Author

Ngiau

Date of Award

12-2000

Degree Name

Master of Science in Engineering

Department

Mechanical and Aerospace Engineering

Department

Mechanical and Aeronautical Engineering

First Advisor

Dr. Daniel Kujawski

Second Advisor

Dr. Philip Guichelaar

Third Advisor

Dr. Jerry Hamelink

Access Setting

Masters Thesis-Open Access

Abstract

It is a common practice in industry today to totally discard the presence of small below-endurance/fatigue limit cycles while calculating damage accumulation using the Palmgren-Miner rule. However, it was experimentally found in a number of studies that small cycles with stress amplitudes below the fatigue/endurance limit do in fact contribute significantly to the damage accumulation in all types of materials. There was a need to investigate cases where small cycles would only be present for a certain percentage of the material's life. An experimental study was done on 2024- T351 aluminum alloy. Loading was done in low and high cyclic fatigue regimes, and subjected to constant-amplitude fully reversed "regular" loading as well as variable amplitude block loading, sequenced either in block-to-constant or constant-to-block patterns. Additionally, the small cycles in those block loads were tested at stress amplitudes of 100% and 50% endurance limit. The results showed that there was significant damage caused by both amplitudes of small cycles, and this damage was influenced heavily by the sequencing pattern, regime, and the cutoff point. There were instances when the small cycles could be safely ignored, but in other cases the damage values from these supposedly harmless small cycles could be as high as 0.81.

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