Dialogue between micromechanics and probabilistic analyses for estimating the residual life of metallic structural elements under fatigue: application to hydraulic structures

This PhD thesis follows a first PhD thesis (Kamal Harb, 2023-26) undertaken as part of the Medelia research chair (Fondation G-INP). It aims to estimate the residual useful life of hydromechanical steel components under fatigue. It proposes a probabilistic fatigue study methodology, based on fracture mechanics, applied to finite element models of simple welded assemblies between metal plates. This numerical work is being carried out in conjunction with SPRETEC, an Artelia Group design office, within the framework of probabilistic fatigue, comparing different international regulations. This work is rather focused on simple welded joints, e.g. of lock gates, the corrosion effects are not taken into account.

The aim of this second thesis is to propose a reliability approach, taking into account monitoring and Non-Destructive-Examination (NDE) data, for quasi-static structures under polycyclic fatigue of the order 103 to 106 loading cycles before failure. The originality of this thesis lies in the development of a methodology applicable to cast or forged structures (valves, pressure parts such as valve bodies in pipelines), and are more oriented towards the propagation of observed defects. The challenge is to use the results of non-destructive examination (NDE, defect tracking over time) to update the problem data: fatigue law, effective defect size, loadings, etc. More precisely, the objective is to assess residual useful life, as well as the probability of failure as a function of time, to help defining monitoring strategies. The secondary objectives of this thesis are to reduce uncertainty on the various random variables by means of monitoring data, using Bayesian modeling. The application filed also covers the domain of low failure probabilities, associated with target reliability indices, in the sense of the reliability modeling strategy made possible by European regulations (Eurocodes). Moreover, an option of an experimental campaign could be conducted within a partnership with CETIM providing new original results in terms of fatigue characterization of ageing steel components in air or fresh water.

FUNDING

Tec 21

EDF Hydro