The residual stresses in a component or structure are self-balanced stresses caused by incompatible internal strains. They may be generated or modified at every stage in the component life cycle. Welding is one of the most significant causes of residual stresses and typically produces large tensile stresses in the weld balanced by lower compressive residual stresses elsewhere in the component.
Tensile residual stresses may reduce the performance or cause failure of manufactured products. They may increase the rate of damage by fatigue, creep or environmental degradation. They may reduce the load capacity by contributing to failure by brittle fracture, or cause other forms of damage such as shape change or crazing. Compressive residual stresses are generally beneficial, but may cause a decrease in the buckling load.
Finite element simulations are often applied to the prediction of residual stresses in structural components. The calculation of residual stresses involves complex non-linear analyses where several assumptions and approximations are required. The accuracy and resulting predictions, therefore, depend on the analyst's level of expertise.
However, the measurement of weld residual stresses also carries a great range of uncertainties. Moreover, it can be very expensive to understand the full distribution of residual stresses based on measurements only. Because both approaches (prediction and measurement) to characterising weld residual stresses have significant limitations they are often combined, and if the results are similar, they can then be confidently used in assessing the failure modes of a structure.
TWI has 45 years of experience in thermo elasto-plastic modelling. In addition, based on TWI's wide experience in welding processes, it is able to provide synergistic analyses of the importance of residual stresses for each case. Respected for its expertise, professionalism, impartiality and confidentiality, TWI works with the most influential companies worldwide across all industry sectors. TWI is part of international networks to advance the knowledge on measurement and prediction of residual stresses.
The safety of nuclear power stations is of paramount importance. TWI was asked for expert advice when primary coolant feeder tubes in a reactor showed intergranular cracking as well as signs of flow accelerated corrosion. These two factors taken together led to a reduction in the thickness by ~0.1mm per year, resulting in a step profile at the root of the weld increasing the likelihood of failure. TWI's study was based on the assumption that the change in thickness affected residual stresses which in their turn affected the incidence and rate of intergranular cracking. TWI created a 3D weld model of a pipe-to-pipe weld and the results were successfully validated against neutron diffraction measurements. A 3D hub-to-pipe weld model was subsequently set up which predicted the residual stress arising from welding and then thinning.
TWI assisted the Atomic Energy of Canada Limited (AECL) to develop methods for the highly specialised repair of an aluminium reactor vessel. TWI reviewed simulations of the repair process and carried out additional modelling highlighting the importance of the welding sequence on the properties of the repair, including residual stresses. With the aid of numerical modelling of the welding process, the repair was successfully implemented over a period of six months.
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