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Spectrum Fatigue of 7075-T651 Aluminum Alloy Under Overloading and Underloading

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

Topics:
Aerospace Aircraft Aircraft structures Alloys Aluminum alloys Defense Extrusion Fatigue Forming Manufacturing processes Materials Materials properties Metals Propellers and rotors Railway vehicles and equipment Tensile strength Vehicles
This Brief includes a Technical Support Package (TSP).
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SPECTRUM FATIGUE OF 7075-T651 ALUMINUM ALLOY UNDER OVERLOADING AND UNDERLOADING

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    Aerospace & Defense Technology Magazine

    This article first appeared in the December, 2016 issue of Aerospace & Defense Technology Magazine (Vol. 1 No. 8).

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    Overview

    The Technical Information Memorandum (Report No. NAWCADPAX/TIM-2015/282) focuses on the study of spectrum fatigue in the 7075-T651 aluminum alloy, a material widely used in aerospace applications due to its high strength-to-weight ratio. The report, authored by E. U. Lee, R. E. Taylor, and B. Pregger, was published by the Naval Air Warfare Center Aircraft Division and acknowledges support from the Office of Naval Research.

    The primary objective of the study is to investigate how the 7075-T651 aluminum alloy behaves under varying loading conditions, specifically overloading and underloading. Spectrum fatigue refers to the material's response to cyclic loading, which is critical for understanding its durability and performance in real-world applications where components are subjected to fluctuating stresses.

    The report outlines the methodology used in the experiments, including the specific loading conditions and the parameters measured during testing. The authors emphasize the importance of understanding the fatigue behavior of materials in order to predict their lifespan and ensure safety in aerospace structures.

    Key findings from the study indicate that the fatigue life of the 7075-T651 aluminum alloy is significantly affected by the loading spectrum. The results demonstrate that both overloading and underloading conditions can lead to different fatigue mechanisms, which in turn influence the material's overall performance. The report provides detailed data and analysis on how these loading conditions impact crack initiation and propagation, which are critical factors in material failure.

    In addition to the experimental results, the report discusses the implications of these findings for engineering practices, particularly in the design and maintenance of aircraft components. The authors suggest that a better understanding of spectrum fatigue can lead to improved predictive models for material behavior, ultimately enhancing the reliability and safety of aerospace structures.

    The memorandum concludes with acknowledgments of the support received from the Office of Naval Research and highlights the importance of continued research in this area to advance material science and engineering practices. The document is approved for public release, ensuring that the findings can be shared with the broader scientific and engineering communities.

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