TWI has 45 years’ experience in thermo-elasto-plastic modelling and, thanks to extensive experience in welding processes, is able to provide synergistic analyses of the importance of residual stresses for each case.
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.
Prediction of residual stresses
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 for the characterisation of weld residual stresses (prediction and measurement) have significant limitations, they are often combined. If the results are similar, they can be confidently used in assessing the failure modes of a structure.
Our work with residual stresses
Respected for our expertise, professionalism, impartiality and confidentiality, TWI works with the most influential companies worldwide across all industry sectors. We are part of international networks to advance the knowledge on measurement and prediction of residual stresses, and have completed several high-profile projects in this area.
TWI was asked for expert advice when primary coolant feeder tubes in a nuclear power station’s 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.
Our study was based on the assumption that the change in thickness affected residual stresses, which in 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.
In a separate project we assisted Atomic Energy of Canada Limited (AECL) in developing 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.
Measurement of residual stresses
Residual stresses may be measured by non-destructive techniques (eg X-ray and neutron diffraction, optical, magnetic or ultrasonic methods); by semi- destructive techniques (eg centre-hole and deep-hole drilling, and the ring core method); and destructive techniques (eg block removal, splitting and layering and the contour method).
Measurement of surface or through-thickness residual stresses in difficult locations or complex geometries is a TWI speciality. Recent applications have included tube-to-plate welds, weld cladding, on-site through-wall stress measurements, and butt welds for a range of thicknesses. This has led to the development of special kits, such as Tubestress and Cornerstress. This equipment has been used in major projects for clients in power generation, defence and nuclear waste disposal industries. The equipment can be customised for measurements in other locations with limited access.
A novel, compact system for residual stress measurement using digital image correlation (DIC) has been developed by TWI. Compared with the conventional method using a strain gauge rosette, DIC avoids the need for extensive surface preparation and precise drilling of the hole. In contrast with conventional DIC, the new system is convenient, easy to use, and does not require extensive expertise.
Additionally, TWI provides residual stress measurements using neutron diffraction and contour technique in collaboration with partner institutes.
TWI stays at the forefront of research in this area, notably via participation in international networks to advance knowledge on measurement and prediction of residual stresses.
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