Estimating fatigue life under variable amplitude loading through quantitative fractography - A case study

Quantitative fractography techniques have been implemented and frequently used in the Failure Analysis Laboratory - LANAF, at the Engineering School of S (a) over tildeo Carlos, for fatigue life estimations of structural components, whose fracture surfaces are subjected to detailed inspection with the aid of scanning electron microscopes. This work describes one of the recent activities in progress at the LANAF, in which fatigue crack initiation and propagation lives have been estimated for an idealized aeronautical part tested in the laboratory, under variable amplitude loading condition VAL, i.e., flight simulation testing. Fractographic reconstitution of sub-critical crack growth has been performed through the identification of marking load patterns left in the wake of the propagating crack, which have been correlated to the significant load levels applied during the fatigue test. A semi-automated procedure to estimate fatigue lives under VAL has been developed and implemented by which a virtual marking load pattern is generated and compared to the real pattern determined fractographically. (C) 2004 Elsevier Ltd. All rights reserved.