les_mom_necking_1    les_mom_necking_2 [1, 2, 3] Necking is local deformation that “begins at a tensile point or ultimate stress point” [a]. After ultimate stress is reached, the cross-sectional area of a small portion of the material decreases. This is a result of uniaxial tension or stretching [b].  This newly smaller area has very large amounts of strain and is seen as an instability, called the “neck”. The act of necking can be shown on a stress-strain diagram.  It is the range on the graph from the ultimate stress point to the point of fracture of the material [a]. Necking takes place after a material passes through the elastic, yielding, and strain hardening region of a material test [c]. Necking is mostly associated with ductile materials, and is common during experimentation of steel in tension in many materials labs. The necking region can take on a cup or cone-like shape in ductile materials. In brittle materials, there is no necking region. The material will simply fracture with a relatively flat plane at the fracture area. Necking has criterion as determined by Considère in 1885: 1) During tensile deformation, the material has a decrease in cross-sectional area, 2) Strain hardening occurs during tensile deformation, and 3) All materials have flaws in their structure [d].

 

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