Anastasis V. Georgiades

Strain measurement in a concrete beam by use of the Brillouin-scattering-based distributed fiber sensor with single-mode fibers embedded in glass fiber reinforced polymer rods and bonded to steel reinforcing bars

The strain measurement of a 1.65-m reinforced concrete beam by use of a distributed fiber strain sensor with a 50-cm spatial resolution and 5-cm readout resolution is reported. The strain-measurement accuracy is +/-15 microepsilon (microm/m) according to the system calibration in the laboratory environment with non-uniform-distributed strain and +/-5 microepsilon with uniform strain distribution. The strain distribution has been measured for one-point and two-point loading patterns for optical fibers embedded in pultruded glass fiber reinforced polymer (GFRP) rods and those bonded to steel reinforcing bars. In the one-point loading case, the strain deviations are +/-7 and +/-15 microepsilon for fibers embedded in the GFRP rods and fibers bonded to steel reinforcing bars, respectively, whereas the strain deviation is +/-20 microepsilon for the two-point loading case.

Modeling of smart composites on account of actuation, thermal conductivity and hygroscopic absorption

The asymptotic homogenization models for smart composite materials are derived and effective elastic, actuation, thermal expansion and hygroscopic expansion coefficients for smart structures are obtained. The actuation coefficients characterize the intrinsic transducer nature of active smart materials that can be used to induce strains and stresses in a coordinated fashion. Examples of such actuators employed with smart composite material systems are derived from piezoelectric, magnetostrictive, and some other materials. The pertinent mathematical framework is that of asymptotic homogenization. The objective is to transform a general anisotropic composite material with a regular array of reinforcements and/or actuators into a simpler one that is characterized by some effective coefficients; it is implicit, of course, that the physical problem based on these homogenized coefficients should give predictions differing as little as possible from those of the original problem. The effectiveness of the derived models is illustrated by means of two- and three-dimensional examples.

Reliability assessment of pultruded FRP reinforcements with embedded fiber optic sensors

Fiber optic strain sensors are successfully embedded in glass and carbon fiber reinforced polymer (GFRP and CFRP) reinforcements during pultrusion. The specific application is the use of the smart composite reinforcements for strain monitoring of innovative civil engineering structures. A comprehensive reliability assessment of the pultruded smart FRP rods with embedded sensors is performed, encompassing mechanical tests at room temperature as well as under conditions of low and high temperatures. In these tests, the strain output from the embedded fiber optic sensors was in good agreement with the output from surface mounted extensometers. The fatigue and short-term creep behavior of the smart composite rods is also examined. Finally, the long-term performance of the smart composites under sustained loads in alkaline environments simulating conditions encountered in concrete structures is assessed. The experiments conducted showed that the strain from the embedded fiber optic sensors conformed well with the corresponding output from the extensometers or foil gages.

The mechanical performance of pultruded composite rods with embedded fiber-optic sensors

Abstract Fiber-optic strain sensors are successfully embedded in glass- and carbon-fiber-reinforced polymer (GFRP and CFRP) tendons during pultrusion. The study of the performance of the embedded Fabry-Perot fiber-optic sensors under conditions of static and dynamic loading when exposed to both low and high temperature extremes, is presented. The experiments entailed subjecting the GFRP and CFRP tendons to sinusoidal and trapezoidal load waveforms of about 11 kN magnitude inside a temperature chamber. The temperature in the chamber was varied from −40 to 60°C in increments of 20°C. The strain output from the embedded sensors was compared to that from externally mounted extensometers as well as to theoretical strain values. It was determined that the performance of the Fabry-Perot sensors was not affected by ambient temperatures falling within the range of −40 to +60°C and the sensor readings conformed very well with the corresponding extensometer and theoretical readings.

On the processing and evaluation of pultruded smart composites

The use of the pultrusion process for the manufacture of fiber reinforced polymer (FRP) composites with embedded fiber optic sensors are discussed. The specific application is the use of smart composite reinforcements for strain monitoring in innovative bridges and structures. The Bragg Grating and Fabry Perot fiber optic sensors are successfully embedded during the pultrusion of FRP rods. The behavior of optic sensors during pultrusion was assessed, and the effect of the embedment of optical fibers and their surface coatings on the mechanical properties of the composite was investigated. Monitoring of the output of embedded fiber optic strain sensors during the pultrusion of composite rods may provide useful information about the formation of process-induced strains within the composite. To verify the operation of the optic sensors embedded in the smart pultruded tendons, mechanical tests were conducted and the output of the fiber optic sensors was compared to that of an extensometer during quasi-static and cyclic tensile tests.

