Aircraft Stress Sequence Development: A Complex Engineering
Process Made Simple
Development of stress sequences for critical aircraft structure
requires flight measured usage data, known aircraft loads, and established relationships
between flight loads and structural stresses. Resulting cycle-by- cycle stress sequences
can be directly usable for crack growth analysis and coupon spectra tests. Often, an
expert in loads and spectra development manipulates the usage data into a typical sequence
of representative flight conditions for which loads and stresses are calculated. For a
fighter/trainer type aircraft, this effort is repeated many times for each of the fatigue
critical locations (FCL) resulting in expenditure of numerous engineering hours.
The Aircraft Stress Sequence Computer Program (ACSTRSEQ),
developed by SwRI under contract to the San Antonio Air Logistics Center, presents a
unique approach for making complex technical computations in a simple, easy to use method.
The program is written in Microsoft Visual Basic for the Microsoft Windows environment.
This environment has become common among users of personal computers and offers a
consistent user interface. The benefit of a common user interface is to reduce the time
required for an individual to learn the operation of a program written for that
The program was originally developed for the T-38 aircraft.
Subsequently, capabilities for T-37, F-5A, F- 5B, F-5E, and F-5F aircraft were added.
Although, ACSTRSEQ performs many complex engineering computations, these complexities are
not apparent to the user. The user is exposed to a number of choices such as aircraft
type, FCL of interest, and type of stress sequence desired. To further simplify the
process for the user, choices are made by a pointing device such as a mouse on either menu
items or screen images of the aircraft structure of interest. Screen images are either
photographic or schematic depending on the information or detail that is required at that
point in the decision process. Various levels of information detail are available to the
user depending on his knowledge or experience level. Less experienced users can choose
structural areas of interest from screen pictures of the wing, fuselage, and empennage.
However, users experienced in aircraft structural considerations and program usage may
select program options from text menus. In addition, schematics are provided for each FCL
which show the general location of the FCL as well as details such as fastener hold
diameters and material thicknesses and other crack parameters for which stresses will be
computed. These structural details are required for the crack growth analysis procedures
for which the stress sequence are produced.
Printed user and programmer documentation is provided with
ACSTRSEQ and is also available as part of the program "help" facility. The
on-line help capability, which includes the full text of the user/programmer logic manual,
allows the user full access to information regarding all program aspects through the use
of index, search, or hypertext linking functions. Included in this documentation are
engineering considerations, equations, and brief tutorials on data sources and results
applications. As such, the on-line documentation provides a basic level of training for
the determination of aircraft loads, stresses, and stress sequences required for crack
growth analyses and laboratory testing.
The Aircraft Stress Sequence Computer Program integrates large,
cumbersome reference information with the complex techniques of stress sequence
development in a graphical user interface (GUI) that is suitable for both novice users as
well as engineers experienced in spectra development. The result is a tool that provides
accurate and repeatable results in a timely fashion to meet the demands of continuous
damage tolerance analyses and testing programs. The concepts exhibited by this computer
program only begin to explore the possibilities of productivity gains that can be obtained
with properly designed state-of-the-art software.
Mechanical and Materials Engineering Publications