Although in the last decades the applicable regulations for the design of steel structures have been deeply improved accounting for modern design approaches and technologies, in a few industrial fields, concepts and formulations derived from centuries-old experience are even nowadays the governing rule. This very often limits the potential offered by the technological innovations in high-performance material, such as quenched and tempered (QT) steels. After having verified in the scientific literature, in the commercial one and in several open material databases that some main data necessary to the comprehensive characterization of QT steel used in building large metal structures are lacking, an extensive experimental campaign was undertaken to achieve an updated and specific fatigue strength characterization for certain types of materials. The aim is to overcome the strict requirements of current regulations on the application of both parent QT materials and welded homogeneous and heterogeneous joints between different steel strength. The experimental campaign allowed filling the knowledge gap as well as providing further understanding whether the use of high-strength steel is effective in the improvement of performances of large steel structures, whose fabrication procedures are typical of building sites rather than workshops. Their extensive application in current building practice is expected to be highly beneficial in terms of weight and costs.

Using the results from testing industrial batches of 23 mm steel heavy plates after thermomechanical rolling and subsequent post-weld heat treatment, the patterns of fatigue crack formation in the fracture specimens during CTOD (Crack Tip Opening Displacement) testing for fracture toughness are investigated. Visual, microstructural, and fractographic studies of the nature of fracture formation and the surface of the secondary separations have been conducted. The probable causes of the manifestation of the potential “pop-in” effect on the load–displacement diagrams of the notch opening displacement are described, as well as its potentially negative impact on the interpretation of test results.

This paper presents the results of a relatively wide experimental campaign, which was conducted with the aim of complementing test data already available in the open literature. A few years ago, several fatigue tension tests were conducted on notched square-section specimens. The purpose of these tests was to provide support for the validation of fatigue assessment approaches based on the notch stress intensity factor (N-SIF) concept, e.g. the strain energy density approach and the peak stress method. However, it should be noted that the run-out was limited to 2·10 ⁶ cycles. With ships and offshore structures as the primary focus of this study, which undergo 10 ⁸ cycles during their operational lifespan, a few series of rotating bending fatigue tests were conducted on variously notched specimens made of different structural steels. These tests were carried out up to 5·10 ⁷ cycles as the run-out target. A total of approximately 100 specimens were prepared and tested, which resulted in a comprehensive data set that also included measurements of local notch geometries. The findings revealed that the notch geometry, specifically the notch opening angle and the radius at the tip, exerted a substantial influence on the fatigue strength. The analysis of the experimental data is further enhanced by the useful comparisons with numerical finite element simulations of specimens by applying the strain energy density approach.