The influence of microstructure of metal on the fatigue crack propagation has to be clarified to improve the fatigue strength of metal. A modulated pattern method to study the micro fatigue crack propagation behaviour was developed, by which every fatigue striation on the fracture surface can be numbered, and the roles of grain boundaries, inclusions and other microstructures were discussed using a precipitate hardening aluminum alloy. Grain boundaries and sub-grain boundaries often worked as resistance for the fatigue crack propagation. Inclusions etc. did accelerate or resist the fatigue crack propagation depending on their mechanical properties. No reverse direction crack growth caused by preceded cracking has been observed.
