2.1 Plastic (Permanent) Deformation

Unlike elastic deformation, during which, for example, a rod under a tensile load returns to its initial length as long as a defined value (elastic limit of the material, R _{p0, 2} limit) is not exceeded, a workpiece which is plastically deformed retains its shape permanently.

For the elastic range, the following applies:

• ? _z in N/mm ² tensile stress

• ? in elongation

• l ₀ in mm initial length

• l ₁ in mm length under the influence of force

• ? l in mm lengthening

• R _m in N/mm ² tensile strength (was ? _B)

• R _e in N/mm ² resistance at the yield point (was ? _S)

• E in N/mm ² modulus of elasticity.

Figure 2.1: Tension test bar change in length under stress

In the plastic range,

a permanent deformation is caused by sufficiently high shear stresses. This makes the atoms in row A ₁ (Fig. 2.2) change their state of equilibrium in relation to row A ₂. The extent of the displacement is proportional to the extent of the shear stress ?.

Figure 2.2: Ideal process of position change of the atoms

If the effective shear stress is less than ? _f ( ? _f yield shear stress) then m < a/2 and after the stress is removed the atoms return to their original position - elastic deformation.

If, however,...