numero26

G. Fargione et alii, Frattura ed Integrità Strutturale, 26 (2013) 143-155 ; DOI: 10.3221/IGF-ESIS.26.14 143 Fatigue characterization of mechanical components in service G. Fargione, D. Tringale Dipartimento di Ingegneria Industriale e Meccanica, Università degli Studi di Catania, Viale Andrea Doria 6, 95125 Catania E. Guglielmino, G. Risitano Università degli Studi di Messina, Dipartimento di Ingegneria Elettronica, Chimica e Ingegneria Industriale, Contrada Di Dio (S. Agata), 98166 Messina grisitano@unime.it A BSTRACT . The quickly identify of fatigue limit of a mechanical component with good approximation is currently a significant practical problem not yet resolved in a satisfactory way. Generally, for a mechanical component, the fatigue strength reduction factor (  i ) is difficult to evaluate especially when it is in service. In this paper, the procedures for crack paths individuation and consequently damage evaluation (adopted in laboratory for stressed specimens with planned load histories) are applied to mechanical components, already failed during service. The energy parameters, proposed by the authors for the evaluation of the fatigue behavior of the materials [1-5], are defined on specimens derived from a flange bolts. The flange connecting pipes at high temperature and pressure. Due to the loss of the seal, the bolts have been subjected to a hot flow steam addition to the normal stress. The numerical analysis coupled experimental analysis (measurement of surface temperature during static and dynamic tests of specimens taken from damaged tie rods), has helped to determine the causes of failure of the tie rods. The determination of an energy parameter for the evaluation of the damage showed that factors related to the heat release of the material (loaded) may also help to understand the causes of failure of mechanical components. K EYWORDS . Mechanical components in service; RRM; Fatigue. I NTRODUCTION n this work it has been applied to mechanical components damaged during the working phase, the procedures of damage assessment which have been already adopted in laboratory on the test specimens stressed with programmed loads history. The energy parameters proposed for years by the authors to estimate the materials fatigue behaviour, have been tested on specimens made out of the bolts of a connection flange of high temperature and head steam pipes which have been subjected not only to the usual proof stress, but also to a steam flux caused by a leak of the seal. The numerical and the following experimental analysis carried out through the measurement of the surface temperature during the static and dynamic tests on the specimens made out of by damaged slings allowed to define the causes for the slings breakage. The determination of a reference energy parameter to evaluate also the damage sustained by the material has highlighted again how the factors linked to the material heat release under stress can help to evaluate also the causes of the failure of the mechanical components. I

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