Tensile Test (Part 2)

If the true stress based on actual cross section area of specimen, it is found that the stress-strain curve increases continually up to fracture.

From engineering stress-strain curve, it can be observe yield point which identifies yield strength and limit of deformation form elastic deformation to be plastic deformation. The stress of elastic deformation region is linearly proportional to strain which is called “proportional limit”. There is a greatest stress the material can withstand without any measurable permanent strain remaining on the complete release of load while the plastic deformation is permanent deformation after load releasing.

Yield stress could be finding in two ways depending on graph characteristic. First, the yield point could be observed clearly, the value on the stress axial is equal with yield stress. In case of unclearly yield point such as in carbon steel with annealing or skinpass rolling, it uses 0.2 percent strain off set by parallel lining with graph at proportional limit region, 0.2 percent strain. The point which is intersection between line and stress-strain curve is yield point or proof stress at 0.2 percent strain offset.

If apply load continuously, it will reach the maximum load or ultimate tensile strength which appear as the peak of curve. After that, necking will occur on some areas of material that effect to stress decreasing rapidly while the strain or elongation is increased until fracture occurs. Total change in length of gage length is used for calculation of percent elongation as mentioned above.

Resource : http://www.isit.or.th/, http://www.key-to-steel.com/, http://a-sp.org/