The increasing size of container vessels is accompanied by an increase of plate thicknesses as well as the extensive use of higher tensile strength steels. For large plate thicknesses as well as high yield strengths, the influence of these parameters on fatigue strength of thermal cut plate edges is not well validated. This paper describes results based on fatigue tests for specimens with yield strengths of 355 to 460 MPa and plate thicknesses of 25 to 80 mm. The specimens were fabricated using gas and plasma cutting as well as usual post-treatment methods. The objective of the investigations was to verify and, if suggested by the results, to modify the current fatigue assessment procedures for thermal cut plate edges.
Due to the trend to use higher strength steel as well as large plate thicknesses in the upper flange of container ships, e. g. for upper deck and coaming, the influence of material strength as well as plate thickness on fatigue strength of thermal cut plate edges was investigated by fatigue tests. Classification societies’ rules and guidelines for fatigue strength of ship structures often provide correction factors for those parameters, where increasing yield strength has a positive effect, while increased thickness is considered to be detrimental for low thermal cut qualities (e.g. DNV, 2021). In addition, the fatigue test results were used for validation of the currently applied characteristic fatigue strength values of thermal cut plate edges (up to FAT 150). The investigations were carried out within a Joint Development Project (JDP) between the Korean Committee of Shipbuilding Steel Development and DNV GL.
During the last decades significant research on fatigue strength has been done, but mainly related to welded joints (e.g. Hobbacher, 2016) which are considered as weak point with respect to cumulative fatigue (Parmentier and Huther, 2013). In general, non-welded elements such as thermal cut plate edges have a higher fatigue resistance but may also become critical due to stress concentrations. Examples in shipbuilding are window corners, lightening holes as well as hatch corners. Especially, thermal cut hatch corners of Ultra Large Container Ships (ULCS) with up to 100 mm thick plates are considered as critical elements with respect to fatigue life (Parmentier and Huther, 2013).
