1. Foam Reinforced Aircraft Fuselage Study
Narasimha Harindra Vedala, Tarek Lazghab, Amit Datye, K.H. Wu
Mechanical And Materials Engineering Department
Florida International University
Miami, Florida

2. Overview Propose a strategy to reduce the effect of fatigue failure
Simulate the failure behavior of fuselages due fatigue using finite element analysis
Determine the effect of reinforcement on fatigue life due to the new proposed strategy

3. Fatigue Failure
It is defined as the failure of a metal structure due to cyclic loading.
Fatigue Failure occurs at load amplitudes much lower than the breaking load of the material

4. Aloha incident in 1988 caused due to fatigue failure
Fatigue Failure in Aircrafts
Aging aircrafts fatigue analysis
Pressurization and Depressurisation of the fuselage results in cyclic stresses
Aloha incident in caused due to fatigue failure

5. How to reduce the effect of fatigue failure?
Using composite materials for manufacturing fuselages.
Using Reinforcement to the fuselage
Pantherskin
Research on polyisocyanurate foam at FIU since Nicknamed as Pantherskin

6. Why Pantherskin? Inexpensive foam Light weight :Density =2 to 4 lb/ft3
High fire resistance

7. Fracture in metals Modes of Cracking in flat bodies
(based on Linear Elastic Fracture Mechanics)
Mode I
Mode II
Mode III

8. Objective
Study the effect of adding foam layer to stress distribution in the fuselage
Generate a Finite Element model to conduct fatigue analysis on fuselage with foam reinforcement
Compare with results from previous methods

9. Fracture parameters Stress Intensity Factors
It is defined a quantity that characterizes the severity of the crack situation
For flat plate with infinite width
Strain Energy Release Rate
E = Elastic modulus

10. Verification study Procedure Verification
Flat plate with central crack
Results were compared with
Rybicki’s (1977) model

12. Finite Element Analysis
Model Definition
Aircraft fuselage can be considered as a cylinder with equal spaced cracks
Considering
symmetric
geometry
a section of the
fuselage is used for
finite element analysis

13. Verification study : Unstiffened curved panel
Procedure
Verification
Compared
with Young’s model
using STAGS FE
package
Longitudinal crack configuration

14. Bulging of crack in the fuselage

15. FEA of Aircraft Fuselage
Boeing B737
obtained
Configuration
R=74.018
t= 0.036
Aluminum
EA=10.5 Msi vA = 0.33
Foam
EF =3000 psi vF = 0.3

16. Components of fuselage

17. Finite Element Model

18. Boundary Conditions and loading

19. Von Mises Stress

20. Stress distribution in the panel when 2 foam is added

23. Fatigue Analysis Coffin-Manson equation Cycles until failure.
Strain range
Slope of elastic strain amplitude vs. fatigue life.
One reversal intercept of plastic strain vs. life line.
On reversal intercept of elastic strain vs. life line.
Slope of plastic strain amplitude vs. fatigue life

24. Fuselage panel with Longitudinal Crack 4.5
Fatigue Analysis
Fuselage panel with Longitudinal Crack 4.5

25. Global model and Submodel
A submodel is generated to study the stress distribution near the crack
Results for the global model are used for the nodes at the boundary of the submodel
Location of submodel in the global fuselage panel

26. Von mises stress Distribution in Submodel

28. Conclusion
Reinforcement of the fuselage skin with foam help to reduce the stresses distribution
18% for 2 " foam layer
25% for 4 " foam layer
A maximum of 32% reduction in stress intensity factor is possible for 4" foam addition
A 3.7 fold increase in life of the aircraft is achievable if 4" foam is used
A parametric is developed which can be used for different fuselage configurations

29. References
Chen, D.,(1991), “Bulging of Fatigue Cracks in a Pressurized Aircraft Fuselage,” Ph. D. Dissertation, Department of Aerospace Engineering. Delft, The Netherlands: Delft University of Technology
Rybicki, E.F., Kannien, M.F.,(1977) “A Finite Element calculation of stress intensity factors by a modified crack closure integral”. Engineering Fracture Mechanics, Vol. 9, pp
Ahmed, A.A, Backukas, J., Jr. and Paul W. Tan, Awerbuch J., (2002) “Initiation and distribution of multiple-site damage (MSD) in fuselage lap joint curved panel”, Sixth Joint FAA/DoD/NASA Conference of Aging Aircrafts, Albuquerque, San Francisco, CA.
Torres, M.J. and Wu, K.H., (1993), “A New Approach to solve aging airplane problems using Polyisocyanurate”, Journal of Cellular Plastics, Vol. 29, N4, p.p 380.
Rahman, A., Backukas J.G., Jr., Paul W. Tan, and Catherine A. Bigelow,(2002), “Bulging Effects on longitudinal cracks in lap joints of pressurized aircraft fuselage”, Sixth Joint FAA/DoD/NASA Conference of Aging Aircrafts, Albuquerque, San Francisco, CA.

30. Future Research Several modeling issues were encountered
Numerical formulation for defining fracture parameters for composite materials is not yet available
Results have to be verified by experimental tests

31. Questions