Comparative study of evolution of residual stress state by local mechanical tensioning and laser processing of ferritic and austenitic structural steel welds.

Complex thermal stresses generated in welded structures are undesirable but inevitable in fusion welding. The presence of residual stresses can be detrimental to the integrity of a welded joint. In this research, redistribution of residual stress magnitude and profile was studied and compared in two multi-pass welded structural alloys (API X100 and 304L stainless steel) after cold rolling and laser processing. The residual stress field was studied by neutron diffraction using the SALSA strain scanner at their reactor neutron source at ILL, Grenoble. In addition to a complex distribution of residual stress state, multi-pass welds also forms dendritic grain structure, which are repeatedly heated, resulting in segregation of alloying elements. Dendritic grain structure is weaker and segregation of alloying elements may result in formation of corrosion microcells as well as reduction in overall corrosion prevention due to depletion of alloying elements in certain areas. The modification of as-welded residual stress state was done by cold rolling which was followed by laser processing to create a recrystallized microstructure to minimise segregation. The main objective of this study is to understand the suitability of this novel manufacturing technique to create a stress free weldment with recrystallised grain structure. Hardness evolution in the welded structures was scanned following welding, post weld cold rolling and cold rolling followed by laser processing. Hardness distribution in both the structural alloys showed a significant evidence of plastic deformation near the cap pass of the weld metal. Residual stress redistribution was observed up to 4 mm from the capping pass for ferritic steel, while in austenitic steel weld, post weld cold rolling was effective in modifying the residual stress redistribution throughout the entire thickness. Laser processing in both cases reinstated the as-welded residual stress distribution and resulted in softening of the strained area

Fatigue of thin periodic triangular lattice plates

A method for predicting the fatigue life of triangular lattices is proposed in this paper by considering fatigue properties of single lattice struts. Fatigue tests of different sizes of lattice plates of aluminium alloy, and tests of single struts with different maximum fluctuating loads, have been conducted to validate this method. It is found that the struts in a triangular lattice break near to strut intersections, where stress and strain concentrations occur. Similar crack propagation paths were observed in different lattice plate specimens: the cracks grew at a 30° angle to the initial edge crack in the upper half of lattice plate. The mixed-mode fatigue crack propagation rate was also studied and expressed using an effective stress intensity factor. A size effect on the crack growth rate of triangular lattice plates was also observed: a fatigue crack will propagate slightly quicker in larger triangular plates than in smaller ones

Investigation of post-weld rolling methods to reduce residual stress and distortion

The mechanisms of post-weld rolling and how it reduces and eliminates residual stress and distortion are poorly understood. Finite element analysis was applied to two different methods of rolling: rolling the weld bead directly with a single roller and rolling beside the weld bead with a dual flat roller. The models showed that both rolling techniques were able to induce compressive stress into the weld region, which increased with rolling load. The distribution of stress was sensitive to the coefficients of friction between the workpiece and the roller and the backing bar. High friction coefficients concentrated the plastic deformation and compressive stress within the centre of the weld bead. Distortion can be eliminated by rolling; however, the experiments indicated that this was only achieved when applied to the weld bead directly