The stress-strain curve describes the behavior of steel bars under loads. It is created by testing steel specimens. A steel specimen is gradually pulled through a testing machine until it breaks, and stress and corresponding strains are recorded. The stresses are then plotted along the vertical axis, and as a result of these stresses, corresponding strains are plotted along the horizontal axis.
There are different mark points on the stress-strain curve that represent various stages that steel specimen passes through prior to fracture. It is very crucial to understand the stress-strain curve in order to be able to understand the response of steel bars when subjected to loads.
When the steel specimen is subjected to load, it behaves as an elastic material which means the stresses and strains are proportional. But as the load is increased, the specimen starts to lose its proportionality and ultimately fails or yields. When the load is increased beyond the yield point, the steel bar goes through stress hardening and can sustain greater stress, after it reaches the fracture point.
Stress-strain Curve of Steel Bars
When the steel specimen is subjected to load, it goes through several stages such as elastic stage, yield point, and fracture. The important stages of the stress-strain diagram are presented below:
Limit of Proportionality
This stage is represented on the stress-strain curve from the initial point until point "A". In this region, the stress is low and does not generate permanent strain. The stress and strain are proportional to each other, so as the stress is removed the steel bar would regain its original shape.
It is located between point "A" and "B" on the curve. When the stress on steel specimen is further increased, it would create elastic strain. The stress and strain are not proportional to each other.
It is the most important point on the stress-strain curve from the design point of view. This point, denoted by letter B on the curve, is considered as the failure point in the design of reinforced concrete structure. So, as the steel bar reaches the yield point in the reinforced concrete element, it would be considered as a failed member.
The yield point is the beginning of steel plastic deformation. The stress and strain are not proportional. The point B is called the upper yield point whereas the point C is the lower yield point.
As the stress is further increased beyond yield point, strain hardening takes place that is represented from point C to D, beyond which necking starts. During strain hardening, the material undergoes changes in its atomic and crystalline structure, resulting in increased resistance of the material to further deformation. The maximum ordinate in the stress-strain diagram i.e. point D is the ultimate strength or tensile strength.
Rupture strength is the strength of the material at rupture. This is also known as the breaking strength. It is the point "E" on the stress-strain diagram.