Effects of crack closure on fatigue crack-growth predictions for 2024-T351 aluminum alloy panels under spectrum loading

Effects of plasticity induced crack closure on the calculated fatigue crack-growth behaviour of middle-crack tension specimens made of 2024-T351 alloy subjected to variable amplitude fatigue loading were examined. The specified loadings were four distinct aircraft spectra: (1) “modified” center-wing spectrum performed as part of this study; (2) fighter aircraft maneuver spectrum; (3) lateral gust spectrum; and (4) pressurized fuselage gust and maneuver spectrum. The experimentally observed crack-growth lives were compared with crack-growth predictions obtained using the crack closure concept in FASTRAN. Fatigue crack-growth material data extracted from the NASGRO materials database and the literature follows two distinct paths possibly due to different transitions from flat-to-slant (single or double shear) fracture. The material baselines for the FASTRAN code were selected to fit the two different paths. The two baseline curves were used to predict crack-growth life under four spectra to assess how the observed differences in the crack-growth rate trajectories affect the results. Using the lower bound baseline curve, which is suspected to be characterized by the double shear fracture mode, the FASTRAN model was able to predict crack- growth behaviour within ±25% of the test data for all four spectra.