FATIGUE-CRACK GROWTH-STUDIES UNDER COMBINATION OF SINGLE OVERLOAD AND CYCLIC CONDENSATION ENVIRONMENT

Fatigue crack propagation is examined using single overload in cyclic condensation environment. Using this environment the growing crack is exposed cyclically to environments with high and low humidity. These environmental changes cause the formation of a periodic pattern on the fracture surface called beach marks when the crack growth is low. A single overload can also cause the formation of beach marks in the overload affected zone when cyclic condensation environment is used. The markings are being formed due to the retardation of the crack growth subsequent to the overload. The markings are also formed close to the stretch zone and the spacing increases with increasing distance from the overload zone; subsequently they stop forming. The microscopic crack growth rate after the overload obtained from measurement of beach mark spacing is compared with the macroscopic one calculated from crack length measurement. The mechanism of beach mark formation after the overload is the same as in the initiation zone of short cracks and near-threshold propagation of the long one. The materials used are centre-cracked-tension (CCT) specimens of 7075-T6, 2024-T3 and 7075 aluminium glass fibre epoxy laminate (GLARE I).