Necking (engineering)

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-->In mechanical engineering, necking is a mode of ductile flow of material in tension. This is visible when applied stress passes a material’s ultimate strength. The material cross sectional area decreases, becoming thinner, and increases in length before it fails completely.

When a ductile material is deformed in tension, it first behaves nearly linearly. Upon further applied stress, past a material’s ultimate strength, the material’s cross sectional area decreases, causing the material to thin. The location of the thinning is dependent on where the greatest number of surface imperfections occur. Since imperfections are structurally weaker, necking will occur at said location.

Examples of necking can be seen by axially and oppositely stretching Silly Putty or pulling a plastic grocery bag.

This technique is also used in the wire-making industry where, by pulling on a thicker length of wire, it becomes thinner, longer, and more ductile.

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