By Guiyi Wu
Welding induced residual stresses affect the defect tolerance of welded structures. They affect the resistance to fracture because they change the crack driving force. They also affect fatigue crack propagation behaviour because they can be either tensile or compressive and therefore can move parts of the applied stress range into or out of regimes of cyclic damage accumulation. The influence of residual stresses on the mechanical behaviour of cracked structures subject to static or cyclic loads is a complex matter.
It is important to understand the as-welded residual stresses as well as the redistribution of residual stresses under pre- or in-service conditions, including the possibility of heat treatment and cyclic loads during which shake-down or ratcheting behaviour may occur. This report provides a comprehensive overview of current practice for assessment and treatment of residual stress for the integrity of welded steel structures and identifies opportunities in this area.
- Many techniques are available for residual stress measurement and no single method can provide a full range of measurement capabilities.
- There are a number of methods to reduce (relieve) welding residual stress. Post-weld heat treatment (PWHT) is an attractive method, but there are limited data on experimentally measured residual stress distributions in welds after local PWHT. There is even less evidence on the local PWHT residual stress distribution measured by various techniques. Recent work performed by TWI provided valuable information regarding the treatment of residual stresses after furnace PWHT and local PWHT in structural integrity assessments.
- Prediction of residual stress can be achieved by finite element analysis, but is generally based on many assumptions and approximations. In order to make accurate predictions, the different properties of the different phases should be taken into account. The phase transformation and constitutive models developed by TWI predict the residual stress in a single-pass ferritic steel weld very well.
- The current classification of residual stress into primary and secondary stresses simplifies the treatment of residual stress in structural integrity assessment, but the guidance on considering elastic follow-up in the current codes is not clear.
- Limited experimental data are available on the relaxation of residual stress due to thermal or mechanical loading, which restricts the application of the relaxation rules suggested in BS7910:2013 when the actual loading condition is multiaxial. Nevertheless, recent work carried out by TWI indicates that the relaxation rule in BS7910 is valid for uniaxial loading but less relaxation is estimated for biaxial loading compared with uniaxial loading.
- Tensile residual stress is detrimental to the fatigue life of a structure and fatigue crack propagation in a residual stress field has been, and is still, a hot research topic. Modifying tensile residual stresses to compressive residual stresses will improve fatigue life of a welded joint.
Linear superimposition of transverse residual stress resulting from welding:
(b) Quenching effect
(c) Phase transformation
[From Macherauch E and Wohlfahrt H, 1977: ‘Different sources of residual stress as a result of welding’, in Residual stresses in welded construction and their effects, Vol 1, R W Nichols (Ed), The Welding Institute]