Failure mode of laser welds in lap‐shear specimens of high strength low alloy (HSLA) steel sheets

In this paper, the failure mode of laser welds in lap‐shear specimens of non‐galvanized SAE J2340 300Y high strength low alloy steel sheets under quasi‐static loading conditions is examined based on experimental observations and finite element analyses. Laser welded lap‐shear specimens with reduced cross sections were made. Optical micrographs of the cross sections of the welds in the specimens before and after tests are examined to understand the microstructure and failure mode of the welds. Micro‐hardness tests were also conducted to provide an assessment of the mechanical properties in the base metal, heat‐affected and fusion zones. The micrographs indicate that the weld failure appears to be initiated from the base metal near the boundary of the base metal and the heat‐affected zone at a distance away from the pre‐existing crack tip, and the specimens fail due to the necking/shear of the lower left load carrying sheets. Finite element analyses based on non‐homogenous multi‐zone material models were conducted to model the ductile necking/shear failure and to obtain the J integral solutions for the pre‐existing cracks. The results of the finite element analyses are used to explain the ductile failure initiation sites and the necking/shear of the lower left load carrying sheets. The J integral solutions obtained from the finite element analyses based on the 3‐zone finite element model indicate that the J integral for the pre‐existing cracks at the failure loads are low compared to the fracture toughness and the specimens should fail in a plastic collapse or necking/shear mode. The effects of the sheet thickness on the failure mode were then investigated for laser welds with a fixed ratio of the weld width to the thickness. For the given non‐homogenous material model, the J integral solutions appear to be scaled by the sheet thickness. With consideration of the plastic collapse failure mode and fracture initiation failure mode, a critical thickness can be obtained for the transition of the plastic collapse or necking/shear failure mode to the fracture initiation failure mode. Finally, the failure load is expressed as a function of the sheet thickness according to the governing equations based on the two failure modes. The results demonstrate that the failure mode of welds of thin sheets depends on the sheet thickness, ductility of the base metal and fracture toughness of the heat‐affected zone. Therefore, failure criteria based on either the plastic collapse failure mode or the fracture initiation failure mode should be used cautiously for welds of thin sheets.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/90305/1/j.1460-2695.2011.01609.x.pd

Influence des conditions de rectification sur la tenue en fatigue d’un acier 42 CD 4

Les essais de fatigue en flexion plane réalisés au Cetim sur un acier 42 CD 4 ont mis en évidence une très nette influence des conditions de rectification (type de meule, profondeur de passe notamment) sur les contraintes résiduelles et sur la limite d’endurance.A l’aide de la méthode par rayons X, on a caractérisé la stabilité en fatigue des contraintes résiduelles engendrées par trois conditions de rectification afin de permettre leur prise en compte dans un calcul prévisionnel de tenue à la fatigue.Cette étude a permis d’améliorer nos connaissances sur l’influence de la rectification en fatigue en séparant l’influence respective de la rugosité superficielle et des contraintes résiduelles. Elle a mis en évidence d’autre part l’importance de la relaxation des contraintes résiduelles en fatigue et la relation de cette relaxation avec le comportement cyclique du métal

Measurement of residual stress in round bars quenched after heating by induction

SIGLELD:8019.3153(PNR--90134). / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Fatigue Design of Notched Components with Stress Gradients and Cyclic Plasticity

Conference: 27th Spring Meeting on Fatigue and Plasticity Paris, FRANCE, 2008International audienceThis paper shows that fatigue strength of notched specimens under cyclic loading can be simply and accurately estimated by using elastic-plastic computations and averaging stress over a critical volume obtained by an optimisation process minimizing the dispersion between experiments and simulations. The Dang Van high cycle fatigue criterion is considered. Fatigue tests (tension-compression, bending and torsion) carried out by the Cetim, are used to calibrate the critical volume