Special Steel

Ultimate tensile strength
Beyond yielding, continuous loading leads to an increase in the stress required to permanently deform the sample as shown in the engineering stress-strain diagram. At this stage, the sample is strain hardened or work hardened. The degree of strain hardening depends on the nature of the deformed materials, crystal structure and chemical composition, which affects the dislocation motion. FCC structure materials having a high number of operating slip systems can easily slip and create a high density of dislocations. Tangling of these dislocations requires higher stress to uniformly and plastically deform the sample, hence resulting in strain hardening.

Fracture strength
After necking, plastic deformation is not uniform and the stress decreases accordingly until fracture. The fracture strength (FS) can be calculated from the load at fracture divided by the original cross-sectional area.

Tensile ductility
Tensile ductility of the sample is represented as % elongation or % reduction in area. % elongation = [(L-L0)/L0]*100 and % reduction are = [(A0-A)/A]*100 where A0 is the original cross-sectional area of the sample and A is the cross-sectional area at fracture.
The fracture strain of the specimen can be obtained by drawing a straight line starting at the fracture point of the stress-strain diagram parallel to the slope in the linear relation. The interception of the parallel line at the x axis indicates the fracture strain of the sample being tested.

Tensile toughness
Tensile toughness (UT) can be considered as the area under the entire stress-strain diagram which indicates the ability of the material to absorb energy in the plastic region. In other words, tensile toughness is the ability of the material to withstand the external applied forces without experiencing failure. Engineering applications which requires high tensile toughness is for example gear, chains and crane hooks, etc.