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A simple method for the rapid assessment of ship bottom structures subjected to grounding over seabed obstructions with large contact surfaces is proposed in this paper. It has been recognised that the shape and size of the seabed obstruction is of crucial importance in relation to the characteristics of bottom damage. Most studies on ship grounding are concerned with 'rock-type' sharp obstructions, where plate tearing is the dominating failure mode. However, very few studies are found related to grounding over blunt obstructions with large contact surfaces such as 'shoals'. Denting rather than tearing is more likely for the bottom plating as observed from actual grounding incidents. The sharp obstruction may cause earlier penetration and result in very unfavourable consequences such as compartment flooding. In contrast, the bottom plating may not fracture when moving over a blunt-type sea floor. But it may threaten the global hull girder resistance and give rise to even worse consequences such as hull collapse. The proposed simple method is established on the basis of a series of closed-form solutions for individual structural members developed from the plastic mechanism analysis. The primary deformation modes for the major bottom structural members are sliding deformation of longitudinal girders, denting and crushing of transverse members and indentation of bottom plating. The effect of friction is considered and estimated in a simple manner. The vertical resistance that governs the vertical ship motion is derived. It is found that the vertical resistance is free of friction. The proposed simple method for bottom strength is verified against large-scale non-linear finite element analyses, where a good correlation is obtained.

Anisotropic cyclic stress-strain curves (CSSCs) and fatigue lives were obtained from fully reversed fatigue tests in strain control on two orientations of an Al-6061-T6 plate (along the rolling direction and the transverse direction). The experiments were conducted at room temperature in air under three loading conditions: tension/compression, torsion, and combined tension/torsion in-phase. Based on the CSSCs data, the anisotropic constitutive relations of the material were obtained by using Hill's anisotropic plasticity theory. Yield loci and flow behavior were determined and compared to theoretical predictions. Two anisotropic effective stress-effective strain criteria were evaluated. During the fatigue tests, fatigue cracking behavior was observed. The damage mechanism of the material was shear dominated. Four multiaxial fatigue life prediction models representing three different concepts were used to correlate the anisotropic fatigue life data of the plate under the biaxial loadings. A new shear cracking model incorporated with material anisotropy coefficients is proposed and correlated with the fatigue life data very well.

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