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Generally speaking, a material that experiences a ductile fracture will exhibit a "cup-and-cone" shape for the two ends of the fracture, as can be seen in many real world examples (and in many photos on the internet). There are a healthy number of textbooks that refer to "completely ductile" materials when discussing ductile fracture alongside cup-and-cone fractures, typically accompanied by an illustration of necking to a single point.

Is this completely ductile fracture something that we have seen reasonable approximations of in the real world? That is to say, on a macroscopic scale, do we have examples of ductile materials failing by necking to "a point"?

The question is whether or not the idea of a completely ductile fracture, necking down to "a point" before the two pieces separate, is something that is only possible in a model or with a perfect, ideal lattice in the metal, or if we have observed reasonable approximations of it in the real world

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  • $\begingroup$ HI Daniel. Are you looking for ductile failure examples other than test specimens? $\endgroup$
    – NMech
    Aug 4, 2021 at 19:16
  • $\begingroup$ @NMech A completely ductile failure example from an actual piece of equipment would be perfectly fine. I would expect that a test specimen would be easier to find an example of, however. $\endgroup$
    – Daniel
    Aug 4, 2021 at 19:17
  • $\begingroup$ ductile failure of bolt. Is that the sort of thing you are after? Are you just looking for images? Because, to be honest, I don't really see a question. $\endgroup$
    – NMech
    Aug 4, 2021 at 19:33
  • $\begingroup$ @NMech The question is whether or not the idea of a completely ductile fracture, necking down to "a point" before the two pieces separate, is something that is only possible in a model or with a perfect, ideal lattice in the metal, or if we have observed reasonable approximations of it in the real world. For instance, the cup-and-cone fracture is both observed and predicted with more advanced models of ductile fracture. $\endgroup$
    – Daniel
    Aug 4, 2021 at 19:36

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Although it is possible to see a perfect example of ductile failure (see below an example with a bolt), in general it requires a specific geometry that will enable the characteristic "cup and cone" that you are seeing in the textbooks

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When you step away from the cylindrical geometry and depending on the properties of the material, then the failure of a ductile material, (sometimes) manifests itself as the failure along the 45 degrees plane. See below examples for steel and aluminium.

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figure 2: ductile failure for different materials (similar specimen) (source MIT LAb report)

The bottom line is that the characteristic "cup and cone" failure is very dependent on the geometry and the loading conditions. So its very difficult to encounter it in general structures.

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I don't know the "scale" you meant, but here are examples of real-world ductile fracture/failure. Note that the cup-and-cone fracture shape is easily identified for the rod/bolt element, but not so clear on the plate element. However, you shall pay attention to the deformation around the bolt holes and the net area reduction across the failure plane of a gusset plate.

Note that the structural engineering world embraces the ductile failure mode, as it is always preceded with elongation due to necking that provides ample warning as opposed to the brittle failure that occurs in a sudden manner without visible physical changes. In addition, plate failure is preferred over bolt failure, so the structural bolt is usually made of stiffer/stronger material than the typical plate and the structural members/shapes, or a higher safety factor is applied to the bolt at design.

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In the five years I worked as a mechanical-test technician, I never saw a completely ductile failure. I did see failures that came close enough to make it difficult to measure the cross-section at the point of failure. (A true ductile failure would have been quite obvious: the test machine would have stopped with the specimen still unbroken, due to the load dropping below the "break" threshold.)

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  • $\begingroup$ I don't know other countries. But in the US, failure through direct tension is safeguarded by the very high safety factor if the application cannot be avoided. For instance, under the hook elements in the rigging industry, the safety factor can be as high as 4-5. $\endgroup$
    – r13
    Aug 6, 2021 at 21:03
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    $\begingroup$ @r13, doesn't matter how big the safety factor is: there were five-ton, 30-ton, and 60-ton test machines in the lab, and a 300-ton test machine I could get access to if needed. If the part you want tested is stronger than that, there's always the option of getting the machine shop to produce a mechanical-advantage test fixture rather than doing a straight pull. $\endgroup$
    – Mark
    Aug 6, 2021 at 21:16
  • $\begingroup$ We are taking two different applications - lab tests and real-world applications. For the test specimen, see the pictures in @NMech's answer which shall be familiar to you if you are using ASTM standards at work. $\endgroup$
    – r13
    Aug 6, 2021 at 21:23
  • $\begingroup$ @r13, I mostly did round and tube specimens, not flat ones, but I also tested a number of finished parts and assemblies. I don't think a crane hook ever came through the lab, but there were some rather memorable trailer hitches. $\endgroup$
    – Mark
    Aug 6, 2021 at 21:27
  • $\begingroup$ No matter round or flat, they are very difficult to break without purposely created defects, also I guess the difficulty in producing uniform stress at the failure plane plays a big role too. Note, no disrespect here, but 5 years into a profession is not long enough to see/experience a great number of cases. One of my friends had worked in the nuclear engineering field for over 40 years, when asked how much he knew the design, he always replied: "3 types of hangers". BTW, he's a real expert on hanger design though. $\endgroup$
    – r13
    Aug 6, 2021 at 21:44
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I have had to break metal straps, copper wires and what not, all of them ductile material with no tools available.

like everybody else I do this by repeated folding and unfolding the part until the break point has lost its ductility to plastic hardening and can't bend anymore and breaks.

I have noticed the break surface is a kind of history record of deformations and stresses on the section. including some miniature cracks and some hard, sharp edges that can cut your fingers if you're not careful.

Totally non scientific but has worked every time.

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    $\begingroup$ It is a scientific experiment called "fatigue" caused by repetitive load and stress reversal. $\endgroup$
    – r13
    Aug 4, 2021 at 21:00
  • $\begingroup$ @r13, specifically, low-cycle fatigue caused by stressing beyond the yield point. $\endgroup$
    – Mark
    Aug 6, 2021 at 20:41
  • $\begingroup$ @Mark I won't object to the suggested calling "low-cycle" but I think in order to distinguish it, a time frame of the repetition needs to be addressed, as well as the maximum and minimum stresses in each cycle and the stress range. $\endgroup$
    – r13
    Aug 6, 2021 at 21:18

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