Most structural members and machine components are subjected in service to cyclic loadings of varying amplitude. The variation in stress level follows either a regular or random pattern. The resulting crack growth is affected by the applied load sequence in the early stage (crack initiation) and in the later stage (crack propagation) of fatigue. The fatigue crack growth is known to be retarded by tensile overloads and accelerated by compressive overloads (underloads). However, the phenomenon and mechanism of the load sequence effects, especially those of overloading and underloading, on fatigue crack growth in different environments remain to be clarified.

Test Set-Up in MTS Machine

Structural components are mostly subjected to variable amplitude or spectrum fatigue loading in various service environments. Blades in gas turbine engines experience low-amplitude, high- frequency vibration during operation, superimposed on a relatively smaller number of cycles of fatigue loading due to start-up and shut-down. Railway tracks are subjected to random loading depending on the frequency and loading conditions associated with the passage of trains. The rotors and bearings of a turbo-generator are subjected to an overload (OL) during every start-up. On the ground, the lower wing skin of the aircraft is under compression. During flight, variable loads due to gust are superimposed on a mean tensile load corresponding to an undisturbed flight. The transition from a compressive load on the ground to a tensile load during flight is an important load cycle in itself and is usually referred to as a ground-air-ground cycle.

The fatigue crack growth under spectrum loading is affected by load interaction, such as crack growth acceleration, retardation or even arrest. Due to the load interaction effects, reliability, and life assessment of structural components entails considerable difficulties under spectrum loading. For instance, high OL peaks cause retardation effects whereas underload (UL) peaks accelerate the crack growth and weaken the preceding retardation effect.

To account for the load spectrum effects, cycle-by-cycle fatigue crack growth prediction models were developed. They are divided into three main groups, Willenborg, Wheeler, and UniGrow. The first and second ones consider that the current cyclic crack tip plastic zone develops inside a larger zone created by the preceding OL. Furthermore, the second one is based on crack closure, and includes plasticity-induced crack closure model and strip yield model. Third group, the unified two parameter model is based on the elasticplastic crack tip stress-strain history.

This research was initiated to clarify the fatigue crack growth behavior of a 7075-T651 aluminum alloy under spectrum loading with periodic OL or UL cycles in different environments. A middle-tension M(T) specimen was machined in L-T orientation from a 7075-T651 aluminum alloy extrusion of 127x127x394 mm (5x5x15.5 in.). It was 102 mm (4 in.) wide, 235 mm (9.3 in.) long and 2 mm (0.086 in.) thick, and its center notch was 3 mm (1/8 in.) long. Its mechanical properties were UTS 538 MPa (78 ksi), YS 446 MPa (65 ksi) and elongation 11 percent.

The fatigue tests were conducted under constant amplitude loading and spectrum loading with periodic OL or UL cycles at ambient temperature in an MTS machine. The loading frequency was 5 Hz, the growing crack length 2a was measured, employing direct current potential drop technique, and the fatigue crack growth rate da/dN was computed. Subsequently, half crack length vs. number of loading cycle a vs. N and fatigue crack growth rate vs. stress intensity range da/dN vs. ΔK were plotted.

This work was done by E. U. Lee, R. E. Taylor, and B. Pregger for the Naval Air Warfare Center, Patuxent River, Maryland. NAWC-0002


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SPECTRUM FATIGUE OF 7075-T651 ALUMINUM ALLOY UNDER OVERLOADING AND UNDERLOADING

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