## Determination of the fatigue limit and initial crack length by means of fracture mechanics

- The fatigue design of metallic components is usually based on two different approaches, namely total life and damage tolerance. The former takes into account both the initiation and propagation stages. The latter is based on fracture mechanics and needs an initial defect, which propagates until the critical size under cyclic loading, provided it is large enough. In this case, a residual life rather than a total life is obtained. The overall lifetime of a cyclically loaded structure involves four consecutive stages: a) crack initiation; b) propagation of microstructurally short cracks; c) propagation of physically/mechanically short cracks; d) propagation of long cracks. Considering the propagation from the short crack regime, the damage tolerance approach can be extended to adequately calculate the total life of a component. The fatigue strength of metallic materials containing defects depends on the non-propagation condition of small cracks emanating from these defects. In this work,The fatigue design of metallic components is usually based on two different approaches, namely total life and damage tolerance. The former takes into account both the initiation and propagation stages. The latter is based on fracture mechanics and needs an initial defect, which propagates until the critical size under cyclic loading, provided it is large enough. In this case, a residual life rather than a total life is obtained. The overall lifetime of a cyclically loaded structure involves four consecutive stages: a) crack initiation; b) propagation of microstructurally short cracks; c) propagation of physically/mechanically short cracks; d) propagation of long cracks. Considering the propagation from the short crack regime, the damage tolerance approach can be extended to adequately calculate the total life of a component. The fatigue strength of metallic materials containing defects depends on the non-propagation condition of small cracks emanating from these defects. In this work, the presence of mechanically short cracks from the beginning of the component's life is considered. Consequently, the methodology of analysis must be able to treat adequately local ligament yielding effects typical for short cracks and must include the thorough description of the crack closure effect up to the long-crack regime. In the analysis based on long cracks, the linear-elastic condition is mostly satisfied which allows the use of the linear elastic parameter ΔK for describing the crack driving force. On the contrary, this assumption is not adequate for mechanically short cracks because the crack depth is in the order of the plastic zone. Instead, an elastic-plastic driving force should be considered. Furthermore, the gradual build-up of the plasticity-induced crack closure effect must be considered, which implies a transition from the intrinsic (effective) threshold value, ΔKth,eff, to the long-crack threshold, ΔKth,LC. The effective component is a material parameter which is dependent on the elastic properties and crystal lattice. The gradual build-up of crack-closure is described by ΔKth,op which is a function of the plastic properties, grain-size, environment conditions, load ratio and crack-depth. The crack closure can be characterized experimentally by the so-called cyclic R-curve. Some other crack closure effects, such as roughness or oxide-debris induced, might be incorporated as well. The knowledge of the cyclic R-curve can be useful to determine the largest non-propagating crack size at the material fatigue limit. This is realised by means of the so-called cyclic R-curve analysis: The tangent criterion between the driving force and the cyclic R-curve define the transition between crack arrest and propagation. In this regard, a0 is defined as that crack depth which will only grow into a non-propagating crack due to the development of crack closure. The procedure outlined briefly here has been successfully applied to the determination of the fatigue limits of steel weldments. Considering the stochastic distribution of the weld geometric parameters (namely, weld toe radius, the flank angle, and the excess weld metal) it is possible to perform a full probabilistic cyclic R-curve analysis and determine this way the statistical distribution of the initial crack size at the weld toe at the fatigue limit.…

Autoren/innen: | Sergio Blasón |
---|---|

Koautoren/innen: | Mauro Madia |

Dokumenttyp: | Vortrag |

Veröffentlichungsform: | Präsentation |

Sprache: | Englisch |

Jahr der Erstveröffentlichung: | 2020 |

Organisationseinheit der BAM: | 9 Komponentensicherheit |

9 Komponentensicherheit / 9.4 Integrität von Schweißverbindungen | |

DDC-Klassifikation: | Technik, Medizin, angewandte Wissenschaften / Ingenieurwissenschaften / Ingenieurwissenschaften und zugeordnete Tätigkeiten |

Freie Schlagwörter: | Cyclic R-curve; Fatigue Strength; Fracture Mechanics; Short Crack Propagation |

Themenfelder/Aktivitätsfelder der BAM: | Material |

Material / Life Cycle von Komponenten | |

Veranstaltung: | Virtual Conference on Mechanical Fatigue |

Veranstaltungsort: | Online meeting |

Beginndatum der Veranstaltung: | 09.09.2020 |

Enddatum der Veranstaltung: | 11.09.2020 |

Verfügbarkeit des Dokuments: | Datei im Netzwerk der BAM verfügbar ("Closed Access") |

Datum der Freischaltung: | 16.09.2020 |

Referierte Publikation: | Nein |