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Thick fabric composite cylinders for nozzle parts in solid rocket motors should be designed to endure the extreme temperature and pressure of combustion gas. As the thickness of the composite cylinder increases, fabricational residual stresses due to the anisotropic thermal expansion or shrinkage of fabric composites also increase, which induces inter-laminar failures. Therefore, the accurate estimation of the residual stresses is indispensable for the development of thick fabric composite cylinders. In this paper, the residual stresses in thick cylinders made of carbon fabric phenolic composites were measured by a new radial-cut-cylinder-bending method. To obtain the residual stresses from the measured relative strains during the radial-cut operation, a bending test of the cylinder with the radial-cut was performed instead of measuring the material properties with respect to radial positions. The thermal residual stresses were also calculated by finite element method considering shear deformation of fabric layers, and compared with the measured residual stresses by the new method, from which it was found that the new simple method estimated the residual stresses pretty well. Also the inter-laminar tensile strength at the position of maximum radial residual stress could be obtained from the bending test.

Carbon/phenolic composites are used in the nozzle parts of solid rocket motors due to their heat-resisting, ablative, and high strength characteristics, which are required to endure the high temperature and pressure of combustion gas passing through the nozzle. But the thick axi-symmetric structure of the composite nozzle induces high thermal residual stresses due to the large difference of coefficient of thermal expansion (CTE) between the in-plane and the out-of-plane. In this work, in order to reduce the through-thickness CTE and the void content, a compression in the thickness direction was applied to the composite prepreg by a compressive jig during manufacturing of composite to supplement the low autoclave pressure. The through-thickness CTE of the fabric composite was calculated by a compaction model and compared with the measured one by thermo-mechanical analysis. The through-thickness CTE changed drastically with respect to the compaction amount, and the void content of the carbon/phenolic fabric composite laminate showed different characteristics from the ordinary fabric laminates with respect to the autoclave pressure and the jig pressure.

Mechanical design can be classified into stiffness design and strength design. In the stiffness design, the stiffness or deformation of members is concerned, and the enhancement of dynamic characteristics such as natural frequency or damping capacity of members or systems is also important. While, in the strength design, the primary concern is the enhancement of load carrying ability of members or systems. Fiber reinforced composite materials offer a combination of strength and modulus that are either comparable to or better than many traditional metallic materials. Because of their low specific gravities, the strength-weight ratios, and modulus-weight ratios of these composite materials are much superior to those metallic materials. Composite materials can be tailored to meet the specific requirements of each particular design. Available design parameters are the choice of materials (fiber, matrix), the volume fraction of fiber and matrix, fabrication method, number of layers in a given direction, thickness of individual layers, type of layer (unidirectional or fabric), and the layer stacking sequence. The greatest disadvantages of composite materials are the costs of the materials and the lack of well-defined design rules, therefore, composite materials should be applied in the right place with appropriate design rules. Up to now, the fiber reinforced composite structures are mainly employed in the strength design such as aircraft, spacecraft and vehicles. In this paper, the novel application examples of composite structures to components for the robots, machine tools and automobiles are addressed considering the stiffness design issues of composite structures.

Substituting composite structures for conventional metallic structures has many advantages because of higher specific stiffness and higher specific strength of composite materials. In this work, one-piece automotive hybrid aluminum/composite drive shaft was developed with a new manufacturing method, in which a carbon fiber epoxy composite layer was co-cured on the inner surface of an aluminum tube rather than wrapping on the outer surface to prevent the composite layer from being damaged by external impact and absorption of moisture. The optimal stacking sequence of the composite layer was determined considering the thermal residual stresses of interface between the aluminum tube and the composite layer calculated by finite element analysis. Press fitting method for the joining of the aluminum/composite tube and steel yokes was devised to improve reliability and to reduce manufacturing cost, compared to other joining methods such as adhesively bonded, bolted or riveted and welded joints. Protrusion shapes on the inner surface of steel yoke were created to increase the torque capability of the press fitted joint. From experimental results, it was found that the developed one-piece automotive hybrid aluminum/composite drive shaft had 75% mass reduction, 160% increase in torque capability compared with a conventional two-piece steel drive shaft. It also had 9390 rpm of natural frequency which was higher than the design specification of 9200 rpm.