Fractographic reconstitution of fatigue crack growth in integrally stiffened panels

Abstract

Application of classical fractographic method of fatigue crack growth reconstitution based on the striation spacing measurement is conditioned by ability to distinguish and measure striations on the fracture surfaces. Other methods for crack growth curve reconstitution such as fractographic marking or textural fractography enable the reconstitution even if the striations are arduously detectable or wholly missing and also in the case of fractured bodies loaded with complex loading spectrum. A research program was focused on design and experimental verification of fracture marking of integrally stiffened panels during the fatigue tests under flight-by-flight spectrum loading and testing the effectiveness of various fractographic methods. The panels represent complex testing models of aircraft wing parts. Three kinds of cyclic loading were used: constant amplitude loading (CAL), flightby-flight type spectrum loading without marking and flight-by-flight spectrum with marking. Different methods were applied for fractographic reconstitution of fatigue crack growth: (a) measurement of striation spacing, (b) identification of beach marks, (c) image textural analysis of SEM images of fatigue fracture surfaces (textural fractography) aimed at morphological features oriented in the direction of crack growth. The obtained crack growth curves enable the study of the influence of loading spectrum on crack growth kinetics and comparison of the different fractographic methods.