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) can be subjected to impact damage during flight and/or during ground maintenance
and/or repair. AFRL/RXLP is developing a reliable and robust on-board sensing/monitoring capability for next
generation thermal protection systems to detect and assess impact damage. This study was focused on two classes of
metallic thermal protection tiles to determine threshold for impact damage and develop sensing capability of the impacts.
Sensors made of PVDF piezoelectric film were employed and tested to evaluate the detectability of impact signals and
assess the onset or threshold of impact damage. Testing was performed over a range of impact energy levels, where the
sensors were adhered to the back of the specimens. The PVDF signal levels were analyzed and compared to assess
damage, where digital microscopy, visual inspection, and white light interferometry were used for damage verification.
Based on the impact test results, an assessment of the impact damage thresholds for each type of metallic TPS system
was made.
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.
KEYWORDS: Sensors, Aluminum, Magnetic sensors, Heat treatments, Magnetism, Calibration, Temperature metrology, Nondestructive evaluation, Process control, Atomic force microscopy
Sensors for online monitoring of the heat treatment of aluminum alloys have been developed. The correlation between heat treatment parameters of Al 7075, the microstructure of the materials and NDE measurements has being studied systematically. By establishing correlations to the process parameters, it was assumed to be possible to devise a technique that provides not only insight into the aging process but also act as a quality control method for process verification. Multi-frequency eddy current allows compensation for environmental influences along with the ability to perform measurements at elevated temperatures and is therefore suitable for process monitoring.
7075 T651 aluminum alloy is frequently used in aircraft applications for its high strength to weight ratio. However, aircraft parts made of this alloy have been plagued by stress corrosion cracking (SCC). Retrogression and re-aging (RRA) is a post T651 two-stage heat treatment that provides improved SCC resistance with minimal loss in tensile strength. In this study, various forms of microscopy and mechanical testing are used to investigate how the RRA process affects the microstructure. The microscopic observations in this paper show that the precipitates in the aluminum alloy coarsen and that the grain boundary regions are depleted of copper and magnesium. The mechanical testing performed shows that the aluminum alloy decreases in strength and increases in conductivity when exposed to longer retrogression times prior to re-aging.
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