Fabien Lefebvre

Effect of Weld Quality and Postweld Improvement Techniques on the Fatigue Resistance of Extra High Strength Steels

High strength steel (HSS) used in lifting equipment structure and metallic framework are known to have high static strength, good stiffness and very good weldability however knowledge of fatigue behaviour of HSS in welded structure is not sufficient in order to have representative and probabilized S-N curves to modify the design codes. In this study, a large fatigue test campaign was performed on cruciform and symmetrical butt welded joints of high and very high strength structural steel plates. Numerous fatigue tests were carried out for each configuration in as welded condition. To improve the fatigue performance, post-welding improvement techniques (burr grinding and TIG dressing) were performed on the welded specimens. All fatigue results were analysed using a statistical approach defined in the Eurocode 3 standard. It was noticed the impact of weld quality on fatigue performance. The application of post-weld improvement techniques (TIG dressing and burr grinding) leads to an additional improvement of the fatigue strength and reduces the scattering (COV), in comparison with the current welding standards or recommendations. The limited influence of the base metal yield stress on fatigue performance is explained by the local microstructure at the weld toe where fatigue cracks initiate; at this location, the mechanical properties are similar whatever the steel grades.

HFMI: understanding the mechanisms for fatigue life improvement and repair of welded structures

Fatigue resistance of high-frequency mechanical impact (HFMI) treatment is investigated on high-strength welded steel S690. Cyclic four-point bending tests are performed on butt weld samples and HFMI samples with different conditions of treatment with or without stress-relieving heat treatment. Microstructural characterisations with electron backscattering diffraction observations and residual stress analyses are presented. The main objective of this paper is to reveal and identify the parameters (residual stresses, geometry, strain hardening, etc.) which have an effect during the HFMI operation and to understand the phenomena involved during fatigue stressing. The optimisation of the geometry of the fatigue specimen allowed to repetitively obtain failures in the treated areas, thereby showing the relationship between the geometry, the strain hardening, the residual stresses and the fatigue strength. The main conclusions of this paper showed that HFMI treatment improves the fatigue performance of welded joints (in keeping with the International Institute of Welding results). The residual compressive stresses are prominent. The microstructure does not seem to have any effect, despite the nanostructured layer. The geometry is of secondary importance despite a significant change in the local geometry. HFMI treatment is also presented as retrofitting treatment.