Due to their unique properties, carbon fiber reinforced polymers (CFRP) are becoming ever more prevalent in today's society. Unfortunately, CFRP suffer from a wide range of failure modes and structural health monitoring methods are currently insufficient to predict these failures. It is apparent that self-sensing structural health monitoring could be advantageous to protect consumers from catastrophic failure in CFRP structures. Previous research has shown that embedded nickel nanostrand nanocomposites can be used to instantaneously measure strain in carbon fiber composites, but these methods have been severely limited and can induce high stress concentrations that compromise the structural integrity of the carbon fiber structure. In this research the strain sensor material and the connective circuitry to the sensor are analyzed to improve the practicality of in situ strain sensing of carbon fiber structures. It has been found that the use of nickel nanostrands embedded directly onto carbon fiber as a strain sensor material has no advantages over a carbon fiber strain sensor alone. Additionally, it has been shown that the circuitry to the strain sensor plays a critical role in obtaining a strong, consistent piezoresistive signal that can be related to strain. The use of nickel coated carbon fiber in the circuitry has been evaluated and shown to reduce the noise in a piezoresistive signal while allowing for remote strain sensing from greater distances away from the strain location. The piezoresistive strain sensing utilized in the tested sensor designs relies on electrons tunneling through an insulting barrier between two conductors. This phenomenon is known as quantum tunneling. Two factors - tunneling barrier height and gap distance - affect the probability of quantum tunneling occurring. Thus, to accurately model and predict the piezoresistivity of nanocomposites these two parameters must be known. Through the use of dielectric spectroscopy the gap distance can be determined. Using nanoindenting, the barrier height for various polymers was also determined. The measured values can be used, in future work, to improve the modeling of nickel nanostrand nanocomposite.

No description available.

This thesis presents the design of multifunctional structures through the optimal placement of nanomaterial additives. Varying the concentration of Carbon Nanotubes (CNTs) in a polymer matrix affects its local effective properties, including mechanical stiffness, electrical conductivity, and piezoresistivity. These local properties in turn drive global multifunctional performance objectives. A topology optimization algorithm determines the optimal distribution of CNTs within an epoxy matrix in an effort to design a set of structures that are capable of performing some combination of mechanical, electrical, or peizoresistive functions. A Pareto-Based Restart Method is introduced and may be used within a multi-start gradient based optimization to obtain well defined multiobjective Pareto Fronts. A linear design variable filter is used to limit the influence of checkerboarding. The algorithm is presented and applied to the design of beam cross-sections and 2D plane stress structures. It is shown that tailoring the location of even a small amount of CNT (as low as 2 percent and as high as 10 percent, by volume) can have significant impact on stiffness, electrical conductivity, and strain-sensing performance. Stiffness is maximized by placing high concentrations of CNT in locations that either maximize the bending rigidity or minimize stress concentrations. Electrical conductivity is maximized by the formation of highly conductive paths between electrodes. Strain-sensing is maximized via location of percolation volume fractions of CNTs in high strain areas, manipulation of the strain field to increase the strain magnitude in these areas, and by avoiding negative contributions of piezoresistivity from areas with differing net signed strains. It is shown that the location of the electrodes can affect sensing performance. A surrogate model for simultaneous optimization of electrode and topology is introduced and used to optimize a 2D plane stress structure. This results in a significant increase in sensing performance when compared to the fixed-electrode topology optimization. / Ph. D. / This dissertation presents a method that allows for the best placement of a limited amount of filler material within a base matrix material to form an optimal composite structure. Adding filler material, in this case Carbon Nanotubes, can change the effective behavior of the composite structure, enhancing the capabilities of the base matrix material by adding structural stiffness, electrical conductivity, and even the ability for the structure to measure its own strains. The degree to which these changes occur is dependent on the amount of filler material present in any given subsection of the structure. The method then is focused on determining how much of the filler to place in different subsections of the structure to maximize several measures of performance. These measures pertain to structural performance, electrical conductivity, and the structure’s ability to sense strains. Steps are taken within the method to remove non-physical designs and also to find the overall best design, called the global minima. The method is applied to several test structures of varying complexity, and it is shown that the optimization method can heavily influence performance by tailoring the filler material distribution. Further electrical and sensing performance gains can be obtained by properly selecting where the electrodes are located on the structure. This is demonstrated by including electrode placement in the design method along with the filler distribution.

