Effect of Fatigue Loads on behavior of 2024 Aluminum for Aircraft Hydraulic Applications

Hydraulic systems in aircraft largely comprise of metallic components with high strength to weight ratios. Some examples of such material include Aluminum and Titanium alloys which are typically chosen for low and high-pressure applications respectively. For aircraft fluid conveyance products, hydraulic conduits are fabricated by axisymmetric turning to support flow conditions. The hydraulic conduits can have grooved interfaced design within for placement of elastomeric sealing components. This article presents a systematic study carried out on common loads experienced by fluid carrying conduits and the failure modes induced. Firstly, a static structural analysis was carried out on each of the geometries of the test articles to identify the locations having areas of high stress concentration. Test articles of various wall thicknesses and internal diameters were pressure impulse tested at different conditions of side loads to identify cycle numbers till failure and failure locations. On the test articles that structurally failed, the failure locations were identified, and the Scanning Electron Microscope (SEM) analysis was carried out to identify the characteristic footprints of failure surfaces and crack initiation. A comparison done between parameters influencing fatigue life like alternating stress, stress concentration factor and backup material show fairly good trends in accordance with first principles. However, the authors note and appreciate a few data points that deviate from expected trends and through this paper an attempt is made to explain the interplay of said parameters leading to the deviation. It was established that side & lateral plug loads have a significant contribution and influence on failure as opposed to hoop loads which are common in aerospace hydraulic components. It was concluded that the presence of axially misaligned cyclical loads caused reduction of fatigue life on axisymmetric adapters of at least 40K and the absence of the same improved fatigue life by almost 200K. Through the findings of this study it is concluded that a complex interplay between material stiffness, amplitude of alternating stress and stress concentration factor influences how the material responds to cyclic loading.