Tarik J. Dickens

Development of a triboluminescence-based sensor system for concrete structures

The triboluminescence phenomenon has been proposed as a sensor system for detecting and monitoring damage in aerospace and civil infrastructure systems (CIS). While significant work is being done in developing such systems for aerospace structures, little or no work is being done in developing triboluminescence-based sensor systems for the critical and aging CIS. This article reports some findings in the work being done by the authors to develop such a sensor system for civil infrastructure applications. A ZnS:Mn-based cementitious patch that emits light when stressed or fractured was developed and its triboluminescence performance under loading characterized. The results show that a ZnS:Mn concentration level of 10% gives the best triboluminescence response without adversely affecting the compressive strength of the patch, while also minimizing the use of the expensive ZnS:Mn crystals. The triboluminescence response increased as the concentration of ZnS:Mn in the system increased. The highest triboluminescence response was obtained at a concentration level of 25% but resulted in significant reduction in the system’s compressive strength. Nonetheless, the presence of ZnS:Mn affects the hydration process by slowing down the conversion of the needle-shaped crystals of calcium sulfoaluminate hydrate (ettringites) into the monosulfate hydrate that makes concrete vulnerable to sulfate attack.

Enabling damage detection: manufacturing composite laminates doped with dispersed triboluminescent materials

Triboluminescent materials are being harnessed to address the gaps in current structural health monitoring systems. Their innate ability to emit light when stressed or broken makes them ideal candidates for the ubiquitous and in situ monitoring of structures. The increasing use of advanced composites in critical structures, where subsurface damage initiation may go unnoticed, further highlights the urgency in developing efficient online monitoring technologies. This work looked at the manufacturing of composite laminates that have been doped with various concentrations (0 to 10 %wt.) of a triboluminescent material (ZnS:Mn). Laminates were manufactured using a vacuum infusion process. Dispersing the ZnS:Mn particulates was cumbersome because their density was higher than the resin that caused settling during resin infusion. The dispersion of ZnS:Mn is critical to their use in the health monitoring of the host structure. As such, a method for mechanical agitation using a rotational vacuum infusion apparatus was developed employing centrifugal motion. The degree of dispersion in the resulting laminates was determined using scanning electron microscopy and the energy dispersive scanning feature of the electron microscope for elemental mapping. A quantitative metric was established by computations of the Euclidean distance of EDS mapping. Studies of the effect of ZnS:Mn concentration on the tensile strength of laminates showed that increasing the ZnS:Mn concentration reduced the tensile strength. Key processing parameters were studied, and determined that curing kinetics were not altered by ZnS:Mn inclusion.

Getting light through cementitious composites with in situ triboluminescent damage sensor

Triboluminescent damage sensors comprising highly efficient triboluminescent materials could allow simple, real-time monitoring of both the magnitude and location of damage. The inability to effectively capture and transmit the triboluminescent optical signals generated within opaque composites like concrete has, however, limited their damage monitoring applications. The in situ triboluminescent optical fiber sensor has been developed to enable the detection and transmission of damage-provoked triboluminescent emissions without having to position triboluminescent crystals in the host material. Flexural tests were performed on mortar and reinforced concrete beams having the in situ triboluminescent optical fiber sensor integrated into them. The intrinsic triboluminescent signals generated in the beams under loading were successfully transmitted through the optical fibers to the photomultiplier tube by side coupling. Successful side coupling will make a truly distributed in situ triboluminescent optical fiber sensor possible when the entire length of the sensor is mostly covered with the triboluminescent composite coating. The results show the viability of the in situ triboluminescent optical fiber sensor for the structural health monitoring of cementitious composites. Real-time failure detection was demonstrated in unreinforced mortar beams, while real-time damage (crack) detection was demonstrated in reinforced concrete beams. Preliminary work on reinforced concrete beams showed that the integrated in situ triboluminescent optical fiber sensor was able to detect multiple cracks caused by loading, thereby providing early warning of structural degradation before failure.

Engineering Crack Formation in Carbon Nanotube-Silver Nanoparticle Composite Films for Sensitive and Durable Piezoresistive Sensors.

We report highly sensitive and reliable strain sensors based on silver nanoparticle (AgNP) and carbon nanotube (CNT) composite thin films. The CNT/AgNP was prepared by a screen printing process using a mixture of a CNT paste and an AgNP ink. It is discovered that the sensitivity of such sensors are highly dependent on the crack formation in the composites. By altering the substrate use and the relative ratios of AgNPs and CNTs, the formation and propagation of cracks can be properly engineered, leading to piezoresistive strain sensors with enhanced sensitivity and robustness.

Tailoring the photocatalytic reaction rate of a nanostructured TiO2 matrix using additional gas phase oxygen

Nanostructured TiO2 was synthesized by the sol–gel method. The titania was supported on nanoporous poly(styrene-co-divinylbenzene) (PS). The samples were characterized by several techniques (scanning electron microscopy, energy-dispersive spectroscopy, X-ray diffraction, and ultraviolet–visible spectroscopy). Three types of TiO2 samples were prepared using various temperatures and were studied for the photocatalytic degradation of methylene blue. The photocatalytic efficiency of TiO2 was observed to increase by activating the TiO2 surface using nanoporous PS. The photocatalytic performance of the synthesized samples showed a higher performance using molecular O2, which was purged through the reactor.

