Crack sidebranches??? 3

[Hercules28]

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

 

[metengr]

Yes, I have seen branching off of main cracks, which was SCC. You have other cracks barely visible that have similar branching characteristics. By the way, it would have been better viewing in an unetched condition.

 

[swall]

Hercules--you don't give us much to go on. To me, it looks like fatigue cracking in sheet/strip material.

 

[Hercules28]

Swall you are correct, it was a cyclic test but I didn't want to direct you to an easy answer just in case you got other ideas.

It does look like a fatigue crack since it is straight.

 

[mcguire]

Branching cracks indicates the underlying phenomenon is SCC based, even if accelerated by fatigue. Branching indicates multiple initiation points ahead of the crack tip. (Think about it) This means the cause was a species diffusing ahead of the crack tip, i.e. hydrogen.


Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[metengr]

Agree with mcguire.

 

[redpicker]

I have to, respectfully, disagree with mcguire in that the branching cracks by themselves are not conclusive evidence of SCC. There _are_ other mechanisms that can create a branch to a crack.

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

 

[Hercules28]

I shall agree here with redpicker. This is a cyclic fatigue test and obviously the piece has a high amount of deformation as you can see from the deformed grains.

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.

 

[metengr]

You keep spoon feeding more information. Plain water test???? You have the potential for hydrogen embrittlement with a heavily cold worked material (ferrite islands) in an aqueous environement. It is called transgranular stress corrosion cracking or better yet EAC. Better look closer.

 

[Hercules28]

It was a cyclic water pressure test up to 1000psi max pressure and it failed at like 3K cycles. cycle time is like 3-5 secs.

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.

 

[Hercules28]

But doesn't hydrogen embrittlement occur in high temps? This was room Temp.

 

[metengr]

No. Don't confuse hydrogen embrittlement wiht hydrogen damage.

 

[mcguire]

It is not necessary to have overt corrosion for hydrogen to accelerate crack growth under fatigue conditions. The presence of deformation shows you are at a stress intensity level at which crack growth should occur. Hydrogen merely accelerates the process.

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/

 

[Metalhead97]

Reponding to the OP.

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.

 

[mrfailure]

One often sees short secondary cracks in corrosion-assisted fatigue. My first thought is secondary cracks propagate away from the primary crack initially because of the duplex structure - for a while it is easier to propagate perpendicular to the primary crack due to resistance of the structure. Eventually, load energy overcomes this resistance, allowing the secondary cracks to propagate for a while parallel to the primary before primary crack geometry stops growth.

Aaron Tanzer

http://www.lehightesting.com

 

[mcguire]

I disagree on the root cause of branching. A new branch of a crack cannot originate from the surface of an existing crack. The surface of a crack is a free surface which has been relieved of stress.

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/

 

[mrfailure]

Secondary cracks form off of the primary crack under corrosion-assisted fatigue. Usually they run a grain or two before they arrest. The cracks continue in the general direction of the primary crack before they arrest or rejoin the primary crack. This is normal, and the feature can be used in metallography to define propagation direction of the primary crack. These cracks are not considered SCC "branching". The unusual feature here is that they grow perpendicular to the primary crack for a while before running parallel and running out, but that is because of the nature of duplex steels - it requires less energy to propagate that direction than to propagate along the phase interface than to push through.

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

http://www.lehightesting.com

 

[metengr]

Very interesting thread. After reviewing all of the comments above I think there is a real case to be made that you have is hydrogen assisted cracking in this environment.

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.