Jeong K. Na

Detection of surface breaking fatigue crack on a complex aircraft structure with Rayleigh surface waves

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.

Ultrasound finite element simulation sensitivity to anisotropic titanium microstructures

Analytical wave models are inadequate to describe complex metallic microstructure interactions especially for near field anisotropic property effects and through geometric features smaller than the wavelength. In contrast, finite element ultrasound simulations inherently capture microstructure influences due to their reliance on material definitions rather than wave descriptions. To better understand and quantify heterogeneous crystal orientation effects to ultrasonic wave propagation, a finite element modeling case study has been performed with anisotropic titanium grain structures. A parameterized model has been developed utilizing anisotropic spheres within a bulk material. The resulting wave parameters are analyzed as functions of both wavelength and sphere to bulk crystal mismatch angle.

Detection of impact damage on thermal protection systems using thin- film piezoelectric sensors for integrated structural health monitoring

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.

Investigation of bond quality effects on piezoelectric sensing of impact damage

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.

Disbonding Effects on Elastic Wave Generation and Reception by Bonded Piezoelectric Sensor Systems

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.

Root-cause estimation of ultrasonic scattering signatures within a complex textured titanium

The nondestructive evaluation of polycrystalline materials has been an active area of research for many decades, and continues to be an area of growth in recent years. Titanium alloys in particular have become a critical material system used in modern turbine engine applications, where an evaluation of the local microstructure properties of engine disk/blade components is desired for performance and remaining life assessments. Current NDE methods are often limited to estimating ensemble material properties or detecting localized voids, inclusions, or damage features within a material. Recent advances in computational NDE and material science characterization methods are providing new and unprecedented access to heterogeneous material properties, which permits microstructure-sensing interactions to be studied in detail. In the present research, Integrated Computational Materials Engineering (ICME) methods and tools are being leveraged to gain a comprehensive understanding of root-cause ultrasonic scatterin...

Effect of irregularity in shape and boundary of a macro-texture region in titanium

Peak amplitudes of mode converted shear wave signals back scattered from macro-texture regions (MTRs) in an aerospace grade titanium alloy material are measured to be about the same level as corner trapped shear wave signals. In addition to the abnormally high shear wave responses, the time of flight data indicates that the MTR signals are back scattered from a location deep in the sample so that the round trip travel time is close to that of corner trapped signals. In this work, these two ultrasonic properties of an MTR in a test specimen cut from a titanium jet engine disk are closely studied to understand the root cause of abnormally high shear wave responses. Based on the amplitude and time of flight data collected in a laboratory condition, a decision has been made to investigate further experimentally and computationally how surface irregularity of an acoustically reflective surface affects incoming shear waves upon reflection. Attempts are made to correlate the localized back scattered signal respo...

CHARACTERIZATION OF DIRECT DEPOSIT PIEZOELECTRIC SENSORS FOR STRUCTURAL HEALTH MONITORING APPLICATIONS

Recent developments of Sol‐Gel based direct deposited lead‐zirconate‐titanate (PZT) piezoelectric transducers prompted an investigation into the consistency of their sensing characteristics. Five samples were evaluated to determine if the manufacturing process creates characteristic differences in performance from sensor to sensor. Tests characterizing the transducers investigated included resonant frequency, harmonic distortion, damping characteristics, energy distribution, and durability measurements. Despite minor physical differences incurred during their fabrication, the transducers displayed consistent properties. The performance of a Lithium Niobate single crystal transducer element and commercial off‐the‐shelf transducers are compared to these spray‐on sensors.

Spatially distributed damage detection in CMC thermal protection materials using thin-film piezoelectric sensors

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.