An asymptotic homogenization model for smart 3D grid-reinforced composite structures with generally orthotropic constituents

A comprehensive micromechanical model for smart 3D composite structures reinforced with a periodic grid of generally orthotropic cylindrical reinforcements that also exhibit piezoelectric behavior is developed. The original boundary value problem characterizing the piezothermoelastic behavior of these structures is decoupled into a set of three simpler unit cell problems dealing, separately, with the elastic, piezoelectric and thermal expansion characteristics of the smart composite. The technique used is that of asymptotic homogenization and the solution of the unit cell problems permits determination of the effective elastic, piezoelectric and thermal expansion coefficients. The general orthotropy of the constituent materials is very important from the practical viewpoint and makes the analysis much more complicated. Several examples of practical interest are used to illustrate the work including smart 3D composites with cubic and conical embedded grids as well as diagonally reinforced smart structures. It is also shown in this work that in the limiting particular case of 2D grid-reinforced structures with isotropic reinforcements our results converge to earlier published results.

Smart pultruded composite reinforcements incorporating fiber optic sensors

The issues of processing, evaluation, experimental testing, and modeling of smart fiber reinforced polymer (FRP) composite materials are discussed. The specific application in view is the use of smart composite reinforcements for a monitoring of innovative bridges and structures. The pultrusion technology for the fabrication of fiber reinforced polymer composites with embedded fiber optic senors (Fabry Perot and Bragg Grating) is developed. The optical sensor/composite material interaction is studied. The tensile and shear properties of the pultruded carbon/vinylester and glass/vinylester rods with and without optical fibers are determined. The microstructural analysis of the smart pultruded FRP is carried out. The interfaces between the resin matrix and the acrylate and polyimide coated optical fibers are examined and interpreted in terms of the coatingss ability to resist high temperature and its compatibility with resin matrix. The strain monitoring during the pultrusion of composite parts using the embedded fiber optic sensors was performed. The strain readings from the sensors and the extensometer were compared in mechanical tensile tests.

Processing and characterization of smart composite reinforcement

The issues of processing and characterization of pultruded smart composite reinforcements with the embedded fiber optic sensors are discussed. These fiber reinforced polymer reinforcements incorporate the optical fiber sensors to provide a strain monitoring of structures. The required modification of the pultrusion processing technology to allow for the incorporation of fiber optic sensors is developed. Fabry Perot and Bragg Grating optical strain sensors were chosen due to their small size and excellent sensitivity. The small diameter of the sensor and optical fiber allow them to be embedded without adversely affecting the strength of the composite. Two types of reinforcement with vinylester resin were used to produce the experimental 9.5 mm diameter rods. The reinforcements were carbon and E-glass fibers. In order to fully characterize the pultrusion process, it was decided to subject the strain sensors separately to each of the variables pertinent to the pultrusion process. Thus, sensors were used to monitor strain caused by compaction pressure in the die, compaction pressure plus standard temperature profile, and finally compaction pressure plus temperature plus resin cure (complete pultrusion process). A strain profile was recorded for each experiment as the sensor travelled through the pultrusion die, and for the cool-down period after the sensor had exited the die.

Micromechanical modeling of smart composite materials with a periodic structure

Comprehensive micromechanical models for smart composite materials with a periodic structure are derived and effective elastic, actuation, thermal expansion and hygroscopic expansion coefficients pertaining to these structures are obtained. The actuation coefficients characterize the intrinsic nature of adaptive structures that can be used to induce strains and stresses in a controlled manner. The effective coefficients replace the rapidly oscillating coefficients inherent to the differential equations that govern the behavior of smart anisotropic materials with a regular array of reinforcements and actuators. The mathematical framework employed is that of asymptotic homogenization that permits the determination of the effective coefficients through solution of unit cell problems. The unit cell problems are shown to be independent of the global boundary value problem. It is implicit of course that the physical model based on these coefficients should give predictions differing as little as possible from those of the original problem. Once determined, the effective coefficients can be utilized in studying different types of boundary value problems associated with a given structure. The effectiveness of the derived models and the use of the effective coefficients is illustrated by means of various two- and three-dimensional examples associated with periodic laminates.