A field deployable hyperspectral imager utilizing chromotomography (CT), with a direct vision prism (DVP)
as the dispersive element, has been constructed at the Air Force Institute of Technology (AFIT). A "shift and
add" reconstruction algorithm was used to resolve spectral and spatial content of the collected data. The AFIT
instrument is currently the fastest known imaging DVP based hyperspectral CT instrument of its type and is
a prototype for a space-based system. The imager captured images at rates up to 900 frames per second (fps)
and acquired data cube information in 55 ms, during testing. This instrument has the ability to capture spatial
and spectral data of static and transient scenes. During testing, the imager captured spectral data of a rapidly
evolving scene (a firecracker detonation) lasting approximately 0.12 s. Spectral results included potassium and
sodium emission lines present during the explosion and an absorption feature as the fireball extinguishes. Spatial
and spectral reconstruction of a scene in which an explosion occurs during the middle of the collection period
is also presented in this paper. The instrument is capable of acquiring data required to identify, classify and
characterize transient battlespace events, such as explosions.
This paper compares and contrasts one-dimensional (1D) and
three-dimensional (3D) scanning laser Doppler
vibrometer (LDV) measurements of Lamb waves generated by lead zirconate titanate (PZT) transducers. Due
to the large cost and capability differences between the previously mentioned systems, this study is provided
to highlight differences between these systems. 1D measurements are defined here as measurements of only
out-of-plane velocities which are well-suited for studying
anti-symmetric Lamb wave modes. 3D measurements
provide both in- and out-of-plane velocities, which are especially important when studying both symmetric and
anti-symmetric Lamb wave modes. The primary reason for using scanning LDVs is that these systems can
make non-contact, accurate surface velocity measurements over a spatially-dense grid providing relatively high
resolution image sequences of wave propagation. These scans can result in a clear understanding of Lamb waves
propagating in plate-like structures and interacting with structural variations.
Lamb waves are being explored for structural health monitoring (SHM) due to their capability of detecting relatively
small damage within reasonably large inspection areas. However, Lamb wave behavior is fairly complex, and therefore,
various computational techniques, including finite element analysis (FEA), have been utilized to design appropriate
SHM systems. Validation of these computational models is often based on a limited number of measurements made at
discrete locations on the structure. For example, models of pitch-catch of Lamb waves may be validated by comparing
predicted waveform time histories at a sensor to experimentally measured results. The use of laser Doppler vibrometer
(LDV) measurements offers the potential to improve model validation. One-dimensional (1D) LDV scans provide
detailed out-of-plane measurements over the entire scanned region, and checks at discrete sensor locations can still be
performed. The use of three-dimensional (3D) laser vibrometer scans further expands the data available for correlation
by providing in- and out-of-plane velocity components over the entire scanned region. This paper compares the use of
1D and 3D laser vibrometer data for qualitatively and quantitatively validating models of healthy metallic and composite plates.
In this study, the feasibility of using a scanning laser vibrometer for detecting hidden delamination in multi-layer
composites is explored. First, Lamb waves are excited by Lead Zirconate Titanate (PZT) transducers mounted on the
surface of a composite plate, and the out-of-plane ultrasonic velocity field is measured using a 1D scanning laser
vibrometer. From the scanned time signals, wave field images are constructed and processed to study the interaction of
Lamb waves with hidden delamination. In order to highlight the defect area in the image, the performance of different
image processing tools were investigated. In particular, the Laplacian image filter was found to accentuate the visual
indications of the ultrasound-defect interaction by suppressing the presence of incident waves in the wave field images.
The performance of the proposed scheme is investigated using experimental data collected from a 1.8 mm thick multilayer
composite plate and a 10 mm thick composite wing structure.
This paper presents an initial study on Lamb wave propagation characteristics in z-pin reinforced, co-cured
composite pi-joints for the purposes of structural health monitoring (SHM). Pi-joint test articles were designed
and created to replicate a co-cured, all composite skin-spar joint found within a typical aircraft wing structure.
