Abstract: The traditional cropping processes have some disadvantages, such as poor surface quality, low yield, waste of materials and high energy-consumption. The precision cropping process with circumferential force low cycle fatigue, which is a new type of precision cropping process method, is studied. According to the stress concentration effect of a V-shape notch, the fatigue crack on the tip of the V-shape notch is prompted to initiate and extend. The precision cropping machine’s working principle is described. The criterion that the crack on the root of V-shape notch initiate or not under the effect of low cycle fatigue is provided. The materials which include 0.2%C steel, H59 copper, 0.45%C steel, 20Cr steel and LY12 aluminum are studied under two control curves. Accelerating the initiation and extension of crack and the good cross section of the metal bar are obtained. The results show that the mean stress of the metal bar in the cropping process can be effectively reduced because of the V-shape notch’s stress concentration. The metal bar’s stable crack growth and fracture can be got when increasing impact displacement constantly at the same time reducing the impact frequency in the cropping process.

Key words: Crack, Low cycle fatigue, Precision cropping, Stress concentration