ABSTRACT:As steel strength increases, the fracture toughness may be quite different from that of normal steel, and the corresponding welded joints can be the critical spots due to possible brittle fracture behavior. Moreover, the design load for high-strength steel structure is larger and the steel with higher stress is more sensitive to defect, which increases the potentials of brittle fracture. The service of steel structures in cold regions increases the crisis of brittle fracture. Therefore, a series of three-point bending tests were conducted at low temperature to investigate the fracture toughness of high-strength steel and its butt weld. Fracture micro-mechanisms were analyzed through Scanning Electron Microscopy of the fractured surfaces in specimens. The fracture toughness indices (critical CTOD values) of high-strength steel and its butt weld all decrease as temperature decreases. The heat affected zone (HAZ) is more critical to fracture than the base material, indicated by much lower critical CTOD values and higher transition temperature. The fracture toughness of high-strength steel is relatively lower than the conventional steels (i.e. 235MPa, 345MPa and 390MPa). The results obtained in this paper provide reference for the fracture resistant design of high-strength steel structures in cold regions.
Fatigue Failure is a common failure mode for offshore steel jacket and the system fatigue reliability analysis for this type of structure has been developed greatly in recent year. Nevertheless none of those methods gives satisfactory results and there are some problems such as long calculating time, low accuracy, etc. To deal with these problems, a Synthesis Searching Method (SSM) and corresponding program for the system fatigue reliability analysis of structure system is proposed in this paper which can identify the dominant fatigue failure paths of the structures more efficiently and much faster. The analytical probability of system failure through these paths has been evaluated. A numerical example is given to illustrate the efficiency of the proposed method. It is demonstrated that this method has great advantage in analyzing the fatigue reliability of large and complex offshore steel jackets.
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