As part of an on-going, multi-year effort focused on developing a practical structural health monitoring (SHM) sensor for
critical structural components in aircraft, a miniature Rayleigh surface wave sensor has been developed and tested. The
sensor was specifically designed to detect localized, deterministic cracking in targeted locations in critical locations
where fatigue cracking is prevalent. A representative aircraft component was used in the present investigation.
Miniature interdigital transducers (IDTs) operating in the low megahertz frequency range were designed, fabricated, and
tested on compact tension (CT) fatigue specimens in the laboratory before they were strategically placed on the structure,
where surface wave signals were monitored in both pitch-catch and pulse-echo detection modes simultaneously. Under a
high-cycle fatigue loading to the structure, the IDT sensors performed well with three of the sensors successfully
detecting the existence of a critical fatigue crack. Visual and eddy current inspection methods subsequently verified the
presence of the crack and its location. In this paper, the entire effort from the design and characterization of the IDT
sensors to the final fatigue test on an actual aircraft part is discussed.
Thermal protection systems (TPS) of aerospace vehicles are subjected to impacts during in-flight use and vehicle
refurbishment. The damage resulting from such impacts can produce localized regions that are unable to resist extreme
temperatures. Therefore it is essential to have a reliable method to detect, locate, and quantify the damage occurring
from such impacts. The objective of this research is to demonstrate a capability that could lead to detecting, locating and
quantifying impact events for ceramic matrix composite (CMC) wrapped tile TPS via sensors embedded in the TPS
material. Previous research had shown a correlation between impact energies, material damage state, and polyvinylidene
fluoride (PVDF) sensor response for impact energies between 0.07 - 1.00 Joules, where impact events were located
directly over the sensor positions1. In this effort, the effectiveness of a sensor array is evaluated for detecting and
locating low energy impacts on a CMC wrapped TPS. The sensor array, which is adhered to the internal surface of the
TPS tile, is used to detect low energy impact events that occur at different locations. The analysis includes an evaluation
of signal amplitude levels, time-of-flight measurements, and signal frequency content. Multiple impacts are performed
at each location to study the repeatability of each measurement.
Thermal protection systems (TPS) are frequently subjected to impacts from micrometeoroids and ground handling during refurbishment. The damage resulting from such impacts can greatly reduce the vehicle's overall ability to resist extreme temperatures. Therefore, it is essential to have a reliable method to detect and quantify the damage resulting from impacts. In this effort, the effectiveness of lightweight thin film piezoelectric sensors was evaluated for impact detection and quantification in CMC wrapped TPS. The sensors, which were adhered to the bottom of the TPS tile, were used to sense impact events occurring on the top of the tile, with the ultimate goal of quantifying the level of impact level and damage state based on the sensed signals. A reasonable correlation between impact load levels and sensed response were observed for load levels between 0.07-1.00 Joules. An increase in signal frequency content was also observed as impact levels were increased, with specific frequency bands occurring in the 2-16 kHz range. A preliminary nondestructive evaluation of the impact damage sites was also accomplished, where a reasonable correlation between the gross damage features (i.e. impact crater dimensions) and signal response was observed.
Surface bonded sensors have significant potential for detecting and characterizing damage in legacy aircraft structures as
part of a Structural Health Monitoring system. In this effort, research has been undertaken to understand the impact of
adhesive viscoelastic properties on the generation of elastic wave energy by surface-bonded sensors in thin plates.
Previous work has shown that bonded sensors can degrade and fail due to exposure to weather, vibration, temperature,
and mechanical loading. In that work, experimental and analytical studies were performed to characterize the transfer of
static load from a structure into a surface-bonded sensor. The results indicated that the sensor should be decoupled from
the nearly static loading induced from the structure to improve its durability. In this effort, we build on that original work
to determine what effect the adhesive has on elastic wave generation and reception in the host structure. The results
indicate that strong coupling with the structure is required for effective generation and reception of elastic waves, where
the elastic material properties of the sensor, bond, and host structure were considered. Although the two goals appear to
be contradictory (sensor durability and elastic wave generation), the use of a strongly viscoelastic adhesive is viewed as
potential solution for both by allowing weak coupling at low frequencies and strong coupling at high frequencies.
