Fatigue and fracture assessment of butt welds

Abstract

The specific project is inspired by the general trend of investigating the effects of various parameters in the sensitive area of structural fatigue, which nowadays has become one of the most important design aspects in the ship and ocean structures building industry. The size of vessels, especially the container ships, has increased significantly during the past years, a development which leads to intensive use of high tensile steels in combination with plates of big thickness. This work has tried to deal with the above matters, i.e. the performance of high tensile steel and the plate thickness effect on fatigue strength of butt welded joints.

In the first part, an evaluation of fatigue test results takes place. The fatigue tests were carried out by Germanischer Lloyd in co-operation with shipyards for butt welded specimens of various thicknesses made of specific high tensile steel, called YP47. The purpose is to investigate the plate thickness effect and the overall performance of YP47. Following the recommendations of International Institute of Welding (IIW), the S – N curves for each series of data were designed and useful statistical information were extracted regarding the scatter of the results and the FAT class of each series. Influence of misalignments was investigated and additionally the results were compared with previous experiments. Generally, a good performance of YP47 was observed, while, in contrast to the relevant rules and guidelines, there was no clear thickness effect, mainly because of the large scatter of the results.

The second part is an assessment based on fracture mechanics concept. Specifically, the parameters C and m of the Paris crack growth equation were investigated. The numerical analysis was performed by the software VERB (failure assessment software), based on experimental results of fatigue tests from a joint development project between GL and Korean shipyards. The specimens used were butt welded 80 mm plates made of higher tensile steel and they were tested under a special two – level load history which resulted in creation of beachmarks on the fatigue crack surfaces. These beachmarks gave valuable information regarding the crack initiation location and the crack propagation phase that was used for the numerical assessment. The obtained results were compared with the recommended values from literature (IIW).

Finally in the third part, the thickness effect of butt welds is investigated in terms of the notch stress approach and fracture mechanics. Specimens of various thicknesses and different weld geometries from the first part are modeled and the notch stress distribution through the thickness is calculated using the software ANSYS. The influence of meshing, the thickness of the plate, the geometry of weld as well as the concept of fictitious notch rounding and undercut on the notch are examined. Moreover, crack propagation calculations are performed using the software FRANC2D (a 2dimensional crack propagation simulation) by initiating cracks of various lengths to the weld toe. Similar calculations are done by VERB, using the notch stress distribution obtained from ANSYS and the results are compared. Finally, the thickness effect in the total life time predicted by notch stress approach and fracture mechanics is investigated and compared with the values given in the references.