Computational Mechanics of welds and welded structures

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Arc welding is a complex manufacturing process in which an electric arc generates heat in an area approximately 1 cm2 while travelling at speeds of approximately 1 to 10 mm/s to form a pool of liquid metal with volume usually slightly less than 1 cm3. As the liquid metal on the trailing edge of the weld pool solidifies in milliseconds, a weld is created. The transient temperature field drives metallurgical phase changes, thermal expansion and changes temperature dependent properties of the materials. The microstructure phases change as it is heated and cooled. The strain due to thermal expansion-contraction, phase changes and restraint of the structure generates stress and plastic deformation. The plastic strain rate leads to residual stress, deformation and damage. During the welding process, various defects such cracks and pores can form. Because cracks and pores can be a function of the stress-strain history, it is important for design engineers to have reliable estimates of the transient stress and strain fields in the neighbourhood of each weld joint to assess the risk of cracking. The residual stress in welds plays an important role in the risk of in-service failure due. This research will train students to develop and apply realistic computer models of the real physics, mechanics and material science of the welding process and the behaviour or welded structures for the following computational models; i) weld pool physics and evolution of microstructure in the fusion zone and heat-affected zone of multi-pass welds, ii) thermalplastic stress analysis of the solid from the melting point in the weld pool to in-service behaviour, iii) fatigue, creep, ductile and brittle fracture failure modes in welds, iv) optimal design of welded structures. These transient 3D non-linear models are integrated into a software framework. The software is used in industry to optimize the design of welds and welded structures and for training welders, welding engineers and inspectors. It is often argued that welds are the least reliable parts of structures such as power and petrochemical plants, ships, bridges and pipelines. The capability to design and build more reliable structures is an important benefit to Canada. Improved training is important for the more than 50,000 members of the Canadian Welding Association. In 2009, when AECL's NRU reactor was shut down and 85% of the worlds isotope production halted, the repair process was welding. Software, based research funded by NSERC, that was used to predict the performance of the design of each weld before welding tests began, played an important role in the success of the repair. The Welding Institute said this was the complex weld repair they had ever seen.