Studies on fatigue crack growth for airframe structural integrity applications |
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Authors: | R Sunder |
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Affiliation: | (1) Structural Integrity Division, National Aerospace Laboratories, 560017 Bangalore, India;(2) Present address: Bangalore Integrated System Solutions, 99, 14th A cross, Malleswaram, 560 003 Bangalore, India |
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Abstract: | A review is made of efforts at the National Aerospace Laboratories in the development of fatigue crack growth prediction technology
for airframe applications. The research was focused on extension of rainflow techniques for crack growth analysis and development
of accelerated crack growth calculation methods for spectrum loading. Fatigue crack closure forms a crucial element of modelling
and fractographic techniques were developed for its study. These, combined with binary coded event registration enabled crack
growth and closure mapping for part-through cracks in metallic materials. Experimental research on short cracks at notches
led to discovery of the hysteretic nature of crack closure, which explains well-known history-sensitive local mean stress
effects in notch root fatigue. Optical fractography of failures obtained under simulated service conditions revealed that
short cracks do not exhibit any more scatter than long cracks at comparable growth rates. The nature of multi-site crack initiation
and growth of small cracks at notches was investigated and the effort extended to lug joints that are widely used in airframe
applications. Results from this work suggest the possibility of modelling crack growth from a size smaller than 50 microns
through to failure, thereby accounting for a major fraction of total life.
The work described in this paper enjoyed the strong support of Dr S R Valluri, Prof R Narasimha and Dr K N Raju. Financial
support for the effort was provided by Aeronautical Research & Development Board, Aeronautical Development Agency and Department
of Science and Technology. |
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Keywords: | Fatigue crack growth rainflow technique airframe structural integrity fractographic techniques |
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