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)

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 29/01/03 M.Koundouros © QinetiQ (2003)

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 29/01/03 M.Koundouros © QinetiQ (2003)

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)

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)

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)

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)

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 29/01/03 M.Koundouros © QinetiQ (2003)

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)

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)

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)

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)

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)

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)

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 29/01/03 M.Koundouros © QinetiQ (2003)

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 CU004-08763. Thanks to Dr. E.S. Greenhalgh for many helpful discussions. 29/01/03 M.Koundouros © QinetiQ (2003)