Optical in-situ analysis method for fatigue cracks and its application in fatigue initiation and growth study on steels with different microstructures

The laser confocal microscope (LCM) with stretching attachment and new sample holders was used firstly to observe the in situ fatigue cracks of steels. A new method for studying the fatigue crack growth behavior was created, which realized the automatic photographing and video recording for the fatigue cracks. The length of fatigue crack was obtained at different fatigue cycles according to the photographs and the video, and then growth rate da/dN was deduced accordingly. Different microstructure steels were studied including polygonal ferrite (PF), polygonal ferrite and martensite (PF + M), acicular ferrite with a few martensite (AF + M), polygonal ferrite and bainite (PF + B). The maximum stress was about 80% of its yield strength of the samples, and the stress ratio of R (R = P-min/P-max) equaled to zero at a frequency of 0.25 Hz in the fatigue test. The results showed that the crack initiation time of these four kinds of steels displayed a negative correlation with the yield ratio Rel/Rm (yield strength Rel to tensile strength Rm).