Structural integrity and damage tolerance of composite t-joints in naval vessels

摘要

In this thesis, the application of composite materials for marine structures and specifically naval vessels has been explored by investigating its damage criticality. The use of composite materials for Mine Counter Measure Vessels (MCMVs) was desirable, especially for producing material characteristics, such as light weight, corrosion resistance, design flexibility due to its anisotropic nature and most importantly stealth capability. The T-Joint structure, as the primary connection between the hull and bulkhead forms the focus of this research. The aim of the research was to determine the methodology to predict the damage criticality of the T-Joint under a pull-off tensile loading using FE (Finite Element) based fracture mechanics theory. The outcome of the research was that the Finite Element (FE) simulations were used in conjunction with fracture mechanics theory to determine the failure mechanism of the T-Joint in the presence of disbonds in the critical location. It enables certain pre-emptive strengthening mechanisms or other preventive solutions to be made since the T-Joint responses can be predicted precisely. This knowledge contributes to the damage tolerance design methodology for ship structures, particularly in the T-Joint design. The results comparison between the VCCT (Virtual Crack Closure Technique) analysis and the experiment results showed that the VCCT is a dependable analytical method to predict the T-Joint failure mechanisms. It was capable of accurately determining the crack initiation and final fracture load. The maximum difference between the VCCT analysis with the experiment results was approximately 25% for the T-Joint with a horizontal disbond. However, the application of the CTE (Crack Tip Element) method for the T-Joint displayed a huge discrepancy compared with the results (fracture toughness) obtained using the VCCT method, because the current T-Join t structure geometry did not meet the Classical Laminate Plate Theory (CLPT) criteria. The minimum fracture toughness difference for both analytical methods was approximately 50%. However, it also has been tested that when the T-Joint structure geometry satisfied the CLPT criteria, the maximum fracture toughness discrepancy between both analytical methods was only approximately 10%. It was later discovered from the Griffith energy principle that the fracture toughness differences between both analytical methods were due to the material compliance difference as both analytical methods used different T-Joint structures.