The current state of the art in structural health monitoring is primarily reliant on sensing deformation of structures at discrete locations using sensors and detecting damage using techniques such as X-ray, microCT, acoustic emission, impedance methods etc., primarily employed at specified intervals of service life. There is a need to develop materials and structures with self-sensing capabilities such that deformation and damage state can be identified in-situ real time. In the current work, the inherent deformation and damage sensing capabilities of carbon nanotube (CNT) based nanocomposites are explored starting from the nanoscale electron hopping mechanism to effective macroscale piezoresistive response through finite elements based computational micromechanics techniques. The evolution of nanoscale conductive electron hopping pathways which leads to nanocomposite piezoresistivity is studied in detail along with its evolution under applied deformations. The nanoscale piezoresistive response is used to evaluate macroscale nanocomposite response by using analytical micromechanics methods. The effective piezoresistive response, obtained in terms of macroscale effective gauge factors, is shown to predict the experimentally obtained gauge factors published in the literature within reasonable tolerance. In addition, the effect of imperfect interface between the CNTs and the polymer matrix on the mechanical and piezoresistive properties is studied using coupled electromechanical cohesive zone modeling. It is observed that the interfacial separation and damage at the nanoscale leads to a larger nanocomposite irreversible piezoresistive response under monotonic and cyclic loading because of interfacial damage accumulation. As a sample application, the CNT-polymer nanocomposites are used as a binding medium for polycrystalline energetic materials where the nanocomposite binder piezoresistivity is exploited to provide inherent deformation and damage sensing. The nanocomposite binder medium is modeled using electromechanical cohesive zones with properties obtained through the Mori-Tanaka method allowing for different local CNT volume fractions and orientations. Finally, the traditional implementation of Material Point Method (MPM) is extended for composite problems with large deformation (e.g. large strain nanocomposite sensors with elastomer matrix) allowing for interfacial discontinuities appropriately. Overall, the current work evaluates nanocomposite piezoresistivity using a multiscale modeling framework and emphasizes through a sample application that nanocomposite piezoresistivity can be exploited for inherent sensing in materials. / Ph. D.

The goals of this research are to develop and validate the use of distributed fibre optic sensors for use in strain monitoring of buried culverts, and to use full-scale experiments to evaluate the performance of both deteriorated steel and reinforced concrete culverts rehabilitated with grouted slipliners subjected to surface loading. Bench scale experiments were conducted to evaluate the use of fibre optic sensors against conventional strain sensors. Then, fibre optic sensors were attached to a full-scale culvert that was tested in a buried state as a proof of concept. Finally, fibre optic sensors were used in two large scale buried pipe tests to explore the performance of rehabilitated flexible and rigid culverts. A deteriorated steel culvert was tested in a buried state under surface loading, then rehabilitated with a grouted high density polyethylene (HDPE) slipliner while still in a buried state and tested under surface loading at 0.9 m and 0.6 m burial depths. The rehabilitated steel pipe was tested under service loading, and up to 1250 kN of applied load. The results suggested that the grouted annulus stiffened the overall structure, and increased the capacity of the system to over 3 times the fully factored design load. A deteriorated reinforced concrete culvert was tested and rehabilitated in a similar fashion. The grout in the annulus penetrated the cracks at the crown, invert and joint of the concrete pipeline. The lined concrete pipe was tested to 1200 kN under single axle loading, and to 800 kN under single wheel loading. The results suggested that while the concrete pipe was stiffened by the grout, it remained the primary contributor to structural capacity, with the liner contributing little to the capacity. Repair reduced the diameter change by an average of 90%, with the capacity reaching approximately 3.3 and 4.2 times the design loads for single axle and single wheel pair loading, respectively. The maximum response was under single axle loading over the barrels of the concrete pipe. In no instance did the structures reach an ultimate limit state, and the tests were stopped after bearing failure of the soil occurred. / Thesis (Master, Civil Engineering) -- Queen's University, 2013-11-28 17:24:50.815

