Sunday, September 2, 2007

Shear stress

Shear stress

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Shear stress is a stress state where the stress is parallel or tangential to a face of the material, as opposed to normal stress when the stress is perpendicular to the face. The variable used to denote shear stress is τ (tau).

The formula for shear stress in a beam is:

\tau = {VQ \over It}

Structural members in pure shear stress are the torsion bars and the driveshafts in automobiles. Riveted and bolted joints may also be mainly subjected to shear stress. Cantilevers, beams, consoles and column heads are subject to composite loading, consisting of shear, tensile and compressive stress.

Shear stresses within a semi-monocoque structure may be calculated by idealizing the cross-section of the structure into a set of stringers (carrying only axial loads) and webs (carrying only shear flows). Dividing the shear flow by the thickness of a given portion of the semi-monocoque structure yields the shear stress. Thus, the maximum shear stress will occur either in the web of maximum shear flow or minimum thickness.

A road destroyed by shear.
A road destroyed by shear.

Also constructions in soil can fail due to shear; e.g., the weight of an earth-filled dam or dike may cause the subsoil to collapse, like a small landslide.

Shear stress is relevant to the motion of fluids upon surfaces, which result in the generation of shear stress. Particularly, the laminar fluid flow over the surface has a zero velocity and shear stress occurs between the zero-velocity surface and the higher-velocity flow away from the surface.

[edit] Unit of measure

The physical quantity of shear stress is pressure, thus force divided by surface area. The SI-UNIT is thereby Pascal (Pa), thus N/m² - Newton per square meter.

[edit] See also

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