Charles E. Bakis

Self-monitoring, pseudo-ductile, hybrid FRP reinforcement rods for concrete applications

Abstract The feasibility of hybrid -fiber-reinforced-polymer rods that demonstrate the important safety features of self-monitoring capability and pseudo-ductility is demonstrated. The rods are intended to be the basis of improved pultruded reinforcements for concrete or other civil applications where safety is of critical importance. The lowest elongation fiber in the seven rods investigated is carbon, which by virtue of its piezoresistivity allows the monitoring of deformation and fracture throughout an entire rod with simple electronic equipment. Resistance measurements obtained during quasi-static tests clearly reveal failure of the carbon fibers. Following this easily detected event, higher loads can be safely sustained by the remaining high-elongation fibers if the carbon tows are dispersed in the cross-section rather than concentrated in one location.

Variable Stiffness Structures Utilizing Fluidic Flexible Matrix Composites

In this research, the capability of utilizing fluidic flexible matrix composites (F2MC) for autonomous structural tailoring is investigated. By taking advantage of the high anisotropy of flexible matrix composite (FMC) tubes and the high bulk modulus of the pressurizing fluid, significant changes in the effective modulus of elasticity can be achieved by controlling the inlet valve to the fluid-filled F2MC structure. The variable modulus F2MC structure has the flexibility to easily deform when desired (open-valve), possesses the high modulus required during loading conditions when deformation is not desired (closed-valve — locked state), and has the adaptability to vary the modulus between the flexible/stiff states through control of the valve. In the current study, a 3D analytical model is developed to characterize the axial stiffness behavior of a single F 2MC tube. Experiments are conducted to validate the proposed model, and the test results show good agreement with the model predictions. A closed/open modulus ratio as high as 56 times is achieved experimentally. With the validated model, an F2MC design space study is performed. It is found that by tailoring the properties of the FMC tube and inner liner, a wide range of moduli and modulus ratios can be attained. By embedding multiple F 2MC tubes side by side in a soft matrix, a multi-cellular F2MC sheet with a variable stiffness in one direction is constructed. The stiffness ratio of the multi-cellular F2MC sheet obtained experimentally shows good agreement with a model developed for this type of structure. A case study has been conducted to investigate the behavior of laminated [+60/0/-60] s multi-cellular F2MC sheets. It is shown that the laminate can achieve tunable, steerable, anisotropy by selective valve control.

The interfacial strength of carbon nanofiber epoxy composite using single fiber pullout experiments.

Carbon nanotubes and nanofibers are extensively researched as reinforcing agents in nanocomposites for their multifunctionality, light weight and high strength. However, it is the interface between the nanofiber and the matrix that dictates the overall properties of the nanocomposite. The current trend is to measure elastic properties of the bulk nanocomposite and then compare them with theoretical models to extract the information on the interfacial strength. The ideal experiment is single fiber pullout from the matrix because it directly measures the interfacial strength. However, the technique is difficult to apply to nanocomposites because of the small size of the fibers and the requirement for high resolution force and displacement sensing. We present an experimental technique for measuring the interfacial strength of nanofiber-reinforced composites using the single fiber pullout technique and demonstrate the technique for a carbon nanofiber-reinforced epoxy composite. The experiment is performed in situ in a scanning electron microscope and the interfacial strength for the epoxy composite was measured to be 170 MPa.

Flexible Matrix Composite Skins for One-dimensional Wing Morphing

Morphing aircraft wings require flexible skins that undergo large strains, have low in-plane stiffness, and high out-of-plane stiffness to carry aerodynamic loads. For some morphing applications deformation and low stiffness in the flexskin is required in one direction. In these cases, a flexible matrix composite (FMC) skin is proposed as a possible solution. A FMC comprises of stiff fibers embedded in a soft, high-strain capable matrix material. The matrix-dominated direction is aligned with the morphing direction. This allows the skin to undergo large strain at low energy cost. However, the high-stiffness in the fiber-dominated direction allows application of pretension along this direction, without rupture, and is critical for the membrane skin to carry out-of-plane pressure loads without excessive deformation. An analysis for a FMC skin panel is developed and validated against experiment. The analysis is used to conduct design studies. Comparison of the FMC skin to a matrix-only skin illustrates the importance of the fiber’s stiffness in tolerating pretension and limiting out-of-plane deformation under load. The other dominant parameter that limits out-of-plane deformation is panel size, with smaller lengths in the non-morphing direction proving beneficial. In general, fiber and matrix modulus has limited effect on out-of-plane deformation of flexskin panels.

