Ifsttar PhD subject

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Title : Increasing life expectancy of welded joints in the case of metallic bridges and steel structures.

Main host Laboratory - Referent Advisor   -

Director of the main host Laboratory   -

PhD Speciality Matériaux, Génie Civil, Mécanique, Structures

Axis of the performance contract 2 - COP2017 - More efficient and resilient infrastructure

Main location Nantes

Doctoral affiliation Université Bretagne Loire

PhD school SCIENCES POUR L'INGENIEUR, GEOSCIENCES, ARCHITECTURE (SPIGA)

Planned PhD supervisor DIENG Lamine  -  Université Gustave Eiffel  -  MAST - SMC

Planned financing Contrat doctoral  - Ifsttar

Abstract

Context and objectives:

Fatigue is, after corrosion, the main pathology of metal structures. In the case of welded joints, the fatigue phenomenon occurs preferentially at the weld joint due to high stress concentration and the existence of residual stresses created during welding operations. Different weld joint processing methods exist and are intended to improve the geometry of the weld seam (grinding, TIG re-melting). Some methods can even reverse the residual stresses present in the assembly by introducing residual compression stresses (hammering, peening, shot blasting). Although the effectiveness of these methods is proven, it is now difficult to quantify and guarantee the gain in lifetime obtained which could subsequently be taken into account in the calculations of service life of new structures existing ones. Conditions of realization of these methods must be provided as well as means of control of the quality of their implementation on site.

The subject of the proposed thesis therefore focuses on the study of the improvement of the fatigue life of welded assemblies provided by different methods of completion. The main objective is to propose an applicable approach aimed at increasing the lifespan of new structures and extending existing structures.

Thesis work program:

A first part of this thesis will focus on the implementation of a large experimental campaign conducted on welded T-shaped test specimens, aged by fatigue then subjected to the three most used finishing techniques: hammering, grinding and TIG remelting. These specimens will be conducted to fatigue rupture in order to quantify the lifetime contribution obtained for each finishing technique. Several levels of ageing may also be considered. The study of residual stresses due to welding and finishing will be carried out by the DRX method available from our partner SONATS. A parametric study on the conditions of realization of the treatments will be carried out in order to determine the influence of certain parameters on the effectiveness of the techniques. Among these parameters we can note: the welding temperature, the number and the order of the passes, the hammering power, the re-flow energy, the angle of inclination of the torch, ...

This experimental work will subsequently improve and validate the existing finite element welding and finishing models in the SMC laboratory, taking into account, in particular, the phase changes that play a major role in the development of residual stresses. In this context, we will focus on the quantification of the increase in life span provided by each method, by studying the modification of internal residual stresses. Different situations will be compared: the case where the treatment is carried out at the as the new structure and the one where the treatment is carried out during its service life.

Finally, a final part will focus on monitoring and / or on-site monitoring of fatigue phenomena before and after treatment. Several methods of monitoring fatigue damage will be tested in the laboratory and then ranked according to several criteria such as ease of implementation, data mining, accuracy, and cost.

The ultimate goal is to provide project owners with a comprehensive approach incorporating recommendations for choice, implementation and control of completion and an assessment of the new fatigue resistance of the structure.

Expected results :

The proposed work is intended to establish a level of confidence regarding the effectiveness of completion treatments performed on welded assemblies already aged (stress cycle history), but also to improve the understanding of the effects and mechanisms involved during the course of the treatment process.

The main expected results are listed below:
- Contribution of "finishing treatments technique" on aged welded joints with respect to their fatigue behaviour.
- Evaluation of the state of residual stresses generated by the technique (relaxation or even inversion of the stresses) and its evolution as a function of the history and fatigue loading of the welded assembly.
- Welding and finishing modelling models to optimize the parametric study of the processing conditions.
- Recommendations for the implementation (mode of operation) of the various completion methods that are the subject of the thesis with the objective of improving the reproducibility of the operations and therefore the level of confidence in the expected lifetime gain.
- Fatigue monitoring solutions for welded joints and evaluation of the new fatigue strength of the structure.

Innovative character:

The innovative character is the quantitative evaluation of the beneficial effect of finishing treatments whether in the case of new structures or in the case of structures that have already undergone fatigue cycles. The modelling tools developed will also make it possible to understand the limits and the phenomena involved for the various finishing methods studied.