Παρατηρείται ότι, το ΙΑΜ έχει ήδη εδρεώσει τη θέση του ανάμεσα στα δομικά υλικά. Όμως για να μπορεί να χαρακτηριστεί ως πολυλειτουργικό υλικό, θα πρέπει να παρέχει και άλλες λειτουργίες μη δομικής φύσεως. Η διατριβή αυτή, επικεντρώθηκε στην ανίχνευση βλάβης σε στοιχεία ΙΑΜ μέσω διηλεκτρικών μετρήσεων και πιο συγκεκριμένα μέσω της μεταβολής της ηλεκτρικής αντίστασης στα στοιχεία αυτά. Τα αποτελέσματα των πειραματικών δοκιμών θα χρησιμοποιηθούν ώστε να θέσουν τις βάσεις για κατάλληλους συντελεστές συσχέτισης μεταξύ της εξέλιξης της βλάβης και της πιεζοαντίστασης σε στοιχεία ΙΑΜ. / This thesis presents the preliminary results of an ongoing experimental program aiming at assessing the piezoresistivity of carbon textile reinforced concrete dumbbell specimens under monotonic tensile loading, along the direction of loading. During testing both longitudinal strain and longitudinal electrical resistivity were recorded; electrical resistivity measurements were realized using a high-precision multimeter. The results of this experimental campaign may be used for setting the ground for establishing appropriate correlation factors between damage progression and piezoresistivity properties for TRC elements.

Multifunctional materials both with damping properties and strain sensing properties are very important. They promise to be more weight-efficient, and provide volume-efficient performance, flexibility and potentially, less maintenance than traditional multi-component brass-board systems. The goal of this dissertation work was to design, synthesize, investigate and apply polyaniline/Multiwall carbon nanotube (PANI/MWCNT) and polyurethane (PU) /MWCNT composites films for structural vibration control and strain sensors using free layer damping methods and static and dynamic strain sensing test methods. The PANI/MWCNT was made by in situ polymerization of PANI in the presence of MWCNT, then frit compression was used to make circular and rectangular PANI/MWCNT composite films. PU/MWCNT composites were made by the layer-by-layer method. Free end vibration test results showed both of PANI/MWCNT and PU/MWCNT have better damping ratios than each of their components. Static sensing test indicated that though there appears to be residual strain in both composite sensors after the load is removed, both the sensor and the foil strain gage react linearly when re-engaged. A drift test of the sensor showed that it is stable. The dynamic sensing test results showed that over the range of 10-1000 Hz, the PANI/MWCNT composite sensor was consistently superior to foil strain gage for sensing purposes since the highest peak consistently corresponded to the input frequency and was much higher, for example, at 20 Hz, 820 times higher than those of the strain gage. Using the same criterion, the PU/Buckypaper composite sensor was comparable to or superior to the foil strain gage for sensing purposes over the range of 10 Hz to 200 Hz. The relationship of loss factor, η, and beam coverage length, L1/L, is discussed for single sided and double sided attachment. For both PANI/MWCNT and PU/MWCNT, the loss factor, η, was found to increase as coverage length, L1/L, increases. The loss factor, η, was found to have a maximum as with coverage length, L1/L, as the coverage length continues to increase. The trend for double sided attachment was found to follow the trends discussed by Rao (2004) and Levy and Chen (1994) for viscoelastic material constrained damping.

