Published: Jan 1993
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A fractographic study was carried out on fatigue crack closure and crack growth rates in an Al-Cu alloy under conditions of monotonic and cyclic yield at the notch root. Comparative tests were also conducted on smooth specimens subject to elastic loading conditions. A notch root fatigue crack can see substantial difference between crack closure and opening stress. This difference is considered responsible for local mean stress effects observed in notch fatigue response to complex load sequences. The difference between closure and opening stresses diminishes to negligible proportions as stress excursions reduce to elastic interval and is therefore hardly observed in long cracks.
To support validation effort on predictions, notch root crack growth rates were determined from optical fractography under a programmed load adaptation of the FALSTAFF load sequence. The results of these measurements suggest that scatter in short crack growth data is similar to that in long cracks at comparable growth rates. Life to initiation of (naturally initiating) multiple notch root cracks is dissimilar. The crack having been in existence longest appears to assume the role of the dominant crack and accelerates others (smaller) cracks.
TEM and optical fractography, fatigue crack closure, crack opening and closure stress, fatigue striations, programmed loading
Scientist, National Aeronautical Laboratory (NAL), Bangalore,
Senior Research Fellow, Indian Institute of Science, Bangalore,
Visiting Scientist (NAL) from the Institute for Strength Problems, Kiev,