Oded Rabinovitch

Experiments and analytical comparison of RC beams strengthened with CFRP composites

This paper deals with strengthening, upgrading, and rehabilitation of existing reinforced concrete structures using externally bonded composite materials. Five strengthened, retrofitted, or rehabilitated reinforced concrete beams are experimentally and analytically investigated. Emphasis in placed on the stress concentration that arises near the edge of the fiber reinforced plastic strip, the failure modes triggered by these edge effects, and the means for the prevention of such modes of failure. Three beams are tested with various edge configurations that include wrapping the edge region with vertical composite straps and special forms of the adhesive layer at its edge. The last two beams are preloaded up to failure before strengthening and the ability to rehabilitate members that endured progressive or even total damage is examined. The results reveal a significant improvement in the serviceability and strength of the tested beams and demonstrate that the method is suitable for the rehabilitation of severely damaged structural members. They also reveal the efficiency of the various edge designs and their ability to control the characteristic brittle failure modes. The analytical results are obtained through the Closed-Form High-Order model and are in good agreement with the experiment ones. The analytical and experimental results are also used for a preliminary quantitative evaluation of a fracture mechanics based failure criterion for the strengthened beam.

Adhesive Layer Effects in Surface-Mounted Piezoelectric Actuators

The role of the adhesive layers in active panels with surface-mounted (bonded) piezoelectric layers is studied. The investigation focuses on the strain transfer mechanism between the active layers and the host structure, the stress concentrations involved, and the influence of the geometrical and mechanical properties of the adhesive layers on the static response of the panel. The analysis is based on the High-Order approach and uses 2D elasticity to model the adhesive layers. The mathematical formulation is derived using variational principles, compatibility requirements, and the piezoelectric constitutive equations. Confirmation of the analytical model is achieved through an experimental study that reveals good agreement between the theoretical predictions and the behavior of the active structure. Numerical results are presented for a typical piezoelectric active panel and compared to detailed 2D finite element analysis. The results reveal the high-order effects and the stress concentrations in the transition zone near the edge of the panel and indicate that a careful selection of the adhesives properties can improve the behavior of the structure and reduce the severity of the stress concentrations involved. The paper concludes with a summary and recommendations for the analysis, design, and use of smart structures with bonded actuators.

A high-order finite element for dynamic analysis of soft-core sandwich plates

The dynamic behavior of soft-core sandwich plates is investigated. A high-order finite element concept that has been developed for the dynamic analysis of multi-layered plate structures with stiff and compliant layers is applied to the soft-core sandwich plate. The application to sandwich plates aims to validate the general model through comparison with experimental and analytical benchmarks and to throw light on the unique structural response of the sandwich plate. The model introduces the core’s three-dimensional stress and deformation fields using a high-order kinematic assumption that is based on the closed-form solution of the static governing equations of the core. The first-order shear deformation laminated plate theory is used for the face sheets. The combination of the high-order theory with the finite element concept aims to extend the application of the theory to more general layouts, to reduce the computation effort needed for a three-dimensional analysis, and to address some of the obstacles due to differences in length scales and elastic properties. The validity and the capabilities of the formulation are examined through comparison with experimental and analytical results taken from the literature. In addition, the static, free vibration, and dynamic behaviors of an ‘L’ shaped sandwich plate subjected to localized loads and boundary conditions are numerically studied. The formulation, the comparison with experimental and analytical benchmarks, and the numerical study highlight the three-dimensional effects and reveal unique aspects of the dynamic response of soft-core sandwich plates.

On the design of piezoelectric smart fins for flight vehicles

A systematic approach for the design of active piezoelectric fins developed for a small-scale flight vehicle is presented. The proposed design approach uses analytical and computational tools that are based on the high-order theory and provides a graphical representation of the response spectrum of the active fin. In addition, it enables the coupling of the structural and aerodynamic analyses and provides a frame in which the results of the two types of analysis are adjoined. A numerical design study of a twist-actuated smart fin is presented and discussed. The results reveal the sensitivity of the structure to a broad range of geometrical, mechanical, and electromechanical design variables and provide guidelines for the optimization of the active structure. A set of normalized design master curves that can be scaled to fit various geometrical layouts of the structure investigated are also presented and discussed.

Free Vibration of Delaminated Unidirectional Sandwich Panels with a Transversely Flexible Core and General Boundary Conditions — A High-Order Approach

A high-order theoretical approach for the free-vibration analysis of delaminated unidirectional sandwich panels with a compressible core is presented. The analytical approach accounts for the flexibility of the core in the vertical direction and the resulting high-order displacement, acceleration, and velocity fields within the core. The existence of delaminated regions along the sandwich panel and the various interfacial stress transfer conditions at the delaminated core-face interface are considered and studied. The analytical model developed in the article is applicable to any type of boundary conditions including different support conditions at the same section. The derivation of the model uses Hamiltons variational principle as well as the beam and lamination theories for the face-sheets and two-dimensional elasticity theory for the modeling of the core. The free-vibration problem is formulated and solved assuming a harmonic behavior in time and using the multiple shooting method along with the Newton—Raphson scheme for the solution in space. Numerical results that emphasize the influence of the boundary conditions, the effect of the delaminated regions, the influence of the contact conditions within the debonded region, and the high-order effects through the depth of the core are presented and discussed. The model and the solution procedure are verified through a comparison with finite elements analyses and with an analytical solution that is based on the Modified Galerkin method. The article closes with a summary and conclusions.

