New Orleans, Louisiana
June 26, 2016
June 26, 2016
August 28, 2016
Fatigue is the most common cause of structural failure in aircraft. Findlay and Harrison (Findlay, S. J. and Harrison N. D., “Why Aircraft Fail,” Materials Today, 5(11), 2002.) report that fatigue accounts for over 60% of structural failures in aircraft components, while simple overload accounts for only 14%. This trend is in direct opposition to typical undergraduate aerospace curricula wherein static strength receives the preponderance of attention. Where fatigue testing is found in undergraduate laboratory courses, it is most often in the form of a rotating fatigue test in which the number of cycles to failure for a pristine specimen is counted. If test are repeated at various load levels, such data can be used to produce a S-N curve. While such an experiment produces valuable introduction to fatigue phenomena, additional insight would result from a laboratory experience that explores the comparison between static and fatigue failures, the influence of stress concentrations on fatigue behavior, and fatigue crack growth processes.
This paper presents the development of a laboratory exercise that has been implemented in an undergraduate Aerospace Structures Laboratory course over the past seven years. The experiment uses two notched aluminum test specimens. Each specimen is formed from rectangular bar stock. The first specimen type has a large centrally-located hole producing a stress concentration factor of about 2.1 while the second specimen type has a pair of machined notches symmetrical located about the center, producing a considerably larger stress concentration factor. Both specimen types have the same nominal cross-section area in the notched section. Specimens of each type are first loaded is quasistatic tension revealing that the ultimate failure load for the two specimen types is essentially the same regardless of the large difference in stress concentration factor. Specimens of the same type are then loaded in zero-to-maximum constant amplitude fatigue under load control at equal nominal stress levels until specimen failure. This produces a surprisingly (to the uninitiated) large difference in fatigue life in the two specimen types. Comparison of data from multiple sections of the course, as well as the historical database of results from previous semesters allows important issues of scatter in fatigue experiments to be examined. The laboratory experience supports concurrent discussion of the topic of fatigue in a classroom Aerospace Structural Design course that provides additional theoretical discussion of the relevant physical phenomena and correlation to real-world examples such as the Comet airliners disasters in which rapid fatigue crack growth from stress concentrations led to the loss of several aircraft and many lives.
Careful selection of test parameters and specimen dimensions is needed to enable the experiment to be finished in the span of an undergraduate laboratory session. This papers describes the development of the experiment and efforts to assess its effectiveness in meeting its pedagogical goals.
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