Alternating bending
Alternating bending is the loading of a - mainly metallic - component by a bending force, the direction of which is periodically reversed by 180 °, so that the component is elastically deformed alternately in opposite directions . A cyclic to-and-fro bending load is called a load cycle. As a result of the changing bending stress on the component, tensions build up - starting from the central plane of the component - which reach their maximum on the two opposing surfaces. There is a constant alternation of tensile and compressive stress .
In terms of construction, the bending force, which changes diametrically in its direction of action, can act on the free end of a component that is clamped on one side, like a cantilever beam . In this case, the maximum tensile stress builds up in each case on the component surface subjected to the bending force, while the maximum compressive stress arises in each case on the opposite component surface that is not subjected to bending force. If, on the other hand, the component is supported at both ends and the to-and-fro bending force acts on the component between the two bearing points, the stress conditions are reversed.
example
A bridge construction with horizontal girders that vibrate vertically due to heavy traffic .
Due to the constantly reversing stress conditions in the rhythm of the load cycles, in particular due to the respective tensile stresses, notches, cracks and the like can occur on the surfaces of the component, which ultimately lead to (premature) breakage of the component in the course of the alternating bending load. The point in time of the break depends on the one hand on the level of the bending force and the resulting extent of the changing elastic deflection of the component and on the other hand on the number of consecutive load cycles. This dependency of the fracture behavior can be graphically represented and illustrated by a Wöhler curve . Here, a is concavely curved first (left) curve branch (the so-called time strength ) that the number of cycles up to fracture to be smaller, the higher the voltage generated by the bending force in the component material. As the load (tension) decreases, the Wöhler curve finally merges approximately tangentially into a second branch of the curve on the right, which has a straight horizontal course. This straight, horizontal branch of the curve is called fatigue strength . In this comparatively low load or voltage range, load cycles of many millions are possible before the component breaks.
If the bending load is correspondingly strong and the bending frequency is high (number of load cycles per unit of time), alternating bending can cause vibration cracking to occur in conjunction with a corrosion medium. This leads to (further) reductions in stress at break and number of cycles at break.
See also
literature
- Dubbels Taschenbuch für den Maschinenbau, 11th edition, Berlin / Göttingen / Heidelberg 1956, vol. 1
- HÜTTE, Des Ingenieurs Taschenbuch, 28th edition, Berlin 1955, Vol. I Theoretical Basics