1. Imperial College
OF SCIENCE TECHNOLOGY AND MEDICINE Department of Aeronautics
Failure Analysis of a Composite Wingbox with Impact Damage:- A Fracture Mechanics Approach
Michael Koundouros
COMPTEST 2003
29 January 2003
Sponsored by QinetiQ
29/01/03 M.Koundouros
© QinetiQ (2003)

2. Introduction A CFRP Wingbox manufactured and tested by QinetiQ
Fixed at Root
Loaded at Tip like a Cantilever Beam
Tapered Skins (4mm7mm)
6 Spars
T800/924 carbon epoxy system
150 J impact damage in skin
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© QinetiQ (2003)

3. Introduction Cont… Possible Failure Modes which may occur are:
Skin Failure
Due to Microbuckling
Due to Delamination
Due to Matrix Cracking
Stiffener Breakage
Due to Buckling
Skin/Stiffener Interface Failure
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4. Analysis Aims Attempt Static linear Analysis of wingbox using FE77
Without any form of damage
Replace the damage zone by an open hole
Compare Surface Strains from FE to Gauge Readings
Extract Direct Stresses in 0Deg Plies for entire Surface
Apply Soutis-Fleck Fracture model to predict the failure load of the wingbox
Compare this result with other failure criteria like:
Max Stress Criterion (with and without damage present)
29/01/03 M.Koundouros
© QinetiQ (2003)

5. Linear Static Analysis – Undamaged Config.
8 Noded Shell Elements
Stiffeners attached to skins via rigid links
Taper in skin modelled by setting thickness of certain layers to zero
Root fully fixed against disp. & rotation.
Concentrated forces at each spar
29/01/03 M.Koundouros
© QinetiQ (2003)

6. Linear Static Analysis – Undamaged Config.
Maximum COMPRESSIVE stresses in fibre direction of 0Deg ply across width of wingbox
High Localised Stresses at root
Max. average stress at 17.2%L
Max. Stress Criteria358 kN or 236 kN if local high stresses are considered.
Reduction of 34% in strength
29/01/03 M.Koundouros
© QinetiQ (2003)

7. Linear Static Analysis – Damaged Config.
Damage caused by 150J low velocity impact replaced by open hole of equivalent area based upon C-Scan.
Tip Deflection = 47mm
Damaged Surface in Compression due to tip load
Extract Stress Variation in 0Deg ply adjacent to hole
29/01/03 M.Koundouros
© QinetiQ (2003)

8. Linear Static Analysis – Damaged Config.
Comparison of Surface Strains between FE77 results and experimental strain gauge readings show that the stiffness of the model is comparable to the wingbox and that a linear analysis is adequate
Location of Strain Gauges
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© QinetiQ (2003)

9. Linear Static Analysis – Damaged Config.
Compressive Stress Distribution adjacent to hole in 0Deg ply in fibre direction
Max. Direct Stress= 1100MPa
29/01/03 M.Koundouros
© QinetiQ (2003)

10. Linear Static Analysis – Damaged Config.
Stress Distribution in 0Deg ply of compressed Damaged Surface
Max. Direct Stress= 1100MPa
29/01/03 M.Koundouros
© QinetiQ (2003)

11. Soutis-Fleck Model – Stable Crack Propagation
Stable crack growth occurs when the average stress over the crack’s length reaches the laminate’s unnotched strength.
Stress Distribution adjacent to hole in 0Deg ply in fibre direction
29/01/03 M.Koundouros
© QinetiQ (2003)

12. Soutis-Fleck Model – Unstable Crack Propagation
The microbuckle at the hole edge is assumed to behave as a crack of the same length, with no traction on the crack surfaces.
The Model assumes that unstable crack growth occurs when the stress intensity factor at the crack tip equals the laminate in-plane fracture toughness
Unstable fracture occurs when:
29/01/03 M.Koundouros
© QinetiQ (2003)

13. Soutis-Fleck Model – Unstable Crack Propagation
Load Reversed so that damaged surface in tension
Crack introduced by disconnecting nodes next to hole
29/01/03 M.Koundouros
© QinetiQ (2003)

14. Soutis-Fleck Fracture Model
Real wingbox failed at a load of 105 kN due to microbuckling in the skin
Stable Crack Growth is STRESS Based:- computed from stress distribution adjacent to hole
Unstable Crack Growth is ENERGY Based:- computed from Stress Intensity Factors
Failure of the wingbox occurs at intersection of these two curves at a load of 99.2 kN, crack=1.8 mm
29/01/03 M.Koundouros
© QinetiQ (2003)

15. Results Summary
Soutis-Fleck Model worked very well for this case because the failure mode was due to microbuckling in the skin
S-F result was conservative due to damage being replaced by an open hole of equivalent volume
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© QinetiQ (2003)

16. Acknowledgements
The authors acknowledge the full support of the MOD Applied Research Programme on Aircraft Materials and Structures (DS4-321).
The work is carried out under QinetiQ contract CU
Thanks to Dr. E.S. Greenhalgh for many helpful discussions.
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© QinetiQ (2003)