Fatigue Analyses of modifications on pressurized aircraft fuselages are both necessary and tedious. Using the Hyperworks software suite and StressCheck, RUAG has developed a fatigue analysis method which streamlines the process from the creation of the spectrum up to the detailed analysis of selected fastener holes and delivers results quickly and efficiently. This method was then used to certify the installation of two large windows in the floor of a single engine turboprop A/C for aerial survey applications. Speakers David Schmid, Manager Structural Analysis, RUAG Schweiz AG

1. Fatigue Analysis of a
Pressurized Aircraft Fuselage
Modification using Hyperworks
and StressCheck
David Schmid
Manager Structural Analysis
RUAG Aviation
Paris, 01.10.2015

2. 07.10.2015RUAG Aviation2
Agenda
 RUAG Aviation at a Glance
 PC12 Aerial Survey A/C Modification
 Fatigue Process Overview
 Spectrum Creation using HyperMath
 Postprocessing
 Fastener Detail Analysis
 Method Validation Example

3. RUAG Aviation
Based in: CH, DE, USA, AU, MY, BR
Net sales: CHF 525 million
EBIT: CHF 38 million
Employees: 2,022
Customers/partners: Swiss Air Force, German
Bundeswehr, Royal Australian Air Force, Bombardier,
Dassault, Embraer, Airbus Group, Airbus Helicopters,
Pilatus, Boeing, General Electric, Northrop
Grumman and Saab
 Manager and integrator of systems and
components for civil and military aviation
 Repairs and maintenance work, upgrades,
and the manufacture and integration of
subsystems on aircraft and helicopters over
their entire lifespan
 Manufacturer of the Dornier 228: Aircraft for
challenging special missions as well as
passenger and cargo transportation

4. Aerial Survey Modification
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Two windows in the lower
fuselage aft of the main
wing (approx. 500 x
500mm & 500x200mm)
for the installation of
survey cameras or
LIDAR

5. Modification – CAD View
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6. Detailed CAD-View
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Unmodified
Modified
New
FWD

7. HyperView
StressCheck
OptiStructHyperMesh
HyperMath
Process Overview – Safe Life Analysis
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DFEM Creation Spectrum Creation
Fatigue Process
S-N Curves
PostprocessingDamage Calculation
Detailed AnalysisCritical Locations
Reporting
𝐷𝑖 =
𝑛𝑖
𝑁𝑖,𝑓
≥ 1.0

8. Fatigue DFEM
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 Fatigue DFEM contains
relevant structure at
bottom fuselage with
element size approx. 4mm
 Fasteners modeled as
CBUSH elements with
stiffness calculated acc. to
Huth formula
 Relevant Load Cases
applied as freebody loads
from GFEM

9. Generate Spectrum
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Advantages Disadvantages
+ Only few A/C specific
parameters required
+ Output can be used directly in
OptiStruct
+ Outputs Maneuver, Gust,
Landing, Taxi and Fuselage
Pressure spectra
+ New missions can be easily
included by adding mission file
in ASCII format
+ Corresponds largely to method
described in AC23-13A
- Asymmetric maneuvers and
VT / HT Gust not included
- Flights all have same severity
- No GUI
 RUAG HyperMath Program to calculate load
spectra for CS / FAR23 A/C

10. Generate Spectrum
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 Exceedance curves from FAA
AC23-13 were used to create
the spectrum
 Method to build spectrum is
stochastic. While the total
number of events per FH is the
same as in the exceedance
curves, the number of cycles in
the individual bins might not
 Figures show that the
exceedance curves calculated
from the generated spectrum
match the basic AC23-13
exceedance curves

11. Fatigue Calculation Setup
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 HyperMesh Fatigue process was used to set up the
model for the first time -> fewer errors & quick setup
time
 Material data (S-N curves) from MMPDS with
appropriate Kt
 Reference load cases (gust, maneuver, landing, taxi
and pressure) assigned to respective spectra
 Safety factors applied on stresses of reference load
cases to account for influence of e.g. blind vs. Solid
rivets
 OptiStruct fatigue solver was used to calculate the
damage

12. PostProcessing
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 HotSpot finder
from the MVP
toolbar was used
to find locations
with damage > 1
within a certain
distance to each
other
 Elements immediately adjacent to fastener elements
were ignored

13. Detailed Analysis
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 Detailed analysis methodology is similar to the one presented
during EATC 2014
 Freebody forces applied to fastener hole StressCheck model
 Stress concentration was calculated for each load case
(pressure, gust / maneuver & landing) using StressCheck model
 Damage was calculated using optistruct using the stresses
calculated with StressCheck

14. Method Validation Example
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 FEM Fatigue model of
coupon using the same
methodology and data as
in the PC12 fatigue
analysis
 Calculated stresses need
to be multiplied by safety
factors depending on
configuration (solid vs.
blind rivet, countersunk
vs. universal head)
 Analytical method gives
conservative results ->
good for global calculation

15. Model Validation Example
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16. Summary
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 Complete fatigue analysis (safe life) of primary structure
of a CS23 aircraft using HyperWorks / APA software
 Structure is certified and flying
 Only one piece of proprietary information was
necessary (joint test data to validate the method), all
other information / data is public
 Works well for sheet metal parts with / or without
fastener holes. Complex parts need a different approach
 Fast approach, largest single item is creating the
detailed fatigue FE model

17. 07.10.2015RUAG Aviation17
Thank you for your attention!