Influence of welded joint microstructures on fatigue behaviour of specimens with a notch in the heat affected zone

Abstract

In this paper, the results of determining the Paris law coefficients, C and m, are presented, based on the experimental data obtained by three-point bending testing of 8 welded joint specimens. The specimens were taken from a welded plate, with P460NL1 used as the parent material and VAC 65 used as filler material. The specimens had a 1.4 mm notch in the heat affected zone, either in the root or the weld face side. After the fatigue crack growth tests performed using the Fractomat 7609/213 (RUMUL), the obtained crack growth rate vs. number of cycles (da/dN vs. N) diagrams were used to calculate the coefficients. The values of these coefficients were then compared to each other, in order to determine the cause of noticeable differences, which were assumed to be a result of different microstructures in different specimens, caused by the increased temperature measured in the specimens near the end of the welded joint. Since the specimens used in the experiment had differences in terms of notch position and temperature, they were divided into four groups. Analysis of the results involved the comparison of obtained a-N curves with the images of microstructures, as seen on the broken specimens, in order to determine whether the changes in the curve's slope correspond to the changes in microstructures along the specimens' cross-sections. The experimental results confirmed this assumption, and it was determined that the crack growth rate was the highest in the weld metal, whereas the lowest rates (highest resistance to fatigue crack growth) were observed in the parent material, with the heat affected zone being between these two value ranges.