Bohrlochintegrität ist unerlässlich für die erfolgreiche und nachhaltige Produktion und Injektion von Fluiden aus Reservoirgesteinen, wie beispielsweise bei der Nutzung von Kohlenwasserstoffen, Geothermie oder Standorten für geologische Speicherung. Um die Integrität einer Bohrung über seine Lebenszeit zu gewährleisten, ist vor allem eine erfolgreiche primäre Komplettierung nötig. Besonders die Zementation der Rohre stellt dabei ein großes Risiko dar, weil durch die natürlichen Gegebenheiten im Bohrloch viele Faktoren Einfluss auf die Zusammensetzung und Verteilung der Zementsuspension haben. Diese Studie untersucht das Monitoring-potenzial von faseroptischer ortsverteilter Dehnungsmessung DSS (distributed strain sensing). Ergänzend zu faseroptischer ortsverteilten Temperaturmessung DTS (distributed temperature sensing), welche seit mehr als zwei Jahrzehnten in der Industrie Anwendung findet, kann jeder Ort einer Glasfaser zusätzlich Informationen über den mechanischen Spannungszustand geben. Experimentelle und analytische Arbeiten wurden durchgeführt, um die Auswirkung von Laständerungen auf einer Faser zu quantifizieren. Desweiteren wurde der Einfluss komplexer mehrschichtiger Bohrlochkabel auf Dehnungsmessergebnisse untersucht. Ein faseroptisches Messkabel wurde im Zuge dieser Arbeit im Ringraum entlang der Produktionsrohrtour einer Bohrung installiert. Die gemessenen Geländedaten zeigen Ergebnisse aus zwei Arbeitsschritten der Fertigstellung der Bohrung - der Filterverkiesung und der Zementation. Aufgrund der Dichtedifferenz von Kies und Bohrspülung wurde am Kabel ein Dehnungseffekt gemessen. Die Teufe, in welcher der Dehnungseffekt auftritt, korreliert mit Wireline Gamma-Gamma-Dichtedaten, welche im gleichen Zeitfenster gemessen wurden. Die anschliessende Kompaktion des Kieskopfes wurde durch das Glasfaserkabel in Form einer zunehmenden mechanischen Belastung erfasst. Während der anschliessenden Zementation der Rohrtour wurde ein Dehnungseffekt in der Mischzone von Flüssigkeiten mit unterschiedlichen rheologischen Eigenschaften gemessen. Anhand eines Experiments konnte bestätigt werden, dass fluidrheologische Parameter (wie die Fluidviskosität) mit einem faseroptischen Messkabel quantifiziert werden können. Hierfür werden Fluidscherspannungen gemessen, welche durch das Fliessen von Fluiden an der Kabeloberfläche hervorgerufen werden (amtliches Zeichen zur Patentanmeldung: EP 19171265.2). DSS-Messungen erweiten das Verständnis von Fluidverdrängungsvorgängen in Bohrlöchern und ermöglichen eine Beurteilung von Komplettierungsvorgängen in Echtzeit. / Borehole integrity is fundamental for the successful and sustainable utilization of hydrocarbons, geothermal energy and sites for geological storage. The success of the primary well completion is necessary to ensure the integrity of a well over its lifetime. In particular, the casing cementation represents a great risk because many factors have an influence on the composition and distribution of the cement suspension due to the natural conditions in the borehole. This study investigates the monitoring potential of fiber-optic distributed strain sensing (DSS) using a measurement cable which is installed in the annulus of a well. Similar to distributed temperature sensing (DTS), which is used for temperature monitoring in industry applications for more than two decades, fibers additionally convey information about their mechanical stress state. Laboratory as well as analytical work was performed to quantify the effect of load changes on a fiber. In addition, the influence of complex multilayered downhole cable on the strain response is examined. The presented field data shows results from two stages of the well completion - the gravel packing and the cementation. Due to the difference in density of gravel and drilling fluid, a deformation is measured on the cable. The depth at which the stretching effect occurs correlates with wire-line gamma-gamma density data measured in the same time window. The subsequent compaction of the gravel head, which was not revealed by the logging measurement, was detected by the fiber optic cable in the form of an increasing mechanical load on the cable. During cement pumping, fluid shear stresses create a measurable load on the cable, especially in the mixing zone of liquids with dfferent rheological properties. Based on this observation, an experiment was designed and conducted which aims at measuring fluid rheological parameters such as fluid viscosity. For this purpose, the fluid shear stresses acting on the fiber optic sensing cable in the flow path are measured (patent application number: EP 19171265.2). DSS measurements extend the understanding of fluid displacements in wellbores and allow an assessment of well completion process in real time.

