Plastic deformation

Plastic deformation is a permanent change in a material’s shape or dimensions that remains after the applied load is removed. It occurs when stress exceeds the material’s elastic limit.

Dislocation: A line defect in a crystal lattice around which atomic planes are misaligned. Dislocation motion lets crystals deform permanently at stresses far below those needed to shear a perfect lattice.
Elastic deformation: A reversible change in shape or dimensions that disappears when the applied load is removed. It is the recoverable response that precedes permanent deformation in many loading cycles.
Metal forming: Manufacturing processes that shape metal by imposing plastic deformation without removing material. It uses controlled plastic flow to produce parts such as sheets, forgings, and extrusions.
Viscoelasticity: Time-dependent material behavior that combines elastic energy storage with viscous flow. Unlike ideal plasticity, viscoelastic strain can recover gradually after unloading.
Tensile testing: A test that pulls a specimen while measuring force and extension to characterize mechanical behavior. Its stress–strain curve shows yielding, plastic flow, and permanent elongation after unloading.
Slip system: A combination of crystallographic plane and direction along which dislocations move in a crystal. The active slip systems determine how a crystal accommodates plastic strain.
Stress (mechanics): Internal force per unit area within a material, expressed as a tensor in general loading. Plastic flow begins when the material’s internal stress state reaches a yield condition.
Forging: A process that shapes metal by compressive forces applied with hammers or presses. The workpiece takes its lasting shape through substantial plastic deformation under compression.
Creep (deformation): Slow, time-dependent deformation under sustained stress, often significant at elevated temperature. Creep can accumulate permanent strain even when stress remains below short-term yield strength.
Stress–strain curve: A graph relating material stress to strain during loading, often including unloading behavior. Its departure from the elastic region identifies plastic strain and material hardening.

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