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Vers une approche locale du phénomène de fatigue mécanique. Application aux matériaux et aux structures en composites à fibres continues

Abstract : The original concept of mechanical fatigue was linked to the failure of structures and was treated at first within the framework of Fracture Mechanics. Models developed to explain this phenomenon must therefore be able to be applied to structures: changing the structure means changes to the model and its identification. It is therefore judicious to develop models capable of treating both the damage processes and also able to be used within a local framework: to this end a method based on Damage Mechanics seems appropriate. This approach has long been employed and requires only the identification of damage processes at the level of the RVE (Representative Volume Element) to be used for any structural geometry. However incoherencies can be encountered such as when the number of load cycles applied to the macroscopic structure does not reflect the internal loading kinetics, leading, for example to effects due to the frequency of loading. This can be found to influence the laws governing the evolution of damage phenomena at the local level, leading to wrong conclusions. Nevertheless this is not inevitable if precautions are taken. This goal of this study has been to introduce rigorous concepts of mechanical fatigue in the local framework of Damage Mechanics and to carry out a local approach to fatigue. The ultimate aim has been to be able to carry out calculations on structures subjected to fatigue loading with the same degree of finesse as under quasi-static conditions.An initial step has been to reveal the shortcomings of existing models and show that at present no model of a structure subjected to fatigue using Damage Mechanics is perfectly satisfactory.As a result the notion of the fatigue problem has been formulated and used as a base for defining a suitable framework for developing a local approach to fatigue.A deliberate goal has been to avoid constructing incremental fatigue laws but to introduce the real effects of loading cycles deduced from the evolution of behavioural laws obtained in quasi-static tests, requiring rules of linearization and a clear definition of the adopted approach.Benefiting from specific problems involving fatigue damage and applying them to this approach, a rigorous description of the phenomena and the identification of a fatigue damage model applied to intra-laminar cracking has been developed. The laws governing the local approach have been described for the case of a structure subjected to fatigue. It has been shown that for a given case of a material and damage processes the proposed approach applied to a structure subjected to fatigue loading can be modelled with the same ease as resolving the effects of quasi-static loading.This study has created very general foundations for a new approach to understanding the fatigue of structures. It clarifies the fatigue concept by defining it within a tightly defined framework with the goal of formulating a local approach to fatigue applied to the calculation of structures subjected to fatigue loading. It has been successfully used for a particular configuration. However for the present it is difficult to assert that it can be used as a local approach for all fatigue phenomena, in all materials and for the most general types of loading. The study has not had the ambition of providing a solution to all these cases but rather to open a new approach to resolve other cases and possibly to provide a guideline to achieving a truly local approach to a general solution of the fatigue of structures.
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https://tel.archives-ouvertes.fr/tel-02063465
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Submitted on : Monday, March 11, 2019 - 11:23:14 AM
Last modification on : Tuesday, March 12, 2019 - 1:12:42 AM
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  • HAL Id : tel-02063465, version 1

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Emilie Duboquet. Vers une approche locale du phénomène de fatigue mécanique. Application aux matériaux et aux structures en composites à fibres continues. Physique mathématique [math-ph]. Université Bourgogne Franche-Comté, 2018. Français. ⟨NNT : 2018UBFCK021⟩. ⟨tel-02063465⟩

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