Question

Define yield strength and briefly explain why it is such an important material property for maes.

Answer 1

Yield strength is the lowest stress that produces a permanent deformation in a material. In some materials, like aluminium alloys, the point of yielding is difficult to identify, thus it is usually defined as the stress required to cause 0.2% plastic strain. This is called a 0.2% proof stress.
When metals are being stressed in tension at relatively low levels, the applied stress is linearly proportional to the induced strain, i.e. elastic deformation. The relationship between the applied stress, s and the strain being induced, e is as follows:

 

s = E e

where E is the modulus of elasticity, or Young’s modulus.

 

Elastic deformation is nonpermanent, i.e. the material returns to its original shape and size after the applied stress (load) is released.

 

When the applied stress exceeds the elastic region, plastic deformation takes place, i.e. the applied stress is no longer proportional to the strain. The point where the nonlinearity of the stress-strain relationship begins is known as the proportional limit. The applied stress is related to the  induced strain in the plastic deformation region by the following equation: 
s = K e n
where K and n are constants that differs between alloys. The parameter n is termed as the strain hardening exponent.