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Unusual fracture pattern in F91 tensile test?

  • 1.  Unusual fracture pattern in F91 tensile test?

    Posted 05-12-2021 11:42
    We recently experienced an interesting fracture pattern on a tensile that we pulled, see attached pictures. Overall it was a typical, ductile cup/cone but the strange thing is that the perpendicular fracture face included a second, X-shaped fracture parallel to the direction of loading. As you can see, it's fairly centered in the tensile and the fractures forming the X are essentially perpendicular to each other. My lab actually took some pictures of this tensile on the machine before it broke where you can see the fractures breaking through to the surface just before rupture.

    I suppose what I am wondering is why the stresses would distribute such to cause a fracture to open up parallel to the direction of loading like this. I understand that ductile shear tends to happen at 45 degrees and brittle fracture tends to happen at 90 degrees, but to me this implies that the greatest stress (at least relative to the material's strength in that direction, which could be anisotropic) was circumferential tension, like hoop stress. Shouldn't the surface be in circumferential compression?

    A few pieces of background information on this:

    • My lab tells me that this is not the first time they have seen this on F91, or for that matter F22. The reason they took pictures and brought it to my attention this time around is that the sample did not break on its own; they needed to manually bump the force up a smidge at the end to get it to rupture. My presumption here is that, since the machine is constantly adjusting its force to produce a constant crosshead speed, it probably overshot a bit and started applying too little force to get it to break.
    • I have no reason to believe that there is an issue with our tensile machine where it's exerting a significant torsional component or anything like that. We get it calibrated regularly and we test plenty of other materials on it which don't show this fracture pattern.
    • The material is standard ASTM A182 F91, normalized and tempered. We forge a test bar to about 2:1 reduction and take the tensile in the longitudinal direction at T/4. Aside from the aforementioned behavior at rupture, the stress/strain graph was perfectly normal, with typical values for YS/UTS/elong/RoA. I have no reason to believe there were any gross inclusions or anything anomalous like that in the material.


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    Sean Piper
    Product / Process Metallurgist
    Ellwood Texas Forge Houston
    Houston TX
    773-524-8985
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  • 2.  RE: Unusual fracture pattern in F91 tensile test?

    Boston Chapter Admin
    Posted 05-12-2021 16:04
    I'd be curious regarding residual stress of the as-machined test piece.  Thank you for sharing the pictures with us!

    Sincerely,

    Dan D.

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    Daniel Denis
    ASM Chapter Council Vice Chair
    Hartford Chapter
    Senior Consultant - Metallurgy & Materials
    Structural Integrity Associates, Inc.
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  • 3.  RE: Unusual fracture pattern in F91 tensile test?

    Posted 05-13-2021 08:03
    I've seen centerline segregation / shrinkage pipe cause similar fractures in strip specimens.

    A longitudinal cut micro , with the cutting plane perpendicular to the longitudinal crack, should verify this or rule it out.

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    Andrew Werner
    Chief Metallurgist - Retired
    Retired
    East Bernard TX
    (832) 563-3489
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  • 4.  RE: Unusual fracture pattern in F91 tensile test?

    Posted 05-13-2021 08:27
    As you stated, the overall fracture is typical for a ductile tensile sample. This is just an semi-educated guess based on the images you provided, but it looks like the parallel fractures are likely just due to local constraint in the sample.
    The material wants to elongate with the applied load but it is constrained on the sides. Something has to give, either by slip or fracture. In this case it looks like the material fractured parallel to the applied load to relieve the lateral stress.
    I imagine that if you look at the fracture surface (the fracture parallel to the load) in that area you would see a lot of brittle cleavage.

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    Edward White
    Ariel Corporation
    Granville OH
    (740) 397-0311
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  • 5.  RE: Unusual fracture pattern in F91 tensile test?

    Posted 05-13-2021 09:09
    Might be worthwhile to make a micro or two behind the fracture.....

