necking: one construct, many systems

In materials science, necking is the onset of localized thinning in a ductile specimen pulled in tension, where deformation concentrates into a narrowing neck once the material can no longer harden fast enough to offset its shrinking cross-section. E. W. Hart formalized the instability condition in Theory of the tensile test in Acta Metallurgica, building on Armand Considère's 1885 criterion. The referent is a mechanical tensile instability in a stretched solid, the same localization geologists read in stretched rock layers.[1]

Unit

One neck, localized at the point of plastic instability

Recovery means

Pull a ductile bar in tension and watch deformation localize into a thinning region at maximum load

Measured by

Detected at the load maximum (Considère construction); neck geometry tracked by strain measurement

In structural geology, necking is the localized thinning of a competent (stiff) rock layer stretched within weaker surrounding rock, producing pinch-and-swell structures that, carried to completion, break the layer into sausage-like boudins. The process is described in Haakon Fossen's Structural Geology in the chapter Boudinage. It is the same extensional localization instability seen in a metal tensile test, here frozen into deformed rock over geological time.[2]

Unit

One neck (pinch) between boudins in a stretched layer

Recovery means

Find a stiff layer thinned into pinches and swells (boudins) within weaker rock

Measured by

Characterized by boudin spacing and extension ratio from outcrop measurement