Structure Cracks due to Vibration/Resonance and Fatigue

[struclearner]

Dears,
When a structure fails, got cracks after a certain number of hours in the field, one possibility is fatigue failure, other is vibration related, there might be resonance occurring. How the failure due to resonance/vibration is quantified. What is the analytical way of relating the frequencies, resonance to the number of hours to failure? Which failure, due to resonance or due to fatigue, will be earlier.
In the Response analysis of the structure, how the test data of vibration be used, and in what format the test date should be, to use for Response analysis.

Thanks a lot for the input/referring to literature/books.

 

[GregLocock]

The material at the crack initiation point cannot tell if a given strain history is caused by loads or by vibration, so they are evaluated the same way.



Cheers

Greg Locock


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[spongebob007]

The wiseacre in me would tell you to give the problem to someone who knows what they are doing. However, I am feeling kind of nice today and I will give you some hints: Fatigue failures will occur at stresses that can be well below material yield. If you are indeed experiencing resonant conditions it is possible that the stresses due to resonance could exceed yield or ultimate strength.

Also, try "Shock and Vibration Handbook" ISBN 0-07-137081-1

 

[bxtsafe]

Ressonance will occur when the loading frequency is near one of the component natural frequencies (mainly close to the 1st natural frequencies).

Fatigue will occur in all problems that have cyclic loading and the higher component stress is above the Fatigue Limit. Obviously, the component stresses during ressonance will be higher and it will failure by fatigue before, then compared to a situation of non-ressonance.

I reccomend you to read about fatigue (for example the chapter 6 of the Shigley's Mechanical Engineering Design; it is a very very simple text on fatigue, just to introduce the readers to the subject) and also a book on vibration (I find good the book "Mechanical Vibrations" by "Singiresu S. Rao").

Regards,