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Necking Analysis of Strain Softening Materials ...

Hi all.

I'm studying the high temperature viscoplastic behavior of titanium alloys. It is well known that at high temperature range, deformation induced dynamic recrystallization (DRX) causes dramatically strain softening. Such strain softening behavior is also treated as plastic instable according to literatures.

During my experimentation of high temperature tensile test of titanium thin sheet, I didnt find necking until the specimen reached very large strain, usually larger than 0.4 when strain rate was set slower than 0.1/s. Before the initiallization of necking, the specimen was already strain softened.My question is, how to analyze such necking phenomena for strain softening materials, or is there any creterion for prediction of the critical strain of necking initialization?

The traditional necking creterion is based on both strain hardening and strain rate hardening hypothesis. Meanwhile such DRX induced strain softening behavior is definitely not the type of damage related softening (microstructure analysis didnt show any kind of void, crack, etc.. ). Any suggestion is appreciated. Thank you.

 

Best regards.

Xiaoteng Wang 

Comments

aslan

Dear Wang

there are plentiful models that describes this phenomenon and if u need detailed informations i can mail the related papers to you. but in my investigations on Ti alloys i didnt saw any microstructural changes due to this flow softening i.e if its DREX why the grains are still elongated. so i prefere you to use bergstorm model for your modeling.

best regards

Dear Aslan,

 Thank you for your kindly suggestions. If you have such papers for the necking analysis of strain softening materials, please send me  some detail information. Thank you again. About the DRX in Ti alloys, I dont know the details about your work, but I dont think the elongation of the grains is a direct evidence of whether DRX begins or not.  The deformation of grains always appears when metals undergo severe hot deformation. But the optical microscope or SEM pictures can not give enough information of how things happend within the  grain. The most strong evidence is using TEM or EBSD to analyze the substructures. In our studies of titanium alloy, we used EBSD to analyze the misorientation of the grains, and the results turns out to prove an initiation of DRX. 

Another interesting topic is the relationship between the microstructrual changes and  the deformation behaviors. I noticed that you mentioned "DRX due to flow softening", but in my opinion the microstructural change should be the reason why stress-strain curves are showing flow softening patterns. Using flow softening to explain why recrystallization happens sounds confusing to me.

Thank you for your suggestions again.

 

best regards.

Wang

Dear Wang

if your EBED resluts proved the appearence of DRX, so the critical strain to initiate flow softening can be calculated. i have papers that calculates the critical strain in hot compression and i think the procedure can be similar for hot tension.

i dont have your E.mail address. please mail to me and then i will send you 3 paperes(aslan_25253@yahoo.com)

Here is some guidance for you

1)Recrystallization and related phenomena Humpherys and sellars

2)workability J.J.Jonas

 

 

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