CCD-based thermoreflectance imaging is a powerful tool for high-resolution, 2D thermal imaging. Given the low signalto- noise ratio, thermoreflectance imaging is typically performed using a lock-in imaging, “4-bucket” algorithm that requires averaging over many (typically 103-104) sample modulation periods, leading to relatively long measurement times. However, averaging over multiple samples in the presence of noise also has the potential secondary effect of stochastic resonance enhancement, in which signals smaller than the bit depth of the camera can be measured, dramatically improving the thermal resolution. In this study, we develop a model of stochastic resonance enhancement of the image “buckets” through additive noise, quantization, and averaging. We demonstrate that for noise amplitudes greater than 1.25 least significant bits (LSB), the root mean square (RMS) error in an image bucket is independent of the input signal amplitude. In addition, we show that for input signals for which the image quantization error is greater than 0.5 LSB, the RMS error in an image bucket is minimized in the presence of small, non-zero, amounts of noise, demonstrating stochastic resonance enhancement. Our simulations confirm earlier results that an image bucket may be modeled as a Gaussian-distributed random variable, but the expected mean is offset due to the flooring quantization scheme. Finally, our results for experimentally reasonable noise and signal levels suggest that for measurements made using low numbers of iterations (<5000), a small tuning of the CCD camera noise could increase the stochastic resonance enhancement of the image bucket.
Composite materials pose a complex problem for ultrasonic nondestructive evaluation due to their unique material properties, greater damping, and often complicated geometry. In this study, we explored acoustic wavenumber spectroscopy (AWS) as a means of rapid inspection of laminate and honeycomb composites. Each aerospace sample was tested at different ultrasonic frequencies using steady-state excitation via a piezo electric actuator. We measured the velocity response of the composite at each pixel via a raster scan using a laser Doppler vibrometer. We were able to detect radial inserts along corners, delamination, and facing-core separation by analyzing local amplitude and wavenumber responses. For each honeycomb composite, we excited the sample at the first resonant frequency of the individual cells. The local mode shape for each cell was extracted from the local amplitude response. Analyzing local amplitude and phase responses for each cell provided an accurate indication as to the presence, size, shape, and type of defect present in the composite. We detected both delamination and deformation of cells within a honeycomb composite. For the laminar composites, we analyzed the non-resonance steady-state response at several excitation frequencies.
This paper explores the use of a steady-state scanning laser Doppler vibrometer (LDV) system for the identification
of transition areas between solid, liquid, and gaseous substances in an enclosed container. This technique images
lateral surface velocity under the excitation of a single-frequency ultrasonic tone, produced by a piezoelectric actuator.
Differences in measured spatial wavenumber at discrete measurement points of a surface scan can be used to detect
the boundaries between solid, liquid and gaseous regions of material. We used the LDV system to compare the relative
distributions of solid wax, liquid wax, and air in a cylindrical container based on local changes in wavenumber.
Through the same methodology, we were able to distinguish the transition between solid and liquid epoxy in a
container. Finally, by repeatedly scanning the container during a phase-changing reaction within the container, we
established that the system can be used to monitor reactions as they progress.
We present a new in-process laser ultrasound inspection technique for additive manufacturing. Ultrasonic energy was introduced to the part by attaching an ultrasonic transducer to the printer build-plate and driving it with a single-tone, harmonic excitation. The full-field response of the part was measured using a scanning laser Doppler vibrometer after each printer layer. For each scan, we analyzed both the local amplitudes and wavenumbers of the response in order to identify defects. For this study, we focused on the detection of delamination between layers in a fused deposition modeling process. Foreign object damage, localized heating damage, and the resulting delamination between layers were detected in using the technique as indicated by increased amplitude and wavenumber responses within the damaged area.
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