Constraint-Based Assessment of Fracture - Application of ISO 27306 to Column-to-Beam Welded Joints -

A method of constraint loss correction of CTOD fracture toughness for fracture assessment of steel components, ISO 27306, has been published in 2009.  ISO 27306 enables the CTOD toughness correction by the equivalent CTOD ratio b (0 < b < 1), defied as the CTOD ratio at an equivalent Weibull stress level between the standard fracture toughness specimen and structural components.  Materials with a high Weibull shape parameter and a high yield-to-tensile ratio give a low b, which leads to a large constraint loss in structural components.

In this paper ISO 27306 has been applied to column-to-beam welded joints.  Numerical analysis indicates that the strength overmatch frequently observed in steel welds has a marginal effect on b, provided that the weld metal strength is 25 % overmatch and less. A HAZ-notched CTOD specimen with a reduced-thickness has been proposed for fracture assessment of column-to beam joints, where unstable fracture occurs from a crack in HAZ in the surface weld pass.  This surface HAZ has a unique toughness different from other HAZ toughness in multi-pass welds, because it will be not reheated by the subsequent weld-pass.  A thickness effect has been implemented into the equivalent CTOD ratio b for the reduced-thick CTOD specimen.

Wide plate welded joints with a surface notch in the HAZ at geometrical discontinuity, simulating column diaphragm and beam connection, have been tested at a low temperature.  The fracture performance predicted from the reduced-thick CTOD specimen consists well with the experimental results.  Based on the equivalent CTOD concept, a discussion is conducted on the HAZ toughness that is necessary to meet the design requirement of a high strain in the column-to-beam joints.