Benjamin D. Buckner

Laser-scanning structural health monitoring with wireless sensor motes

A highly compact laser-scanning device that can be used as an in situ detection and monitoring device for metal fatigue is described. A prototype is built on a Mica2 wireless sensor mote platform with TinyOS-based firmware. This device is shown to be capable of detecting fatigue-related changes in the surface bidirectional reflectance distribution (BRDF) of an aluminum test coupon, and detectable changes in BRDF are measured at a very early stage of fatigue development, when no cracks larger than 50 µm in length have yet developed at the monitored location. The system power requirements are compatible with standard sensor mote architectures such as the Mica and Telos series, enabling the potential for multiyear lifetimes without battery replacement. Such an in situ optical fatigue sensor could be used in a variety of structural health monitoring applications, including aerospace, rail transport, and civil structures.

Digital synchroballistic schlieren camera for high-speed photography of bullets and rocket sleds

A high-speed digital streak camera designed for simultaneous high-resolution color photography and focusing schlieren imaging is described. The camera uses a computer-controlled galvanometer scanner to achieve synchroballistic imaging through a narrow slit. Full color 20 megapixel images of a rocket sled moving at 480  m/s and of projectiles fired at around 400  m/s were captured, with high-resolution schlieren imaging in the latter cases, using conventional photographic flash illumination. The streak camera can achieve a line rate for streak imaging of up to 2.4  million lines/s .

Thermally assisted electroluminescence: a viable means to generate electricity from solar or waste heat?

It has been proposed recently that thermally assisted electroluminescence may in principle provide a means to convert solar or waste heat into electricity. The basic concept is to use an intermediate active emitter between a heat source and a photovoltaic (PV) cell. The active emitter would be a forward biased light emitting diode (LED) with a bias voltage, Vb, below bandgap, Eg (i.e., qVb < Eg), such that the average emitted photon energy is larger than the average energy that is required to create charge carriers. The basic requirement for this conversion mechanism is that the emitter can act as an optical refrigerator. For this process to work and be efficient, however, several materials challenges will need to be addressed and overcome. Here, we outline a preliminary analysis of the efficiency and conversion power density as a function of temperature, bandgap energy and bias voltage, by considering realistic high temperature radiative and non-radiative rates as well as radiative heat loss in the absorber/emitter. From this analysis, it appears that both the overall efficiency and net generated power increase with increasing bandgap energy and increasing temperature, at least for temperatures up to 1000 K, despite the fact that the internal quantum yield for radiative recombination decreases with increasing temperature. On the other hand, the escape efficiency is a crucial design parameter which needs to be optimized.

Laser scanning technique for fatigue damage evolution detection

Metal components subjected to cyclic stress develop surface-evident defects (microcracks, slip bands, etc). Monitoring the formation and evolution of these fatigue damage precursors (FDPs) with increasing numbers of cycles can be an effective tool for determining the fatigue state of the component, which can be used in remaining fatigue life prognostics. In this paper a laser scanning technique (LST) for FDP detection is described and experimental results from examination of specimens made of nickel-based superalloy and aluminum are presented. The proposed detection technique is based on scanning a focused laser beam over the specimen surface and detecting variations in spatial characteristics of the scattered light signal. These variations indicate the presence of surface abnormalities and therefore can be associated with incremental fatigue damage formation. The studies performed show that the proposed LST can serve as a basis for design of a portable non-contact instrument for in situ structural health monitoring.

Power generation by thermally assisted electroluminescence : like optical cooling, but different

Thermally assisted electro-luminescence may provide a means to convert heat into electricity. In this process, radiation from a hot light-emitting diode (LED) is converted to electricity by a photovoltaic (PV) cell, which is termed thermophotonics. Novel analytical solutions to the equations governing such a system show that this system combines physical characteristics of thermophotovoltaics (TPV) and the inverse process of laser cooling. The flexibility of having both adjustable bias and load parameters may allow an optimized power generation system based on this concept to exceed the power throughput and efficiency of TPV systems. Such devices could function as efficient solar thermal, waste heat, and fuel-based generators.

