Fatigue damage detection in 2024 aluminum alloy by optical correlation
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A. F. HIEBER,
AND
R. K. MUELLER
The use of optical correlation techniques to monitor fatigue damage in 2024-T3 aluminum alloy is described. Topographical information from the surface of a sheet specimen is recorded holographically and compared with the actual surface by measuring correlation intensity as fatigue damage accumulates. The hologram is recorded on a thermoplasticphotoconductor device that can be developed and e r a s e d in situ, so that repeated r e c o r d ings can be made easily during a fatigue test. The results are presented as curves of correlation intensity (Ic) v s fatigue cycles (N) for unnotched specimens fatigued in tensiontension. The curves exhibit three regions, namely a decelerating loss of log I c v s N over the first portion of the fatigue life (Region A), followed by a linear loss (Region B), and finally by an accelerating loss over the final portion of the life (Region C). Changes in the initial surface finish of the specimen do not affect the general form of the correlation curve. Metallographic evidence indicates that the three regions correspond to fatigue-induced changes in the specimen surface, although dimensional changes in the specimen may also contribute to correlation losses. Region A appears to correspond to a period of initial cyclic strain adjustment, and Region B to a period of crack initiation and/or early growth. Region C corresponds to the presence of growing fatigue cracks, and the transition from B to C occurs when the cracks exceed about 20 pm in length. Thus, the correlation intensity data provide a sensitive indication of accumulating fatigue damage and impending failure in individual specimens.
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work of Marom and Mueller 1,2 indicates that a technique employing holography (wave front reconstruction) may be a sensitive method of monitoring the development of fatigue damage in metals. Their experiments showed that, unlike conventional inspection techniques, the holographic method may provide an early warning of impending fatigue failure in individual specimens or components. Bond and coworkers ~ also used this technique to study fatigue (in Fe-3Si) and came to similar conclusions. The technique is one of optical correlation (pattern recognition) in which the overall structure of the surface is monitored for changes. Information about the specimen surface topography is stored in the form of a hologram (spatial filter) at some time tl. The stored information is then compared with information coming from the specimen surface at a later time t~, after a certain number of fatigue cycles. The method is based on the observation that fatigue damage (crack initiation and subcritical flaw growth) generally begins at or near the surface of the metal where the material is unconstrained and the s t r e s s e s are often highest. Consequently, if the topographical information at time t2 is different from that stored in the spatial filter at tl, one infers that fatigue damage has occurred. No detailed study of the relationship between holoW. L. HAWORTHis AssociatePro
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