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I have a scenario where we have encountered a cracked steel plate where the crack has propagated within the plate longitudinally for an extensive length. The plate thickness is 1", and the crack length is approximately 12" with somewhat of a bend horizontally, yet staying within maybe 3/8" max variation propagating right between the top and bottom of the plate. Stresses are complex but the highest load is likely a vertical load, and may be close to 10-20 tons. Impact factor typically ranges between 3.5-5.5 for this load case, but it is complex and not precisely known for the location for the crack, which is suspected to have received only perhaps loads upward to an impact factor of 2-3. Precise stresses for failure are unknown. Steels in question is 4140. Growth marks have been observed throughout most of the entire crack, with somewhat of a < shape, which seems to indicate that it cracked both top and bottom and then finally through perhaps? But that Seems strange that it would crack just right near the middle of the plate thickness for such an extensive length when loads are not applied linearly.

I would like to review the theories how the crack propagated based on the described observations. What can a < crack surface tell me? Can someone list that out and provide appropriate literature backup?

Picture showing grow patterns

Overall details

Overall details

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    $\begingroup$ Would it be possible to share pictures of the actual plate? If the cracks aren't easily visible in pictures, could you share the pictures with the cracks "drawn" over it? I'm having a bit of trouble interpreting your images. $\endgroup$ – Wasabi Jan 21 at 22:53
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At the time of 1994 Northridge earthquake in California, I along with many other engineers inspected several four and five story damaged buildings to assess the urgent safety situation.

We observed many torn base plates of various thickness and sizes up to 2" thickness. The most probable cause was shear rupture under large pendulum motion of the columns.

Those observations are collected in a document available online.

Google, 1994 Northridge earthquake- gpo.gov.

This failure of shear plates is a complex area and propagation of cracks has to do with dynamic repetitive loading, fatigue, exposure to chemicals, incorrect welding, defective product, structural design problems. It may have to be investigated by teams of different expertise.

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Your diagram shows a typical brittle fracture propagation , likely caused by an impact load. The chevrons point to the origin of the fracture. The impact strength of 4140 depends on the heat-treatment; it can be brittle at room temperature. In failure analysis, the chevron area ( purple ) is of no particular interest other than it points to the origin. Refer to LEFM ( linear elastic fracture mechanics) in Wikipedia; a relatively good summary.

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  • $\begingroup$ While I agree with your assessment somewhat, but I came across this, which I thought is interesting. While the steel I have is not X100, it seems like it is not 100% true that it can only propagate in one direction? researchgate.net/publication/… $\endgroup$ – Isa Feb 20 at 22:15
  • $\begingroup$ DCB coupons with side notches stressed at specific rates in a hydraulic test frame ? : sort of like saying a Ford pickup truck is a lot like as Ferrari. Also I tend not to be impressed by grad student work when most of their references are to their professors publications, especially when they have very few references to the "masters" like Irwin. Although I see no need to change my answer I enjoyed the paper very much. The controlled rolled microalloys with Cb/ Nb ( and Ti) are a very impressive and tough type of steel ; I object when the authors referred to them as "brittle". $\endgroup$ – blacksmith37 Feb 22 at 1:02

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