Luders band in steel - localised yield, work hardening and propagation

A material that has a yield drop, i.e., where the stress to initiate plasticity is greater than that for propagating the plastic zone, exhibits elongation at a roughly constant stress until the plastic/elastic interface is halted. In other words, there is an upper and lower yield strength. Luders elongation begins at a stress concentration, which hardens that particular region. In ferritic steels, the yield drop is associated with the pinning of dislocations by interstitial atoms (C, N). The unpinning stress is greater than the stress for free dislocations to move, leading to the yield drop. In this story, the work of the Tsuji Group has shown that the same effect can be induced in austenitic stainless steel by martensitic transformation at the plastic-elastic front during tensile testing.
www.phase-trans.msm.cam.ac.uk

[@hai2410]

This is the best explanation of Luders extension I can find on the internet. Thanks a lot!

[@bhadeshia123]

Thank you, you are welcome.

[@kaptantsubasa8110]

Prof. Badeshia many metallurgy and materials engineer must congrulate you. I am from Turkey and I have learnt alot of things from your videoes and your web site I really appreciate to you👍

[@bhadeshia123]

It makes me happy that you use the materials, and you are very welcome.

[@purnimakumari4832]

Thanku so much Sir, it happened to understand in easy way. And also, thanks a lot for quick response from you, whenever we ask questions.

[@bhadeshia123]

It's my pleasure

[@kartikkanwar7107]

hatts off to you for your immense efforts🙌🏼

[@bhadeshia123]

Thank you so much 😀

[@Methuselah_]

Thank you.

[@Methuselah_]

What is the scale on the right hand of the video in min 3:02?

[@johanwestraadt8178]

I really enjoyed the whiteboard video and the videos of the DIC tensile tests. How long did it take to make the video and which software did you use?

[@bhadeshia123]

Thank you. It took me most of Sunday to compile the video, using "Videoscribe", which is a superb whiteboard software. Most of the time is on making a coherent story rather than in using the software, which is really easy to use. I then combined the whiteboard component with the tensile test videos using iMovie. The music is available copyright-free from the KZbin Audio library kzbin.infomusic?nv=1

[@bobstevens7028]

Dr. Bhadeshia, it is said that necking (the plastic instability) occurs in a tensile test when the rate of local specimen thinning (and hence the local stress increase rate) exceeds the rate of increase in load bearing capability that arises from strain hardening. If that is true, then the specimen should theoretically begin to neck whenever the stress strain curve exhibits either a flat or a negative slope since the defined condition should be met when this slope is flat or negative. However, we know this necking does not happen right after the upper yield point is reached and we also know it does not occur with Q&T low alloy steels during the flat region (slope near 0) of the stress strain curve in the region where the offset yield strength is measured. Is there any explanation for why this is so? Clearly this would seem to prove that the text book explanation of why a plastic instability occurs does not actually apply universally to all cases wherein it should apply as per the given explanation.

[@bhadeshia123]

The initial event that leads to necking is a stress concentration locally in the tensile specimen. That is why a thinning of the specimen can begin at that location. How that thinning then develops depends on the work hardening rate. If the sample is homogeneous, there will be no stress concentration and no development of necking even if the work hardening rate is small. Incidentally, the upper yield point is followed by the propagation of a Luders band which then traverses the specimen, and then work hardening becomes visible in the engineering stress-strain curve. And eventually necking. So the yeild point is not synonymous with necking. If the slope you mention is zero, then the work hardening rate just compensates for thinning so that would not lead to necking unless there is a local concentration of stress due for example to a defect.

[@bobstevens7028]

@@bhadeshia123 Thanks for the explanation. It is said that annealed low carbon steels tend to exhibit the yield point drop phenomenon discussed in the video. Would we expect low carbon steels with a primarily bainitic microstructure (developed for example via either austempering or quenching at a bainite producing cooling rate) to also exhibit the yield point drop phenomenon?

[@Methuselah_]

What is the coating on the specimen? What happened to the sample in min 4:02?

[@bhadeshia123]

No coating, digital image correlation

[@hbbabu4791]

Hello sir....after turning away the dislocations from intertials, stress drops then Curve has extended zigzag pattern at lower yield point. ?

[@bhadeshia123]

Not sure about this.

[@mostafaamirjani8330]

Hi. (min 2:42) The derth of martensite?

[@bhadeshia123]

Dearth implies scarcity. There is no shortage of martensite.

[@SandeepKumar-ux8pg]

Please make a video of Zinc Coating On steel sheet on microstructure level how Fe-Zn phase form all the zinc bath chemistry .......Make a good video and explain well..Thanku...Ur all video is really useful thanku once again

[@bhadeshia123]

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