Fractographic Study on the Use of Rainflow Counting for Small and Long Cracks in AA7050

Abstract:

Using a da/dN equation to predict fatigue crack growth for a variable amplitude loading sequence, requires converting the sequence into an equivalent series of constant amplitude cycles, which is sometimes achieved using the rainflow cycle counting technique. Rainflow counting views small intermediate cycles as an interruption to a larger cycle, in effect, the crack tip remembers the state of the larger cycle. This has been shown to be an effective technique in predicting fatigue growth rates for long cracks, but has not been extensively investigated for use in predicting the growth of small cracks. An investigation was made into the applicability of rainflow cycle counting for predicting the crack growth of small and long cracks created with variable amplitude fatigue loading in AA7050-T7451 plate, a common modern aircraft material. A series of coupons were tested with a number of different variable amplitude loading sequences which had distinct marker bands inserted to separate the individual segments of loading and enable them to be identified fractographically. For the sequences examined, which covered varying numbers of interrupted cycles and a staircase of three steps, the baseline and the rainflow loading segments within each sequence showed effectively the same rate of growth for the same stress intensity range in both the small and long crack coupons, demonstrating that rainflow cycle counting was a suitable cycle counting technique for both small and long cracks.

• Periodical: Advanced Materials Research (Volumes 891-892)
• Pages: 687-692
• DOI: https://doi.org/10.4028/www.scientific.net/AMR.891-892.687
• Online since: March 2014
• Authors: Paul White*, David S. Mongru
• Keywords: Fatigue Crack Growth (FCG), Fractography, Rainflow Cycle Counting, Variable Amplitude

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