WHAT HAPPENS AT THE YIELD POINT? (ELASTIC-PLASTIC DEFORMATION)
Reversible Deformation vs. Irreversible Deformation.
Experimental stress analysis (ESA) of structural materials sometimes requires testing to complete failure. In such cases, particularly with ductile materials, failure is often preceded by large local strains, the magnitudes of which are of interest to the test engineer.
The yield point is the point on a stress–strain diagram which indicates the end of the elastic region and the beginning of the plastic region. In principle, when the stress (load) before the yield point is removed, the material should return to its original shape. After the yield point, the stress divided by the strain is no longer constant.
Understanding the yield point is vital when designing a structure, since it generally represents an upper limit to the stress (load) that can be applied.
The value of yield strength is important in the design of any structure or object in civil or mechanical engineering, because it limits the safe operating range.
Materials with higher yield limits can safely sustain higher stress levels.
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 σ, and the strain Ɛ= ΔL/L being induced, is as follows: σ = EƐ (E is the modulus of elasticity, or Young’s modulus).
In terms of material failure mechanisms (as shown in this video), there are basically two types, namely brittle fracture and ductile fracture.
Ductile fracture is preferred over brittle fracture in structure testing.
This is mainly due to the fact that brittle fractures often happen suddenly and without any advanced visible warnings.