Bryan R. Loyola

Detection of spatially distributed damage in fiber-reinforced polymer composites:

This work describes a novel method of embedded damage detection within glass fiber–reinforced polymer composites. Damage detection is achieved by monitoring the spatially distributed electrical conductivity of a strain-sensitive multiwalled carbon nanotube thin film. First, thin films were spray-deposited directly upon glass fiber mats. Second, using electrical impedance tomography, the spatial conductivity distribution of the thin film was determined before and after damage-inducing events. The resolution of the sensor was determined by drilling progressively larger holes in the center of the composite specimens, and the corresponding electrical impedance tomography response was measured by recording the current–voltage data at the periphery of the monitored composite sample. In addition, the sensitivity to damage occurring at different locations in the composite was also investigated by comparing electrical impedance tomography spatial conductivity maps obtained for specimens with sets of holes drilled at different locations in the sensing area. Finally, the location and severity of damage from low-velocity impact events were detected using the electrical impedance tomography method. The work presented in this study indicates a paradigm shift in the available possibilities for structural health monitoring of fiber-reinforced polymer composites.

Spatial Sensing Using Electrical Impedance Tomography

The need for structural health monitoring has become critical due to aging infrastructures, legacy airplanes, and continuous development of new structural technologies. Based on an updated structural design, there is a need for new structural health monitoring paradigms that can sense the presence, location, and severity with a single measurement. This paper focuses on the first step of this paradigm, consisting of applying a sprayed conductive carbon nanotube-polymer film upon glass fiber-reinforced polymer composite substrates. Electrical impedance tomography is performed to measure changes in conductivity within the conductive films because of damage. Simulated damage is a method for validation of this approach. Finally, electrical impedance tomography measurements are taken while the conductive films are subjected to tensile and compressive strain states. This demonstrates the ability of electrical impedance tomography for not only damage detection, but active structural monitoring as well. This paper acts as a first step toward moving the structural health monitoring paradigm toward large-scale deployable spatial sensing.

Analysis of Volatile and Non-Volatile Biomarkers in Human Breath Using Differential Mobility Spectrometry (DMS)

Exhaled human breath contains thousands of chemicals that are potential biomarkers of disease or chemical exposure. Although many bench-top analytical instruments could measure concentrations of these chemicals, small and portable systems have the best advantage of being used in a clinical point-of-care environment or in a field setting. Here, we demonstrate coupling a miniature differential mobility spectrometer (DMS) with both a gas chromatograph (GC) and separately an electrospray ionization (ESI) module to analyze exhaled breath condensate. Our combined GC/DMS and ESI/DMS instrument systems are capable of measuring an extremely large number of chemical analytes contained in exhaled breath condensate. We have established methodologies for detecting single compounds and approximate the limits of detection for our systems. The detection limit and analytical power are clinically relevant for many potential biomarkers, and suggests our device may have many applications for disease diagnostics in human breath analysis.

The electrical response of carbon nanotube-based thin film sensors subjected to mechanical and environmental effects

Fiber-reinforced polymer composites are a popular alternative to traditional metal alloys. However, their internally occurring damage modes call for strategies to monitor these structures. Multi-walled carbon nanotube-based polyelectrolyte thin films were manufactured using a layer-by-layer deposition methodology. The thin films were applied directly to the surface of glass fiber-reinforced polymer composites, with the purpose of structural monitoring. This work focuses on characterizing the sensitivity of the electrical properties of the film using time- and frequency-domain methods under applied quasi-static and dynamic mechanical loading. In addition, environmental effects such as those of temperature and humidity are varied to characterize the sensitivity of the electrical properties to these phenomena. (Some figures may appear in colour only in the online journal)

Temperature changes in exhaled breath condensate collection devices affect observed acetone concentrations.

Chemical analysis of exhaled breath condensate (EBC) is an emerging method to non-invasively identify and measure potential biomarkers of disease. Various EBC collection methods have been proposed, each with strengths and weaknesses. Recent evidence in the literature suggests that sample collection methodologies could introduce potential artifacts in biomarker measurements. In this study, we tested the effect of thermal changes during condensate collection on measured EBC chemical concentrations. Using both actively-cooled and passively-cooled devices, we measured distinct differences in the amount of condensate that can be collected over discrete time periods. We also found that concentrations of acetone varied with the thermal profile changes in the collection devices, in apparently identical EBC samples. Together, this evidence suggests that great care should be taken to standardize EBC collection methods, and that small deviations in the thermal properties of the collection devices could contribute to confounding EBC measurement artifacts. This has implications for the design and development of future portable breath analysis systems, especially miniature hand-held devices.

