what if luders bands do not show up in the stress-strain graph as expected?

[LorenzoUK]

Hello,

Luders Bars are expected to show up in the stress-strain graph when testing at room temperature tensile specimens made of low alloy steel: the graph will therefore CLEARLY show an Upper Yield Point and a Yield Point Elongation between the elastic and the elastoplastic phases.

But what if this doesn't happen, and the graphs only shows elastic and plastic phases, one after the other? This is currently happening in numerous occasions during a huge testing campaign on low allow steels.

Which are the possible interpretations or reasons? Should the test be invalidated? Should the alloy be considered NOT suitable?

Please note that NOT ALL the samples coming from the same block of steel and extracted one near the others show this behaviour. Nevertheless, in some cases it happened. It is likely that proximity has a role then, therefore there might be something locally in the material which can justify this behaviour. But What exactly?

Last information: yield and ultimate strength values don't show any significant difference in respect of the presence or not of the Luders Bands.

Thanks in advance for your help

Kind Regards

Lorenzo

ATTACHMENT:

http://files.engineering.com/getfil...-4c83-b7cd-547f8fd1e104&file=Luders_Bands.gif

 

[metengr]

The presence of Luders bands will be a function of prior thermal/mechanical processing history of the steel and is not an indicator of material acceptability, given all things being equal for tensile testing.

 

[weldstan]

Agree with metengr.

 

[LorenzoUK]

Thanks metengr,

but what exactly changes, in the prior thermal/mechanical processing history of the steel, making Luders Bands NOT showing up in some cases?

thanks

 

[metengr]

Steel that has been supplied normalized versus annealed or hot formed.