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Some studies into the fatigue properties of 2024-T3 sheet aluminum White, Robin Tristram

Abstract

A large number of axially loaded specimens of 2024-T3 aluminum sheet alloy were fatigued to failure in alternating tension. The stress ratio was kept constant at .05 and maximum stresses were chosen to cause failure in the upper part of the S-logN curve where failure is by progressive hardening. The specimens were prepared in three ways, unpolished in which the rough edges were rounded, chemically polished in which the central section was polished chemically, and mechanically and chemically polished in which the central section was rubbed with emery paper then polished chemically. Macroscopic and microscopic examinations of chemically polished specimens tested at a maximum stress of 47.5 ksi were also made. Statistical analysis was used to fit experimental frequency distributions to the life values at each stress level, to determine the effect of the different polishing methods and to determine the effect of a light oil coating. Of the two tried, the Lognormal and the Weibull, the Weibull distribution provided a better fit and was easier to use than the Lognormal. For the three different polishing groups, the mechanically and chemically polished specimens gave the longest lives at all stress levels followed by the chemically polished then the unpolished specimens. Also both groups which received a final chemical polish showed S-logN curves with a much rounder knee than did the unpolished group. Coating the specimens with a light non-corroding oil resulted in all cases in an increase in life and in more scatter in the life values. The increases ranged from 20% to 62%, At the knee of the S-logN curve, all groups showed a bimodal. distribution in the life values. This was felt to be the result of a change in the failure mechanism such as the one postulated by Wood. Examination of the fracture surface and of small cracks which formed near the edges indicated that the failure occurred in two stages; nucleation and growth of a small non-distorting fatigue crack followed by transition to a rapidly propagating ductile crack. Observations of the failures showed that it took approximately 1000 cycles from the appearance of the first small crack to when the ductile crack had propagated through the section. Microscopic examination showed that the fatigue cracks were intercrystalline and grew on the surface along slip marking which formed ahead of the tip of the crack. The only effect the crystal structure had was in the direction of the slip band formation. There was no indication that the mechanism by which the cracks initiated was different than the one by which they propagated. The cracks initiated at the surface and grew through to the other side. Once they reached the other side a ductile crack formed and final failure soon followed.

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