Materials Engineering Department
BS in Materials Engineering
In order to investigate the effects of grain orientation on the high cycle fatigue properties of 7050-T7452 aerospace grade aluminum, rotating-bending fatigue tests were performed on longitudinal (LG) and short transverse (ST) specimens taken from two sample sets (Method A and Method B) of open die forged 7050 aluminum. The fatigue specimens were machined to a 0.2" gage diameter and 4" length and subsequently hand polished to a mirror finish to minimize potential crack initiation sites. The tests were conducted on a Fatigue Dynamics RBF-300 HT tester in the fully reversed loading condition, with stress levels varying from 20 ksi - 45 ksi in 5 ksi increments, except the 25 ksi stress level. The applied stress amplitude and number of cycles to failure for each specimen were plotted to generate S-N curves comparing both Method A and B. The fatigue life was comparable between Method A and Method B sample sets and the fatigue life of longitudinal samples was greater than the fatigue life of short transverse samples. Runout tests were stopped at 107 cycles and set as 20 ksi for both orientations. There was no significant statistical difference between Method A and Method B on fatigue life of 7050-T7452 aluminum. There was a significant statistical difference between LG and ST specimen orientations on fatigue life due to the precipitate alignment and grain flow from forging.