Structural resistance of ship and offshore structures exposed to the risk of brittle failure
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- Institutt for marin teknikk 
The thesis aims to present the possibility of brittle failure in ship and offshore structures at low temperatures, and propose a material failure criterion for the prediction of ductile-brittle fracture transition in large-scale structures with shell elements. The thesis also includes a test framework for cryogenic spill accidents on steel plates which is to investigate the structural behaviour during cryogenic conditions, thereby enabling evaluation of the reliability of the proposed material failure criterion through numerical analysis of the test. Partly due to global warming in the past few decades, navigation in the Northern sea route (NSR) has been pioneered. Structures operating and traveling in the NSR are in harsh environmental conditions, and particularly in sub-zero temperatures. It is a well-known phenomenon that steel, in general, becomes more brittle as the temperature decreases while the initial yield stress, the flow stresses and the tensile strength increase. This fact reminds us that the structures in the NSR, where the average temperature is -40℃ in the winter, potentially have the risk of brittle failure caused by abnormal events such as ship-ship and ship-iceberg collisions. Liquid natural gas (LNG) vessels, which carry cryogenic cargo at a temperature of -163℃, may also experience unexpected brittle failures during accidents. If a leakage of LNG occurs by human errors or accidents, it results in the embrittlement and initial damage (failure) of the steel. This may further lead to progressive degradation, which is normally called cascading damage, of the structural integrity after the first accident. The research work is divided into two main parts. Part 1, “Modelling of the ductile-brittle fracture transition in steel structures with large shell elements” where first, the possibility of brittle failure in marine structures from abnormal accidents is examined by performing nonlinear finite element analysis (NLFEA) of Charpy-V tests. The energy absorption of the Charpy specimen predicted by NLFEA shows a tendency comparable to that of the tests only when the shear failure criterion for ductile fracture and the RKR criterion for brittle fracture are simultaneously implemented. The numerical study well presents the potential occurrence of brittle fracture for actual marine structures exposed to low temperatures. The feasibility of the RKR criterion for the numerical prediction of brittle failure is evaluated, whereas a need for another failure criterion with coarse shell elements to predict the ductile-brittle fracture transition (DBFT) is encountered. In order to predict the ductile-brittle fracture transition with coarse shell elements, the strain energy density (SED) failure criterion is proposed. To this end, the local simulations of a fictitious plate discretized by very fine solid elements are performed with the combined use of the Gurson model for ductile damage and fracture and the Richie-Knott-Rice (RKR) criterion for brittle fracture. Based on that, the critical values of the SED criterion are calibrated for a range of temperatures and plane stress states. The numerical analysis for drop tests on steel-plate structures is carried out to evaluate the SED failure criterion, and this verifies that the proposed failure criterion is an efficient and robust for the prediction of DBFT. In Part 2, “Structural behaviour of imperfect plate subjected to cryogenic spills”, the cryogenic effects on the structural behaviour is examined by carrying out experiments and computational analyses. First, a series of cryogenic spill tests were conducted with imperfect plates. An artificial crack as small as possible was intentionally created in the middle of the plates for the imperfect condition. Liquefied nitrogen was used as a cryogenic material and a spill scenario where the cryogenic fluid formed a horizontal pool in the middle of the steel plate was adopted. The cryogenic spill tests, it was found that the thermally induced stresses were large enough to trigger crack initiation and growth. Further, the weld and heat-affected zone introduced with certainty a reduced fracture resistance. Numerical simulation of the tests was performed using shell elements. The computational study comprised initial heat transfer analysis and subsequent stress analysis. The heat transfer analysis predicted a temperature change that was comparable to the test data. The stress analysis used the temperature-dependent von Mises material model with the SED failure criterion proposed in Part 1. The comparison of experimental and numerical results for stresses, fracture and its path showed satisfactory agreement, thus the numerical study indicates that the SED failure criterion is a useful concept for practical design of ship and offshore structures subjected to sub-zero temperatures, i.e. exposed to the risk of brittle failure.