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    Anthony Giammarise
    consulting metallurgist
    MOSTLY METALS
    Erie PA
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  • 6.  RE: Unusual fracture pattern in F91 tensile test?

    Posted 05-13-2021 10:50
    Thank you for sharing the pictures. These fractures appear to exhibit delamination or splitting. A triaxial stress state develops in the necked region of the tensile specimen, which includes a circumferential tensile stress. If there is a weak fracture plane, e.g. due to segregation or texture, then this stress can induce fracture along these planes producing the type of fracture you have shown. This is sometimes seen in plate and pipeline steels, though it can certainly be present in other steel products due to factors listed above.

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    Kip Findley
    Colorado School of Mines
    Golden CO
    (303) -273-3906
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  • 7.  RE: Unusual fracture pattern in F91 tensile test?

    Posted 05-13-2021 23:11
    Hi Sean,
    I'm not sure if these comments will be enough to narrow down the cause but here goes some brain-storming.  A bit of hand-waving can be expected.

    -Did the material properties measured fall within the "typical" values?  Not asking whether they met minimums, but whether they were remarkable.  The recent discussion of unusually high tensile values in a 15-5PH comes to mind.  Are there tracking statistics on this material, historical data to compare with?

    -When a tensile bar necks down quite substantially there is a very high state of triaxial stress.  I don't recall ever seeing quite as regular a set of 90 degree splits but I think I saw some with internal nearly vertical flat faces within the fracture zone, sometimes called a star pattern.  Which leads me to wonder if there could have been a plane of weakness along the material axis due to an alignment of inclusions or perhaps an unusual microstructural feature from an imbalance of composition or heat treatment.  A planar arrangement of ferrite, or nonmetallics, where the high triaxial stresses then caused a tensile failure across such a plane that had lower ductility (local / directional fracture toughness, essentially)?  An unusual microstructure from a borderline failed heat treatment, or an improper temperature during forging?

    -Another commenter asked about machining.  The near-surface residual stress state, if there was some abnormality in the machining process, can be affected.  Could the surface have ended up being "burnished", similar to a residual stress improvement treatment, due to problems in an automated specimen prep routine?  Or the opposite, an excessive depth feed or bad machining tool contour causing micro-tearing that wasn't apparent after the final specimen was polished down with emery prior to testing?

    It depends on how far you want to go.  You could look for asymmetry in the specimen's hardness by doing a ring of indentation hardness tests around the specimen circumference, that would be pretty inexpensive.  Magnetic particle inspection to see if there is a continuation of any sort of feature beyond the cracked lines, fairly inexpensive.  You could look for microstructural issues by doing metallography, at a slightly greater expense.  A quick review of furnace charts and chemistry would seem warranted, particularly if the tensile results were remarkable in any way.

    If you come to any conclusion, it would be interesting to hear about it.

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    Paul Tibbals
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  • 8.  RE: Unusual fracture pattern in F91 tensile test?

    Posted 05-15-2021 12:19
    I have seen this type of fractures in testing control-rolled steel plate.  Depending on the thermomechanical treatment the plate received (i.e., % reduction at a certain temperature range) it can create a texture that under the triaxiality condition near the end of the test, leads to cracking.  The solution was to change the reduction schedule and / or the temperature at which these reductions were taken.  Interestingly enough, there was never a reduction in the strength measured or the toughness of the product, but it was a concern in the case of a failure for other reasons the appearance of the perpendicular cracks created the impression of a defective material.  Now in your case you have two sets of cracks at 90 degrees to each other, which I suspect is due to the fact that your material is forged.  Perhaps your reductions take place along two main planes located at 90 degrees from each other (?).  Anyway, I agree with Professor Findley's comment about the cracks being the result of texture formation during processing.  Hope this helps.

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    Nassos Lazaridis
    Owner
    NAL Consulting, retired
    Granger IN
    (574) 273-1979
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