What Are the Effects of Fatigue?

Fatigue strength refers to the maximum stress that a material can undergo under alternating load multiple times without causing damage, and is called fatigue strength or fatigue limit. In fact, it is not possible to conduct an infinite number of alternating load tests on metallic materials.

Mechanical parts such as shafts,

In 1954, two successive crashes in the world's first commercial airliner, de Havilland Comet, caused

The fatigue life of a part is related to the stress and strain levels of the part. The relationship between them can be expressed by the stress-life curve (-N curve) and the strain-life curve (-N curve).

The mechanism of fatigue can be divided into three interrelated processes:

Designers often consider the most important safety factor to be the overall strength of a component, assembly, or product. In order for the design to achieve overall strength, the engineer needs to enable the design to carry the ultimate load that may occur and add one

According to the analysis of fatigue failure, the crack source is usually generated in the place with stress concentration, and the reduction of the durability limit of the component is largely due to the stress concentration effect caused by various influencing factors. Therefore, trying to avoid or reduce stress concentration can effectively improve the fatigue strength of components. The fatigue strength of components can be improved from the following aspects.

Fatigue strength

Reasonable fatigue strength profile

The more drastic the change of the section of the member, the larger the stress concentration factor. Therefore, the method of changing the external dimensions of components is often used in engineering to reduce stress concentration. If a larger transition fillet radius is used, the section change should be as slow as possible. If the fillet radius is too large to affect assembly, a spacer ring can be used. It reduces the stress concentration without affecting the assembly of the shaft and the bearing. In addition, concave fillets or unloading grooves can be used to achieve gentle stress transitions.

When designing the shape of components, try to avoid sharp holes and grooves. When the cross-section size changes suddenly (stepped shaft), when the structure needs a right angle, the unloading groove or the undercut groove can be opened on the larger diameter shaft section to reduce the stress concentration; when the shaft and the hub adopt a static fit, the The load reduction groove is opened upward or the diameter of the shaft of the mating part is increased, and a rounded corner transition is adopted, so that the stiffness difference between the hub and the shaft can be reduced, and the stress concentration at the edge of the mating surface can be reduced.

Fatigue strength improves component surface quality

Generally speaking, the stress on the surface layer of a component is very large. For example, in a component subjected to bending and torsion, the maximum stress occurs on the surface layer of the component. At the same time, due to processing reasons, tool marks or damages on the surface of the component will cause stress concentration. Therefore, for components with high fatigue strength requirements, finishing methods should be used to obtain higher surface quality. Especially for materials that are sensitive to stress concentration, such as high-strength steels, the processing is more delicate.

Fatigue strength improves component surface strength

Commonly used methods are ^[5] two methods of surface heat treatment and surface mechanical strengthening. Surface heat treatment usually adopts high-frequency quenching, carburizing, cyanidation, nitriding and other measures to improve the fatigue resistance of surface materials of components. Surface mechanical strengthening usually uses rolling, shot peening, etc. on the surface of the component to form a pre-stress layer on the surface of the component to reduce the tensile stress that is most likely to form fatigue cracks, thereby improving the strength of the surface layer.

Hawker Technology

The current product transformation of Hawker Technology is reflected in welding stress relief equipment and surface finishing equipment. Among them, this technology can eliminate tensile stress on the metal surface and preset compressive stress, so that the stress is easily released in the part where the metal is cracked, and no cracking will occur Happening.