Crack sidebranches???
Crack sidebranches???
(OP)
Dear all,
There is a feature in this crack that I find puzzling and I am looking into some ideas. Has anybody seen sidebranching like this on a crack? Material is duplex stainless steel. formed, non annealed.
Thanks
There is a feature in this crack that I find puzzling and I am looking into some ideas. Has anybody seen sidebranching like this on a crack? Material is duplex stainless steel. formed, non annealed.
Thanks





RE: Crack sidebranches???
RE: Crack sidebranches???
RE: Crack sidebranches???
It does look like a fatigue crack since it is straight.
RE: Crack sidebranches???
Michael McGuire
http://stainlesssteelforengineers.blogspot.com/
RE: Crack sidebranches???
RE: Crack sidebranches???
A crack will branch (bifurcate) when the energy available to the crack tip is enough both propagate the existing crack and initiate a new crack. The most common example of this is a SCC phenomena where diffused hydrogen lowers the energy to initiate a new crack and new cracks form in advance of the crack tip. However, similar situations can also occur with very high strain rates when the strain at the crack tip increases faster than the crack can propagate. Eventually, with high enough strain rates, you can have enough energy at the crack tip to both propagate the crack and initiate a new crack. This can be seen with explosive loading.
Also, with highly ansitropic matierals, you can have situations where the highest crack resistance is perpendicular to the principle stress. In general, crack formaiton will occur perpendicular to the principle stress. However, if the crack resistance in another direction is low enough, you can get a crack to form in that other direction. This may be what is occuring in the OP, I don't believe there is enough information given.
rp
rp
RE: Crack sidebranches???
So there is a lot of strain in the material. There is no corrosion environment here. Plain water test.I shall look into explosive loading. Thanks redpicker, i feel that's the right direction.
RE: Crack sidebranches???
RE: Crack sidebranches???
Before that it had not seen any service. I have read about hydrogen embrittlement but would that occur so fast? Maybe you got a point here as if I remember correctly hydrogen embritlement occurs during the repassivation ..right?
I did see fatigue striations under the SEM. Could this go hand in hand with the EAC?
I would also expect a more random crack network if that was the case. In this case we see that the sidebranches are on one side of the crack and the crack runs straight. So the sidebranches may be related to the loading of the structure.
I will read more into SCC.
RE: Crack sidebranches???
RE: Crack sidebranches???
RE: Crack sidebranches???
The binding energy of hydrogen to dislocations in the deformation zone, when assisted by hydrostatic tension due to the crack configuration, is sufficient to dissociate water, as the hydrogen source. This is why the seemingly benign environment causes accelerated cracking and the branched cracking.
There doesn't seem to be explosive deformation, so I exclude that possibility, but we're all dealing with imperfect information here. No certainties exist. We're all just suggesting explanations.
Michael McGuire
http://stainlesssteelforengineers.blogspot.com/
RE: Crack sidebranches???
What I see is a rolled/drawn microstructure. The majority of the fracture is transgranular with branching. The zig-zag pattern of the top piece indicates cyclic loading. However, branching is could be caused from many different things. I've had cases where branching was directly caused by sectioning of the crack.
RE: Crack sidebranches???
Aaron Tanzer
www.lehightesting.com
RE: Crack sidebranches???
A crack can change direction for the reasons you cite, Aaron.
But, branching must originate from separately nucleated cracks growing and meeting is my contention, excluding explosive events. I welcome anyone's thoughts on my un-proven contention.
Michael McGuire
http://stainlesssteelforengineers.blogspot.com/
RE: Crack sidebranches???
Hercules28 has already told us they were created in a corrosion-assisted fatigue test. SCC occurs over time usually in static loading. Duplex stainless steels are not prone to SCC from mere water exposure.
Finally, I would be curious to see a micronmarker on the photo so we can get a sense of how long these secondary cracks really are.
Aaron Tanzer
www.lehightesting.com
RE: Crack sidebranches???
Cold worked material + water immersion + cyclic stresses = hydrogen assisted cracking. I consider this a subset of SCC. If you remove any one of the three factors above, you only have fatigue crack propagation.
Annealed duplex stainless steel would not have this type of corrosion fatigue crack propagation. I have seen many corrosion fatigue cracks and branched cracks that I have seen run adjacent and parallel to the main crack propagation because of tensile stress direction.
Yes, the perpendicular secondary cracks are straight in the OP picture from the main crack, and so are transgranular cracks induced by hydrogen embrittlement or delayed cracking in ferrous materials. More examination would be required of this specimen unetched, and I would bet more cracks will be present.