INVESTIGATION OF SMALL FATIGUE CRACKS .1. PLASTIC-DEFORMATION ASSOCIATED WITH SMALL FATIGUE CRACKS

Small fatigue crack growth has been studied in two 7000 series aluminium alloys possessing different grain sizes. As it has been suggested that the relatively large plastic deformation associated with small fatigue cracks may explain their fast growth rates compared to long cracks, the work focused on experimental measurement of the plastic zone size (PZS) accompanying small cracks. The characteristics of plastic zone development and the relationship between PZS and crack growth rate have been studied through periodic plastic zone size measurement of growing small fatigue cracks. It was found that small fatigue crack growth is indeed accompanied with comparatively large plastic zone sizes. It is suggested that this is mainly due to a near surface effect. It was also found that an exponential relation exists between small crack growth rates and their plastic zone sizes for a given material.