Simultaneous measurements of density field and wavefront distortions in high speed flows

This paper presents results from simultaneous measurements of fluid density and the resulting wavefront distortions in a sonic underexpanded jet. The density measurements were carried out using Rayleigh scattering, and the optical distortions were measured using a wavefront sensor based on phase shifting interferometry. The measurements represent a preliminary step toward relating wavefront distortions to a specific flow structure. The measured density field is used to compute the phase distortions using a wave propagation model based on a geometric-optics approximation, and the computed phase map shows moderate agreement with that obtained using the wavefront sensor.

Focusing schlieren systems using digitally projected grids

Schlieren imaging has been an essential method for studying aerodynamic effects, particularly thermal convection, shock waves, and turbulent flows. This paper describes a compact portable digital focusing schlieren system that can be used to visualize relatively large fields for applications in ventilation design and aerodynamics research. Visualizing large fields is difficult using classical schlieren systems that employ collimated light because their field of view is limited by the size of the mirrors or lenses. Background-oriented schlieren systems are well-suited for visualizing large fields, but their sensitivity is limited by the need to simultaneously maintain focus on the background pattern and the test area. Lens and grid-based focusing schlieren systems are essentially hybrids between classical and background-oriented systems. They can visualize fields that are much larger than possible with classical schlieren systems, while providing more sensitivity than background-oriented schlieren systems. Using commercially available camera lenses and optics, fields up to several square meters can be visualized. A key innovation in the system presented here is that digital display devices are used to display the background pattern, which simplifies the optical system and reduces its size. To calibrate the system, proprietary software is used to analyze images acquired by the system’s digital camera, and then a background pattern is computed that is complementary to the cutoff grid. The calibration software also provides real-time background subtraction and contrast enhancement. The schlieren system is portable enough that it can be set up quickly in industrial facilities.

Digital focusing schlieren imaging

Since its invention in the 19th century, schlieren imaging has been an essential method for studying many aerodynamic effects, particularly convection and shock waves, but the classical method using parabolic mirrors is extremely difficult to set up and very expensive for large fields of view. Focusing schlieren methods have made large- area schlieren more feasible but have tended to be difficult to align and set up, limiting their utility in many applications We recently developed an alternative approach which utilizes recent advances in digital display technology to produce simpler schlieren system that yields similar sensitivity with greater flexibility.

Eliminating atmospheric optical noise through digital holography

Systems that attempt to image or project optical energy or information through a turbulent atmosphere are limited by aberrating, refractive index variations. The processes can be improved in a variety of ways if the complex wave function of the aberrated wave can be recorded, reconstructed and analyzed at a sufficient speed. This paper describes application of digital holography for recording, reconstructing, and processing complex wave functions to complement methods such as adaptive optics and lucky imaging. Having the complex waveform provides all of the information required by adaptive optical procedures and also enables improved image processing that is not applicable to real images. Unlike intensity averaging, when complex wave functions are averaged, the random fluctuations in the phase cancel since phase terms include both positive and negative values. In this paper we describe the application of digital holography for recording, reconstructing, and processing complex wave functions of atmospherically aberrated wave functions and report demonstrations in correcting for atmospheric turbulence.

Optical detection and discrimination of fatigue and damage in engineering materials

Optical detection of material damage, especially fatigue, has obvious importance in many aspects of engineering. Recent work in the area has shown that scanning light scattering is capable of detecting signs of fatigue and foreign-object damage on many types of surfaces; however, the persistent problem with practical implementation of this technique is the presence of background noise signals, false defect counts. Several new approaches to discriminating fatigue damage signals from non-fatigue-related signals have been developed, including filtering of 2-dimensional scattered intensity maps, peak detection and correlation among multiple channels, and exploitation of surface reflectance and scattering properties to control the information received by the sensor. Similar techniques are also shown to enable highly robust detection of foreign object damage (FOD) on turbine blade edges