|MadSci Network: Physics|
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
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