Vinod S. Agarwala

Assessment of incipient material damage and remaining life prediction with nonlinear acoustics

Subjected to a wide spectrum of operational and environmental loads, engineering structures exhibit first signs of material deterioration at micro and meso scales. In contrast to nondestructive assessment of the macro‐scale damage that resulted from material fracture, a limited number of technologies have tackled the complex task of detecting and monitoring the small‐scale incipient damage. This paper discuses application of the nonlinear acoustic vibro‐modulation technique for assessment of the micro/meso scale fatigue damage and remaining life prediction of metallic samples. A nonlinear acoustic damage index (DI), which indicates strength of the damage‐induced nonlinear interaction of the high‐frequency ultrasonic wave and the low‐frequency structural vibration, was monitored during a set of strain‐controlled fatigue tests. Stable growth of the damage index was observed at an increasing number of fatigue cycles showing correlation between DI and the micro/meso scale damage accumulation. This correlation...

Experimental Study on Hidden Corrosion/Delamination Detection with Ultrasonic Guided Waves

Hidden corrosion detection is critical in the aerospace industry. Occurring on the inside surfaces or at the interfaces of an aircraft’s skin, the corrosion must be detected from the outside surface. Surface waves are, therefore, not suitable for detecting such defects/failures. Ultrasonic bulk wave methods can be used to detect the corrosion caused thinning in the wall or a delamination of a structure [1,2]. However, since the method is based on point-by-point testing, it becomes a tedious time consuming procedure for large area inspection. Guided wave methods are being developed to tackle this problem [3–6]. An experimental study of hidden corrosion/delamination detection in single/multiple layer aluminum plates is conducted with specially selected ultrasonic guided wave modes. Both corrosion simulation specimens by machine cutting, and real corrosion specimens by electrochemical processing, Two-layer specimens have been prepared with such corroded sheets to form an artificial interface corrosion/delamination. Various wave modes are subsequently generated on these specimens to examine the implications of thinning on mode cutoff, group velocity changes, mode frequency shifts, and transmission and reflection amplitudes. Finally, a practical problem of skin to honeycomb core delamination detection with guided waves is also addressed.

Corrosion Fatigue in 7075-T6 Aluminum: Life Prediction Issues for Carrier Based Operations

Fatigue experiments performed on 7075-T6 Al alloy at various frequencies and stress levels and under alternating wet and dry environments indicate that simultaneous action of corrosion and fatigue substantially accelerates crack initiation and growth rates when compared to pure fatigue (dry air) conditions. In experiments performed under alternating wet and dry conditions, fatigue crack growth rate was observed to increase rapidly in the presence of mildly corrosive (salt solution) and decrease sharply when subjected to noncorrosive dry air. Crack arrest of various durations was observed at transition points between dry and wet cycles. Lowering the frequency of fatigue loading significantly reduced crack initiation and overall life time. S-N curves showed a continuous downward trend without reaching a plateau or threshold. These observations led to the conclusion that aircraft structural integrity can be seriously compromised even under mildly aggressive environments and at subcritical stress levels when cracks are present regardless of changing of environments and/or test frequency.

Micro/meso scale fatigue damage accumulation monitoring using nonlinear acoustic vibro-modulation measurements

Monitoring the incipient damage at the earliest possible stage is essential for predicting structural performance and remaining life of structural components. Existing prognostic methodologies incorporate conventional SHM and NDE techniques responsive to cracks and delaminations resulted from the irreversible material fracture and disintegration at the macro-scale. There is an increasing need for technologies that could allow for monitoring material degradation at the micro/meso scale before the onset of the macro-scale fracture. In this contribution, we report results of the real-time monitoring of the material micro/meso scale degradation using the nonlinear acoustic vibro-modulation technique. The technique explores nonlinear acoustic interaction of high frequency ultrasound and low frequency structural vibration at the site of the incipient damage. The indicator of the damage severity, nonlinear acoustic damage index (DI), was measured in real time during the strain-controlled three-point bending fatigue test of aluminum and steel specimens. Nondestructively, degradation of the specimen was revealed through the increase in the DI, which correlated well with the respective decrease in the specimens stiffness. Destructive SEM examination confirmed sensitivity of the DI to the incipient micro/meso scale damage and advocated for utilizing the vibro-modulation approach for assessment of material degradation before fracture.

Fatigue Damage Monitoring and Remaning Life Assessment using Nonlinear Vibro‐Modulation Technique

Assessment of structural deterioration due to in‐service and environmental loads is an essential element in ensuring safety, operability and long life of various structures and structural components. We applied vibro‐acoustic modulation technique to monitor material degradation at the micro/meso scale before the onset of the macro‐scale fracture. The technique explores nonlinear acoustic interaction of high frequency ultrasound and low frequency structural vibration at the site of the incipient damage. It is shown that micro/meso scale degradation increases the material nonlinearity leading to modulation of the high frequency ultrasonic signal by low frequency vibration, quantified by the nonlinear acoustic Damage Index (DI). Numerous tests with hundreds of test coupons proved high sensitivity of the developed technique to micro/meso scale fatigue damage accumulation. The study further demonstrated that the remaining life of the fatigued material can be predicted using phenomenological damage accumulation...

Damage Assessment With Nonlinear Vibro-Acoustic Modulation Technique

Assessment of structural deterioration due to in-service and environmental loads is an essential element in ensuring safety, operability and long life of various structures and structural components. There is an increasing need for technologies that could allow for monitoring material degradation at the micro/meso scale before the onset of the macro-scale fracture. In this contribution, we report results of the real-time monitoring of the material micro/meso scale degradation using the nonlinear acoustic vibro-modulation technique. The technique explores nonlinear acoustic interaction of high frequency ultrasound and low frequency structural vibration at the site of the incipient damage. It is shown that micro/meso scale degradation increases the material nonlinearity leading to modulation of the high frequency ultrasonic signal by low frequency vibration, quantified by the nonlinear acoustic damage index (DI). The correlation of DI measured for hundreds of test coupons with material degradation was confirmed using traditional ultrasonic and SEM examinations.Copyright