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PHD Thesis : Ultra-high speed imaging for the validation of accoustic wave analysis for early detection of cracks.

This offer is available in the following languages:
Français - Anglais

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General information

Reference : UMR6183-CANMER-001
Workplace : NANTES
Date of publication : Thursday, June 25, 2020
Scientific Responsible name : Julien RETHORE
Type of Contract : PhD Student contract / Thesis offer
Contract Period : 36 months
Start date of the thesis : 1 September 2020
Proportion of work : Full time
Remuneration : 2 135,00 € gross monthly

Description of the thesis topic

Fatigue, i.e the failure of mechanical structures under cycling loading, has been studied from the
beginning of the industrial revolution, more than 150 years ago, owing to its importance in naval
and rail industries. This failure mechanism is one of the most dangerous for engineering
infrastructures and remains nowadays a considerable technological challenge as it can occur
unexpectedly when the structure is operating apparently in a safe and steady state regime,
without apparent sign of mechanical damage. Even if lifetime and safety prediction methodologies
based on the non-destructive monitoring of mechanical properties have been proposed, the early
detection and monitoring of fatigue crack growth remains a crucial challenge. A common limitation
of these non-destructive methods is that they detect a modification of a physical property
averaged at the scale of the structure or the component, hence are hardly sensitive to the effect of
a single (or few) crack in its early stages of propagation, and faced with classical signal/noise issues.
This can lead to too late alarms. We have recently reported, for the first time, in different metallic
materials, the detection of acoustic emissions specific of fatigue crack growth during cyclic loading
[Deschanel 2018a, Deschanel 2018b, Deschanel 2017]. These so-called acoustic multiplets are
characterized by highly correlated waveforms, signature of a unique source. We interpret these AE
multiplets as the specific signature of the slow, incremental propagation of a fatigue crack at each
cycle or the rubbing along its faces.
The main objective of the proposed thesis is to understand the origin of these multiplets, identify
the physical processes involved, for different materials, under different loading conditions by
means of Imaging techniques. To achieve this goal, controlled crack growth tests will be
performed, in different conditions, with combined AE and DIC (digital image correlation)
monitoring. Associated with Ultra-high speed imaging of the displacement fields, we should be
able to follow and analyze, from the crack tip to the AE sensor, the wave formation arising from a
single crack propagation event. Several points will be developed:
• develop model experiments for analyzing single crack propagation event
• use high speed imaging to analyze experimentally, using full-field measurements, the origin
of the signal acquired by AE sensors
• obtain data allowing for the validation of AE source models
• confirm that AE originate from the same mechanisms during cyclic loading

Work Context

GeM is a Joint Research Unit supervised by Centrale Nantes, the University of Nantes and the CNRS.

Founded in 2004, the objective was to bring together within the same laboratory all the skills in the Nantes Saint-Nazaire area in the fields of civil engineering, materials and processes, modelling and simulation in structural mechanics. GeM is very involved in research training with around one hundred PhD students and several master programmes.

The research topics of the six teams focus on Materials, Processes and Structures. Internal skills include experimental techniques, modelling and numerical simulation.

Six research teams:
o E3M: Mechanical State and Microstructure of Materials (IUT St Nazaire)
o IEG – Water-Geomaterial Interactions (IUT St Nazaire)
o MEO – Materials-Environment-Structures (ECN)
o PMM – Processes and Mechanics of Materials (ECN)
o MS – Modelling and Simulation (ECN)
o TRUST – Health Monitoring, Reliability and Structural Computation (UFR Sciences and Techniques)

You will join the MS team (Modeling and Simulation). The proposed thesis will take place in the GeM laboratory, at the Centrale Nantes.

This thesis is part of the e-WARNINGS ANR project. The scientific and industrial consortium is
composed of 4 laboratories (Mateis - INSA Lyon, GeM – EC Nantes, ISTerre – UGAGrenoble
and LaMCoS - INSA Lyon) and 1 industrial partner (Mistras Group, SAS, Sucy
en Brie).

Constraints and risks

This thesis does not involve any particular risk.

Additional Information

- The candidate must hold an engineering degree and/or a master's degree in Mechanical or Civil Engineering.

- The position requires a solid knowledge of solid mechanics, experimental mechanics, image processing. Skills in transient analysis and fracture mechanics are also expected.

- Written and oral communication skills in both English and French will be required to present the work in scientific conferences.

- The candidate will be part of a research project involving several partners. Teamwork skills will therefore be important for the success of the project.

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