Low Charpy V-Notch Test Result in Steel Beam Subjected to Flexion

[Logan82]

Hi,

I have a mandate in which there is an industrial bridge for a gantry crane. The bridge is over 100 years old. The bridge is seldom used, so fatigue is not an issue. The bridge is built of A7 Steel with a low fracture toughness resistance (Charpy V-Notch Test Result: 8 J at 0°C). There is no beam redundancy in this bridge (only 2 main beams). The main beams are almost used at 100 % in flexion (so there is significant tension at the bottom of these beams).

Some engineers in this mandate argue that this bridge can be used above 0°C, saying that this test is necessary only when the beams are subjected to temperature under 0°C. However, I can't find anything backing this claim.

Are there some arguments that could justify using this structure above 0°C? So far, I am leaning toward recommending a replacement of the main beams.

Thank you in advance!

 

[Lomarandil]

Sure, the argument is that the beam (apparently) calcs out at less than 100% in flexure, and that a low fracture toughness is not particularly relevant to ordinary bridge crane use, unless there is any sort of existing flaw or stress riser present.

Has a coupon test been performed that would indicate any concerns with other physical properties of the material?

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just call me Lo.

 

[Logan82]

Thank you for your answer Lomarandil! I also wanted to get back to you regarding fracture mechanics from a previous post. However, the conditions of my problems have changed since the previous thread (in addition to having almost 100 % shear, I have almost 100 % tension). Link of previous thread:

https://www.eng-tips.com/viewthread.cfm?qid=496646.

You have really given me an interest for the field of fracture mechanics. I have since obtained several books on the subject. I think you are right that there are some practical applications in structure for this field. Are tests on steel coupons necessary to determine KIC? The last time you used fracture mechanics in structure, did you have to perform a KIC test?

To get back on this thread, yes, there has been a coupon taken from the steel. The ultimate tensile strength has been measured and is normal for this steel.

[...] that a low fracture toughness is not particularly relevant to ordinary bridge crane use, unless there is any sort of existing flaw or stress riser present.

Why would you say it is not particularly relevant for ordinary bridge crane unless there is any sort of existing flaw or stress riser present?

Would it be possible to argue that due to the fragility of the steel, a small crack (or a small standard defect, since steel is never perfect) could possibly not be detected, stress through standard use could accentuate the crack and eventually bring the bridge to fail catastrophically (reaching KIC)?

 

[Lomarandil]

Why would you say it is not particularly relevant for ordinary bridge crane unless there is any sort of existing flaw or stress riser present?

I say that because fracture toughness (and more generally ductility) is related to dynamic loading and energy absorption/dissipation at higher rates of strain. Typical loading of a bridge crane girder (at least with a skilled operator and excluding accidents like a dropped load) is not particularly dynamic -- strain energies are applied slowly and relatively evenly.

Of course, a crack/flaw/stress riser of some nominal size combined with the low fracture toughness could cause catastrophic failure. A fracture mechanics analysis would identify that critical size threshold.

Someone more frequently using fracture mechanics could probably provide an order-of-magnitude estimate for your conditions. I am not that person, but I suspect that it would indicate that a (trained) visual inspection would be sufficient to identify any critical flaw.

You also might consider whether the cost-benefit tradeoff might mean that a more thorough inspection method (ultrasonic, perhaps) would provide adequate confidence against fracture-related issues (by increasing confidence identifying the largest flaw present) more economically than performing the full detailed analysis.

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just call me Lo.

 

[EdStainless]

KIC (and KIIC and KIIIC) are measured in tests.
These are related to crack propagation and often used in conjunction with calculations of critical flaw size.

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P.E. Metallurgy, consulting work welcomed

 

[Logan82]

Thank you Lomarandil and EdStainless.

Lomarandil:
- Thank you for the strain rate mention.
- The only stress raisers would be bolt holes.

To all:
- I know there is ASTM E399 to measure KIC, but do you know what are the tests for KIIC and KIIIC? I believe they refer to the modes I, II and III as shown in the following picture:

Source: Jean-Paul Baïlon. Des matériaux. 2019.
- Once you find KIC of your material, my biggest question is how do you determine "α", the corrective factor in the equation KIC = α*σ*sqrt(pi()*a), where "σ" is the nominal stress, to isolate the crack length "a"? My application would be for a beam. Source: Jean-Paul Baïlon. Des matériaux. 2019.

 

[Logan82]

- Once you find KIC of your material, my biggest question is how do you determine "α", the corrective factor in the equation KIC = α*σ*sqrt(pi()*a), where "σ" is the nominal stress, to isolate the crack length "a"? My application would be for a beam. Source: Jean-Paul Baïlon. Des matériaux. 2019.

I have found my answer to this question in the book John M. Barsom. Stanley T. Rolfe. Fracture and Fatigue Control in Structures: Applications of Fracture Mechanics. Third Edition. 1999.

 

[Logan82]

I have found that the strain rates are very slow through the help of this post

https://www.eng-tips.com/viewthread.cfm?qid=497472.

Based on fracture mechanics, say the results are the following:
• A crack of the fiber tension of 7 mm would cause a fragile rupture of the beams at 0°C.
• A crack of the fiber tension of 3 mm would cause a fragile rupture of the beams at -20°C.

I believe I could detect a 3 mm crack during an inspection, but if it reaches that point it will be too late, as the beam will have a fragile rupture. Under 3 mm, it is hard to detect.

Say it is not possible that the structure is overloaded. Then I have the following questions on how the mechanic of the rupture would work:
• Say there is already a 1 mm crack, it would be very hard to detect it. Overtime, could it develop into a 3 mm crack and rupture even if the fatigue indicates that there is no issue?
• How can I be sure that a crack with a length higher than 3 mm would not be formed after 1 load due to the fragility of the material?
• Should I still go for a minimum operation temperature of 0°C to give me a certain range so that if there is a crack, I can detect it before a fragile failure?

 

[Lomarandil]

[ul]
[li]The idea of the fracture analysis is that given the level of stress (and geometry) you have considered, a 3mm or larger crack will grow incrementally with each loading cycle. Any crack smaller than 3mm (e.g. your 1mm crack) would require a higher level of stress to grow.[/li]
[li]If a crack/flaw does not exist, and standard structural analysis does not indicate an overstress given the loading, a crack/flaw will not magically develop.[/li]
[li]This depends on your inspection method and frequency (and consequence of failure). I would expect that a 3mm flaw should be apparent during a detailed visual inspection of a relatively clean beam, but if there is a lot of other material or clutter that makes inspection more difficult, you may choose to take a safer limit.[/li]
[/ul]

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just call me Lo.