Features: the stress is repeated alternation; the crack source is usually the stress concentration on the surface of gear, such as the fillet of tooth root, machining tool mark and material defect; the tail end is sharp and thin, and the micro structure mainly shows transgranular growth, and its general trend is vertical to the main stress.

Cause: the alternating stress level is too high, and the material defect and stress concentration source have serious influence.

Preventive measures: control the alternating stress level, avoid too small root fillet in the design, and control the process factors to reduce the surface and internal defects of materials.