Fatigue strength refers to the maximum stress of a material that does not cause damage under an infinite number of alternating loads, called fatigue strength or fatigue limit. In fact, metal materials are not likely to be tested in an infinite number of alternating loads.

Conventional fatigue strength calculations are based on nominal stress and can be divided into infinite life calculations and finite life calculations.

Mechanical parts, such as shafts, gears, bearings, blades, springs, etc., periodically change the stress at each point during the working process. The stress that changes periodically with time is called alternating stress (also called cycle). stress).

Under the action of alternating stress, although the stress on the part is lower than the yield point of the material, the phenomenon of cracking or sudden complete fracture after a long period of work is called metal fatigue.

Fatigue damage is one of the main causes of mechanical component failure. According to statistics, about 80% of mechanical parts failures are fatigue damage, and there is no obvious deformation before fatigue failure. Therefore, fatigue damage often causes major accidents, so the shaft, gear, bearing, blade, spring, etc. are subjected to alternating load. Parts should be made of materials with better fatigue strength.