09:00
Session 4: Life extension and management of ageing fleets
Chair: Daniele Fanteria
09:00
20 mins
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The inherent need for holistic structural integrity application and progress
Kimberli Jones, Bradley Kuramoto, Bryce Harris, David Hoeppner
Abstract: The holistic structural integrity process (HOLSIP) has been gaining attention within structural integrity communities worldwide in recent years. HOLSIP involves accounting for elements beyond the safe-life and damage tolerance design paradigms, including physics-based and probabilistic model creation and use of advanced nondestructive inspection techniques. Material microstructure, crack growth phases, Initial Discontinuity States (IDS), residual stresses, and environmental considerations are incorporated within this paradigm. Maintenance and unintentional damage also are a factor within the holistic framework. The end goal of HOLSIP is to maintain structural integrity by accounting for these factors and predict potential failures, including environmental impacts and time-based degradation, throughout the useful life of a system and is applicable to many industries, not just aerospace applications.
HOLSIP’s practices are becoming accepted as problem solving methodologies to many of the aging issues affecting United States Air Force legacy aircraft fleets. The proposed presentation will detail some of the holistic approach advances used in ASIP and other structural integrity organizations and focus on the progress which has been made in these efforts over the past 50+ years. Examples from the authors’ experiences in F-16 sustainment will be used to explain how time-based degradation and failure modes must be accounted for throughout the life of the aircraft.
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09:20
20 mins
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Towards deriving loads spectra representing operational life: Equivalent flight profile versus single flight profiles
Juliana Diniz Mattos, Mariano Arbelo, Frank Machado
Abstract: Deriving the loads spectra is an essential task for structural sizing to achieve an acceptable economical life for a product. Furthermore, compliance with the certification requirements shall be based on typical loading spectra expected in service. The task that arises consists in deriving loads spectra to represent an operational life comprised by different flight profiles whose features may vary widely from one operator to another. The current work aims to compare two approaches used to address this issue: either treating each flight profile distinctly or deriving one equivalent flight profile that intends to accommodate all the former ones. A study case defined for a hypothetical loading scenario is presented and a stress analysis is performed to compare the fatigue results from both approaches. The benefits and risks that result from adopting one approach or another are also evaluated. Finally, this work provides resources to optimize the application of the different proposed loading spectra along the overall aircraft design process.
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09:40
20 mins
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On the development of statistical knock-down factors for WFD assessment
Wagner Rissardo, Carlos Chaves
Abstract: The nucleation of multiple fatigue cracks over large areas, how they interact, and whether they can lead to a catastrophic event has been for a long time a concern, not only for the OEMs and the Regulatory Agencies, but also for the entire community involved in aircraft fleet management, as long as the fleets age.
The phenomenon of Widespread Fatigue Damage (WFD) has been investigated for decades, and to demonstrate freedom of WFD up to the LOV for large transport aircraft structures became mandatory in the latest amendments of the regulations.
During the ICAF Symposium in 2013, Safarian [1] has proposed an approach to assess WFD based on fatigue life estimation and the application of a set of knock-down factors.
The present work provides an in-depth investigation for each of these factors, bringing a deduction or an explanation for them based on statistical approaches, together with some examples of application.
By understanding each and every factor’s origin and eventual built-in conservativeness, the engineer in charge can propose alternative approaches and come out with more realistic results, eventually saving weight and resources, without compromising safety.
[1] P. Safarian, (2013), Widespread Fatigue Damage Evaluation Method, Proceedings of the 27th ICAF Symposium, Jerusalem.
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