The silicone‐epoxy and polyurethane topcoats applied in a simulated sea splash environment were studied according to ISO20340 standard, and their properties were analyzed by differential scanning calorimetry (DSC), scanning electron microscopy (SEM), X‐ray photoelectron spectroscopy (XPS), and electrochemical impedance spectrum (EIS). Glass transition temperature (Tg) of silicone‐epoxy polymer is 7.7 °C higher than that of polyurethane polymer. Measurement of gloss and color difference demonstrates that anti‐ageing property of silicone‐epoxy topcoat is better than that of polyurethane topcoat. It is known from SEM and XPS analysis that silicone‐epoxy topcoat can retain the good morphological structure and ageing resistance after accelerated cycle ageing test. The results from EIS analysis indicate that the silicone‐epoxy topcoat has good anti‐corrosion properties. Measurement of contact angles indicates that surface tension of silicone‐epoxy topcoat is smaller than that of polyurethane topcoat before and after corrosion test.
The improvement of the welding speed becomes extraordinarily difficult for the friction stir welding (FSW) of Al-Li alloys with medium thickness (5-10 mm). In this study, 2195-T8 Al alloy plates with a thickness of 6 mm were subjected to FSW under different welding speeds of 200-800 mm/min with the objective of seeking an optimised welding parameter. The tensile strength of ∼ 422 MPa at a relatively high welding speed of 400 mm/min with a tool rotational speed of 800 rpm was obtained, which reached a comparable level to those of the joints with low welding speeds. The combination of moderate tool rotational speed and moderate welding speed was a feasible optimising direction for FSW of Al-Li alloy with medium thickness.
Polymethyl methacrylate (PMMA) polymer is widely used in various fields today. In order to reveal the structural impact performance of PMMA materials in underwater engineering thoroughly, this paper firstly proposed a simplified plate model for a spherical shell hull under concentrative impact loading. Then, to simulate the hyper-elastic material properties of PMMA in the impact process, the Johnson–Cook constitutive model and damage failure model were adopted. And the least squares method was used to confirm accurately the J–C constitutive and damage failure model parameters of PMMA through material test data. Moreover, the dynamic process of the steel bullet impacting the PMMA plate structure was analyzed by the finite element software ABAQUS. The calculation results show that the numerical simulation results in this paper have a good convergence, and the residual velocities at different initial velocities and thicknesses of plates are in good agreement with the experimental test data. Therefore, the feasibility and accuracy of the impact analysis of PMMA structures based on J–C constitutive and damage failure models in this paper are verified accordingly. Finally, based on the presented finite element model, the structure response and the variation of residual velocity of the bullet with the PMMA plate thickness was analyzed in depth; that is, the results show that the residual velocity of the bullet has a certain linear relationship with the thickness, even in an underwater environment, and even in an underwater environment will increase both with a thicker structure or a higher pressure.
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