1. Crack Trajectory Prediction in Thin Shells Using FE Analysis
6th International Conference on Computation of Shell and Spatial Structures
Cornell University and NASA Langley Research Center
A.D. Spear1
J.D. Hochhalter1
A.R. Ingraffea2
E.H. Glaessgen3
1 Graduate Research Assistant, Cornell University
2 Principal Investigator, Cornell University
3 Grant Monitor, NASA Langley Research Center

2. Outline Motivation & objectives Relevant past work
Point-source damage: HOW TO LAND SAFELY?
Fatigue damage: HOW MANY MORE FLIGHTS?
Relevant past work
Improvements in physics-based modeling
Incorporating the nano- & micro-scales
Current technical challenges

3. Point-source damage: HOW TO LAND SAFELY?
Airbus A300 damaged by surface-to-air missile

4. Point-source damage: Objectives
Develop finite element-based analyses to predict growth of point-source damage within airframe structures under realistic conditions and in real-time
Interface real-time damage assessment with control systems to provide a damage-dependent flight envelope to restrict structural loads in the presence of severe damage

5. Point-source damage: Technical approach
Generic aircraft component damaged by surface-to-air missile
Stiffeners
Skin
Idealized
Damage
Airbus A300 damaged by surface-to-air missile
Integrate information from on-board sensors to characterize damage
Develop interface with control system
Reduced model
Response surface
Recast structural component as a lower order model (i.e. equivalent plate)
Get the sensor description of inflicted damage and compute updated allowable load in real-time
Parameterize damage configurations
Store a response surface of computed allowable load given the damage configuration and query in real-time
Response surface
Damaged Area

6. Point-source damage: Predicting damage configurations
Before Impact
Projectile
Projectile
0 degrees
45 degrees
After Impact
T. Krishnamurthy and J.T. Wang, NASA Langley Research Center

7. Point-source damage: Response surface method
Original
Load Allowable
Global Finite Element Model
Decrease Load Allowable
YES
Store New Load Allowable in Response Surface
Extract Local Boundary Conditions
Catastrophic Crack Growth? NO
Parameterized Damage
State
Local Finite Element Model
Explicit Crack Growth Simulation

8. Outline Motivation & objectives Relevant past work
Point source damage: HOW TO LAND SAFELY?
Fatigue damage: HOW MANY MORE FLIGHTS?
Relevant past work
Improvements in physics-based modeling
Incorporating the nano- & micro-scales
Current technical challenges

9. Fatigue damage: HOW MANY MORE FLIGHTS?
April 28, Aloha Airlines Flight 243
levels off at 7,000 meters...
Small cracks start at each rivet hole…
25 mm
…then link to form a lead crack
The plane, a B , had flown
89,680 flights, an average of 13 per day over its 19 year lifetime. A “high time” aircraft has flown 60,000 flights.

10. FRANC3D-ABAQUS interface for crack growth simulation
Displacement in
z-direction
Fatigue damage: Relevant past work
[inch]
-0.16
FRANC3D-ABAQUS interface
for crack growth simulation
Global-Local
Hierarchical Modeling
Maximum tangential stress for crack trajectory
Experimental determination of phenomenological material constants:
- crack tip opening angle, CTOA
- critical radius, rc
Initial crack z
-1.10
measured
Internal cabin pressure, P
What about
-slanted crack growth?
influence of fundamental fatigue damage mechanisms?
the inherent stochastic nature?
predicted ui ui

11. Improvements in physics-based modeling: Modeling crack front with 3D finite elements
Shell-to-solid couple

12. Improvements in physics-based modeling: Modeling crack front with 3D finite elementsRD RD
SEM’s of 7075-T651 (R. Campman, CMU)

13. Improvements in physics-based modeling: Considering common damage mechanisms
Loading Direction
(a) Incubation – the process that leads to the first appearance of a cracked particle
(b) Nucleation – the appearance event of a crack in the matrix
(c) Propagation – the process of crack extension governed by microstructural heterogeneities
Stage I – Slip along a single band
Stage II – Slip along multiple bands, causing crack propagation subnormal to the global tensile direction
(a)
(b)
(c) b
10 mm a c
250 mm
Stage I/II illustration from:
C. Laird, 1967.
100
Cycle:
Cycle: 1
Cycle:
3000
Cycle:
SEM/OIM courtesy of Northrop Grumman Corporation

14. Improvements in physics-based modeling: Incorporating the nano- & micro-scales
FCC polycrystal plasticity for grains & linear elastic, isotropic for particles
1 1% strain in simple tension, along RD-axis

15. Improvements in physics-based Modeling: Incorporating the nano- & micro-scales
Molecular dynamics simulation
Grain Boundary
Crack
Incubated crack

16. Technical Challenges
Incorporating nano- & micro-scale simulation in a computationally feasible manner
Determination of damage configurations and assessment during flight
Better physical understanding of the governing mechanisms for crack growth
Why does CTOA appear to work?
Interpolating between damage states
Development of real-time interface with control system