An investigation of fatigue crack growth mechanism on the basis of observations with AFM and SEM (1st report, examination from striation formation in single crystals of a Fe-3.2%Si alloy with (110) [001]orientation)

Fatigue crack growth tests were carried out on thin plates of a Fe-3.2%Si single crystal with an appropriate orientation for striation formation. To clarify the mechanism of crack growth, behaviors of slips near a crack tip during the loading and unloading parts of a fatigue cycle were observed in-situ with an atomic force microscope and a scanning electron microscope and fracture surfaces were also analyzed. Following results are obtained. (a) Striations on a pair of fracture surfaces reveal peaks to valleys matching other than any kinds of symmetrical matching. (b) Profiles of striations reveal shallow ripples. (c) During loading, a pair of concentrated slips activated alternately at a crack tip lead to crack opening and thus crack advance. (d) During unloading, reverse slips which lead to crack closing are activated in a distributed manner. (e) At much higher growth rate, both opening and closing processes by a number of alternating slips are observed. (f) Vacuum environment facilitates a number of alternating slips: this is one of the causes for observing no distinctive striations on fracture surfaces in vacuum. In result, a fundamental model for fatigue crack growth is proposed on the basis of observations mentioned above.