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Damage tolerant design criteria and the current use of random or flight-by-flight loading methods in the full-scale testing of aircraft structures have necessitated the development of new techniques for acquiring accurate fatigue crack growth information from fracture surface analysis. This paper describes a technique that utilized a 64-cycle constant-amplitude “marker block”, in an otherwise random load spectrum of 4469 cycles, to derive fatigue crack growth data from an aluminum alloy spar cap and two wing skin rivets. The presence of an undetected incipient crack in the spar cap, from a previous full-scale test, provided an opportunity of accounting for the entire random loading crack growth history in terms of the “marker band” spacings. It was possible to identify the marker bands in the scanning electron microscope at relatively short crack lengths of 2.6 mm for the spar cap and 0.5 mm for the rivets. A total of 89 of the 90 marker blocks applied in the test were accounted for on the spar cap fracture surface. Examination of the rivet fracture surfaces revealed 12 and 8 marker bands, respectively, over crack lengths of some 3 mm.
quantitative fractography, full-scale aircraft fatigue testing, random, spectrum, flight-by-flight loading, marker blocks, marker loads, marker bands, fatigue crack propagation, aluminum alloys, scanning electron microscopy
Senior Technical Officer, Structures and Materials Laboratory, National Aeronautical Establishment, National Research Council of Canada, Ottawa, Ontario