Impact of residual stress and elastic follow-up on fracture

The presence of tensile residual stress in cracked structures combined with external loading
leads to circumstances where a structure may fail at a lower applied load than when residual stresses are
not present. This is taken into consideration in the fracture assessment codes which are usually invoked to
determine whether a cracked structure is fit-for-purpose. These codes typically attempt to decompose the
stresses present in the structure under consideration into either “secondary” or “primary” components, in
order to simplify the assessment and avoid the need for detailed numerical modeling. It is acknowledged
that whether a given residual stress field should be classified as “secondary” or as “primary” is dependent
on the level of associated elastic follow-up EFU. However, although there is a significant body of work
related to the influence of EFU on the high temperature creep behavior of uncracked structures, the EFU
concept has not yet been rigorously applied to the fracture assessment of cracked structures. This paper
represents a first step towards a more rigorous application of the EFU concept to the fracture assessment
of cracked structures containing residual stresses. Insight is provided into the influence of residual stress
and EFU on fracture by considering the behavior of a simple three-bar assembly. Having introduced the
concept, a three-bar type test rig capable of generating fit-up residual stresses with varying levels of EFU
in a compact-tension fracture-specimen is presented. Results, produced using this test rig, from two cases
with identical levels of initial residual stress but different levels of associated EFU are considered. It is
concluded that EFU is important in determining how the residual force in the specimen changes and
therefore how the component of crack driving force associated with the residual force changes as damage
accumulates in the specimen subsequent to fracture initiation.