Fractographic reconstitution of fatigue crack history – Part I

Abstract

A B S T R A C T Quantitative fractography of components which have failed under cyclic loading can offer very valuable information usually unattainable by any other means -in particular, a detailed description of fatigue crack growth (FCG) in space and in time. The reliable characterization of crack propagation provides data important in engineering applications.The main source of information on the relationship between fractographic findings and macroscopic crack growth rate (CGR) is striation spacing. This quantitative microfractographic characteristic was the basis for the methods described in the paper. An application of the fractographic reconstitution of FCG is illustrated by an example of an aircraft wing spar fatigued during a full-scale test.Keywords fatigue crack growth; quantitative fractography; striation spacing. N O M E N C L A T U R Ea = crack length D = relation between macroscopic crack growth rate and striation spacing N = number of loading cycles p s = area percentage of striation patches on fracture surface v = macroscopic crack growth rate s = striation spacing K = stress intensity factor range K eff = effective value of stress intensity factor range ϑ = angle between local & macroscopic crack growth direction I N T R O D U C T I O NFractography is a potent and useful tool for studying failure processes in a broad spectrum of applicationsfrom fundamental research on relations between material microstructure and failure micromechanisms to practical failure analysis of service events. General capability of the fractography is based on a simple axiomatic presumption: the information on a failure process history is encoded in the fracture surface, and possibly in its vicinity. The task of fractography is to decode this information, and to apply it in the fields of fracture & damage mechanics, materials science and other linked disciplines.Our interest is to discuss the information encoded in a new surface created by a fatigue crack in a cyclically loaded body. Fatigue cracks can propagate by many fracture micromechanisms. The character of the failure proCorrespondence: I. Nedbal.

Fractographic reconstitution of fatigue crack growth in integrally stiffened panels

“…This fact supports the applicability of the empiric relationship between macroscopic crack growth rate and striation spacing established during our previous experiments (see, e.g., [3] , [4]). …”

“…The fractographic reconstitution was based on striation spacing measurement (see [3]). Application of this method results in crack growth curves with more than 100 datapoints as striations were clearly visible on the majority of the crack surface area (see Fig.…”

Structural overloading evaluation based on the identification of subcritical crack increments

“…In the absence of knowledge of service loading, it is practically impossible to determine the crack growth curves and interaction among cracks using methods of reconstruction of crack growth based on microfractographic data via quantitative fractography according to Refs. [10,19,20]. Determining the fatigue crack growth history is often possible only by speculative estimation based on the occurrence of tear bands.…”

FRACTURE MECHANISMS OF SIX TOP CORROSION RESISTANT Fe AND Ni ALLOYS AT FATIGUE AND STRESS CORROSION CRACKING

“…Generally, their formation accelerates crack propagation and thus inequality D = v/s > 1 is valid (see, e.g. [14,15]). b) Austenitic steels (Uranus SB8, Uranus B26): High toughness of these steels is as usually accompanied by the lower yield stress.…”