Fibrillar Network Adaptive Structure with Ion-transport Actuation

The overall objective of this research is to create a new actuation system, emulating the ability of plants to generate large strains while carrying significant structural loads. Specifically, the authors aim to create high-authority active structures by exploring a revolutionary combination of two innovative ideas inspired by the mechanical, chemical, and electrical properties of the plants. The first idea, inspired by the fibrillar network in plant cell walls, is to create a high-mechanical-advantage actuator structure based on flexible matrix composites (FMCs). Through fiber—matrix tailoring of FMC tubes, one can cause the structure to actuate in certain desired directions when pressurized. Second, the actuator concept is combined with a novel electroosmotic (EO) transport mechanism to regulate pressure inside the FMC tube, inspired by the ion-transport and volume-control phenomena in plant cells. By adjusting the applied voltage across a charged porous membrane, one can control the internal pressure and actuator response. The performance of the system (pressure, response time, stroke, load, etc.) can be tuned by proper selection of the membrane (e.g., pore size, surface charge, membrane pore area, etc.) and FMC (materials, fiber angle, etc.) properties. The new system can use natural seawater (ideal for naval applications) or a small amount of onboard solution with appropriate properties for electroosmotic pumping. This approach has several advantages over traditional actuators, such as large stroke/force, design flexibility/scalability, and electrical activation with quiet operation and no moving parts. In this research, the FMC structure and EO pump (EOP) models are developed and validated, and the integrated model is analyzed to provide guidelines for designing the overall actuation system.

Test Methods for FRP-Concrete Systems Subjected to Mechanical Loads: State of the Art Review

This report addresses the methodology for determining the long-term behavior of bridge structures reinforced with fiber reinforced plastic (FRP) rods. In particular, a review of the existing literature on what has been done and what needs to be done for the development of accelerated test methods, with emphasis on mechanical loads, is presented. The focus of the report is on new, bonded FRP reinforcement for concrete bridge structures. Companion reports address stand-alone FRP systems subjected to mechanical loads, and stand-alone FRP and FRP-concrete systems subjected to environmental loads.

Analysis of bonding mechanisms of smooth and lugged FRP rods embedded in concrete

Abstract The bond behavior of two fundamental types of FRP reinforcement for concrete—smooth rods and lugged rods—has been investigated by examining experimental data and developing detailed finite-element models of these simple rod/concrete systems. The bond controlling parameters, identified by direct pull-out tests on embedded rods, were built into the models so that analytical studies of the effect of each parameter on bond could be carried out. The viability of the finite-element models, created by the use of commercial finite-element programs, was verified by comparing the predicted and experimentally obtained load versus slip and/or load versus longitudinal strain data. The models were used to predict the behavior of environmentally degraded rods. The moire method of strain analysis was used to investigate full-field, local strain distributions in flat, lugged rods embedded on the surface of concrete.

Damage detection and conductivity evolution in carbon nanofiber epoxy via electrical impedance tomography

Utilizing electrically conductive nanocomposites for integrated self-sensing and health monitoring is a promising area of structural health monitoring (SHM) research wherein local changes in conductivity coincide with damage. In this research we conduct proof of concept investigations using electrical impedance tomography (EIT) for damage detection by identifying conductivity changes and by imaging conductivity evolution in a carbon nanofiber (CNF) filled epoxy composite. CNF/epoxy is examined because fibrous composites can be manufactured with a CNF/epoxy matrix thereby enabling the entire matrix to become self-sensing. We also study the mechanisms of conductivity evolution in CNF/epoxy through electrical impedance spectroscopy (EIS) testing. The results of these tests indicate that thermal expansion is responsible for conductivity evolution in a CNF/epoxy composite.

Damage detection via electrical impedance tomography in glass fiber/epoxy laminates with carbon black filler

The conductivity of glass fiber reinforced polymers with nanocomposite matrices can be leveraged for structural health monitoring. Since nanocomposite matrices depend on well-connected networks of conductive nanofillers for electrical conductivity, matrix damage will sever the connection between fillers and result in a local conductivity loss. Monitoring composite conductivity changes can therefore give insight into the state of the matrix. Existing conductivity-based structural health monitoring methods are either insensitive to matrix damage or employ large electrode arrays. This research advances the state of the art by combining the superior imaging capabilities of electrical impedance tomography with conductive networks of nanofillers in the composite matrix. Electrical impedance tomography for damage detection in glass fiber/epoxy laminates with carbon black nanocomposite matrices is characterized by identifying a lower threshold of through-hole detection, demonstrating the capability of electrical impedance tomography to accurately resolve multiple through holes, and locating impact damage. It is found that through holes as small as 3.18 mm in diameter can be detected, and electrical impedance tomography can detect multiple through holes. However, sensitivity to new through holes is diminished in the presence of existing through holes unless a damaged baseline is used. Finally, it is shown that electrical impedance tomography is also able to accurately locate impact damage. These research findings demonstrate the considerable potential of conductivity-based health monitoring for glass fiber reinforced polymer laminates with conductive networks of nanoparticles in the matrix.

Effects of interfacial friction on the damping characteristics of composites containing randomly oriented carbon nanotube ropes

This article presents a model for describing the damping characteristics of uniaxially stressed polymeric composites filled with randomly oriented single-wall nanotube (SWNT) ropes. A close-packed lattice consisting of seven nanotubes in hexagonal array is used to present the nanoropes. The composite is described as a three-phase system composed of a resin, a resin sheath acting as a shear transfer zone, and SWNT ropes. The concept of ‘stick-slip’ motion caused by frictional contacts is proposed to describe the load transfer behavior between individual nanotubes and between a nanotube rope and a sheath. The results of the analytical study show that both the Young’s modulus and the loss factor of the composite are sensitive to stress magnitude. Also, to show the inter-tube sliding effects due to nanotube aggregation, the Young’s moduli and the loss factors of composites filled with aligned SWNTs, aligned nanoropes, and randomly oriented nanoropes are compared.