The thesis is particularly interested in the durability of metal structures assembled by welding that have undergone or not finishing operations. These structures can be onshore (metal bridges, wind turbines, etc.) or offshore (platforms, wind turbines, ships, etc.).
Previous work on the study of fatigue in welded high yield strength steel (HYS) assemblies in offshore structures has led to the development of experimental methods to study the influence of certain finishing processes (typically stress blasting) on the fatigue life of welded joints. The results available in the literature show, for a load ratio R=0.1, that the cracking kinetics are influenced by the residual stress field resulting from the welding operation. Interpretation of the results was made possible after existing residual stress measurements in the welded joint.
An improved welding procedure has been proposed for corner welds in HYS welded assemblies. This procedure, which provides practically, geometric and metallurgical advantages, minimises global and local stress concentrations and reduces residual stress due to the welding operation, in the vicinity of the weld bead foot. The endurance tests on welded joints showed a very good resistance to fatigue, especially in the field of large numbers of cycles, attributed to a significant gain in crack initiation time.
Prestressing shot peening has been studied as a ""global"" finishing treatment to improve the fatigue life of welded joints. The choice of technological parameters (ALMEN intensity, shot size...) is based on a preliminary study of the stresses introduced by different shot peening parameters: stress intensity and depth affected. In addition, a small enough shot size must be provided to treat all drawstring foot defects. The fatigue results showed a significant improvement in the fatigue performance of shot welded joints, particularly in the area of long service lives.
This experimental work, although very interesting, is limited to welding and initial finishing of HYS steels and this for a single loading ratio.
This thesis is intended to complement this work and extend it to conventional steels for all types of loading by the use of numerical tools capable of simulating and predicting on the one hand the thermomechanical behaviour from the welding phase to the cooling (source of generation of residual stresses) and on the other hand the blasting processes.
This thesis project thus consists in quantifying the beneficial effect of two methods of post-treatment of the weld and building a base of exploitable results during a dimensioning / calculation/ verification of fatigue. He is interested in both new structures (i.e. before commissioning), welded and finished, as well as structures already in service and undergoing fatigue ageing.
To meet this general objective, numerical and experimental work will be carried out. The definition of the welding and finishing procedure will be carried out based on the recommendations of the international welding institute, the know-how developed at the SMC laboratory and the feedback of our industrial partner SONATS Europe Technologies (for US hammering).
To meet these objectives, several scientific questions must be answered:
Construction of a multiphysical FEM welding model. The law of multiphase behavior will be introduced into the model either through a subroutine or through a UMT.
Integration of behavioural laws adapted for each method of completion. In the case of hammering, the chosen laws of behaviour must be able to follow the mechanical state of the structure under repeated impacts. The choice of the type of work hardening of the material (kinematic and/or isotropic) is thus a key step for the modelling of hammering. In the case of TIG remelting, the proposed multi-phase behaviour law for welding will be reused.
Evaluation of residual stresses present in the volume before completion. Experimentally, an X-ray diffraction (DRX) analysis will be conducted on our test pieces to describe their residual stress state and validate the proposed numerical models.
Quantitative evaluation of the beneficial effect of completion methods, as well as an understanding of the relationship between the effects of completion (Stress Introduction/Geometry Modification) and post-treatment life.
To begin this thesis, a bibliographical review must be conducted in order to assimilate all the basic notions that will be developed throughout our study. It is necessary to have a good knowledge of the finishing methods studied, of the fatigue aspects of a welded assembly and of the main conclusions of the literature studies on the improvement of the fatigue performance of welded assemblies. Then, the project will be organized according to the sub objectives to be achieved over the three years of the thesis according to a schedule
Characterization of the mechanical state of a T-welded structure: integration of a thermomechanical and metallurgical law behaviour taking into account all the mechanisms at the origin of residual welding stresses, in the case of S355 steel;
Choice of the completion method and translation of the process parameters into input data for the simulations;
Construction and validation of the numerical model proposed. It will be capable to predict the residual stresses of a completed welded structure;
Experimental and numerical evaluation of the fatigue life of completed assemblies before and after surface treatment: monitoring of stress relaxation/stabilization mechanisms;
Recommendations for the implementation of completion on new and existing structures.
The thesis will take place at the SMC-Lab at the Gustave Eiffel University in France. This laboratory has strong skills on the subject, with an experienced coaching team and experimental means adapted to the theme of the thesis subject (instrumentation, mechanical test equipment, welding and finishing skills, modelling equipment). This project will thus enable the doctoral student to acquire knowledge in the field of structures and structures essential to the energy transition, to contribute to the sobriety of resources by aiming to increase the life of structures. The candidate will develop numerical skills and conduct laboratory tests.
Funding category: Financement de l'Union européenne
PHD title: Doctorat en génie civil
PHD Country: FranceOffer Requirements Specific Requirements
cf. https: // euraxess.ec.europa.eu/jobs/846767Contact Information