Fatigue Crack Propagation Behavior of the Welded Steel of a Railway Bridge

Abstract

In the context of a R&D project concerning the new Alcácer do Sal composite railway bridge, a study of the fatigue crack growth on samples of its base material and weldments was performed. For this purpose, tests were carried out on CT specimens designed according to ASTM E647 standard, using the approximate thickness (B) of a structural detail of interest, B=32mm. The choice of B led to a relatively large specimen and was justified by the desire to better simulate service conditions, which would not be possible with smaller specimens, particularly in the case of weldments. The test matrix used included three values of R ratio (maximum/minimum load), 0.1, 0.4 and 0.7, and three material conditions, namely base material (BM), heat affected zone (HAZ) and weld metal (WM). When the nominal range of the stress intensity factor (DK) is used, the measured data displays a strong effect of the weldments on the FCG rates, with the base material presenting higher da/dN values. An evaluation of opening load behaviour was carried out, and it showed extensive closure caused by residual stresses in the HAZ and WM specimens. The investigation included the full field measurement of the residual stress perpendicular to the crack plane, using the contour technique. When the opening load effect was taken into consideration it was found that the da/dN vs. ∆K of the BM, HAZ and WM specimens is approximately identical. Furthermore if loading effects are considered, no significant difference is found for the three R values used, even if, as expected, higher R corresponds to higher da/dN values.