On March 14, 2022,Hugo TRYLA will defend his thesis work conducted at the MSMP laboratory, entitled "Evolution of residual stresses in welded assemblies under mechanical stress - Study and modeling," on the Aix-en-Provence campus (lecture hall J001, 10:30 a.m.).
Summary
Arc welding processes with filler material are widely used in the automotive industry for assembling steel underbody components (LAS). Welding generates significant thermal, metallurgical, and mechanical property gradients. These gradients cause residual stresses that affect the service life of welded structures. The objective of this thesis is to study their evolution under mechanical fatigue stresses.
Based on critical case studies involving the dimensioning of LAS components, three simple welded structure geometries were designed. The endurance limits of each structure were determined at one million cycles under pure tension (R=0.1). Interrupted fatigue tests, in the polycyclic domain, were conducted on the three structures at the previously determined endurance limits. Maps of the residual stress fields on the surface, before and after loading, were produced. These analyses highlight the evolution of residual stresses in the polycyclic range. At the base of the weld, a decrease of around 40% was observed, while in certain areas of the structures, stresses remained. In-situ tests were used to identify the kinetics of stress evolution at the structural level. In the direction of loading, stresses evolve under quasi-static loading, with changes detected for loads of around 10% of the endurance limit. Stress analyses conducted on the three structures show that the changes in residual welding stresses are due to local and structural effects.
In order to integrate the phenomenon of residual stress evolution into Stellantis' LAS component design methodology, a phenomenological model has been identified that can predict the residual hydrostatic pressures to be included in fatigue calculations.
Thesis supervisor: L. Barrallier
Supervisor: Lorène Héraud
Keywords
Welded structures, Residual stress evolution, X-ray diffraction, Polycyclic fatigue
Composition of the jury
- Mr. Sylvain CALLOCH, University Professor, IRDL, ENSTA Bretagne (rapporteur)
- Mr. Manuel FRANCOIS, University Professor, LASMIS, UTT (rapporteur)
- Mr. Louis CASTEX, Professor Emeritus, INSA Toulouse (examiner)
- Mr. Mohamed EL MAY, Doctor, I2M, ENSAM (examiner)
- Mr. Bastien WEBER, Doctor, ArcelorMittal, Maizières-lès-Metz (examiner)
- Mr. Philippe BRISTIEL, Doctor, Stellantis, Poissy (examiner)
- Ms. Lorène HERAUD, Senior Lecturer, MSMP, ENSAM (examiner)
- Mr. Laurent BARRALLIER, University Professor, MSMP, ENSAM (examiner)
