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finding the load by working backwards
2

finding the load by working backwards

finding the load by working backwards

(OP)
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

RE: finding the load by working backwards

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

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RE: finding the load by working backwards

(OP)
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

RE: finding the load by working backwards

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

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RE: finding the load by working backwards

(OP)
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

RE: finding the load by working backwards

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

Stephen

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