TWI Knowledge Summary

Residual stresses

The residual stresses in a component or structure are stresses caused by incompatible internal permanent strains. They may be generated or modified at every stage in the component life cycle, from original material production to final disposal. Welding is one of the most significant causes of residual stresses and typically produces large tensile stresses whose maximum value is approximately equal to the yield strength of the materials being joined, 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 cause a decrease in the buckling load.

Data on residual stresses may be obtained from published literature, measurements or numerical modelling. Standard residual stress distributions for common welded joint geometries are included in BS 7910:2005 Annex Q, British Energy R6 Rev.4 Section IV.4 and API 579 Appendix E. The standard distributions were obtained by fitting upper bounds to published data, and are available for butt welds, T-joints, girth welds in cylinders and weld repairs.

Residual stresses may be measured by non-destructive techniques, including X-ray diffraction, neutron diffraction and magnetic and ultrasonic methods; by locally destructive techniques, including hole drilling and the ring core and deep hole methods; and by sectioning methods including block removal, splitting, slicing, layering and the contour method. The selection of the optimum measurement technique should take account of volumetric resolution, material, geometry and access.

Prediction of residual stresses by numerical modelling of welding and other manufacturing processes has increased rapidly in recent years. Modelling of welding is technically and computationally demanding, and simplification and idealisation of the material behaviour, process parameters and geometry is inevitable. Numerical modelling is a powerful tool for residual stress prediction, but validation with reference to experimental results is essential.

Allowing for residual stresses in the assessment of service performance varies according to the failure mechanism. It is not usually necessary to take account of residual stresses in calculations of the static strength of ductile materials. Design procedures for fatigue or buckling of welded structures usually make appropriate allowances for weld-induced residual stresses, and hence it is not necessary to include them explicitly. Residual stresses have a major effect on fracture in the brittle and transitional regimes, and hence the stress intensity, K, or energy release rate, J, due to residual stresses must be calculated and included in the fracture assessment. K or J may be obtained as a function of stress distribution, crack size and geometry by various methods, including handbook solutions, weight functions, and finite element analysis.

Residual stresses in as-welded structures may be minimised by appropriate selection of materials, welding process and parameters, structural geometry and fabrication sequence. Residual stresses may be reduced by various special welding techniques including low stress non-distortion welding (LSND), last pass heat sink welding (LPHSW) or inter-run peening. They may be relaxed by thermal processes including postweld heat treatment and creep in service, or by mechanical processes including proof testing and vibratory stress relief. Different stress relief treatments are appropriate in different applications. The effectiveness of the treatment may be reduced or the residual stresses may be increased if the treatment is not applied properly. Specialised processes are available for inducing beneficial compressive residual stresses, including peening, shot blasting, induction heating stress improvement (IHSI), low plasticity burnishing (LPB) and mechanical stress improvement procedures (MSIP).

Further information

Other related items covering residual stresses include:

Welds, their quality and inspection capability for high integrity structures and components (April 1999)

Tubestress' measures residual stresses in parts that other equipment cannot reach

FAQ: What are the residual stresses in a dissimilar metal weld?

Job knowledge for welders: Distortion - types and causes

Numerous Member reports and Bulletin articles on residual stresses are also available to TWI Industrial Members.

You can use the Weldasearch literature database to supplement what you find in JoinIT.