Adam Cooney

Characterization of Bonded Piezoelectric Sensor Performance and Durability in Simulated Aircraft Environments

Significant progress has recently been reported in the area of integrated structural health monitoring, with many sensor systems being deployed in actual operational environments. A key question that needs to be addressed and answered with regard to successfully implementing structural health monitoring technologies in aerospace systems involves the long‐term operability, durability, and survivability of integrated sensor systems and their associated hardware. In this activity, the performance characteristics of surface‐bonded piezoelectric sensors have been studied under accelerated exposure conditions typically found in an operational aircraft environment. In particular, sensor performance was studied for freeze‐thaw, moderate heat levels, humidity, electrochemical attack, substrate bend and tensile strains, and dynamic vibration conditions. The sensor performance was characterized using displacement‐field imaging, pitch‐catch signal transmission, and pulse‐echo signal transmission. Evidence of general ...

Evaluation and improvement in sensor performance and durability for structural health monitoring systems

For aerospace applications, the successful transition and use of integrated structural health monitoring systems will require durable sensors that can perform in their intended environment for many years. For legacy aircraft the primary means of implementing a sensor system will be through surface mounting or bonding of the sensors to the structure. Previous work has shown that the performance of surface-bonded piezo sensors can degrade due to environmental effects such as vibrations, temperature fluctuations, and substrate flexure motions. This performance degradation included sensor cracking, disbonding, and general loss of efficiency over time. In this activity, the bond and piezo material characteristics of a typical surface-bonded piezo sensor system were studied to understand and improve the long-term durability and survivability of the sensor system. Analytic and computational models were developed and used to understand stress-strain relationships for the bonded sensor system, with a special emphasis being place on coefficient of thermal expansion issues. Accelerated environmental testing was accomplished for simple bonded piezo sensor systems, where a displacement-field imaging technique was used to understand the piezo sensor performance. Future activities will focus on identifying the optimal bond conditions and piezo material type, with the ultimate goal of improving the robustness of health monitoring systems through improved sensor system design and packaging.

Characterization of sensor performance and durability for structural health monitoring systems

A key question that needs to be addressed and answered with regard to successfully implementing Structural Health Monitoring technologies in Air Force systems involves the long-term operability, durability, and survivability of integrated sensor systems and their associated hardware. Whether a sensor system is fully integrated within a structural material, or surface-bonded to the structure, a number of environmental and system level influences will tend to degrade the sensor system’s performance and durability over time. In this effort, an initial sensor durability study was undertaken to better understand the performance and degradation of piezo wafer active sensor (PWAS) systems under adverse mechanical, temperature, and moisture conditions. A novel displacement-field imaging approach was utilized to understand the vibration characteristics of PWAS transducers placed in accelerated vibration, temperature-cycling, and moisture-cycling conditions. The results showed damage in the form of PWAS sensor cracking events, bond degradation and failure, as well as indications of performance variation and reduction due to the accelerated exposure levels. Future activities will focus on identifying critical durability and survivability issues through advanced sensor modeling and additional accelerated testing efforts, with the ultimate goal of improving the robustness of health monitoring systems through improved sensor system design and packaging.

Tunable Narrow Band Difference Frequency THz Wave Generation in DAST via Dual Seed PPLN OPG

We report a widely tunable narrowband terahertz (THz) source via difference frequency generation (DFG). A narrowband THz source uses the output of dual seeded periodically poled lithium niobate (PPLN) optical parametric generators (OPG) combined in the nonlinear crystal 4-dimthylamino-N-methyl-4-stilbazolium-tosylate (DAST). We demonstrate a seamlessly tunable THZ output that tunes from 1.5 THz to 27 THz with a minimum bandwidth of 3.1 GHz. The effects of dispersive phase matching, two-photon absorption, and polarization were examined and compared to a power emission model that consisted of the current accepted parameters of DAST.

Advanced imaging of hidden damage under aircraft coatings

The external coating systems of nearly all military aircraft are stripped to bare metal during programmed depot maintenance cycles. This paint stripping process has become cost prohibitive in recent years, and is expected to continue to be a major and escalating problem for the sustainment of an aging Air Force fleet. Although a number of competing factors come into play, the key reason behind current paint stripping practices is centered on requirements for visual inspection of the aircraft structure to determine if corrosion and/or fatigue damage is present. In recent years, a number of advancements have been made in the area of nondestructive evaluation (NDE) that provide new inspection capabilities for aircraft skins without the requirement for protective coating removal. In this effort, several advanced imaging methods are evaluated for hidden damage detection and quantification through typical aircraft coating systems. A number of measurement examples are provided for engineered and realistic aircraft reference standards with variations in coating type, coating thickness, hidden damage type, and component complexity being considered. A comparison of measurement sensitivity, resolution, area coverage, ease-of-use, quantitative assessment, data processing requirements, and inspection speed are also made. It is anticipated that the use of one or more of these advanced NDE methods for thru-paint inspections will provide an enabling capability for long-life coating systems and condition based maintenance practices resulting in significant reductions in hazardous waste generation, dramatic cost savings, and enhanced readiness levels for a wide variety of Air Force systems.