Toward Triboluminescent Sensor Realization for SHM: Statistical Modeling of Triboluminescent Composites

Triboluminescence (TL) is a mechanical and luminescent phenomena enabling damage sensing capabilities in materials. Depending on material compound, various excitation mechanisms result in emissions stimulated by rubbing or fracture, and give an indication of internal stress. Design of Experiments helped ascertain experimental knowledge of the multiphase composite system containing ZnS:Mn phosphors (0 - 40%) and vinyl ester resin (VER). This statistical approach proffered an empirical model used to validate triboluminescent production. Data shows concentration compiled with impact energy has a significant effect on the luminous intensity. Light intensity was measured by a photomultiplier tube and a photo-voltaic detector. The signal intensity range was determined for each. The photovoltaic detector acts as a low-light sensor in the range of 0.61 - 0.116 A for impacts less than 0.4 J. Microscopy revealed plates with reasonable dispersion and view of micro-structural inclusions. DMA indicates the inclusion of ZnS:Mn produces a moderate change in Youngs modulus and thermo-kinetic properties.

Triboluminesence multifunctional cementitious composites with in situ damage sensing capability

Structural health monitoring of civil infrastructure systems like concrete bridges and dams has become critical because of the aging and overloading of these CIS. Most of the available SHM methods are not in-situ and can be very expensive. The triboluminescence multifunctional cementitious composites (TMCC) have in-built crack detection mechanism that can enable bridge engineers to monitor and detect abnormal crack formation in concrete structures so that timely corrective action can be taken to prevent costly or catastrophic failures. This article reports the fabrication process and test result of the flexural characterization of the TMCC. Accelerated durability test indicated that the 0.5 ZnS:Mn/Epoxy weight fraction ITOF sensor configuration to be more desirable in terms of durability. The alkaline environment at the highest temperature investigated (45 °C) resulted in significant reduction in the mean glass transition and storage moduli of the tested ITOF thin films. Further work is ongoing to correlate the TL response of the TMCC with damage, particularly crack opening.

Mimicking the human nervous system with a triboluminescence sensory receptor for the structural health monitoring of composite structures

The human nervous system (HNS) provides one of the most advanced examples of how to monitor the structural state of a complex system. In attempts to mimic the HNS, a key component has been the development of the sensory receptor. This paper reports on the development of a triboluminescence (TL)-based sensory receptor that converts mechanical energy from fatigue or impact loads and cracks propagation, into optical signals. This sensor system has potential for wireless, in-situ and distributed sensing (WID). The approach differs from existing fiber optic methods in that it does not require any external light source to function. The optical signal is generated through mechanical excitation of the highly triboluminescent ZnS:Mn. It is then transmitted through optical fibers to photomultiplier tubes (PMT) for detecting, quantifying and locating (with further analysis), intrinsic damage in critical engineering structures like concrete bridges. The TL sensory receptor consists of a sensitized portion of a polymer optical fiber (POF) coated with epoxy containing ZnS:Mn crystals. The sensory receptors were then incorporated into cementitious and polymer samples. Results from preliminary investigation showed that the TL sensory receptor gives repeatable responses under multiple impact loads. The triboluminescent intensity of the signal is directly related to the magnitude of the impact load. Results from dynamic mechanical analysis show a reduction in the Tg of the ITOF coating (TSR) with higher concentration of the triboluminescent (ZnS:Mn) crystals for the epoxy system used. There was however significant enhancement of the modulus with increase in the TL crystals. High-performance epoxy system with the principles of particulate composites would be applied in subsequent work to optimize the properties and performance of the TL sensor system.

Triboluminescent Sensors for Cement-Based Composites

Cementitious composites like concrete are one of the most widely used materials for civil infrastructural systems like bridges, tunnels, and dams. The degradation of these critical concrete structures occurs through the formation and propagation of cracks. Consequently, monitoring cracks in concrete is an effective way to assess the structural condition or “health” of concrete structures. Triboluminescence-based sensor systems have the potential for wireless, in-situ, real time and distributed (WIRD) sensing that can enable continuous monitoring of civil and aerospace structures. This chapter focuses on progress made in the development and application of triboluminescence-based sensors for cementitious composites. Two broad application methodologies namely direct dispersion of triboluminescent crystals (ZnS:Mn) in cementitious composites and the use of the proprietary in situ triboluminescent optical fiber (ITOF) sensor are discussed. The application of the ITOF sensor for damage sensing in unreinforced, reinforced, and retrofitted CC structures is discussed.

Solid-State Dye Sensitized Optoelectronic Carbon Nanotube-Wires: An Energy Harvesting Damage Sensor With Nanotechnology Approach

A novel preparation method of solid state photovoltaic carbon nanotubes (CNT) yarns has been successfully developed by depositing and grafting TiO2 thin films on CNT yarn substrates using a simple sol–gel method and designed for use in structural health monitoring (SHM) applications. The interaligned, ultrastrong and flexible CNYs display excellent electrical conductivity, mechanical integrity and their catalytic properties have been successfully used as working and counter electrodes. The TiO2 nanoparticles have been found to form a homogeneous thin film on the yarn surface, which shows efficient photovoltaic properties with remarkable stability when exposed to simulated solar light (AM 1.5). The yarns’ structure is not altered upon sol-gel treatment and light exposure. The TiO2 film is firmly anchored and the photovoltaic performance is retained even after multiple irradiation cycles. This preparation technique can also be applied to CNT yarn reinforced composite for an innovative in-situ and real-time self damage-sensing properties with infused triboluminescent (TL) materials.Copyright