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Session: Session 7: Fatigue crack growth and life prediction methods  III
Session starts: Tuesday 27 June, 13:30
Presentation starts: 13:50
Room: Theatre room: plenary

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James Newman (Mississippi State University)
Kevin Walker (QinetiQ Pty Ltd)

Abstract:
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 stress-intensity 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 constraint-loss 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 constraint-loss behavior compared well with all tests.