ASIP Conference 2022

(Aircraft Structural Integrity Program)

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The ASIP dates back to a 1950's Air Force publication on structural integrity requirements. It was known from an early stage that ASIP was a vital program in prolonging the life and ensuring the structural safety of all aircraft. Meetings began in the 1970's, but it wasn't until 1984 that it was reshaped into the current conference format. Incidents like the 1988 Aloha Flight 243 Air Disaster highlighted the importance of ASIP requirements and the contributions of the ASIP community, to preclude the recurrence of such tragedies in the future. The ASIP Conference helps to accomplish this through the personal interactions of its attendees, resulting in the exchange of vital ideas and technology.

Stehlin Engineering Contribution to ASIP Conference 2022

Mr Thierry Stehlin has been selected to write a presentation in name of RUAG for the ASIP Conference 2022. The presentation shows the development of the Hybrid Crack Growth Analysis (HCGA) as a method used in the Swiss F/A-18 program to partially address the lack of short crack growth material data.

--------------------- Presentation Abstract ---------------------

The Swiss company RUAG AG is responsible to support the service life extension of the Swiss F/A-18. The development and implementation of this program is challenging because in several areas the fatigue usage of the Swiss F/A-18 is more severe to the original design spectrum and as well compared to other F/A-18 fleets. Indeed, although the Swiss fleet is not the “oldest” in terms of flight hours, it is the world fleet leader for some components in terms of fatigue usage.

In this context, the use of classical analytical methodologies to perform fatigue and damage tolerance assessments is clearly not sufficient to achieve the required Swiss fleet availability and airworthiness goals. Therefore, alternative approaches are being defined/developed, such as total life analysis and risk assessment using probabilistic Crack Growth (CG) analysis. To be successful, these alternative approaches require CG analyses starting at very short initial crack sizes, while most of commercially available CG material data is applicable to long cracks only.

The Hybrid Crack Growth Analysis (HCGA) method developed at RUAG partially addresses the issue of lack of short crack growth material data.

 The method is based on observations made on many Quantitative Fractography (QF) investigations made on natural cracks developing under typical fighter aircraft spectra, amongst other published by the Defense Science and Technology Group (DSTG). One of the main observation made is that the crack growth is fairly log-linear up to several millimeters.

The HCGA method uses a combination of standard CG analysis and a log-linear back extrapolation to estimate the crack growth rate in the short crack regime with the goal to derive a total life CG curve.

A first validation step, made by comparing the results obtained with the HCGA method and those obtained using short crack growth material data developed by DSTG for an aluminum temper for different spectra, show that the new method is able to predict the total life astonishingly well.

A second validation step, which is still ongoing, was initiated with the aim of comparing the HCGA results with QF data available from various projects. The general observations are that the HCGA delivers acceptable results under certain conditions, globally delivers conservative results and may be applicable to aluminum in general, while restrictions may be applicable for steel and titanium.

In absence of reliable short crack material data, the HCGA method provides RUAG an additional tool to support the service life extension by performing total life analyses and risk assessments for the Swiss F/A-18 ensuring airworthiness, while limiting the burden of maintenance work. As well, it is believed that the HCGA could provide a useful simplified method to support the ASIP of aging aircraft and the design of new aircrafts.

The methodology may also be useful for the derivation of CG master curves for subsequent probabilistic damage tolerance analysis or for the assessment of Widespread Fatigue Damage. The presentation covers the background, assumptions, methodology, validations, limitations, usages and possible future developments of the HCGA method.