Under the action of cyclic stress, the maximum stress value of a metal material that can withstand countless cycles without breaking is called fatigue strength. The measurement of fatigue strength of CNC machine tool materials is usually carried out on a rotationally symmetrical bending fatigue testing machine. The machine tool usually uses the aN curve to describe the relationship between the cyclic stress 1 of the material and the number of fracture cycles N.
It can be seen from the curve that the lower the stress value, the more cycles before fracture. When the stress of CNC machine tool is reduced to a certain value, the aN line is parallel to the abscissa axis, which means that when the stress is lower than this value, the material can withstand countless stress cycles without breaking. The stress value at this time is the fatigue limit or strength. When the cyclic stress of CNC machine tools is symmetrical, the fatigue limit is symbolized a. Express. In fact, it is impossible for CNC machine tools to carry out numerous fireflying experiments on various materials, so the machine tool has made corresponding regulations on the number of cycles N according to different materials.
The N of steel is 10 7 times, while non-ferrous metals often take the los term.
There are many factors affecting the fatigue limit. In addition to reducing the stress concentration of the parts in the design, improving the surface roughness of the parts and performing surface heat treatment (such as high-frequency induction hardening, surface deformation strengthening, chemical heat treatment and various surface composite strengthening), It is also a good way to change the residual stress state on the surface of the part and improve the fatigue limit.
There is a certain empirical relationship between the fatigue limit of CNC machine tool materials and its tensile strength, such as carbon steel cJ. – (0. 4～0 5) Gong, gray cast iron a,, -0. 40 “, non-ferrous metal a.=(0 3～0 4) alum, so under the same other conditions, the fatigue strength of the material increases with the increased with an increase in tensile strength.
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