In physics, the stress at a point in a material is the applied force per unit area. The stress unit is the Pascal (symbol Pa). To be exact, the stress at a point may be determined by taking the limit of the load being carried by a particular cross section, divided by that cross section, as the area of the cross section aproaches zero. In general the stress may vary from point to point, but for simple cases, such as circular cylinders with pure axial loading, the stress is constant and equal to the crosssectional area divided by the applied load.
For instance, if we have a steel bolt with a diameter of 5 mm, it has a crosssectional area of 2*10^{5}m^{2}. Suppose that the load is 50k N, the stress (force distributed across the crosssection) is about 2.5 MPa.
That means each m^{2} of bolt would support 2.5 MN of the total load.
In another bolt with half the diameter, and hence a quarter the crosssectional area, carrying the same 50 kN load, the stress will be quadrupled (10 MPa).
The ultimate tensile strength[?] of a material is the value of the stress causing the material's fracture. The yield strength[?] is the value of stress causing plastic deformation. These values are determined experimentally using the measurement procedure known as the tensile test[?].
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