Ayo O. Abatan

Application of commercial finite element codes for the analysis of induced strain-actuated structures

For the past decade, much research effort has been devoted to finite element formulation for the electromechanical coupling effects of piezoelectric materials, and yet, fully elec tromechanical coupled piezoelectric elements have just recently become available in commercialized finite element analysis pack ages. This paper surveys the capabilities of the piezoelectric ele ments provided by commercial FEA codes. Two major packages, ANSYS by Swanson, Inc. and ABAQUS by HKS, Inc., have been reviewed. The finite element formulation of the piezoelectric ele ments are outlined and available element and analysis types are summarized. A simple case of static and dynamic finite element analysis involving piezoelectric and structural coupling has been performed. It has been shown that the piezoelectric elements in the commercial FEA codes give results comparable with those ob tained from proven analytical methods.

Development of an electrical time domain reflectometry (ETDR) distributed strain sensor

In this paper, a detailed description for the ETDR distributed strain sensing mechanism of a coaxial cable was presented, and a signal calibration algorithm for interpreting ETDR signal waveforms was developed. In addition, a prototype coaxial ETDR distributed strain sensor with improved signal sensitivity was presented. The ETDR signal responses of the prototype sensor subjected to a concentrated lateral compression load and distributed axial tension load were experimentally tested. The test results showed that the prototype sensor has a substantially improved signal sensitivity over a commercial RG-174 cable of comparable size. It was also shown that the relation between the impedance change of the sensor and the applied axial tensile strain depends primarily on the mechanical stress–strain response of the sensor. From the test results, it was demonstrated that this relation could be empirically established with the aid of the calibration algorithm.

Crack damage detection of concrete structures using distributed electrical time domain reflectometry (ETDR) sensors

Electrical time domain reflectometry (ETDR) stress/strain sensing technique has been successfully used in geotechnical applications to detect rock deformation and longwall movement. The ETDR sensing method appears to be practical for health monitoring applications of civil concrete structures since durable sensor media can be used. In this paper, feasibility of using an embedded coaxial ETDR cable to detect crack damages in a structure is investigated. Tension and bending tests were performed on edge-notched photoelastic epoxy specimens with embedded coaxial ETDR sensing cables. The test results show that crack lines passing through the embedded sensing cable can be detected. The TDR signal response of the sensing cable reveals not only the location of the crack damage site but also indicates the relative magnitude of the crack opening. The results of the current study strongly suggest that the ETDR sensing technique possesses a great potential for the application of health monitoring of large civil concrete structures.

Transverse shear response monitoring of concrete cylinder using embedded high-sensitivity ETDR sensor

Promising results have been shown in recent studies using commercial coaxial ETDR sensing cables for health monitoring application of concrete beam structures subject to bending load. Although distributed strain monitoring and crack damage detection capabilities of the sensors were demonstrated, the low signal-to-noise ratio of the sensors smears the details of the strain measurement that the ETDR signal waveform can convey. A high-sensitivity coaxial sensor prototype specifically designed for distributed strain sensing application has been recently developed in-house. It has been shown that the prototype sensor has a much superior sensitivity in terms of ETDR signal response to applied loads than commercial coaxial cable counterpart. In this paper, the effectiveness of using an embedded high-sensitivity coaxial sensor prototype to monitor transverse shear response of a concrete cylinder is investigated. Both single and double plane transverse shear tests were conducted. The test results show that the embedded high-sensitivity ETDR prototype sensor is capable of detecting the on-set as well as monitoring the growth of shear-induced crack damages in concrete cylinder specimens.

High-sensitivity electrical TDR distributed strain sensor

Electrical time domain reflectometry (ETDR) distributed strain sensing technique has been successfully used in the health monitoring application of civil concrete structures to detect crack damages and in geotechnical application to monitor rock deformation and longwall movement. Although promising results of using commercial ETDR coaxial sensing cables have been shown in recent studies, the low signal-to-noise ratio of the sensors is a research issue needs to be addressed. Since all the commercial coaxial cables are specifically designed for the transmission of electrical signals, the cable configuration is to sustain a virtually constant electrical property under environmental loading effects. For structural strain sensing application, on the other hand, the electrical impedance of the sensor is required to proportionally vary with respect to externally applied loading. Thus, the commercial coaxial cables are indeed not in an optimal configuration for strain sensing application. In this paper, a newly developed high-sensitivity ETDR coaxial strain sensor prototype is presented. The construction of the prototype sensing cable as well as its electrical properties will be described in details. Experimental characterization of the high-sensitivity prototype coaxial sensor was also conducted. Test results of the sensitivity and tension responses of the ETDR signal of the prototype sensor are presented and compared with those of commercial coaxial cables. It is shown that the prototype sensor has a much superior sensitivity and properties for distributed strain sensing application.