Elastic waves generated by foreign materials impacting surfaces of aerospace vehicle can be used to detect and quantify
the severity of damage. Passive acoustical emission sensors, made of piezoelectric elements, are typically used as impact
signal detection devices. In this study, we have concentrated on characterizing the bonding qualities of piezoelectric
sensors in terms of various bonding materials and adhesion conditions such as bond strength, bond stiffness, partial
bonding, and disbonding. The experiment has been performed with an automated impact testing setup under controlled
bonding and disbonding conditions in an attempt to establish a standardized sensor bond quality inspection methodology.
Durable integrated sensor systems are needed for long-term health monitoring evaluations of aerospace systems. 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 research
effort, the bonding state of a piezo sensor system was systematically 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 elastic wave generation and reception performance for various states of sensor disbond. Experimental studies
were also conducted using scanning laser vibrometry, pitch-catch ultrasound, and pulse-echo ultrasound methods to
understand elastic wave propagation effects in thin plate materials. Significant performance loss was observed for
increasing levels of sensor disbond as well as characteristic frequency signatures which may be useful in understanding
sensor performance levels for future structural health monitoring systems.
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.
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.
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.
KEYWORDS: Sensors, Structural health monitoring, Digital imaging, Environmental sensing, Semiconducting wafers, Active sensors, Aluminum, Imaging systems, Sensor performance, System integration
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.
Optical generation and detection of surface acoustic waves provides a non-contact, remote means of characterizing microscopic surface-breaking cracks in aerospace and industrial materials. Surface ultrasonic displacement fields generated by a non-destructive, laser induced thermoelastic mechanism and detected with an interferometric probe allow for an all optical and spatially adjustable (spot-size, beam shape, beam separation) utlrasonic NDE system. Location of surface-breaking cracks is achieved through observation of a near-field intensification of the detected ultrasonic signal in the vicinity of the crack. The near-field intensification was found to be optimized by scanning both the source and detection beams with specific spatial variables. This non-contact, laser beam scanning NDE technique, allows for imaging capabilities to further enhance surface-breaking crack characterization.
The paper will give a brief overview on techniques that have been developed or are in progress for high resolution characterization of materials at the Center for Materials Diagnostics, University of Dayton. Acoustic microscopy is used to characterize coating systems and localized defects like corrosion pits. Significantly higher resolution is provided by Ultrasonic force microscopy, which allows the imaging of elastic inhomogenities in materials for example, studying nano-grain structures in copper films and nano precipates in aluminum alloys. Several optical high-resolution techniques have been developed or are in progress. These include interferometric imaging of the response of acoustic MEMS transducers, imaging of acoustic wave structures and early detection of crack initiation. Microellipsometric and NSOM imaging techniques are in development for imaging of surface structures significantly smaller than the optical wavelength. White light interference microscopy is frequently used to characterize surface topography with nanometer resolution for example, to quantify fretting damage or stress fields in front of fractures.
A considerable amount of work has recently been applied to the development of laser processing techniques for a wide variety of applications. With regard to aging aircraft, laser processing techniques could play a role in inhibiting crack growth and extending the life of structural aircraft components. The basic concept involves the application of a sharply-focused, moderate power laser beam to a local microscopic defect site that has been detected through advanced NDE techniques. The defect could be pitting corrosion site, a fretting region, or even a microcrack site. The laser would be raster-scanned across the defect, re-melting the site locally to a level where the sharp features of the defect are smoothed out, or perhaps re-melted completely to eliminate the flaw site altogether, thereby reducing stress concentration levels in the material. In order to test the feasibility of this basic concept, a series of measurements were made to study the effect of microscopic laser treatments applied to artificial defects in Al-2024-T3 aluminum and Ti-Al6-4V titanium. The major results of the study showed a moderate to significant level of fatigue life enhancement for engineered notches in the 1 mm size range. The laser treatmen approah may provide an opportunity for 'healing' structural defects in aerospace materials that would otherwise require expensive and time-consuming part replacements in aging aircraft structures.
