Influence of microstructural inhomogeneities on the fatigue crack growth behavior under very low amplitudes for two different aluminum alloys

In the present paper the aluminum alloys EN AW-6082 (peak-aged and overaged) and EN AW-5083 (solution annealed) were investigated regarding the long fatigue crack growth behavior in the range of very low amplitudes and therefore very high number of load cycles. The cracks were initiated at micro notches, prepared by means of focused ion beam technology and examined in situ by a long distance microscope. In first experiments the threshold for each material condition was defined. Subsequently the tests were carried out at constant Delta K values. Further analysis such as electron backscatter diffraction (EBSD) and confocal microscopy were executed to analyze the fatigue crack growth behavior. A microstructural barrier function of the primary precipitates could be detected for each material condition. Grain boundaries seem to influence the crack growth only in case of the work hardening alloy (EN AW-5083), which is the material with smaller average grain size compared to EN AW-6082.