Stress (physics)

In physics, stress is the internal distribution of forces within a body that balance and react to the loads applied to it. Stress is a tensor quantity with nine terms, but can be described fully by six terms due to symmetry. Simplifying assumptions are often used to represent stress as a vector for engineering calculations.

Related Topics:
Physics - Force - Tensor - Vector - Engineering

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The stress in an axially loaded bar is equal to the applied force divided by the bar's area (see also pressure). Stresses in a 2-D or 3-D solid are more complex and need to be defined more rigorously. The internal force acting on a small area dA of a plane that passes through a point P can be resolved into three components: one normal to the plane and two parallel to the plane. The normal component divided by dA gives the normal stress (usually denoted by σ), and the parallel components divided by the area dA give the shear stress (usually denoted by τ). These stresses are average stresses, as the area dA is finite; but when the area dA is allowed to approach zero, the stresses become stresses at the point P. In general, the stress may vary from point to point, but for simple cases, such as circular cylinders with pure axial loading, the stress normal to the cross section is constant.

Related Topics:
Pressure - 2-D - 3-D - Normal - Normal stress - Shear stress - Cross section

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Since stresses are defined in relation to the plane that passes through the point under consideration, and the number of such planes is infinite, there appear an infinite set of stresses at a point P. Fortunately, it can be proven by equilibrium that the stresses on any plane can be computed from the stresses on three orthogonal planes passing through the point. The three planes are normally chosen to be the x-, y- and z-planes. As each plane has three stresses, the stress tensor has nine stress components, which completely describe the state of stress at a point. By using Mohr's circle method or stress tensor transformation, the stresses on an arbitrary plane through P can be computed from the stress tensor at P.

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Stress can occur in liquids, gases, and solids. Liquids and gasses support normal stress (pressure), but flow under shear stress (see viscosity). Solids support both shear and normal stress, with brittle materials failing under normal stress, and plastic or ductile materials failing under shear stress.

Related Topics:
Viscosity - Brittle - Plastic - Ductile

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