Proof stress of a material can be defined as the stress at which the material undergoes plastic deformation. Proof stress is determined as the stress corresponding to 0.2% of strain from the stress-strain curve of the given material. This is the stress value determined for the materials like aluminum, magnesium and copper whose yield point cannot be easily defined.
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Here the importance of proof stress and its determination are explained briefly.
Importance and Features of Proof Stress
The role of proof stress comes into play during the material selection process. As proof stress gives the value of stress beyond which the material undergoes permanent plastic deformation, it decides the value of external stress that can be applied to it. Any increase of stress beyond the proof stress results in the occurrence of plastic deformation hence resulting in fracture.
The proof stress of a material is large enough to stretch the atomic bonds of the materials. This causes the atoms to move across dislocations. This movement causes permanent deformation.
As proof stress is determined for the materials whose yield point cannot be easily defined ( Aluminium, Magnesium, Copper), it is called as offset yield stress. Hence proof stress or offset yield stress are more readily measured for such materials over yield stress.
In no case be the proof stress be confused with ultimate stress value. Proof stress is the value at which the material undergoes plastic deformation that is irreversible while ultimate stress is the value beyond which the material undergoes failure.
Determination of Proof Stress
The stress-strain curve of certain ductile materials from their tension test does not provide a specific value for yield stress and yield point. In such case proof stress or offset yield stress is determined.
For this, in the stress-strain graph, a line parallel to the linear portion of the graph is plotted from 0.2% strain value as shown in graph-1.
The strain at 0.2% is called Permanent Plastic Strain and the intersection of the parallel line at the curve is called as a proof point. The stress corresponding to the proof point is called as proof stress.
Proof stress hence forms design stress for some of the metals. It tests the properties of the materials, the metallurgical state or their microstructure.