Microfabricated differential mobility spectrometers for breath analysis

We have demonstrated the use of a novel micromachined differential mobility spectrometer (DMS) to analyze human breath condensate samples for applications in disease diagnostics. This miniature device is small, portable, low power, and potentially fieldable as a point-of-care clinical diagnostic instrument. To date, we have shown that our instrument system is capable of measuring a higher number of chemicals from breath condensate samples than traditional analytical instruments, such as gas chromatography mass spectrometry (GC/MS). We have also found that we can detect extremely low levels of specific chemicals of interest, such as acetone, down to the single digit parts-per-billion level. The device has also been used to characterize exhaled breath condensate (EBC) samples from individuals, and we have used machine learning algorithms to differentiate between persons based on the measured chemicals in their breath alone. The detection limits and analytical power are clinically relevant for many potential biomarkers, and suggests our device may have many applications for disease diagnostics in human breath analysis.

Characterizing the viscoelastic properties of layer-by-layer carbon nanotube?polyelectrolyte thin films

Many have sought to take advantage of carbon nanotubes for the fabrication of multifunctional thin films. However, before these carbon-nanotube-based films can be used for these intended functionalities, a complete understanding of the mechanical properties is crucial. While most studies have focused on characterizing just the thin films stress–strain performance, their time-dependent mechanical properties still need to be investigated. In this study, free-standing layer-by-layer carbon-nanotube-based thin films have been assembled, and the objective is to investigate their viscoelastic stress relaxation and creep response. First, the results from stress relaxation tests are numerically fitted to two different models, namely the standard linear solid (Zener) model and the linear increasing viscosity-modified Zener (LIV-Zener) model. The results of the fitting suggest that the LIV-Zener model is better suited for describing thin film stress relaxation behavior, which also means that these nanocomposites exhibit time-dependent viscosity. Second, the experimental results from creep testing are also fitted to the LIV-Zener model. The results from both fittings are consistent and suggest that the LIV-Zener model is more capable of describing carbon-nanotube-based thin film viscoelasticity.

In situ phase change characterization of PVDF thin films using Raman spectroscopy

A process for the design and optimization of a morphing aileron using flexible matrix composite (FMC) actuators is developed. Design requirements were based on the use of a morphing aileron with an existing medium size commercial transport aircraft limiting the planform to the existing conventional aileron. The design uses two sets of contracting FMC actuators operating independently to actuate the deformable aileron in both trailing edge up and down cases while matching coefficient of lift values of a conventional aileron. The design was optimized by developing a series of response models which were then used to understand the sensitivity to material, geometric and loading design variables while minimizing required force from the FMC actuators. Using a combination of response models and sequential quadratic programming algorithm, a design is chosen that matched the conventional ailerons performance at multiple flight conditions.

Static and dynamic strain monitoring of GFRP composites using carbon nanotube thin films

Fiber-reinforced polymers (FRP) composites are widely used in aerospace and civil structures due to its unique material properties. However, damage can still occur and typically manifests itself from within the composite material that is invisible to the naked eye. So as to be able to monitor the performance of FRPs, numerous sensing systems have been proposed for embedment within FRP composites. One such methodology involves the embedment of carbon nanotube-based thin films within FRP laminates for strain monitoring and potentially even damage detection. Unlike other sensors, these piezoresistive thin films possess small form factors (and thus do not serve as stress concentration or damage initiation points) and can be easily integrated during composite manufacturing. In this study, a series of laboratory tests have been conducted to characterize the static and dynamic strain sensing performance of these nanocomposites for monitoring glass fiber-reinforced polymer (GFRP) components. Specifically, monotonic uniaxial, cyclic, and fatigue tests have been conducted, while both time- and frequency-domain measurements have also been obtained. The characterization results obtained from this study indicates bi-functional strain sensitivity to monotonic loading until failure, which is found to be reproducible in cyclic dynamic loadings to amplitudes in both functional ranges.

Characterizing the self-sensing performance of carbon nanotube-enhanced fiber-reinforced polymers

The increased usage of fiber-reinforced polymers (FRP) in recent decades has created a need to monitor the unique response of these materials to impact and fatigue damage. As most traditional nondestructive evaluation methods are illsuited to detecting damage in FRPs, new methods must be created without compromising the high strength-to-weight aspects of FRPs. This paper describes the characterization of carbon nanotube-polyelectrolyte thin films applied to glass fiber substrates as a means for in situ strain sensing in glass fiber-reinforced polymers (GFRP). The layer-by-layer deposition process employed is capable of depositing individual and small bundles of carbon nanotubes within a polyelectrolyte matrix and directly onto glass fiber matrices. Upon film fabrication, the nanocomposite-coated GFRP specimens are mounted in a load frame for characterizing their electromechanical performance. This preliminary results obtained from this study has shown that these thin films exhibit bilinear piezoresistivity. Time- and frequency-domain techniques are utilized to characterize the nanocomposite strain sensing response. An equivalent circuit is also derived from electrical impedance spectroscopic analysis of thin film specimens.