Threshold stress
In the strength calculation, a swell load , also called a swell load , is a dynamic (time-variable) component load in which it is periodically only subjected to tensile or compressive loads. The amount of stress fluctuates between two positive values or between a positive value and zero. If the component is alternately subjected to tensile and compressive loads, this is known as alternating stress . The stresses are then alternately positive and negative.
Effects of dynamic component loads
A dynamic load is more critical for a component than a static load because the component is constantly deforming. It is macroscopically elastic ; However, due to fracture mechanical processes, stresses above the elastic limit can occur at fault locations in the component , which cause local plastic deformations. This can lead to embrittlement in the affected area due to work hardening and the resulting cracking. Depending on the design of the construction and depending on the material, components are differently susceptible to such cracks.
Regulations
One of the most important standards used for verifying the fatigue strength of dynamically loaded components is DIN 15018 cranes . This defines a maximum permissible component stress. This depends on an application-related stress group, the component geometry (notch numbers), the number of load changes and, last but not least, on the type of dynamic load that is reflected in the stress ratio and a stress collective (frequency of high or low stress).
The stress ratio is determined as the quotient of the load change between the maximum calculated tensile stress and the maximum calculated compressive stress in the component. A negative tension ratio means, because of the change in sign, a tension-compression-swelling load, a positive tension ratio a pure tension or compression-swelling load. The occurrence of compressive stresses reduces the permissible component stress more than the occurrence of tensile stresses. The maximum permissible component stress can be calculated from these parameters. In order to avoid component failure caused by surge loads, the structural design of components is of the utmost importance. The diagrams used to determine the notch numbers in DIN 15018 provide the designer with important information on avoiding unfavorable component geometries.
See also
Individual evidence
- ↑ DIN 15018 Cranes: Section 1.4 Procedure and Section 1.5 Tables