While the well-established parameters CTOD and Q sets the size scale over which large stresses and strains develop, and characterise the constraint level due to geometry with no stresses present, respectively, the new R parameter provides a basis for more reliable fracture mechanics analyses in which the influence of external forces and residual stresses in front of a crack tip can be taken into account.

The difference between a solution accounting for residual stresses and the small scale yielding (SSY) field at r/(J/σ₀) = 2 and θ = 0 defines R: 

where J stands for the so-called J-integral.

• CTOD can be used as a fracture toughness parameter provided that the driving force (CTOD) is corrected for the residual stress induced constraint (R).
• Since the geometry constraint (Q) and the residual stress induced constraint (R) are additive, the use of standardised constraint correction methods is possible.
• Critical CTOD values established from small scale specimens can still be used as a fracture toughness parameter provided that the driving force (CTOD) is corrected for the residual stress induced constraint (R).
• The validity of the R parameter has been investigated by means of a parameter study.