What is plastic deformation?
Plastic deformation is permanent change of shape. Load a material hard enough to pass its yield point and it will not return to its original form once the load is removed — part of the deformation stays.
The opposite is elastic deformation, where the material springs all the way back. Every structure deforms elastically under load; that is normal and allowed for in design. Plastic deformation is a different regime.
The stress-strain diagram
Pull a steel bar and plot stress against strain and you first see a straight line: the elastic range, where stress and strain are proportional (the slope is the modulus of elasticity). At the yield point the line bends over. From there, strain grows far faster than stress — the material yields. Eventually necking and fracture follow.
For structural steel, the yield strength of the common grade S235 is about 235 N/mm² and that of S355 about 355 N/mm². Those numbers are literally in the designation.
Why it is useful in construction
Plastic deformation sounds like failure, but for safety it is often exactly what you want:
- Advance warning. A yielding steel beam sags visibly before it collapses. A brittle material (glass, cast iron, unreinforced concrete in tension) breaks without notice.
- Redistribution of forces. In a statically indeterminate structure, a plastic hinge can shift load onto parts that still have capacity left.
- Energy absorption. Under seismic and impact loading, energy is absorbed by permanent deformation rather than by fracture.
Where it occurs unintentionally
- An overloaded steel section that sags permanently.
- An aluminium or lead component that “creeps” under repeated thermal movement.
- Settlement of the subsoil: the ground under a foundation also deforms largely permanently.
- Plastics or bitumen at high temperature, causing insulation under a roof membrane to be pressed in.
Rule of thumb on site: if a steel component is visibly bent, stretched or dented after unloading, the yield point has been exceeded. Bending it back restores the shape but not the properties — replacement is then almost always the sensible choice.
Related terms
- Elasticity
- Modulus of elasticity
- Reinforcing steel
- Settlement
- Concrete shrinkage
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