Crack initiation in metal

[Wazy01]

G'day all,

There are different definitions for crack initiation size in the literature. I run into N.E. Dowling papers (and his book) who went through this topic
However for the same material, and the same notch size (radius of curvature), Does the crack initiation size changes under (a) different loading level, (b)loading states (simple tensile, tensile+shear..etc)

Your help is appreciated.
open discussion and suggestion are more than welcome

Thanks

 

[EdStainless]

Depending on the loading (both load and mode) the critical crack length will likely be different.
In one case you get propagation, but with the same initial crack different loading may not result in propagation.
Just look how different the values are for KIc, KIIc and KIIIc.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[rb1957]

g'day to you too !

"different definitions of crack initiation size" ? maybe different definitions on when a crack-like "thing" (like a set of intergranular dislocations) becomes a crack.

Basically different loads (magnitude and types) will cause different crack initiation and growth ... that's the "sigma" term in stress intensity, and there's also different modes (I, II, and III) of crack growth driven by different load types.

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

 

[Wazy01]

EdStainless, rb1975
Thank you for your comments.

I should clarify that my question is about the crack initiation stage. no propagation yet. I like the way rb1975 put it: when the crack-like become crack?.

Therefore, as you know the fracture parameter here the plastic strain/metal at this stage and based on damage parameter (Smith Watson Topper, SWT; Morrow; D.F. Socie..etc) I expect to get an indication on crack initiation life for different cases.

However, the strain-life for smooth specimens that common to be adopted for estimation the life for crack-like case (simulate) have been collected for either unknown crack initiation size or varied initiation size (different data for different metals). Dowling emphasise the effect of assuming different crack initiation size.


In my case, I have inclined sharp notch under axial loading. So there is biaxial state of stresses (that what I think) at the notch varied with the angles.
for simple quick analysis, it seems the crack initiation life so marginal (very high Kt) when the case is mode-I. as the case is mixed-mode/inclined notch that will be change and the initiation life should have a larger contribution of the total life.

I am interested to see how this would change among the different angels. As you can see here is my concern :
for such comparison is it fair to assume the cases have same initiation crack size provided that they have the same geometry, material, and loaded under the same level but local stresses changes.

Thanks

 

[rb1957]

all you need is a consistent definition of when a crack has initiated ... when you can detect it (as a crack) with the naked eye, a magnifying glass, scanning electron microscope ?

Sorry, was thinking this was more a research project, but re-reading you seem to have more practical interests.

If you Have to have a sharp feature (and a high Kt) then the only way to control crack initiation is with stress. But remember you're probably using LEFM (linear elastic fracture mechanics) or simple s/n fatigue to calculate the life of your feature ... but the root of your feature is almost certainly yielded ... so your predictions are probably "off".

But then that starts you (well, me) thinking about other things. Do you Have to machine your sharp feature, or can you use a different manufacturing process (sort of like rolled threads) to improve the stress state at the root of your feature.

Could you have a controlled overload to intentionally yield the sharp feature ? A bit of testing involved but nothing "ridiculous"; a 20% overload would have a significant effect on life. This could be repeated at intervals (sort of like the validation of the F111 pivot fitting).

Can you modify your sharp feature adding a small root relief radius (small, but better than you current design) and still accomplish the design intent ?

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

 

[Wazy01]

Thanks, rb1957 for following up

This is the challenge, no information about the crack initiation- has not been monitored in tests!
You have to predict the total fatigue life (knowing the test data, can help in checking the accuracy for the assumption). also, as you state there is obvious plasticity so e-N curve to use for stage-1. LEFM for crack propagation prediction-stage 2.
to sum the 2 stages, an initiation crack size needed to identify the end of stage one, the start of stage -2.

I agree this is not the smartest way, as monitoring for 1-stage and measuring crack size seems the common practice.
The assumption of equal initiation size for different cases is the simplest option but I am not sure how long can it stand. Assuming varying sizes faces the difficulty of defining the change in the definition of the crack initiation size.

Thanks

 

[rb1957]

most people would use fatigue for crack initiation and DTA for inspection.

The difference being fatigue doesn't assume a crack, and assumes that the crack growth life is small compared with crack initiation.

Your biggest analytical issue is, IMO, accounting for the plasticity at the root of the notch. NL FEA would probably be the best course, maybe with FEA based fatigue (like ncode).

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

 

[Wazy01]

Thanks, rb1975
I agree with you regarding plasticity effect, and I had a look at few FEA based fatigue software that deals with such problems.

My question is
if I assumed crack initiation size from a notch of different orientations is the same, provided that the notch radius, loading level, material properties are same. Do you think this assumption could be valid to some extent, or the case is totally the opposite (should be different with no idea how much this size is differentiated)?

Thanks

 

[rb1957]

I think you can define a critical (or almost critical) direction based on FEA. This is an aspect of DTA that I don't normally dwell on, but I've thought there were special FEA codes that could deal with multi-mode cracking (input crack growth and toughness properties for the different modes and have at it). Plasticity is going to be, by far, the largest factor.

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