Crack propagation and arrest of structural steels and pipelines

Crack arrest of fast running cracks is an important issue for the safety of steel structures. Crack arrest design can prevent fatal damage of large structures by restricting the influence of the incidents. Therefore crack arrest design is important especially for very large structure, where accidents may cause huge economical and social losses. Propagating shear fracture, long running ductile crack propagation in pipelines and brittle crack propagation in heavy thick shipbuilding steels have been investigated. For the propagating shear fracture issue, a new HLP simulation model, applicable to various backfill conditions, including underwater backfill, was developed. The proposed backfill model can be applied to the prediction of the crack arrest of propagating shear fracture under various backfill conditions. The new HLP simulation was successful in estimating full-scale burst tests with various backfill depths. The new HLP simulation for underwater pipelines indicated that propagating shear cracks are easily arrested in offshore pipelines compared to onshore pipeline. The toughness requirement of the line pipe for preventing propagating shear fracture can be significantly smaller in underwater pipelines compared to onshore pipelines. The margin of the underwater pipeline for propagating shear fracture was clarified by the new model developed in this thesis. For brittle crack arrest, an empirical approach for long crack propagation has been conducted. Several large-scale crack arrest tests were carried out in order to investigate the long crack arrestability of heavy-thick shipbuilding steel plates. All plates and their welded joints used in this study satisfied the Charpy toughness requirement for EH-grade shipbuilding steels and welded joints. A brittle crack ran along a welded joint and penetrated through the test plate under a stress exceeding 200MPa, despite the presence of longitudinal stiffeners across the test weld. A brittle crack in a base plate model test with longitudinal stiffeners propagated through the specimen under the maximum design stress. These results suggest that EH-grade cannot ensure arrestability even in the base plate, in case of heavy thick shipbuilding steel. Two crack arrest tests were conducted in order to demonstrate the possibility of long crack arrest using high arrestability steels. The test results indicated that the crack arrest concept could be achieved by a combination of high arrest toughness materials and a special fabrication method which can derive a brittle crack into the base plate of high arrestability plate. The crack arrest design can be used in practice even for large-scale structures using heavy thick steel plates. The effective K concept, as proposed by Machida, et al.31), provides the boundary of the “arrest” and “propagate” data in the large-scale tests conducted in this work. The effective K concept was found to be valid even for long crack propagation of such a heavy thick steel plate as investigated in this paper.