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In materials or mechanical engineering, necking is a mode of ductile flow of a material in tension. This is visible when applied stress passes a material’s ultimate strength. The material's 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. While the cross-sectional area decreases, the amount of stress within that certain area actually will increase, since stress is given by:
.
where σ is the stress, F is the force being applied, and Ainstant is the instantaneous cross-sectional area. The decreasing cross-sectional area causes the stress to increase although the force being applied remains the same.
Necking usually occurs due to either pre-existing flaws or flaws that are caused by stress. The usual order for failure by necking is:
- Neck formation
- Cavity void formation.
- Void coalescence to form cracks caused by dislocation movement
- Crack propagation through neck
- Final shear failure.
The two sides of the fracture point can be matched up fairly well after the material has broken. This type of fracture is known as a cup-and-cone type fracture and is fairly common in failures due to necking. The cup-and-cone appearance of the fracture surface is caused by the ductility of the material.
See also
Wikipedia content modification information:
- This page was last modified on 16 October 2008, at 17:03.
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