Small fatigue crack growth in high strength aluminium alloys

Abstract

This work presents an investigation of small fatigue crack growth behaviour in high strength aluminium alloys. Variation in slip distribution in Al-Zn-Mg-Cu alloys caused by the addition of different volume fractions of dispersoids, was not found to noticeably affect small fatigue crack growth rates. Small fatigue cracks grew more rapidly than long fatigue cracks under the same nominal AK and exhibited the familiar growth pattern of deceleration and acceleration. The selected area electron channelling pattern (SAECP) method was used to measure the plastic zone size (PZS) and shape of small fatigue cracks. A novel experiment enabling PZS measurements on growing small fatigue cracks was developed to investigate the development of plastic zone size and shape with crack growth and the relation between PZS and small crack growth rates. It was revealed that small crack growth was accompanied by relatively large plastic zone sizes, with the ratio of plastic zone size to half crack length ranging from 0.8 to 0.2 and that small crack growth rates were proportional to their plastic zone sizes. It was observed that small fatigue cracks began to decelerate when their relatively large plastic zones, not their crack tips, were blocked by grain boundaries. Both plastic zone size and shape were found to be dependent on crack length and growth morphology. Naturally initiated fatigue cracks were predominantly crystallographic in nature and were accompanied by a relatively long, slender plastic zone shape. However, during subsequent growth over 1-2 grain diameters this shape evolved, firstly to a semicircular shape, and finally to the lobed configuration typically found associated with long cracks. The plastic deformation associated with small cracks was characterized by the introduction of a variable friction stress into the ECS dislocation model and the calculated PZS was used to correlate small crack growth. Crack deceleration was described by blocked PZS and the recovery of the growth was expressed by the necessity of a stress concentration within the plastic zone to overcome the grain boundary barrier to operate a dislocation source in the next grain. The prediction was compared with the experimental results and showed good agreement.