Laser ultrasonic imaging of surface acoustic waves on a material surface provides a non-contact and sensitive method for detecting and characterizing defects and anomalies in aerospace and industrial materials. It has recently been shown that the surface acoustic wave interactions with sharp material discontinuities such as surface-breaking cracks provides an additional benefit of an intensification of the displacement field in the immediate vicinity of the crack site. This near-field intensification can be exploited by scanning, point-by-point, a laser-ultrasonic system to create detailed microscopic images of the surface breaking cracks. In this effort, a number of laser ultrasonic system parameters were studied to better understand the optimized conditions to imaging surface breaking cracks using ultrasonic generation in the thermoelastic regime, and laser detection of ultrasound fields using heterodyne interferometer. In particular, laser beam sizes, separation distances, and motives were varied. In addition, several different time-gating analysis methods were studied, which had a significant effect on both the characteristics as well as the quality of the resultant crack images.
KEYWORDS: Holography, Interferometry, Ultrasonics, Imaging systems, Nondestructive evaluation, Holograms, Visualization, Holographic interferometry, Signal to noise ratio, Near field
Two advanced nondestructive evaluation systems are developed for imaging surface-breaking cracks in aerospace materials. The systems use scanning heterodyne interferometry and frequency-translated holography principles to image ultrasonic displacement fields on material surfaces with high resolution and sensitivity. Surface-breaking cracks are detected and characterized by visualizing near-field ultrasonic scattering processes, which in turn results in local intensification of ultrasonic displacement fields in the immediate vicinity of a crack. The local intensification permits cracks to be easily distinguished from background levels, and creates unique displacement field images that follow the contours and morphology of the cracks with microscopic precision. The interferometric and holographic imaging approaches each provide noncontact and near optical-diffraction-limited measurement capabilities that are essential for probing ultrasonic displacement fields in the immediate vicinity of cracks. Several representative crack-imaging results are provided, along with detailed descriptions of both experimental techniques, and the capabilities and limitations of each method. The resulting systems provide simple yet very powerful tools for evaluating surface-breaking cracks in detail.
Liquid crystal polymer (LCP) is a new and innovative material being used as an alternative to polyimide in the flexible circuit industry. LCP has many intrinsic benefits over polyimide including lower moisture absorption and improved dimensional stability. However, LCP is very resistant to chemical milling or etching. As a result, other methods for processing the material are being investigated including laser micromachining. In this paper, three frequency converted diode-pumped solid-state (DPSS) Nd:YVO4 lasers at 355 nm were used to micromachine a LCP film and a copper/LCP laminate. Of them, two are Q-switched lasers operating in the nanosecond regime and the other a mode-locked laser in the picosecond regime. The Q-switched lasers can be operated at pulse repetition rates of 1 to 300 kHz while the mode-locked system is operated at 80 MHz. The micromachining experiments consisted of cutting the 50 μm thick LCP film, cutting the 18 μm thick copper on the film, and drilling micro-vias through both the copper coating and the film substrate. The laser/material interactions and processing speeds were studied and compared. The results show that, compared to polyimide film of the same thickness, LCP film can be more efficiently processed by laser micromachining. In addition, each laser has a unique advantage in processing LCP based flexible circuit materials. The Q-switched lasers are more capable of processing the copper coating while the mode-locked laser can cut LCP film faster with the smallest kerf width.
KEYWORDS: Corrosion, Ultrasonics, Interferometry, Nondestructive evaluation, Near field, Aluminum, Scattering, Digital imaging, Near field scanning optical microscopy, Heterodyning
A detailed microscopic characterization of stress-corrosion cracking (SCC) processes has been conducted for electro-chemically pitted AA 2024-T3 aluminum dogbone specimens in a high-cycle fatigue environment. The measurements were done in-situ using scanning laser ultrasonic detection of Rayleigh waves propagating along the material surface. Detailed microscopic NDE evaluations of crack extent and position, crack growth rates, and local crack depth were made based on near-field ultrasonic scattering signatures. A variety of electro-chemically generated corrosion pits were studied, where variations of pit depth, pitting surface area, and pit volume loss were correlated to fatigue life, crack initiation, and crack growth rates. The measurement technique provided an advanced crack 'imaging' capability that proved to be a very useful NDE tool for the micro-characterization of crack growth processes, and provided a wealth of information regarding the micro-features of the cracks whcih are currently not available with any other advanced NDE technique.
