Re: The stress-strain curve for iron rises, then dips, and rises again.

John,

That’s a good question, and it demonstrates a pretty cool aspect of 
Metallurgy.  The stress (Stress = Load / Cross-Sectional Area) versus 
strain (Strain = Change in Length / Initial Length) plots for iron and 
most low carbon steels show what is known as a “Sharp Yield Point”.  For 
this curve, there is only elastic deformation (recoverable strain) and 
neglegible plastic deformation (unrecoverable strain) up until the Upper 
Yield Point.  At this point, the material begins to yield plastically, and 
the stress drops down to the Lower Yield Point.  This drop occurs because 
plastic deformation of iron causes the metal at the yield site to soften, 
increasing the magnitude of the plastic deformation.  As more stress is 
applied, the zone of plastic deformation then widens into adjacent areas, 
forming what are called “Luders bands”.  You’ll see these bands as 
diagonal lines (not to be confused with slip lines) across your specimen, 
at approximately 50 degrees from the applied stress.  After the Lower 
Yield Point, the material will begin to work harden, making the stress 
strain curve rise again.

I hope this helps!
Jeff Yap
Mad Scientist

References:
Reed-Hill, R & Abbaschian, R; Physical Metallurgy Principles Third 
Edition; PWS Publishing; Bostion, MA, USA; 1994; pp. 284-286