A fatigue test has been performed on 7075-T651 aluminum specimens which were bonded with polyimide coated optical fibers with discrete Bragg gratings. These fibers were bonded with AE-10 strain gage adhesive. The results indicate that lower strain amplitudes do not produce cause for concern, but that larger strain amplitudes (on the order of 3500 μ) may cause some sensors to become unreliable. The strain response of acrylate coated optical fiber strain sensors bonded to aluminum specimens with AE-10 and M-Bond 200 strain gage adhesives was investigated with both axial and cantilever beam tests. These results were compared to both the strain response of conventional strain gages and to model predictions. The results indicate that only about 82.6% of the strain in the specimen was transferred through the glue line and fiber coating into the fiber. Thus, multiplying by a strain transfer factor of approximately 1.21 was sufficient to correct the optical fiber strain output. This effect was found to be independent of the adhesive used and independent of the three-dimensional profile of the glue line used to attach the fiber. Finally, this effect did not depend on whether the fiber had a polyimide or an acrylate coating. Further investigation was conducted on the feasibility of using optical fiber strain sensors for monitoring subcritical damage (such as matrix cracks) in fiber reinforced composite materials. These results indicate that an array of optical fibers which monitor the strain profile on both sides of a composite panel may be sufficient for these purposes / Master of Science

At present, structural health monitoring efforts focus primarily on the sensors and sensing systems for detecting instances and locations of damage through techniques such as X-ray, micro CT, acoustic emission, infrared thermography, lamb wave etc., which only detect cracks at relatively large length scales and rely heavily on sensors and sensing systems which are external to the material system. As an alternative to conventional commercially available SHM techniques, the current work explores processing-structure-property relationships starting from carbon nanotube (CNT) based nanocomposites to particulate composites with nanocomposite binder/matrix materials, i.e. hybrid particulate composites to investigate deformation and damage sensing capabilities of inherently sensing materials and structures through their piezoresistive (coupled electro-mechanical) response. Initial efforts focused on controlling the dispersion of CNTs and orientation of CNT filaments within nanocomposites under dielectrophoresis to guide design and fabrication process of nanocomposites by tuning CNT concentration, applied AC electric field intensity, frequency and exposure time. It is observed that a combination of exposure time to AC electric field and the AC field frequency are the key drivers of filament width and spacing and that the network for filament formation is much more efficient for pristine CNTs than for acid treated functionalized CNTs. With the knowledge obtained from controlling the morphological features, AC field-induced long range alignment of CNTs within bulk nanocomposites was scaled up to form structural test coupons. The morphology, electrical and mechanical properties of the coupons were investigated. The anisotropic piezoresistive response both for parallel and transverse to CNT alignment direction within bulk composite coupons under various loading conditions was obtained. It is observed that control of the CNT network allows for the establishment of percolation paths and piezoresistive response well below the nominal percolation threshold observed for random, so called well-dispersed CNT network distributions. The potential for use of such bulk nanocomposites in SHM applications to detect strain and microdamage accumulation is further demonstrated, underscoring the importance of microscale CNT distribution/orientation and network formation/disruption in governing the piezoresistive sensitivities. Finally, what may be the first experimental study in the literature is conducted for real-time embedded microscale strain and damage sensing in energetic materials by distributing the CNT sensing network throughout the binder phase of inert and mock energetic composites through piezoresistive response for SHM in energetic materials. The incorporation of CNTs into inert and mock energetic composites revealed promising self-diagnostic functionalities for in situ real-time SHM applications under quasi-static and low velocity impact loading for solid rocket propellants, detonators and munitions to reduce the stochastic nature of safety characterization and help in designing insult tolerant energetic materials. / Ph. D.