References :

1. K. Guiot, Amélioration de la tenue en fatigue des assemblages métalliques soudés, Stage de Master mécanique numérique des structures UFR Sciences et techniques de Nantes, 2012
2. S. Chataigner, L. Dieng, Y. Falaise, M. Grasset, (2015, December). Evaluation des contraintes résiduelles dans les assemblages soudés et amélioration de leur durée de vie par techniques de parachèvement/appraisal of residual stresses within welded assembly and increase of their fatigue life using fatigue treatment methods. In Annales du Bâtiment et des Travaux Publics (Vol. 67, No. 5, p. 14). Editions ESKA.
3. Chataigner S., Dieng L., Guiot K., Grasset M., "Improving welded joint fatigue life using shot peening or grinding", TRB 2013, Washington, 2013.
4. Bernoala V., , "Modélisation des opérations de soudage et de parachèvement, Stage Master mécanique numérique des structures UFR Sciences et techniques de Nantes, 2013.
5. Bernoala V., Dieng L., Chataigner S., Falaise Y., "Improvements of welded joints using shotpeening: modelling of the welding process to determine the initial residual stresses", Eurosteel, Naples, 2014.
6. S. Chataigner, L. Dieng, K. Guiot, M. Grasset, "Improving Welded Joint Fatigue Life Using Shot Peeningor Grinding". Proceeding Transportation Research Board 92nd Annual Meeting, Washington DC, Date: 2013-1-13 to 2013-1-17. http ://worldcat.org /oclc /13-1721
7. L. Dieng, D. Amine, Y. Falaise, S. Chataigner, "Parametric study of the finite element modeling of shot peening on welded joints” Journal of Constructional Steel Research, Volume 130, March 2017, Pages 234-247.
8. L. Dieng, J.Y. Thieuleux, S. Chataigner, Y. Falaise, "Utilisation du grenaillage de précontrainte pour la remise à niveaux en fatigue de structures soudées", JOA Metz, 2017.

Keywords : Welding, finishing, grinding, hammering, TIG remelting, Residual stress, Numerical modeling, Fatigue

