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A shear plane is the plane in which shear stress occurs. Like normal stress, shear stress is a measure of force per unit area. At any given point in a structure, there are many possible planes that can be defined to measure stress. Accordingly, whichever plane contains the area in question is the shear plane. The shear plane is useful for engineers who analyze the internal stresses in structures.
Stress has the same units as pressure: force per area. Normal stress results when a component is put into tension or compression. If a metal rod is stretched vertically, an internal stress will tend to resist further deformation. This stress will occur in horizontal cross sections of the bar. The stress is said to be normal because it is directed at a right angle to, or normal to, the horizontal stress planes.
Shear stress is similar to normal stress in that it has the same units. The direction of stress, however, is parallel to its stress plane. This kind of stress could result if a different set of forces were applied to the same metal bar; namely, if someone held the bottom of the bar stationary while trying to move the top of the bar to the right. The resulting internal stress is called shear stress, because parts of the bar are trying to slide, or shear, past each other.
Under these loading conditions, the stresses on a horizontal plane located in the center of the bar would be shear stresses directed horizontally. The bottom of the bar would be trying to move to the left; it would subject leftward forces in a horizontal shear plane. The top of the bar would be trying to move to the right; it would subject rightward forces on the same shear plane.
Analyzing the shear plane is important in preventing the mechanical failure of a structure. Every material has a limit on how much stress it can endure. This limit is a property of the material itself, rather than the shape of the object. For example, two rubber bands could be made in different sizes but from the same material. The larger one can withstand more force before breaking, but only because it has more cross-sectional area to distribute the force; the internal stress will be the same at failure for both sizes.
Likewise, components can break because of too much shear stress. If a component fails by shear stress, parts of it will literally slide past each other. Old metal bolts, for example, commonly fail in this way. The limit on shear stress, like with normal stress, is a material property.