Neutron diffraction investigations on residual stresses contributing to the fatigue crack growth in ferritic steel tubular bridges

Fatigue crack growth observed in tubular K-joint specimens, typical of tubular bridge structures, always initiates at the chord crown toe locations whether the applied stress range is tensile or compressive. Even though other locations around the weld have highest hot-spot stresses, chord crown toe locations are still the most critical. This raises the question about the relevant tensile residual stress level at that location. The results of residual stress investigations, obtained using neutron diffraction measurements highlight that the direction and location of the maximum tensile residual stresses in K-joints is substantially different from those in the more usual tubular butt joints. Indeed, it is shown that the highest tensile residual stresses are oriented perpendicular to the weld direction, which is also the main orientation of the loading stresses applied in K-joints. This paper demonstrates that it is the complex geometry of the K-joint that causes the superposition of critical stresses, making these joints susceptible to fatigue cracking. Therefore, transverse residual stresses play a crucial part in the fatigue crack growth behaviour that applied stresses alone cannot explain.

► We measure the 3D residual stresses in tubular joints using neutron diffraction.
► We identify direction and location of the maximum tensile residual stresses.
► K-joint geometry will induce a non-usual orientation of residual stresses.
► Fatigue crack propagation is affected by this critical stress orientation.