finding the load by working backwards 2

[enfield]

Finding the load from a crack

Hello there, Could some one with a bit more experience guide me here, Is it possible to arrive at a figure for loading from a cracked part.

I have a poorly designed bracket , that has cracked, What I was thinking was if the crack was to to a shear failure , then this would be at 90 deg to the principle stress, would it be possible to work backwards to arrive at the load the material saw when it failed.
I could look at the crack and determine if it was a once only overload failure or a fatigue failure
Are there any methods that one can use , without resorting to strain guages etc

Stephen

 

[CoryPad]

There are some techniques available. Cracking usually implies tensile stresses 90 degrees to the crack plane. You can measure the crack length, estimate the material's fracture toughness, and use a stress intensity factor equation to find the force.

As far as distinguishing overstress vs. fatigue, the best way is to observe the crack plane with a microscope. The patterns are indicative of the crack propagation mechanism.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[enfield]

Yes I an sticking on this point to , need to examine the crack ..
I am un aware of how to get from the crack length to the toughness test .. I will have to do a bit of noseying around , the books I have arnt much help here ( Mechanical , by shigley )

Thanks it gives me something to think about
cheers Stephen

 

[CoryPad]

You are correct, Shigley won't help. You need a book on Fracture Mechanics. Two good ones are Deformation and Fracture Mechanics of Engineering Materials by Hertzberg and Fracture Mechanics by Anderson.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[enfield]

Ha
now you have done it an excuse to purchase another book ...she who must be obeyed will shoot me
Still its all in a good cause
Thanks so much for all the help , I did a bit of checking and came up the same ally as u suggested , So i will look into that book ( please let it be cheap at Amason!!!)
Kind Kind Regards
Stephen

 

[gearguru]

enfield,
on the address below you'll find best price for any book.

http://www.bestwebbuys.com/books/

 

[enfield]

Thanks to those replys I now have a way of doing it ...thanks once again

Stephen