Crack Propagation

[sushi75]

Hello all!

I'm working on crack propagation, and I have a few question about propagation.

From my understanding, fatigue will iniate a crack along the shear plane (so 45 degrees for tensile loading).
Then a crack will propagate perpendicular to the loading.

Then stress intensity factor is evaluated and the crack becomes unstable when K&c is reached.

But what I don't understand is that the loading is always assumed cyclic, ie fatigue for crack prop.

So the question is, what happens if the load is static rather than cyclic?
I guess paris'law is no longer valid, but is there a way of assessing propagation?

Is it related to the R-curve?

Thanks a lot for your help!!

Cheers

 

[jayrod12]

If the loading is static and not cyclic then why would there be fatigue? The joint wouldn't be worked back and forth with static loading.

 

[sushi75]

Thanks for your quick reply!

Well I was wondering if there is a pre existing crack, and the loading is not cyclic, is there a way to predict a potential crack propagation?

 

[rb1957]

a static load can cause crack growth if it is close to the residual strength of the material. This is normally called "dynamic" crack growth; research "R-curve".

in most cases it is fatigue crack growth that we're interested in ... an extension of traditional fatigue analysis. Fatigue analysis tells us that small cyclic tension stresses can cause failure. Traditional fatigue doesn't (expressly) consider the active crack growth phase (since it is so much shorter than the crack initiation phase). Damage tolerance says assume a crack exists, how quickly will it propagate?

another day in paradise, or is paradise one day closer ?

 

[sushi75]

Thanks a lot for your replies!

I understand better the idea and the influence of fatigue.

So if I want to investigate on a potential crack propagation under constant loading, then I should have a look at residual strength of the material with the crack, is it right?

Cheers!

 

[Lomarandil]

It's been a long time since my class in this subject...

but if you're looking at static load, that's technically a fracture problem, not a fatigue problem.

If I recall, fracture problems are governed by stress intensity inside the "singularity dominated zone" at the tip of a crack. There were a bunch of ways to solve for the stress intensity (K_I?).. I remember using Westergaard's solution a lot, and Irwin?

Hope that helps

 

[rb1957]

most people don't investigate crack propagation under static loads ... maybe you have a special application ? The airworthiness authorities usually don't require assuming flaws exist for static loads ... apply your loads, if MS is +ve then ok. The only time static loads meet cracks is during residual strength, when the crack needs to support limit load. This is usually met by showing stress intensity < toughness of material (Kc).

another day in paradise, or is paradise one day closer ?

 

[boo1]

At some value of stress or at some length of crack, the crack will become unstable and propagate quickly until the component fractures or the remaining cross section is reduced to the point that rupture occurs. That critical stress intensity is called the fracture toughness.