(Aircraft Structural Integrity Program)
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Overview
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.
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