Evaluation of the prior corrosion effects on the fatigue behavior of 7075-T651 and 2024-T351 aluminum alloy specimens with and without cold worked fastener hole

The following dissertation describes research evaluating the effects of prior corrosion damage on the fatigue crack propagation behavior of cold worked fastener holes on two aluminum alloys, viz. 2024-T351 and 7075-T651. The observed behavior is particularly valuable to aid in developing an optimum amount of cold working for a particular amount of corrosion damage found in aging aircraft structures manufactured from aluminum alloys.; Twenty-four fatigue experiments were conducted. All experiments were conducted at two different maximum fatigue stress levels (σ_max ) viz. 25 ksi and 35 ksi and an R value of 0.1 at 10 Hz frequency. The prior corrosion damage was introduced on the precracked side of the center section of the dog-bone specimens using an accelerated corrosion process. When tested at a maximum fatigue stress level (σ_max) of 35 ksi, both 2024-T351 and 7075-T651 aluminum alloy cold working before exposure to corrosive environments (Cx-PC) specimens exhibited longer fatigue life when compared to cold working after exposure to corrosive environments (PC-Cx) specimens. This may indicate that if some corrosion damage is present around fastener holes, the increased stress concentration around the fastener holes because of the presence of corrosion damage in addition to the increased applied fatigue stress level will result in the reduced fatigue life of PC-Cx conditions even though the cold working process was performed around the fastener holes. However, at a reduced stress level σ _max = 25 ksi, PC-Cx specimens resulted in longer fatigue life when compared to the Cx-PC specimens. This indicates that at reduced applied fatigue stress level there can be a compromise between a “small” amount of corrosion damage and the amount of cold working in PC-Cx conditions. In other words, even if a “small” amount of corrosion damage is present around the fastener holes it may not considerably affect the fatigue life of cold worked fastener holes. However this may not be the case for a “large” amount of corrosion damage present around the fastener hole even at a reduced fatigue stress level. The amount of corrosion damage that would either not affect the fatigue life or would result in fatigue life reduction of cold worked aluminum alloy specimens is a question that needs further clarification since this research shows effects of prior cold working in improving life.; From the fractographic study, it was observed that fracture was primarily by the lower energy quasicleavage mechanisms. This observation provides some evidence to support the hypothesis that embrittlement occurred during the prior corrosion process.