A study of fretting fatigue damage transition to cracking in 7075-T6 aluminum alloy

Fretting fatigue can occur in any contacting structural members; the extensive significance can be felt especially in the areas of economic cost and safety. The complex nature of the fretting fatigue phenomenon has limited the amount of development in this field. The aviation industry is one area in which fretting fatigue failures are commonly observed.; The purpose of this study was to understand the fretting fatigue mechanism by characterization of the fretting damage transition to cracking in 7075-T6 aluminum alloy. Determination of the influence of fretting on fatigue life and identification of the damage threshold for 7075-T6 aluminum alloy were also performed. An additional goal of this study was to identify causes and mechanisms related to fretting fatigue by fractographic inspection of surfaces using scanning electron (SEM) and confocal microscopy. A fretting map was created for 7075-T6 aluminum alloy fatigue specimens fretted on 7075-T6 aluminum alloy pads by varying displacement amplitude and normal force.; Results indicated that a fretting fatigue damage threshold exists in this material that varies with respect to applied axial and normal stresses and the microstructure of the material. The exposure to fretting while under cyclic loading substantially reduces specimen life compared to fatigue.; The fretting damage transition to cracking was tracked through interrupted tests. SEM and confocal images of the specimens revealed that fretting plays a role in crack nucleation. Another observation was that pit depth was not the most critical factor in the crack nucleation due to the fretting. Many other factors play important roles in crack formation from fretting damage zones. The presence of subsurface constituent particles beneath the fretting damage also assisted crack formation and ultimately fracture. Fretting degradation depends on the axial stress, normal stress, and microstructure of the material. A larger grain size is more susceptible to increased fretting degradation and a smaller grain size has more resistance to fretting.