A multi-tiered memetic multiobjective evolutionary algorithm for the design of quantum cascade lasers

Recent advances in quantum cascade lasers (QCLs) have enabled their use as (tunable) emission sources for chemical and biological spectroscopy, as well as allowed their demonstration in applications in medical diagnostics and potential homeland security systems. Finding the optimal design solution can be challenging, especially for lasers that operate in the terahertz region. The production process is prohibitive, so an optimization algorithm is needed to find high quality QCL designs. Past research attempts using multiobjective evolutionary algorithms (MOEAs) have found good solutions, but lacked a local search element that could enable them to find more effective solutions. This research looks at two memetic MOEAs that use a neighborhood search. Our baseline memetic MOEA used a simple neighborhood search, which is similar to other MOEA neighborhood searches found in the literature. Alternatively, our innovative multi-tiered memetic MOEA uses problem domain knowledge to change the temporal focus of the neighborhood search based on the generation. It is empirically shown that the multitiered memetic MOEA is able to find solutions that dominate the base-line memetic algorithm. Additional experiments suggest that using local search on only non-dominated individuals improves the effectiveness and efficiency of the algorithm versus applying the local search to dominated individuals as well. This research validates the importance of using multi-objective problem (MOP) domain knowledge in order to obtain the best results for a real world solution. It also introduces a new multitiered local search procedure that is able to focus the local search on specific critical elements of the problem at different stages in the optimization process.

CHARACTERIZATION OF MATERIAL DEGRADATION IN CERAMIC MATRIX COMPOSITES USING INFRARED REFLECTANCE SPECTROSCOPY

Ceramic matrix composite materials for thermal protection systems are required to maintain operational performance in extreme thermal and mechanical environments. In‐service inspection of materials capable of assessing the degree and extent of damage and degradation will be required to ensure the safety and readiness of future air vehicles. Infrared reflectance spectroscopy is an established material characterization technique capable of extracting information regarding the chemical composition of substances. The viability of this technique as a potentially powerful nondestructive evaluation method capable of monitoring degradation in thermal protection system materials subjected to extreme mechanical and thermal environments is analyzed. Several oxide‐based and non‐oxide‐based ceramic matrix composite materials were stressed to failure in a high temperature environment and subsequently measured using infrared reflectance spectroscopy. Spectral signatures at locations along the length of the samples were ...

Quantum Cascade Terahertz Emitters for Subsurface Defect Detection

The transmission and penetration capability of electromagnetic waves with terahertz frequencies promises nondestructive subsurface inspection capabilities using low energy, non‐harmful radiation. The development of compact and portable terahertz frequency radiation sources and detectors is crucial to the practical implementation of future terahertz based nondestructive evaluation tools for aerospace, medical, security, and electronic industries. Recent progress in the bandstructure engineering of multi‐quantum well heterostructures offers the potential to design miniature, efficient, high power, and direct terahertz emitters using standard semiconductor materials and fabrication techniques. Using a recently developed terahertz spectrometer, combined with near and mid‐infrared spectroscopy data, we report on the transmission characteristics of aerospace coatings over a broad spectrum from near‐infrared to terahertz frequencies. The design of quantum cascade emitters based on the obtained transmission data ...

Nondestructive characterization of micromachined ceramics

The aerospace, automotive, and electronic industries are finding increasing need for components made from silicon carbide (SiC) and silicon nitride (Si3N4). The development and use of miniaturized ceramic parts, in particular, is of significant interest in a variety of critical applications. As these application areas grow, manufacturers are being asked to find new and better solutions for machining and forming ceramic materials with microscopic precision. Recent advances in laser machining technologies are making precision micromachining of ceramics a reality. Questions regarding micromachining accuracy, residual melt region effects, and laser-induced microcracking are of critical concern during the machining process. In this activity, a variety of nondestructive inspection methods have been used to investigate the microscopic features of laser-machined ceramic components. The primary goal was to assess the micromachined areas for machining accuracy and microcracking using laser ultrasound, scanning electron microscopy, and white-light interference microscopic imaging of the machined regions.