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.

A Contribution to the Physical Interpretation of the Morphology of Fatigue Fracture Surfaces

“…As in foregoing studies made on alumina alloys [2,5], a very good agreement between fractographic textural reference model and real data was obtained for steel AISI 304 L. Presence of the reference texture -a common set of morphological features of fatigue crack surfaces created under various loading regimes -was proved again. Also in this case, reference features (texture & crack rate) may be regarded to be governed by proposed magnitude w M (the mean local size of the residual major cyclic plastic zone).…”

“…The Reference concept [3,4,5] deals with the phenomenon of the reference texture -a textural component in fatigue fractographs which is common to fractures caused by various loading regimes. The reference texture is unambiguously related with the reference crack growth rate v ref , a product of the conventional crack rate v and the reference factor B, .…”

An analysis of fatigue crack features at various crack lengths through mating fracture surface pairs

“…A prominent method that has been developed over the past decades is that of fracture surface topography analysis (FRASTA) 6 . This method has allowed the recognition of markings formed by overloads on fracture surfaces post‐failure, 7 the calculation of fatigue crack parameters such as crack tip opening displacement, 8 and the correlation of surface texture and features with fatigue crack growth rates 9,10 . However, FRASTA is generally not appropriate for making detailed mechanistic based observations due to the comparatively large scale of these features compared with the cycle‐by‐cycle grow rates that are growing the crack.…”

Comparison of Bayesian and other approaches to the estimation of fatigue crack growth rate from 2D textural features

“…For this purpose, e.g. a multilinear regression model (Sekeresova and Lauschmann, 2008;Kunz et al, 2010) or a neural network may be used. Of these two, the neural network allows us to analyze and describe the structure of the obtained modeland better imagine the textural subset which is mutually related with the CGR (Lauschmann and Goldsmith, 2009).…”