Impact Resistance Modeling of Hybrid Laminated Composites

This paper discusses the analytical and numerical results for the impact responses of two types of plates with equal areal weights: hybrid laminated composite plates and homogeneous metallic plates. The loadings and the boundary conditions for the two types of plates are the same, and an impact load was applied at the center of each plate. An analytical solution based on Fourier expansion was developed to obtain the impact responses of the plates. The finite element method (FEM) was used to perform numerical analyses to verify the analytical solutions. The response patterns from the results were compared and a preliminary conclusion made on the impact resistance ratio between the hybrid laminates with proper interfacial bonding and that of equivalent homogeneous metallic plates. The effect of different shapes of pulse on the laminated composites was also examined. Three loading cases were considered, each being a function of time. The effect of different pulse shapes on the impact response was evaluated. Finally, a solution based on shear deformation theory was developed, and the comparison between classical plate theory results and shear deformation theory results indicates that for the problem examined classical plate theory predicts relatively accurate results.

Effect of Cross Section Material Distribution on Impact Response of Hybrid Composites

A linear analytical model is proposed to evaluate the magnitude of the impact force as a function of the velocity of the impactor. This model provides a simple tool for estimating the magnitude of the impact load from the impact energy. For hybrid composites subjected to low- and medium-velocity impacts where elastic deformation is assumed, the effect of cross section material distribution on impact response was investigated. For a hybrid metal/polymer laminate and a hybrid titanium composite laminate, the relative position of the laminate plies has a significant effect on the plate deflection under impact loads. For equal a real weight plates, the number of layers in a hybrid composite laminate does not appear to affect the impact resistance. However, the relative material ratio of metal to polymer (or metal to polymeric composite) in hybrid composites significantly affects the impact response. Furthermore, the relative ply thickness in a laminate does not have significant effect on its impact resistance for plates with equal a real weight, but the research shows that the total relative material distribution does significantly affect the impact response. The analysis also shows that the effect of cross section material distribution can be qualitatively evaluated by checking the bending stiffness of the plate.

Electrical time domain reflectometry sensing cables as distributed stress/strain sensors in smart material systems

Electrical time domain reflectometry (ETDR) stress/strain sensing technique has been successfully demonstrated in geotechnical applications to detect rock deformation and longwall movement. The ETDR sensing method appears to be practical for health monitoring applications of civil engineering structures since durable sensor media can be used. To use the ETDR sensing technique for structural stress/strain measurements, the coupling mechanism between applied loads and TDR signal response of the sensors need to be understood and modeled accurately. In this paper a theoretical model capable of describing the relation between applied loads and TDR signal response of a coaxial cable was developed. The accuracy of the model was verified by comparing theoretical results with those of finite element analyses. Parametric study to investigate the effects of the physical and structural parameters of the cable was also performed using the theoretical model.

Effect of Resin Interlayer on Fracture Behavior of Composite Laminates

In this work, the effect of resin interlayer on the fracture behavior of composite laminates is evaluated. For cracks in elastic laminates, the closed-form solutions for energy release rate and mode ratio are presented. These solutions are applicable to structures where classical laminated plate theory may be used to predict deflections and strain energies. For a typical graphite/epoxy composite laminate containing a free edge delamination bounded by an isotropic resin interlayer, it is found that the energy release rate is not sensitive to the Youngs modulus of the resin interlayer, but the mode ratio is very sensitive. However, the energy release rate is insensitive to the Poissons ratio of the resin interlayer. It is also found that within a certain range, the thickness of the resin interlayer does not have much effect on the energy release rate. The limitations of the current results are the assumption of elastic deformation and the applicability of classical laminated plate theory.

Experimental characterization of ETDR sensors for crack monitoring in concrete structures

A novel approach for health monitoring of civil infrastructural systems using electrical time domain reflectometry (ETDR) sensors has been established. In this paper, experimental characterization results obtained when a coaxial cable is used as an ETDR sensor to monitor cracks in reinforced concrete structures will be provided. The first part of the paper shows simulation of cracks due to high pressure on the sensor when it is embedded in concrete. The results show that as the pressure on the sensor increases, the ETDR technique was able to detect exactly the location on the sensor where the high pressure is applied. The technique is able to detect the crack location and magnitude of pressure to an application point in a harsh environment. Furthermore, the ETDR technique was able to distinctly detect the locations on the sensors when multiple compressive high-pressure forces were applied on the sensor within a spatial resolution of less than one inch. This multiple sensing ability confirms the ETDR sensing approach as a fully distributed sensing technique. The second part of the paper will show the simulated effect of high temperature on the sensor when embedded in concrete. The results show that the ETDR sensor is reliable and durable with significant increases in temperature variations.