According to the analysis of the fracture surface, the screw fracture is a low-cycle fatigue failure. Three sections are clearly visible on the cross section: the source of fatigue cracks; the fatigue crack propagation region; and the transient fracture region. Fatigue crack sources can occur at the root of the screw with stress concentration, surface cracks, and internal inclusions. The fatigue crack source of the screw is very small and is the core area of ​​the fatigue crack. It can be clearly seen from the photo that the fatigue crack of the screw is at the stress concentration of the screw root. The defective screw root has a circumferential length of 12mm, an axial width of 3mm, a depth of more than 2mm, and a total surface of more than 20mm2, but less than 10% of the fracture area.

The nitrided hardened layer is cracked. Fatigue cracks slowly propagate in this area. Initially, the fracture surface is rubbed smoother and brighter due to repeated alternating stress, and then a shell pattern appears. The shell pattern is generally perpendicular to the crack propagation direction, advancing toward the entire hollow circumference. The space between shells (fatigue arcs) varies. Under repeated effects of alternating stresses of varying sizes, the macroscopic traces left by the discontinuous changes in the crack propagation process are caused by the load change of the extruder during startup, parking, and hard objects blocking the screw. The area of ​​the fatigue crack growth zone usually accounts for 98% of the fracture area, which is in line with the basic characteristics of fatigue cracks. When the shells on both sides of the circumference (fatigue arc) are close to tangency, and the stress of crack propagation to the net section reaches the breaking stress of the screw, the screw is cut off. The cross section has obvious steps, and the area of ​​this part accounts for about 1% of the entire fracture area.