Main host Laboratory - Referent Advisor	-
Director of the main host Laboratory	-
PhD Speciality	Matériaux, Génie Civil, Mécanique, Structures
Axis of the performance contract	2 - COP2017 - More efficient and resilient infrastructure
Main location	Nantes
Doctoral affiliation	Université Bretagne Loire
PhD school	SCIENCES POUR L'INGENIEUR, GEOSCIENCES, ARCHITECTURE (SPIGA)
Planned PhD supervisor	DIENG Lamine - Université Gustave Eiffel - MAST - SMC
Planned financing	Contrat doctoral - Ifsttar
Abstract Context and objectives: Fatigue is, after corrosion, the main pathology of metal structures. In the case of welded joints, the fatigue phenomenon occurs preferentially at the weld joint due to high stress concentration and the existence of residual stresses created during welding operations. Different weld joint processing methods exist and are intended to improve the geometry of the weld seam (grinding, TIG re-melting). Some methods can even reverse the residual stresses present in the assembly by introducing residual compression stresses (hammering, peening, shot blasting). Although the effectiveness of these methods is proven, it is now difficult to quantify and guarantee the gain in lifetime obtained which could subsequently be taken into account in the calculations of service life of new structures existing ones. Conditions of realization of these methods must be provided as well as means of control of the quality of their implementation on site. The subject of the proposed thesis therefore focuses on the study of the improvement of the fatigue life of welded assemblies provided by different methods of completion. The main objective is to propose an applicable approach aimed at increasing the lifespan of new structures and extending existing structures. Thesis work program: A first part of this thesis will focus on the implementation of a large experimental campaign conducted on welded T-shaped test specimens, aged by fatigue then subjected to the three most used finishing techniques: hammering, grinding and TIG remelting. These specimens will be conducted to fatigue rupture in order to quantify the lifetime contribution obtained for each finishing technique. Several levels of ageing may also be considered. The study of residual stresses due to welding and finishing will be carried out by the DRX method available from our partner SONATS. A parametric study on the conditions of realization of the treatments will be carried out in order to determine the influence of certain parameters on the effectiveness of the techniques. Among these parameters we can note: the welding temperature, the number and the order of the passes, the hammering power, the re-flow energy, the angle of inclination of the torch, ... This experimental work will subsequently improve and validate the existing finite element welding and finishing models in the SMC laboratory, taking into account, in particular, the phase changes that play a major role in the development of residual stresses. In this context, we will focus on the quantification of the increase in life span provided by each method, by studying the modification of internal residual stresses. Different situations will be compared: the case where the treatment is carried out at the as the new structure and the one where the treatment is carried out during its service life. Finally, a final part will focus on monitoring and / or on-site monitoring of fatigue phenomena before and after treatment. Several methods of monitoring fatigue damage will be tested in the laboratory and then ranked according to several criteria such as ease of implementation, data mining, accuracy, and cost. The ultimate goal is to provide project owners with a comprehensive approach incorporating recommendations for choice, implementation and control of completion and an assessment of the new fatigue resistance of the structure. Expected results : The proposed work is intended to establish a level of confidence regarding the effectiveness of completion treatments performed on welded assemblies already aged (stress cycle history), but also to improve the understanding of the effects and mechanisms involved during the course of the treatment process. The main expected results are listed below: - Contribution of "finishing treatments technique" on aged welded joints with respect to their fatigue behaviour. - Evaluation of the state of residual stresses generated by the technique (relaxation or even inversion of the stresses) and its evolution as a function of the history and fatigue loading of the welded assembly. - Welding and finishing modelling models to optimize the parametric study of the processing conditions. - Recommendations for the implementation (mode of operation) of the various completion methods that are the subject of the thesis with the objective of improving the reproducibility of the operations and therefore the level of confidence in the expected lifetime gain. - Fatigue monitoring solutions for welded joints and evaluation of the new fatigue strength of the structure. Innovative character: The innovative character is the quantitative evaluation of the beneficial effect of finishing treatments whether in the case of new structures or in the case of structures that have already undergone fatigue cycles. The modelling tools developed will also make it possible to understand the limits and the phenomena involved for the various finishing methods studied. References : 1. K. Guiot, Amélioration de la tenue en fatigue des assemblages métalliques soudés, Stage de Master mécanique numérique des structures UFR Sciences et techniques de Nantes, 2012 2. S. Chataigner, L. Dieng, Y. Falaise, M. Grasset, (2015, December). Evaluation des contraintes résiduelles dans les assemblages soudés et amélioration de leur durée de vie par techniques de parachèvement/appraisal of residual stresses within welded assembly and increase of their fatigue life using fatigue treatment methods. In Annales du Bâtiment et des Travaux Publics (Vol. 67, No. 5, p. 14). Editions ESKA. 3. Chataigner S., Dieng L., Guiot K., Grasset M., "Improving welded joint fatigue life using shot peening or grinding", TRB 2013, Washington, 2013. 4. Bernoala V., , "Modélisation des opérations de soudage et de parachèvement, Stage Master mécanique numérique des structures UFR Sciences et techniques de Nantes, 2013. 5. Bernoala V., Dieng L., Chataigner S., Falaise Y., "Improvements of welded joints using shotpeening: modelling of the welding process to determine the initial residual stresses", Eurosteel, Naples, 2014. 6. S. Chataigner, L. Dieng, K. Guiot, M. Grasset, "Improving Welded Joint Fatigue Life Using Shot Peeningor Grinding". Proceeding Transportation Research Board 92nd Annual Meeting, Washington DC, Date: 2013-1-13 to 2013-1-17. http ://worldcat.org /oclc /13-1721 7. L. Dieng, D. Amine, Y. Falaise, S. Chataigner, "Parametric study of the finite element modeling of shot peening on welded joints” Journal of Constructional Steel Research, Volume 130, March 2017, Pages 234-247. 8. L. Dieng, J.Y. Thieuleux, S. Chataigner, Y. Falaise, "Utilisation du grenaillage de précontrainte pour la remise à niveaux en fatigue de structures soudées", JOA Metz, 2017.
Keywords :	Welding, finishing, grinding, hammering, TIG remelting, Residual stress, Numerical modeling, Fatigue

List of topics

Applications closed