Help with NASGRO crack propagation equation (K_threshold)

[wilhelmjacob]

Hi to all from Chile!!

I´m looking for crack propagation equations as precise as possible without neccesarly using advance software like AFGROW or NASGRO. My idea is to use Franc3D to get the stress intensity factors of certain parts with their cracks and then use Paris, Walker or Nasgro equations so to predict the corresponding remaining life of the part and hopefully compare the three. Paris and Walker are OK but for the Nasgro equation there is the K_threshold property I can`t fully figure out. Searching though open papers I found certain correlations for Kth which are cool and OK to implement but there is a constant Ko which is defined as Kth @ R=0. But all those relations give me that at that point Ko = Kth (which is actually pretty logical) but no way to calculate the value of Ko.

So my question is, where can I get the value of Ko considering that I lack of the laboratory to get it?

Thanks!!

 

[rb1957]

there is no way (at least that i'm aware of) for calculating Kth, just like i don't think you can calculate Kc.

I thought there was some threshold data available for common alloys, like for 2024 it's about 3ksi.in^0.5.

I do my crack growth with excel, using da/dN data available and crack geometry solutions from handbooks. I don't bother with threshold, I also don't use "canned" Paris/Forman/Walker expressions, but fit piece-wise linear curve-fit to the data; three lines gives me sufficient fidelity ... you get the nice "Paris" growth for the mid-range, and a faster curve as the crack approaches critical, and a slower curve for near threshold.

Quando Omni Flunkus Moritati

 

[Stressbro]

Rb1957

Could you recommend some good literature on fatigue, damage tolerance and crack growth? I have Niu but would like a little more detail. Thanks

 

[rb1957]

USAF DTA handbook,
david broek's book is a good primer
any old papers by tom swift


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[boo1]

In order to conduct a fracture analysis, several pieces of information are needed:
1. Load spectrum
2. Minimum detectable flaw size
3. Crack propagation curve
4. Determination of critical crack length

Try some of the NASA documents:
NASA Structures Manual, Fracture Mechanics, Section E2, 1972
ect..

 

[Stressbro]

Thanks everyone,

I just purchased:

Broek's Practical Use of Fracture Mechanics and also
Metal Fatigue in Engineering by Fatemi, Ali

Hopefully these two texts combined will be a good introduction before using tools such as NASGRO.

 

[rb1957]

look into MIT's opencourseware offerings.

do you have someone there to mentor you? working with a person is (IMO) much better than reading a book.

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