Stress-Strain Curve of Mild Steel is very important since it allows us to know how the Structural Steel will behave under various sets of loading, and forms the constituent of Code Book for Design. The Curve is plotted for mild steel under gradually increasing load, and the Upper and Lower Yield points depend upon the rate of loading.
This curve is known as Engineering Stress-Strain Curve. There is no appreciable change of cross-section of Steel Members in a Steel Structure under Service Load, and thus justifies the use of initial cross-section for Stress-Strain Curve.
Initially, from point “O” to “A” the curve is straight, that is Steel behaves such that the Stress is proportional to Strain ( It follows Hook’s Law .) Thus, the point “A” is called the Proportionality Limit. From “A” to “B” the Steel Still behaves as an Elastic Material and regains its shape after the removal of load. Thus, point “B” is called Elastic Limit, and Steel at this point carries the maximum Stress, after removal of which the Steel regains its original shape. In actual practice, the points “A” and “B” are close enough to be considered to coincide. After point “B”, the Steel is in Plastic State and hence does not regain its original shape after the removal of Load. The point “C`” forms the upper yield point after which Stress decreases up to point “C”, which forms the Lower Yield Point. The Upper Yield points are dependent on the shape of the cross-section and the equipment on which the test is performed. If the rate of loading is slow, then Lower Yield Point is observed and if it is rapid then Upper Yield Point is observed. From point “C” to “D” there is a definite increase in yielding without an increase in stress, thus representing the Plastic Region. From the point, “D”, Strain Hardening takes place in the Steel, and Stress reaches its maximum value at “E”, which is the Ultimate Stress the Mild Steel can bear.
After “E” Necking takes place in Steel, which corresponds to decreasing rate of Stress with increasing Strain. At last, the Mild Steel reaches its failure point “F”, where it fractures with Cone and Cup formation. This formation depicts the Ductile Failure of Mild Steel.
For, designing Steel Structures or RCC Structures, the Lower Yield Point is taken into consideration, which corresponds to 250 N/mm2 of Stress, and is called Yield Stress (fy). At this point Strain is about 0.00125. Also, Ultimate Stress (fu) at point “E” is taken into consideration for Design as Minimum Ultimate Tensile Strength of Steel. At this point Strain is about 0.2.
Suggested Material:-
Bauchinger Effect on Youtube.
Effect of Residual Stresses on Stress-Strain Curve, in the book, Limit State Design of Steel Structures by S.K. Duggal
Ultimate Strength, Characteristic Strength, Ductility, Toughness and Idealized Stress-Strain Curve, in the book, Limit State Design of Steel Structures by S.K. Duggal
References:-
Limit State Design of Steel Structures by S.K. Duggal
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