The detection and microscopic characterization of hidden corrosion has recently been a focus of several advanced NDE research efforts. A variety of approaches have been suggested, with laser ultrasonic (LU), scanning acoustic microscopy (SAM), thermography,and x-ray systems being four of the most promising NDE techniques. In this effort, a side-by-side comparison of each of these four techniques was conducted with the goal of assessing the detailed microscopic features of engineered and realistic hidden pitting corrosion reference samples. The reference samples included laser-etched cutouts and electro-chemically created surface pits ranging in size for 250 μm to 5 mm in surface extent, and depths of 25 μm to 1 mm. The effects of material loss/topography, corrosion-byproduct, and paint thickness levels were all addressed. Variations in measurement sensitivity, detectivity, and spatial resolution were studied, with particular attention being focused on the ability of the NDE technique to not only detect the hidden corrosion, but to provide any additional information regarding the microscopic nature of the corrosion area, its roughness, material loss levels, and pitting sharpness. In all cases, the NDE techniques provided an 'image' of the hidden corrosion areas, with some capability for assessing the internal structures of the pits from the measured signal levels or brightness levels of the measured image fields.
Recent advancements in small, microscopic NDE sensor technologies will revolutionize how aircraft maintenance is done, and will significantly improve the reliability and airworthiness of current and future aircraft systems. A variety of micro/nano systems and concepts are being developed that will enable whole new capabilities for detecting and tracking structural integrity damage. For aging aircraft systems, the impact of micro-NDE sensor technologies will be felt immediately, with dramatic reductions in labor for maintenance, and extended useable life of critical components being two of the primary benefits. For the fleet management of future aircraft systems, a comprehensive evaluation and tracking of vehicle health throughout its entire life cycle will be needed. Indeed, micro/nano NDE systems will be instrumental in realizing this futuristic vision. Several major challenges will need to be addressed, however, before micro- and nano-NDE systems can effectively be implemented, and this will require interdisciplinary research approaches, and a systematic engineering integration of the new technologies into real systems. Future research will need to emphasize systems engineering approaches for designing materials and structures with in-situ inspection and prognostic capabilities. Recent advances in 1) embedded / add-on micro-sensors, 2) computer modeling of nondestructive evaluation responses, and 3) wireless communications are important steps toward this goal, and will ultimately provide previously unimagined opportunities for realizing whole new integrated vehicle health monitoring capabilities. The future use of micro/nano NDE technologies as vehicle health monitoring tools will have profound implications, and will provide a revolutionary way of doing NDE in the near and distant future.
One of the most critical aspects of developing and optimizing capacitive micro-machined transducer systems involves the introduction of appropriate stress levels in the membrane structures during the manufacturing process. Subtle variations in the elastic modulus levels and mechanical coupling can dramatically alter the dynamic vibratory response of the MEMS for ultrasonic applications. In this effort, two different optical interferometric NDE approaches were used to evaluate the static and dynamic characteristics of individual MEMS elements in an ultrasonic transducer array system for variations of applied stress. The interferometric techniques provided a detailed microscopic characterization of the physical motions and local microscopic positions of the MEMS transducer membranes. It was found that the flexural response levels of individual MEMS membrane structures due to increased electrostatic forces was directly coupled to the dynamic response of the micro-transducer, and could potentially be used for optimizing the efficiency and dynamic motion extent of the MEMS transducer array. The optical interferometric techniques both proved to be valuable micro-NDE characterization tools, and were perfectly suited for characterizing the dynamic and static responses of the MEMS ultrasonic transducer systems.
It is important to determine the onset of damage as well as the extent of the damage area when materials are illuminated by intense laser radiation. In this work, an optical interferometric technique was used to assess laser-induced damage in semiconductor materials based on the three-dimensional, topographic characteristics of the damage site. Both antireflection coated and uncoated materials were evaluated for variations of fluence level, focused spot size, and laser repetition rate. The interferometric technique was non-contact and nondestructive in nature, providing a high-resolution capability of assessing damage levels on the surfaces of the materials.
