evidence of damage. On the contrary, negligible indication of damage was yielded by the responses of the transducers located on the other shear span, where no diagonal cracking occurred.

Nondestructive Wireless Monitoring of Early-Age Concrete Strength Gain Using an Innovative Electromechanical Impedance Sensing System

Monitoring the concrete early-age strength gain at any arbitrary time from a few minutes to a few hours after mixing is crucial for operations such as removal of frameworks, prestress, or cracking control. This paper presents the development and evaluation of a potential active wireless USB sensing tool that consists of a miniaturized electromechanical impedance measuring chip and a reusable piezoelectric transducer appropriately installed in a Teflon-based enclosure to monitor the concrete strength development at early ages and initial hydration states. In this study, the changes of the measured electromechanical impedance signatures as obtained by using the proposed sensing system during the whole early-age concrete hydration process are experimentally investigated. It is found that the proposed electromechanical impedance (EMI) sensing system associated with a properly defined statistical index which evaluates the rate of concrete strength development is very sensitive to the strength gain of concrete structures from their earliest stages.

Creep analysis of V-notched metallic plates: boundary element method

Abstract Application of the domain boundary element method is made to the creep analysis of two-dimensional viscoplastic structures containing V-notches under plane stress. Stress and strain distributions are obtained for tensile specimens with a single edge notch. The formulation assumes small-strain and small-rotation for a viscoplastic medium. The combined creep-plasticity constitutive model of Hart is used. Numerical examples are solved for different load histories.