On the crystallographic, stage I-like, character of fine granular area formation in internal fish-eye fatigue cracks

The crystallographic character of fine granular area (FGA) formation from internal particles in martensitic high strength steel has been revealed by an assessment of the plastic zone size at the FGA border. This plastic zone size corresponded to about 3 times the martensite lath width. Tests at different temperatures (20°C, 200°C and 400°C) revealed a decreasing FGA size with increasing temperature at constant applied stress amplitude. As a consequence, the critical stress intensity factor varied as the FGA decreased with temperature. In contrast, the critical plastic zone size remained constant and equal to the sizes of microstructural features. This represents a strong similarity between crystallographic, stage I-like, crack propagation and FGA formation in a vacuum. ….Read the article

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Failure Analysis of Aircraft Aluminum Alloy Structures in Coastal Environments

Un cas de corrosion intergranulaire sur aluminium provoqué l’environnement marin aux îles Fidji

Abstract

The failure of aluminum alloy stringer on the tour airplane servicing in the coastal environment of the Fiji Islands were analyzed. Through the analysis of surface topographies and failure characteristics and the consideration of service environment, the evolvement of failure process and the failure mechanisms were analyzed. Findings showed that ultraviolet radiation, temperature, high-humidity and salt spray caused the aging failure of structure coating and then pitting corrosion occurred in the oxide film on aluminum alloy. With the ongoing corrosion along the grain boundary, intergranular corrosion appeared. When the intergranular corrosion continued developing to the adjacent layers, denudation occurred. The volume expansion effect of corrosion products caused the stress concentration on the tip of corroded area and resulted in the initiation of cracks.

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