Question

Describe the similarities and differences in the phenomena of Luders band formation in low-carbon steels and tensile drawing of a polymer.

Answer 1

Solution

Similarity

Drop in yield strength in both

The shear band or Luder band inclined (at an angle) to tensile pulling direction

Non uniform stress strain

Difference

No serrations in polymer tensile curve only shear band

Polymer doesn’t have interstitial, hence no Serrations

Strain softening for polymer but not for steel

Stress-strain- low carbon steel Stress-strain curve polymer material Upper yield Upper yield point Drop in yield strength due to 'strain softening' Elastic Luders band explained Yield Shear band point Luder's band Plastic region elonga tion • Luder's band formation in steel happens due to interstitial atom and dislocation interaction (strain aging) • Within yield point region above, the serrated curve indicates Sequential resistance of dislocation by the interstitial (rise in stress) and overcome the resistance by continuous pulling in tension • The above phenomenon results in formation of the Luder's band • • • • • In Polymers, the tensile drawing stress-strain curve is shown above The curve shows similar drop in the strength after upper yield point However, the serrations in the curve are not noticed The drop in yield is due to 'strain softening' with continuous tensile loading The strain softening leads to formation of 'shear bands'