fatigue crack tools

[Nuno28]

Could anyone tell me where can i find the latest information on analysis of crack propagation for composites? From my understanding, we still over design the composite components in aircraft due to not have complete confidence in the analysis tools for crack growth in composite material. Therefore, we prevent the crack growth by preventing the initiation of cracks with a bulk composite design.

Any help and direction will be welcomed.

Thanks

 

[SWComposites]

What kind of cracks? matrix, delamination, thru-thickenss?
What specific type of structure?
What material system?
What are you specifically trying to invesigate?

Composite aircraft structure is typically designed for "no damage growth" because there are currently no practical NDI methods for detecting and monitoring growth of cracks/delaminations.

There are lots and lots of papers on "cracks" in composites. Try searching for NASA/Langley documents at

http://ntrs.nasa.gov/search.jsp

or a general literature search thru your library.

 

[Compositepro]

Lots of composite parts are used that have microcracks. These are cracks that are between filaments but are stopped by fiber in cross-plies. Fibers are crack-stoppers and is what makes composites so useful. Micro-cracks are not desireable but are often acceptable. Delaminations, or cracks between plies, are the Achilles heal of fiber composites. These cracks will grow and there is not much to stop it. Through thickness stitching and Z-pinning are some approaches to help, but the expense and knockdown in initial strength are drawbacks.

 

[SWComposites]

Delaminations do not always grow. Whether or not they grow depends greatly on the applied load levels and design details. Commercial aircraft structure is certified using a "no-growth" approach, where fatigue tests are conducted to demonstrate that damage (including delaminations) from impacts, manufacturing flaws, etc do not grow over several aircraft lifetimes.

 

[Nuno28]

I understand that the crack growth within the matrix can be estimated. What seems hard to estimate is once it reaches the fibre, does it change direction along the fiber (delaminating it) or does it stop there temporarily creating a stress concentration point. Do we have fracture mechanics capabilities in general composites structures and components to determine if a crack is allowed to grow to a certain size before critical flaw size?