Fatigue crack growth on several materials under single-spike overloads and aircraft spectra

icaf2023 Tracking Number 1

In the mid-1960’s, the phenomenon of flat-to slant crack growth was studied by many in the aircraft industry. At low stress-intensity factors, a crack surface is flat, and the behavior is referred to as the tensile mode. The stress state in the crack-front region is under plane-strain conditions (high constraint). As the crack grows with higher stressintensity factors, a 45-degree shear lip starts to develop at the intersection of the crack front and free surfaces. With further crack extension, a complete shear failure occurs through the thickness of the sheet or plate. This behavior is the shear mode, which is under low constraint or plane-stress conditions. In 1965, Schijve found that the transition from flat-to-slant crack growth on a 2024-T3 aluminum alloy over a wide range in stress ratios (R) occurred at a constant crack-growth rate. Hudson and Newman also showed the same behavior on 7075-T6 and Ti-6Al-4V alloys. The materials considered herein are 2024-T3, 7075-T6 and 9310 steel. Four areas of research are presented: (1) constraint loss during plane-strain to plane-stress crack-growth behavior, (2) fracture behavior, (3) single-spike overload-underload behavior, and (4) simulated aircraft spectrum loading. The FASTRAN crack-closure based life-prediction code was used to correlate the constant-amplitude crack-growth-rate data over a wide range in stress ratios (R) and rates from threshold to fracture, and to calculate or predict the crack-growth behavior on the single-spike overload-underload tests. Crack-closure behavior is strongly dependent upon the level of constraint. The main objectives were to see if the constraint-loss region can be experimentally measured and whether constraintloss behavior is the primary reason for crack-growth delays after single-spike overloads. If the current crack-growth models cannot predict the delay cycles from a simple spike overload, then questions arise about their accuracy under more complex aircraft spectrum loading. Tests were also conducted on the two aluminum alloys under the Mini-TWIST (standard European) transport wing spectrum. Crack-growth analyses using crack-closure theory without constraint loss was unable to predict crack growth under spike overloads or simulated aircraft spectra. However, predicted crack length against cycles with constraintloss behavior compared well with all tests