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Stress - Strain Curve

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Stress - Strain Curve - Lesson Summary

The relationship between stress and strain of a deformed body depends on the material of the body. The actual stress-strain graph for a body depends on many factors such as its material, its temperature and whether it has previously undergone any deformation. The way in which strain changes with stress for any material can be studied experimentally. The corresponding values of stress and strain can be then plotted on a graph. This graph is called the stress-strain curve.
In a typical experiment to study the relationship between tensile stress and strain, the material in the shape of a rod or wire is held fixed at one end. The other end of the rod is subjected to a load, which is increased in stages. The elongation of the rod is measured at each stage.
A machine used for stress testing is shown on the screen. The two ends of the rod are held between the top and bottom grips. One end is fixed and the other end is pulled to apply a precise load. The stress is calculated by dividing the load (F) by the original area of cross-section (A) of the rod.
The strain is calculated by dividing the elongation D L / original length (L) of the rod.
A typical stress-strain graph is shown.
Mild steel experiences large strains for very little change in stress in the plastic region. Such materials have high ductility.
So we can say that materials that strain by large amounts before fracturing are ductile, whereas those that rupture at low values of strain are brittle. Accordingly, the study of stress-strain curves for different materials can help scientists identify appropriate material to be used in the design of engineering structures.
The stress-strain curve is a straight line for low values of strain, and Hooke’s Law is obeyed.
Beyond the elastic limit, a material cannot regain its original size on removal of the load.
 Materials that fracture at small values of strain are brittle and those that fracture at higher values of strain are ductile.


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