A detailed micro-characterization of a MEMS ultrasonic transducer was done using a scanning heterodyne interferometry technique. Both temporal and spatial measurements were made of the out-of-plane displacement levels of the transducer under normal operating conditions. Spatial resolution levels approaching the optical diffraction limit of 1 mm were achieved, which allowed characterizations of individual micro-transducer elements to be made. The resonance characteristics of individual transducer membranes were evaluated for drive frequencies between 1 MHz and 7 MHz. Although the majority of transducer elements showed nearly identical frequency response characteristics, several of the MEMS elements showed evidence of shifted resonance response features, which dramatically altered their performance level. Displacement levels in excess of 100 nm were observed for peak DC and AC drive voltage input levels. Time-sequenced measurements of the oscillating MEMS structures were also studied, and showed phase-reversal effects near the edges of transducer membranes. The scanning interferometry technique proved to be a very useful NDE tool for micro-characterization, and provided a wealth of information regarding the micro-features of the MEMS ultrasonic transducer which are currently not available with any other advanced NDE.
Accurate knowledge of the decay rates of optically generated charge carriers in bulk semiconductor materials is important for various infrared applications. Most of the published decay rates of free carriers generated with above band-gap energy light, in materials such as InAs and InSb are obtained from measurements in thin films. Stronger attenuation of above band gap energy light in these materials prevents the probing in samples thicker than a few microns. To study the decay of free carriers in the bulk semiconductor wafers, we use two-photon absorption of below band gap energy light (obtained from a pulsed CO2 laser). This method generated charge carriers throughout the bulk of the material used (typically having thickness of 1 - 2 mm). The decay of the charge carriers is then probed with a low power cw infrared laser (also with photon energy below the band gap). The decay rates measured at different temperatures are compared with calculations that include Auger and defect-assisted Shockley-Reed-Hall (SRH) recombination processes. Calculation of various recombination processes indicate that the lifetimes are limited by SRH mechanism in InAs samples.
Innovative optical NDE techniques are being developed for the full-field detection and evaluation of surface defects and defect precursors in titanium and aluminum based alloys. The techniques are based on frequency-translated holography and optical correlation principles, and use bacteriohodopsin (bR) holographic films and temporal correlation techniques for real-time storage and retrieval of Surface Acoustic Waves (SAW) features and embedded surface defect information. The SAW waves induced on the material surface being studied are made to interfere with optical light waves, and fringes are produced that are a function of optical Doppler shifts induced by phonon-photon interaction on the surface of the materials. Visualization of these SAW patterns allow for NDE characterization of features on and near the surface of the materials, including defect and defect precursor sites. Preliminary results are provided for real-time bR holographic recordings of acoustic patterns induced on Al2024-T3 material surfaces.
An advanced electronic phase stepped interferometry (EPSI) system is described for quantitative out-of-plane displacement and surface topography measurements for NDE applications. Image processing algorithms were developed using novel techniques to extend the sensitivity of EPSI and provide near real-time measurement capability. EPSI is known to provide out-of-plane displacement and surface topography measurements on the order of 1/20-1/100 microns. Noise can be a limiting factor, however, and robust phase unwrapping, which is required, remains an open and active research area. A prototype EPSI system was developed and tested using simulated and real data sets of known displacement fields. A robust maximum-likelihood binary-tree (MLBT) phase unwrapping technique was developed, providing greater robustness in the presence of noise than standard techniques. In addition, the MLBT algorithm lends itself to efficient and fast parallel implementations for near-real time implementation. The use of these advanced algorithms improves the capabilities of existing EPSI techniques, providing more robustness and near real-time measurements. Results are provided for deformations in Ti-6Al-4V and Al2024-T3 aerospace materials.
This paper describes particle image velocimetry measurements obtained in a Mach 6 flow field. The measurements were performed in the Langley Mach 6 High Reynolds Number Tunnel facility. A wedge model oriented at an angle-of-attack of -15 degree(s) was used to generate an oblique 22.7 degree(s) bow shock. Using 1.0-micrometer aluminum oxide powder as the seed material, PIV photographs in the vicinity of the bow shock region were taken on the centerline of the model at a location 110 mm from the leading edge. Using the two- dimensional velocity fields obtained from analysis of the photographs, normal and tangential components of velocity with respect to the shock angle were obtained. These velocity components were then used to infer the aerodynamic particle sizes present in the tunnel. Results indicated that the 1.0 micrometer seed material introduced into the tunnel had an aerodynamic size of approximately 1.0 - 2.0 micrometers. Differences were noted between the measured and predicted normal component of velocity downstream of the shock after full particle relaxation had occurred. Using qualitative flow visualization, it was determined that due to cavity flow along the test section walls, reflected shocks in the vicinity of the PIV measurements could account for these differences.
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