Shear stress

As shear or slip that type of deformation of a body is referred to by the action of a force at which the force is applied to parallel - in parallel with the inner or outer surfaces of a body. Surfaces are moved relative to each other. This corresponds to the deformation of the body of the geometrical figure of a shear in three-dimensional space (→ for the mathematical description of this figure, please refer shear ( geometry) ).

Formulas

To illustrate, one can imagine a book: you will move the book cover parallel to each other, forming the spine and side stacking an angle equal to 90 °.

When shearing the shear force F is related to the shear stress (or shear stress ) τ and the area A in relation

The shear stress is the dimension of a print, it is a force per unit area, but the force is applied along the surface. The SI unit is thus the Pascal (Pa ), ie N / m² - Newton per square meter.

The tangent of the shear angle by which the edges are tilted is called slip and is proportional to the acting force:

The proportionality constant G is the shear modulus (also: shear or shear modulus ).

In addition, the book top has moved by a distance Ax. Due to the conditions in the right triangle ( trig ) the slip is defined as the ratio of change in length Ax and height l of the sheared body:

For small angles can be simplified to a first approximation

Be set.

Shear in materials

Move the case of crystalline materials at shear stress beyond the elastic limit crystal planes against each other. Such transfer surfaces can also arise new. The cohesion initially remains. At even higher load fails, the cohesion and the workpiece is sheared (see also shear failure, shear cutting).

Shear stress occurs not only under external load, but also for example in thermal cycling (temporary voltages) due to latent tensions ( inhomogeneous solidification, eg during welding ) or permanent voltages ( for example, a coating that shrinks differently than the base material ).

Shear in the Geology

In geology, it differs pure shear (also coaxial shear or pure compression) and simple shear ( non- coaxial shear) between the two end members. A simple shear includes an additional component of rotation as compared to pure shear. These two components are also used for discussion of tectonics transpressiver ( compression plus lateral ) and transtensiver (extension plus lateral ).

Shear in internal combustion engines

Combustion engines are lubricated with oil. By the shear forces occurring in the engine, such as the oil pump, or between the piston rings and cylinder wall, the oil molecules are destroyed in the course of time - the oil ages. The same applies for the additives.