Polymeric composites exhibit highly nonlinear and rate dependent behavior during loading and unloading in off-axis directions. In this study, off-axis specimens were used to study this behavior in AS4/PEEK thermoplastic composite. The equilibrium states of stress during loading and unloading were determined by stress–relaxation and stress–recovery tests, respectively. It was found that the equilibrium stress–strain curves for loading and unloading are basically the same for the same loading–unloading rate. A one-parameter plasticity model was used to generate a master equilibrium plastic stress–strain curve for both loading and unloading. Stress relaxation curves and recovery curves were obtained during loading and unloading cycles, respectively, for various strain rates. Similarity between relaxation and recovery curves was noted, and the effective stress defined from the one-parameter plastic potential function was able to describe both curves.

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It is useful to have a quick analysis program in early design process for feasibility studies of composite cylinder because it takes long time and is not cost effective by commercial programs. In this paper, a GUI program is developed to calculate the stress distribution in a fast manner with the properties, the orientation angle and the stacking sequence of composite material using LabVIEW. The stress distributions of an autofrettaged cylinder and a composite cylinder with internal pressure are compared with the results by MSC Nastran/patran. The stress distribution of steel/composite cylinder is compared with the values of existing studies, and is proved. Furthermore, by calculating the stress distribution of an autofrettaged steel/composite cylinder, the stress distribution is estimated, and the program will be useful in an early design phase for feasibility studies.

This paper describes the design of dual-structured and thick-walled cylinder made of composites and special steel. The structure of special steel and composites reduce the weight of a product maintaining its property which is proper to the characteristics. Hence they are used in the fields, such as various ground weapons, aerospace and sports industries, where high elasticity and low weight are required. Thus in this study, the analysis was conducted to find the most proper composite application method changing its types and angle of laminates for the design. Through the comparison of the results, we suggest the method for composite application which is the most appropriate to the designing purpose of this study.