Because pi-joints exhibit various complex damage modes, formal studies are required if SHM systems are to be
developed to monitor these types of joints for potential damage. Experiments were conducted on a undamaged
(healthy) and damaged test articles where Lamb waves were excited using one lead zirconate titanate (PZT)
transducer. A three-dimensional (3D) scanning laser Doppler vibrometer (LDV) was used to collect high-density
scans of both the in-plane and out-of-plane velocity measurements. In the damaged test article, where delamination,
matrix cracking, and fiber breakage can clearly be seen, changes in both the fundamental antisymmetric
A<sub>0</sub> and symmetric S<sub>0</sub> Lamb wave modes are apparent. In both test articles, the effects of narrow geometry,
discontinuity due to the attachment of the web, and thickness has detectable effects on Lamb wave propagation.
From the comparisons between Lamb waves propagating through the undamaged and damaged test articles, it is clear that damage can be detected using Lamb waves in z-pin reinforced, co-cured composite pi-joints for this case of extensive damage.
This experimental research investigates the effects of adding z-pins to a carbon fiber reinforced plate (CFRP)
on Lamb wave propagation, such as mode conversion and reflections. The motivation for this study is derived
from the current and expected future use of z-pins in aircraft structures coupled with the requirement to design
structural health monitoring (SHM) systems for detecting damage in regions of composite structures with z-pins.
This experimental study is conducted on two 4.8 mm thick CFRP test articles, where one plate has a 20 by 279
mm2 band of z-pins and the other does not. The z-pins have an average diameter of 0.28 mm and are inserted
through the thickness of the panel with an area density of 4% before curing. A three-dimensional (3D) laser
Doppler vibrometer (LDV) was employed to collect velocity measurements over a 1 mm uniformly-spaced grid
of 17,899 scan points. Time-sequenced 3D LDV scans are presented to show that adding this relatively small
amount of z-pins to a 4.8 mm thick CFRP has few measureable effects on Lamb wave propagation.
This paper presents results of an experiment designed to determine the impact of repeated strain cycles on lead
ziconate titanate (PZT) transducers affixed to an aluminum test specimen. The goal of this research effort is to
determine the impact of three cyclic strain levels on PZTs affixed with two different glue types. PZT transducers
are evaluated because they are one of the leading health monitoring technologies used in aircraft structures due to
their ability to transmit and receive Lamb Waves. Analysis of changes in the received signals can indicate the
presence of structural damage. This monitoring paradigm can only be successful if signal changes due to exposure
to aircraft environmental factors (temperature/strain/pressure cycles, etc) over time can be clearly identified and
characterized. This paper presents the results and initial analysis of experiments to determine the changes in signal
responses due to cyclic mechanical strain. Results indicate cyclic strain at 800 με has no effect to 510K cycles,
while cyclic strain at 1700 and 2600 με both cause signal loss to varying degrees.
This paper presents a set of results from an experiment that is designed to evaluate a damage detection approach for
through-thickness fatigue cracks emanating from a rivet hole in a high-performance aircraft bulkhead. Because fatigue
cracks have been found through depot-level visual-inspections at the same location in several aircraft bulkheads, a "hot-spot"
approach to monitor this area with Lamb waves generated from surface-mounted lead ziconate titanate (PZT)
transducers is evaluated. Detecting these fatigue cracks is challenging because the cracks propagate through an area of
restricted geometry - a small plate-like area surrounded by thick webbing - which results in the interference of reflected
wave components with the direct path wave components when using a pitch-catch approach. To minimize this
interference, time-of-flight windows are applied to remove the reflected signals, and to increase probability of detection,
Lamb wave mode tuning is used. Finally, to make the crack easier to detect, various static loads are applied to open the
crack, but new challenges are presented when attempting to detect damage under a static load.