Fatigue design criterion for welded structures

For continuously welded structures subjected to cyclic loading, the highly stressed zones where cracks initiate and lead to failure are usually located at weld toes. At these critical points, called hot‐spots, the very local stress states are difficult to determine so that standard fatigue criteria are very difficult to apply for fatigue life prediction.This work presents a fatigue design criterion for continuously welded thin sheet structures, based on a unique S‐N curve. The approach, which refers to the hot‐spot stress concept, defines the design stress S as the geometrical stress amplitude at the hot‐spot.In practice, the geometrical stress state is calculated by means of the finite element method (FEM) using thin shell theory. Meshing rules for the welded connection, which can be applied methodically to any welding situation, allow the hot‐spot location, and therefore the design stress of any structure, to be determined.Experimental data and FEM calculations show that a unique S‐N curve can be obtained whatever the geometry of the welded structure and the loading mode

LASER CLADDING ON ALUMINIUM BASE ALLOYS

laser cladding is often performed on iron or titanium base alloys. In the present work, this method is employed on aluminum alloys ; nickel or silicon are added by powder injection. Addition of silicon leads to sound surface layers, but with moderated properties, while the presence of nickel induces the formation of hard intermetallic compounds and then to an attractive hardening phenomena ; however a recovery treatment has to be carried out, in order to eliminate porosity in the near surface region

Assessment of Welded Structures by a Local Multiaxial Fatigue Approach

A new local multiaxial computing method for the fatigue assessment of welded structures is presented. This approach is based on the use of an equivalent stress derived from the multiaxial fatigue criterion proposed by Dang Van for plain machined specimens. Application to fillet welds allows a better description of the influence of local geometries and residual stress states. These factors are related to the weld process and the quality of the weld. This approach, associated with definite shell finite element meshing rules in order to take account of the increase of local rigidity introduced by the weld, is also successfully used to assess the fatigue resistance of welded automotive structures

Microstructure and poro-mechanical performance of Haubourdin chalk

International audienceLarge chalk deposits are to be found in the North of France, and, as a result, developing chalk as a load-bearing aggregate is considered by French road building companies. Due to its limited mechanical performance, chalk is subjected to a medium heat treatment process (up to 500 °C). In this context, microstructure and poro-mechanical performance of Haubourdin chalk are characterized before and after heat treatment. The investigation of the so-called water weakening effect is associated to that of heat-treatment, mainly to show to what extent the strength of Haubourdin chalk decreases at a given water saturation state, prior to or after heat-treatment. In terms of microstructure analysis, SEM and FIB/SEM observations show the initial weak cementation of Haubourdin chalk, in direct relation with its moderate mechanical performance. After heat treatment, minor re-crystallisation of calcium carbonate is observed, which should be confirmed further. The specific behavior of Haubourdin chalk is highlighted under partial water saturation and after heat treatment, by using uniaxial compressive tests, and triaxial drained and undrained compressive tests. Pore collapse is quantified directly under increasing hydrostatic stress by measuring coupled gas permeability and porosity change. A heat-hardening effect is identified, which is counter-balanced by the water-weakening effect. The latter makes natural Haubourdin chalk unsuitable as load-bearing road aggregate, without further binding matter