Geometrically nonlinear behavior of piezoelectric laminated plates

The geometrically nonlinear behavior of piezo-laminated plates actuated with isotropic or anisotropic piezoelectric layers is analytically investigated. The analytical model is derived using the variational principle of virtual work along with the lamination and plate theories, the von Karman large displacement and moderate rotation kinematic relations, and the anisotropic piezoelectric constitutive laws. A solution strategy that combines the approach of the method of lines, the advantages of the finite element concept, and the variational formulation is developed. This approach yields a set of nonlinear ordinary differential equations with nonlinear boundary conditions, which are solved using the multiple-shooting method. Convergence and verification of the model are examined through comparison with linear and nonlinear results of other approximation methods. The nonlinear response of two active plate structures is investigated numerically. The first plate is actuated in bending using monolithic piezoceramic layers and the second one is actuated in twist using macro-fiber composites. The results quantitatively reveal the complicated in-plane stress state associated with the piezoelectric actuation and the geometrically nonlinear coupling of the in-plane and out-of-plane responses of the plate. The influence of the nonlinear effects ranges from significant stiffening in certain combinations of electrical loads and boundary conditions to amplifications of the induced deflections in others. The paper closes with a summary and conclusions.

Lateral Out-of-Plane Strengthening of Masonry Walls with Composite Materials

The structural behavior of masonry walls laterally strengthened with externally bonded composite materials to resist out-of-plane loads is theoretically and experimentally studied. Hollow concrete block masonry walls and solid autoclaved aerated concrete (AAC) block masonry walls are examined. A theoretical model that accounts for the cracking and the physical nonlinear behavior, the debonding of the composite layers, the arching effect, the interfacial stresses, and the unique modeling aspects of the laterally strengthened wall is presented. The experimental study includes loading to failure of 4 laterally strengthened masonry walls and 2 control walls. The experimental and analytical results point at the unique aspects of the lateral strengthening of masonry walls with composite materials. In particular, they reveal and explain the premature shear failure in laterally strengthened hollow concrete blocks walls and, on the other hand, demonstrate the potential of lateral fiber-reinforced polymer strengthe...

Nonlinear Analysis of Masonry Arches Strengthened with Composite Materials

The nonlinear behavior of masonry arches strengthened with externally bonded composite materials is investigated. A finite-element (FE) formulation that is specially tailored for the nonlinear analysis of the strengthened arch is developed. The FE formulation takes into account material nonlinearity of the masonry construction and high-order kinematic relations for the layered element. Implementation of the above concept in the FE framework reduces the general problem to a one-dimensional nonlinear formulation in polar coordinates with a closed-form representation of the elemental Jacobian matrix (tangent stiffness). A numerical study that examines the capabilities of the model and highlights various aspects of the nonlinear behavior of the strengthened masonry arch is presented. Emphasis is placed on the unique effects near irregular points and the nonlinear evolution of these effects through the loading process. A comparison with experimental results and a discussion of the correlating aspects and the ones that designate needs of further study are also presented.

Sensory carbon fiber based textile-reinforced concrete for smart structures

This article investigates the feasibility of intelligent textile-reinforced concrete structural elements with sensing capabilities. The concept is based on dual use of glass and carbon fiber textiles as reinforcement and, at the same time, as a sensory agent. Experimental investigation demonstrates the feasibility of the concept in two applications: detecting strains in a mechanically loaded textile-reinforced concrete beam and monitoring the interaction of the structural element with a wet environment. By detecting the changes to the integrative electrical resistance of the carbon tow, the ability of the textile to sense strain and exposure to water is demonstrated. For strain sensing, the hybrid reinforcing textile provides electro-mechanical sensing with a gauge factor of the order of 1 and a detectable correlation with the load, strain, and displacement responses. For the detection of wetting, the implementation of the carbon tow in a Wheatstone bridge detects fractional resistance changes in the order of 10−5, a figure that is effectively detected by monitoring the voltage across the bridge. The response to wetting, which is conditioned by the cracking of the beam and the exposure to ionic conductive solutions, provides a mean to monitor the functionality and the structural health of the textile-reinforced concrete beam.

Piezoelectric Control of Edge Debonding in Beams Strengthened with Composite Materials: Part I – Analytical Modeling

The use of piezoelectric materials for the control of the edge stresses and the edge debonding failure in reinforced concrete beams strengthened with externally bonded composite materials is analytically investigated. A mathematical model that incorporates layers of piezoelectric active materials embedded or surfacemounted on the composite strengthening strip is developed. The model is derived using the electromechanical analog for the variational principle of virtual work along with the compatibility conditions, the piezoelectric constitutive laws, and the closed form solutions for the stresses and displacements in the adhesive. The response of a full-scale strengthened beam to mechanical loads and to different schemes of piezoelectric actuation is investigated in terms of the localized stresses near the edge of the bonded composite strip. The results show that in spite of their limited size and actuation capabilities, the piezoelectric actuators can induce shear and vertical normal stresses that are of similar magnitude but opposite sign to the stresses induced by the mechanical load. Thus, it contributes to the prevention of the debonding failure that characterizes the response of the strengthened beam. The original contribution of the study is in addressing the challenge of using piezoelectric smart and active materials in full-scale civil engineering structures, in developing the methodologies and the tools for the quantitative evaluation of the response of such advanced structural member, and in presenting a potential solution to the edge-debonding problem.