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The purpose of the development of KC-1 LNG cargo containment system is reduction in royalty and increase in competitiveness of shipbuilding industry. An assessment of structure safety for LNG cargo containment system under sloshing load due to ship motion has become an important design element. The ideal way is to implement fully interaction of the fluid domain and the cargo containment system. However the irreg-ular sloshing pressure were idealized in the form of a triangular wave for safety assessment because the flu-id-structure interaction analysis is taken the extensive computation time and difficult to ensure the accuracy of the results. In this study, the sloshing load was assumed to be a triangular wave with a maximum pressure of 10 bar during 15/1000 seconds. In the analytic results, the basic insulation panel of KC-1 LNG cargo contain-ment system was assessed to be structurally safe for sloshing load.Key words : KC-1 LNG cargo containment system, insulation panel, sloshing, structure safety

This paper describes the structural analysis and optimization of stiffeners used in inner tanks for liquid natural gas (LNG) storage, so that the costs can be minimized while the critical buckling load of the inner tank still exceeds the external pressure exerted by the perlite. The original calculation of perlite pressure applied to the inner tank was based on Zick's code, which led to the overestimation of the external pressure, and consequently, an oversized stiffener. In this study, the effects of the material properties of perlite on the external pressure distribution are scrutinized, and the optimum dimensions of a single stiffener are finally obtained through a series of parametric studies. A 15% decrease in the cost of the stiffener compared with the original design is achieved.

Many tests are required to predict the fatigue life of composite laminates made of various materials and having different layup sequences. Aiming at reducing the number of tests, a methodology was presented in this paper to predict fatigue life of composite laminates based on fatigue life prediction of constituents, i.e. the fiber, matrix and interface, using micromechanics of failure. For matrix, the equivalent stress model which is generally used for isotropic materials was employed to take care of multi-axial fatigue loading. For fiber, a maximum stress model considering only stress along fiber direction was used. The critical plane model was introduced for the interface of the fiber and matrix, but fatigue life prediction was ignored for the interface since the interface fatigue strength was presumed high enough. The modified Goodman equation was utilized to take into account the mean stress effect. To check the validity of the theory, the fatigue life of three different GFRP laminates, UDT[], BX[]S and TX[]S was examined experimentally. The comparison between predictions and test measurements showed good agreement.

A micromechanics-based constituent progressive damage model was proposed in this study to predict macroscopic failure behavior of composite laminates under multi-axial mechanical loadings as well as thermal influences. For this purpose, a micromechanics-based failure theory, named the micromechanics of failure, has been further developed not only to account for the constituent failure but also to progress damage. We first modeled the unit cell of the microstructure of a UD lamina both to derive ply properties from constituent properties and to calculate micro stresses within fiber and matrix from macro-level ply stresses using stress amplification factors. Independent failure criteria dedicated to each constituent are employed to judge failure modes of fiber, matrix, and fiber-matrix interface. Good agreement between theoretical predictions and test data was achieved.

The micromechanics of failure was developed to predict the failure of continuous fiber reinforced composites. A micromechanical approach using unit cells of square and hexagonal arrays was employed to compute the micro stresses of constituents and at the fiber—matrix interface, which were used to determine the failure initiation of a unidirectional ply. The constituent properties include two tensile and compressive strengths of fiber and matrix, plus normal and shear strengths at the interface. The matrix and interfacial dominated strength properties are determined by matching the micro stresses at the constituent levels with the observed transverse tensile and compressive strengths on the macro ply level. The longitudinal shear failure is then expected to be a result of damage progression after initial failure. Based on the current MMF, in the graphite/epoxy considered in this study both transverse tensile and compressive failure are expected to occur via matrix failure. However, in the glass/epoxy the transverse tensile and compressive failures are respectively caused by matrix failure and interfacial tensile failure. These predictions are compared with predictions from other widely used failure criteria as well as experimental data. Lastly, we predicted the failure of laminates. Instead of using a unidirectional ply-based failure theory, starting with the fiber, matrix, and their interface will lead to a much simpler, more generic theory.

Micromechanical approaches are employed to investigate the influence of different fiber arrangement on the mechanical behavior of unidirectional composites (UD) under various loading conditions. A micromechanical model with a random fiber array is generated and used in a finite element analysis together with two frequently used representative volume elements (RVE), or unit cell models of square and hexagonal arrays. The algorithm for generating the random fiber array is verified by comparing the comprehensive performance of a unit cell based on our random array and that of a unit cell based on a real fiber distribution in the UD cross-section. Performance of the random and regular fiber arrays is also evaluated through frequency distributions of stress invariants in matrix and tractions at the fiber—matrix interface due to various loading types. The effects of different loading angles on the overall response of regular arrays to various loading conditions are investigated thoroughly. Finally, the Weibull distribution of the maximum normal interfacial traction in random array is compared with the cumulative probability distribution of transverse strength data acquired from experiment, and good agreement is achieved.

A three-dimensional finite element analysis is performed to investigate the effects of fiber arrangements on the residual thermal stresses in unidirectional composites of various fiber volume fractions (FVFs). The fiber arrangements include the regular fiber arrays (square and hexagonal arrays) and a random fiber array. Normal, tangential, and shear stresses at the fiber–matrix interface are first obtained using unit cells of the regular square and hexagonal fiber arrays. To simulate better real fiber arrangements, random fiber distribution is modeled and analyzed using a finite element analysis. Statistical distributions of residual thermal stresses are obtained for various FVFs and compared with the results from regular fiber arrays. The effects of constituent material properties of composites on thermal stresses are also taken into consideration. The results indicate that the random arrangement of fibers has a significant influence on residual thermal stresses especially at high FVFs. The mode stresses of the random fibers are well estimated using the square array whereas the mean stresses are better predicted from the hexagonal array. It is shown that predicted coefficients of thermal expansion are not influenced by the microstructure of composites.

A multi-scale fatigue life prediction methodology of composite pressure vessels subjected to multi-axial loading has been proposed in this paper. The multi-scale approach starts from the constituents, fiber, matrix and interface, leading to predict behavior of ply, laminates and eventually the composite structures. The life prediction methodology is composed of two steps: macro stress analysis and micro mechanics of failure based on fatigue analysis. In the macro stress analysis, multiaxial fatigue loading acting at laminate is determined from finite element analysis (FEM) of composite pressure vessel, and ply stresses are computed using a classical laminate theory (CLT). The micro-scale stresses are calculated in each constituent (i.e. matrix, interface, and fiber) from ply stresses using a micromechanical model. Micromechanics of failure (MMF) was originally developed to predict the strength of composites and now extended to prediction of fatigue life. Two methods are employed in predicting fatigue life of each constituent, i.e. an equivalent stress method for multi-axially loaded matrix, and a critical plane method for the interface. A modified Goodman diagram is used to take into account the generic mean stresses. Damages from each loading cycle are accumulated using Miner’s rule. Each fiber is assumed to follow a probabilistic failure depending on the length. Using the overall micro and macro models established in this study, Monte Carlo simulation has been performed to predict the overall fatigue life of a composite pressure vessel considering statistical distribution of material properties of each constituent and manufacturing winding helical angle.