It is said that necking (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 the 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.
Your assessment is indeed accurate. Yet, basing remarks on necking behavior solely through observation of the stress-strain plot might not be prudent. It is advisable to refrain from commenting definitively on plastic instability unless observable necking occurs in the specimen. The alteration in slope could stem from the interplay between mechanisms of softening and hardening. Should plastic instability arise, it's when geometric softening, involving localized alterations in dimensions, gains prominence over hardening, consequently initiating necking. Therefore, it was a conventional practice, especially in the preceding two to three decades, to supplement stress-strain plots with accompanying pictures of the tensile specimens.
We know that annealed low carbon steels tend to exhibit the yield point phenomena. But if a low carbon steel has a primarily bainitic microstructure, either via austempering, or via undergoing a quench rate which produces primarily bainite, will it exhibit the yield point phenomena?
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Very nice explanation.
These videos are absolutely amazing. Thank you!
Very comprehensive explanation about the cottrell atmosphere.
Thanks a lot. I hope it is useful to you.
Great explanation... Thank you sir for this valuable content
Dear Sai Ganesh, thanks a lot for your kind words. I am glad that the video is useful.
Man u r videos are great for understanding
Glad you like them!
It is said that necking (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 the 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.
Your assessment is indeed accurate. Yet, basing remarks on necking behavior solely through observation of the stress-strain plot might not be prudent. It is advisable to refrain from commenting definitively on plastic instability unless observable necking occurs in the specimen. The alteration in slope could stem from the interplay between mechanisms of softening and hardening. Should plastic instability arise, it's when geometric softening, involving localized alterations in dimensions, gains prominence over hardening, consequently initiating necking. Therefore, it was a conventional practice, especially in the preceding two to three decades, to supplement stress-strain plots with accompanying pictures of the tensile specimens.
Very usefull for physics students also....sir could you plz tell us few reference books
We know that annealed low carbon steels tend to exhibit the yield point phenomena. But if a low carbon steel has a primarily bainitic microstructure, either via austempering, or via undergoing a quench rate which produces primarily bainite, will it exhibit the yield point phenomena?
Wow very good explanation how can i contract you sir ?
Thanks. You can reach me at nchawake@iitk.ac.in
Sir I need Fe3C diagram video
Are there any specific topics related to the Fe-Fe3C diagram or just its explanation?