Specific strength
The breaking length, and specific tensile strength, is a characteristic material property. It is to that length, wherein a cross-section of the freely suspended area (for example, a wire ) of a material by its own weight at the attachment tearing. The tear length may be calculated from the measured in the tensile strength and density. Accordingly, the material fails, if the burden of the weight of A LR g is equal to the force RMA that can accommodate the material. For this equilibrium of forces
Is obtained by solving for the LR breaking length to
It is defined as the ratio of tensile strength to the product of density and gravitational acceleration. The breaking length is usually given in kilometers. In the textile industry, the term tear kilometers with the abbreviation Rkm is common. The breaking length is independent of the size and shape of the cross-sectional area, because not only the resistance increases linearly with the cross-sectional area, but also the mass. A tube and a cylinder of the same material have, independently of their cross-sectional area, the same breaking length.
Importance
The breaking length is then a useful measure when the mass of a component is important. Because of the equivalence of inertial and gravitational mass that is the case, when a load by its own weight or by inertial forces caused.
For example, the loading of a picture hook by its own weight due to the mass is negligible and the strength sufficient as a parameter for a given shape. In a bridge, the load may exceed the load caused by use by its own weight. Then, the material of higher tenacity is preferable.
However, the mass of a component not only induced by the gravitational a burden, but may also by their inertia. Therefore, the tensile length becomes important when components are exposed to strong acceleration large inertial forces. This is the case with turbine blades or rods.
In practice, the breaking length in the material selection but often fades into the background, as other criteria such as cost, ease of processing or stability dominate. Is then about the material steel alternative, the strength is important, since the density of steel varies hardly. Therefore, the breaking length is more representative of the physical-technical understanding than the actual work of a designer.
Examples
Calculation example:
Ex wood with Rm = 100 N / mm ² and a density of 500 kg / m³ ( gravitational acceleration g = 10 m/s2):